Overview of Power Electronics Devices

POWER ELECTRONICS MODULE I Linear V/S Power Electronics, Power Electronic Devices– Power DIODE – Power BJT– Power MOSFET-– IGBT (Insulated Gate Bipolar transistor) – GTO (Gate Turnoff Thyristor), SCR Features of power electronics devices Power Electronics is the art of converting electrical energy from one form to another in an efficient, clean, compact, and robust manner for convenient utilisation. • With “classical” electronics, electrical currents and voltage are used to carry information, whereas with power electronics, they carry power • The electric power that power electronics devices deals with is usually much larger than that the normal electronic devices does In Power Electronics all devices are operated in the switching mode - either 'FULLY-ON' or 'FULLY-OFF' states. LINEAR ELECTRONICS • In linear Electronics devices are operated in ACTIVE region. • Since the device is ON for the entire cycle, Power loss will be more • Efficiency will be low • Dealing with circuits having power rating from few milli watts to few watts POWER ELECTRONICS • In Power Electronics devices are operated in SATURATION and CUT OFF region. • Since the device is not ON for the entire cycle Power loss will be less • Efficiency will be more • Dealing with circuits having power rating from few milli watts to Mega watts POWER DIODE A power diode is a type of diode that is commonly used in power electronics circuits. The power diode is a simple semiconductor device that includes three layers, two terminals , a single junction & conducts current in one direction. In order to increase the power handling capacity we use power diode by small changes in the structure of its low power counterparts (Signal Diodes). Symbol Fig 1.2 : Symbol 1 Power Diode Power diode having three layers the P+ layer, n– layer and n+ layer. The upper layer is the P+ layer, it is heavily doped and acts as an anode, the thickness of this layer is 10 μm & the doping level is 1019 cm-3. The lower layer is n+ layer, it is also heavily doped and acts as a cathode, the thickness of this layer is 250-300 μm & the doping level is 1019 cm-3. The middle layer is n– layer, it is lightly doped. It acts as a drift layer, the doping level is 1014 cm-3 and the thickness of this layer mainly depends up on the breakdown voltage. The heavily doped upper and lower (P+ layer and n+ layer) is called terminal layers and terminal layers is always heavily doped to increase the conductivity of the device. Once drift layer width increases then breakdown voltage will be increased. Due to its light doping concentration of drift layer with increased thickness (the thickness of the depletion region increases with a decrease in doping concentration) helps the diode to block larger reverse- biased voltage and hence have a greater breakdown voltage. VI characteristics of power diode The V-I Characteristics of Power Diode is shown in figure it is just similar to signal diode. When we increase applied forward voltage the forward current increases linearly. In forward biased condition when anode is positive w.r.t. cathode the forward current increase linearly with an increase in forward voltage. 2 Reverse recovery time of power diode If a diode is initially driven in forward bias, and the polarity suddenly switches to reverse bias, the diode will still remain conducting for some time. The time required for conduction to settle into the reverse bias state is the diode's reverse recovery time. Reverse recovery time (trr) : After the forward diode current decays to zero, the continue to conduct in the reverse direction because of the presence of the stored charges in the two layers . The reverse flow for a time called reverse recovery time (trr). The diode retains its blocking capability until reverse recovery current decays to zero. The reverse recovery time is defined as the time between the instance forward diode current become zero and the instant reverse recovery current decays to 25% of its reverse peak value Irm. The reverse recovery time is the combination of two segment of time ta and tb i.e. trr = ta + tb where time ta is the time between zero crossing of forward current and peak reverse current Irm. During the time ta, charged stored in depletion region is removed. Time tb is measured from the instant of Irm to the instant where 0.25 Irm is reached. During tb charge in two semiconductor area is removed. Softness factor or S-factor :- The ratio of ta/tb is called softness factor or S-factor. The diode with softness equal to one is called soft recovery diode and diode with softness factor less than one is called snappy recovery diode or fast recovery diode. Where • Time ta : Charges stored in the depletion layer removed. • Time tb : Charges from the semiconductor layer is removed. S-factor: measure of the voltage transient that occurs during the time the diode recovers. S-factor = 1 ⇒ low oscillatory reverse-recovery process. (Soft –recovery diode) S-factor <1 ⇒ large oscillatory over voltage (snappy-recovery diode or fast-recovery diode). Power diodes now exist with forward current rating of 1A to several thousand amperes with reverse recovery voltage ratings of 50V to 5000V or more. Types of Power Diodes Standard Diodes or General Purpose Diodes: Standard or general purpose diodes have a comparatively high reverse recovery time, when compared to other diodes. Due to this reason they are used in applications which are not time sensitive and generally run on low speeds. Usually the reverse recovery time for general purpose diodes varies between 20 micro seconds to 30 micro seconds which is quite a lot. Typical low speed 3 applications for general purpose diodes include the power diode being used as a rectifier or in a converter, where the frequency input is quite low. Fast Recovery Diodes: As their name suggests, these are the type of power diodes which have a relatively faster reverse recovery time, which usually varies from 2 micro seconds to 5 micro seconds. With such a fast recovery time, they can be easily used in high speed switching applications where the time is of great importance. Due to their property of fast reverse recovery, they are also comparatively expensive as compared to the general purpose diodes. Schottky Diodes: Schottky Diode: It has an aluminum-silicon junction where the silicon is an n-type. As the metal has no holes, there is no stored charge and no reverse-recovery time. Therefore, there is only the movement of the majority carriers (electrons) and the turn-off delay caused by recombination process is avoided. It can also switch off much faster than a p-n junction diode. As compared to the p-n junction diode it has: (a) Lower cut-in voltage (b) Higher reverse leakage current (c) Higher operating frequency Application: high-frequency instrumentation and switching power supplies. Advantages and Disadvantages of Power Diode The advantages and disadvantages of power diode include the following. • The PN-junction region of this diode is large & can supply huge current, however, the capacitance of this junction can also be large, which works at a lower frequency & it is generally used for rectification only. • It will resolve AC at high current and a high voltage. • The main disadvantage is its size & probably needs to be fixed to a heat sink while conducting a high current. • It needs specialized hardware for installing and insulating from the metal frames which are available in the surrounding. Applications :- The applications of power diode include the following. • This diode provides uncontrolled power rectification • It is used in different applications like DC power supplies, for charging the battery, inverters and AC rectifiers. • These are used like snubber networks and free-wheeling diodes due to their characteristics like voltage & high-current. • These diodes are used as feedback, freewheeling diodes, and high-voltage rectifier. POWER Transistors Power transistors are three terminal devices which are composed of semiconductor materials. it can handle large voltage and current. The structure and construction of a power transistor is entirely different from that of a signal transistor but their characteristics and operation in almost same. Power transistors are available in different types with different power and switching speed ratings. 4 Types of Power transistor Power transistors are classified into the following types: 1. Bipolar Junction Transistors (BJTs) 2. Metal Oxide Semiconductor Field-Effect Transistor (MOSFETs) 3. Insulated Gate Bipolar Transistor (IGBTs) BIPOLAR JUNCTION TRANSISTORS (BJTS) Bipolar Junction Transistors can either be used as an amplifier or switch. These are mainly used to control the current, the process of controlling includes the act of amplification, switch-on, and switch-off. Power BJT has 3 terminals — collector, base and emitter. It is a current controlled device. It is the base current that controls the device. Bipolar Junction Transistors are of two types NPN transistor and PNP transistor. Symbol of Power BJT (a) NPN BJT (b) PNP BJT Power BJT NPN Structure The construction of the Power Transistor is different from the signal transistor as shown in the figure. It has three terminals labelled as Collector, Base, and Emitter. A Power BJT has a vertically oriented four layer structure of alternating P and N type doping as shown in above NPN transistor. This is maximising the cross-section area results in current rating of BJT, minimize the on-state resistance, and thus reduce the power losses The doping of emitter layer and collector layer is quite large typically 1019 cm-3. A thin p-layer with doping level of 1016 cm 3is sandwiched between two n-layers as shown in figure. A special layer called the collector drift region (n-) has a light doping level of 1014 cm-3. The thickness of the drift region determines the breakdown voltage of the transistor. The n layer is added in the power BJT which is known as drift region. The characteristics of the device are determined by the doping level in each of the layers and the thickness of the layers. The base thickness is made as small as possible in order to have good 5 amplification capabilities, however if the base thickness is small the breakdown voltage capability of the transistor is compromised. V-I Output Characteristics of a Power Transistor • The below graph represents various regions like the cut-off region, active region, hard saturation region, quasi saturation region. • For different values of VBE, there are different current values IB0, IB1, IB2, IB3, IB4, IB5, IB6. • Whenever there is no current flow, it means the transistor is off. But few current flows which are ICEO. • For increased value of IB = 0, 1,2, 3, 4, 5. Where IB0 is the minimum value and IB6 is the maximum value. When VCE increases ICE also increases slightly. Where IC = ßIB, hence the device is known as a current control device. Which means the device is in active region, which exists for a particular period. • Once the IC has reached to maximum the transistor switches to the saturation region. • Where it has two saturation regions quasi saturation region and hard saturation region. • A transistor is said to be in a quasi saturation region if and only if the switching speed from on to off or off to on is fast. This type of saturation is observed in the medium-frequency application. • Whereas in a hard saturation region the transistor requires a certain amount of time to switch from on to off or off to on state. This type of saturation is observed in the low frequency applications. Switching characteristics of power BJT Switching characteristics of power BJT is shown in Figure. Due to internal capacitances, the transistor does not turn on instantly. VB rises from zero to V1 and the base current rises to IB1, the collector current does not respond immediately. The delay is due to the time required to charge up the base– emitter junction capacitance to the forward bias voltage VBE(0.7V). This time is known as the delay time(td) . For t > td, collector current starts rising and VCE starts to drop with the magnitude of 9/10th of its peak value and the collector current rises to the steady value of ICS and this time is called rise time, tr, required to turn on the transistor. The transistor remains on so long as the collector current is at least of this value. When the input voltage is reversed from V1 to -V2, the reverse current –IB2 helps to discharge the base. The base current required during the steady-state operation is more than that required to 6 saturate the transistor. Thus, excess minority carrier charges are stored in the base region which needs to be removed during the turn-off process. The time required to nullify this charge is the storage time, ts. Without –IB2 the saturating charge has to be removed entirely due to recombination and the storage time ts would be longer. Collector current remains at the same value for this time. After this, collector current starts decreasing and base-to-emitter junction charges to the negative polarity; base current also get reduced. Once the extra charge is removed, base–emitter junction charges to the input voltage –V2 and the base current falls to zero. That time is called fall time, tf . tf depends on the time constant which is determined by the reverse biased base–emitter junction capacitance Turn-On and Turn-Off Characteristics of BJT ADVANTAGES OF BJT’S • BJT’s have high switching frequencies since their turn-on and turn-off time is low. • The turn-on losses of a BJT are small. • BJT has controlled turn-on and turn-off characteristics since base drive control is possible. • BJT does not require commutation circuits. DEMERITS OF BJT • Drive circuit of BJT is complex. • It has the problem of charge storage which sets a limit on switching frequencies. • It cannot be used in parallel operation due to problems of negative temperature coefficient Power Bipolar Junction Transistors has the following characteristics: • Bipolar Junction Transistors are large in size and hence allow maximum current to flow. • Bipolar Junction Transistors have high breakdown voltage. • BJTs have high handling capability as well as current carrying capacity. • Mainly seen in high power applications. APPLICATIONS OF POWER BJT 7 • SMPS(Switch mode power supply) commonly used in computers. • Final audio amplifier in stereo systems. • Power amplifiers. • DC to AC inverters. • Relay and display drivers. • AC motor speed controllers. • Power control circuits. MOSFET- Metal Oxide Semiconductor Field-Effect Transistor MOSFET which handles high levels of power is known as Power MOSFET. As compared to normal MOSFETs in the less voltage range, these MOSFETS works much better by exhibiting high speed of switching. Its operating principle is the same as general MOSFETs. The power MOSFET frequency is high like up to 100 kiloHertz. The power MOSFET symbol is shown below. Power MOSFET Symbols Operating Principle Similar to normal MOSFETs, these types of MOSFETs will switch & control the flow of current in between the two terminals like source & drain through changing the voltage on the gate terminal. Once the voltage is applied to the gate terminal, then a channel can be formed in between the source & the gate terminals which allows the flow of current. 8 Initially there is no path for any current to flow between the source and the drain terminals. However, application of a positive voltage at the gate terminal with respect to the source will convert the silicon surface beneath the gate oxide into an n type layer or “channel”, thus connecting the Source to the Drain. The operation of MOSFET divides into two parts formation of the depletion layer and Creation of Inversion Layer The gate region of a MOSFET which is composed of the gate metallization, the gate (silicon) oxide layer and the p-body silicon forms a high quality capacitor. When a small voltage is application to this capacitor structure with gate terminal positive with respect to the source (note that body and source are shorted) a depletion region forms at the interface between the . SiO2and the silicon Further increase in VGS causes the depletion layer to grow in thickness. MOSFET Switching characteristics: Fig 3.12 Switching characteristics of power MOSFET The switching characteristics of a power MOSFET are influenced to a large extent by the internal capacitance of the device and the internal impedance of the gate drive circuit. Switching waveforms for a power MOSFET are shown in Fig . At turn-on, there is an initial delay tdn during which input capacitance charges to gate threshold voltage VGST . Here tdn is called turn-on delay time. There is further delay tr , called rise time, during which gate voltage rises 9 to VGSP , a voltage sufficient to drive the MOSFET into on state. During tr , drain current rises from zero to full on current ID . Thus, the total turn-on time is ton=tdn+tr . The turn-on time can be reduced by using low-impedance gate drive source. As MOSFET is a majority carrier device, turn-off process is initiated soon after removal of gate voltage at time t1 . The turn-off delay time, tdf is the time during which input capacitance discharges from overdrive gate voltage V1 to VGSP . The fall time, tf is the time during which input capacitance discharges from VGSP to threshold voltage. During tf, drain current falls from ID to zero. toff=tdf+tf . So when VGS≤VGST, MOSFET turn-off is complete. Advantages of a power MOSFET 1. The GATE driving circuit of MOSFET is simple. 2. It can operate at the high switching frequency. 3. It has better thermal stability because the temperature coefficient of MOSFET is positive. 4. Easy to turn ON and OFF. 5. On-state resistance is low. 6. The second breakdown does not take place. Demerits of a power MOSFET 1. The on-state voltage across the MOSFET is very high. So, on-state power dissipation is high. 2. It has the asymmetric blocking capacity. They can block a high forward voltage but they cannotblock high reverse voltage. Therefore, we need to connect a diode to protect the MOSFET. Application of Power MOSFET 1. Uninterrupted Power Supplies (UPS) 2. Switch Mode Power Supplies (SMPS) 3. High-frequency inverter 4. Motor control application 5. Display driver 6. In power amplifier 7. Industrial applications 8. Relay driver 10 IGBT- Insulated Gate Bipolar Transistor The IGBT or Insulated Gate Bipolar Transistor is the combination of BJT and MOSFET. Its name also implies the fusion between them. “Insulated Gate” refers to the input part of MOSFET having very high input impedance. It does not draw any input current rather it operates on the voltage at its gate terminal. “Bipolar” refers to the output part of the BJT having bipolar nature where the current flow is due to both types of charge carriers. It allows it to handle very large currents and voltages using small voltage signals. This hybrid combination makes the IGBT a voltage-controlled device. It is a four-layer PNPN device having three PN junctions. It has three terminals Gate (G), Collector(C) and Emitter (E). The terminal’s name also implies being taken from both transistors. Gate terminal as it is the input part, taken from MOSFET while the collector and emitter as they are the output, taken from the BJT. Construction of IGBT IGBT is made of four layers of semiconductor to form a PNPN structure. The collector (C) electrode is attached to P layer while the emitter (E) is attached between the P and N layers. A P+ substrate is used for the construction of IGBT. An N- layer is placed on top of it to form PN junction J1. Two P regions are fabricated on top of N- layer to form PN junction J2. The P region is designed in such a way to leave a path in the middle for the gate (G) electrode. N+ regions are diffused over the P region as shown in the figure. The emitter and gate are metal electrodes. The emitter is directly attached to the N+ region while the gate is insulated using a silicon dioxide layer. Working of IGBT (https://www.youtube.com/watch?v=h9qMQFB8_iQ) The two terminals of IGBT collector (C) and emitter (E) are used for the conduction of current while the gate (G) is used for controlling the IGBT. Its working is based on the biasing between Gate-Emitter terminals and Collector-Emitter terminals. 11 The collector-emitter is connected to Vcc such that the collector is kept at a positive voltage than the emitter. The junction j1 becomes forward biased and j2 becomes reverse biased. At this point, there is no voltage at the gate. Due to reverse j2, the IGBT remains switched off and no current will flow between collector and emitter. Applying a gate voltage VG positive than the emitter, negative charges will accumulate right beneath the SiO2- layer due to capacitance. Increasing the VG increases the number of charges which eventually form a layer when the VG exceeds the threshold voltage, in the upper P-region. This layer form N-channel that shorts N- drift region and N+ region. The electrons from the emitter flow from N+ region into N- drift region. While the holes from the collector are injected from the P+ region into the N- drift region. Due to the excess of both electrons and holes in the drift region, its conductivity increase and starts the conduction of current. Hence the IGBT switches ON. V-I Characteristics of IGBT Unlike BJT, IGBT is a voltage-controlled device that requires only a small voltage at its gate to control the collector current. However, the gate-emitter voltage VGE needs to be greater than the threshold voltage. Transfer characteristics of the IGBT show the relation of input voltage VGE to output collector current IC. When the VGE is 0v, there is no IC and the device remains switched off. When the VGE is slightly increased but remains below threshold voltage VGET, the device remains switched off but there is a leakage current. When the VGE exceeds the threshold limit, the I-C starts to increase and the device switches ON. Since it is a unidirectional device, the current only flows in one direction. 12 Switching Characteristics of IGBT Switching Characteristics of IGBT is basically the graphical representation of behavior of IGBT during its turn-on & turn-off process. The turn-on time is defined as the time between the instant of forward blocking to forward conduction mode. Here, forward conduction means the device conducts in forward direction. Turn-on time (ton) is basically composed of two different times: Delay time (tdn) and Rise time (tr). Therefore, we can say that ton = tdn + tr. The delay time is defined as the time for the collector-emitter voltage (VCE) to fall from VCE to 0.9VCE. This simply means that, the collector-emitter voltage drops to 90% in delay time and hence the collector current rises from initial leakage current to 0.1IC (10%). Thus, delay time may also be defined as the time period during which collector current rises from zero (in fact a small leakage current) to 10% of the final value of collector current IC. The rise time tr is the time during which collector-emitter voltage falls from 0.9VCE to 0.1 VCE. This means, during rise time collector-emitter voltage falls to 10% from 90%. Therefore, the collector current builds up to final value of collector current IC from 10%. After time ton, the collector current becomes IC and the collector-emitter voltage drops to very small value called conduction drop (VCES). A typical Switching Characteristics of an IGBT is shown below. You may corelate the delay time, rise time and turn-on time. 13 General Comparison with BJT and MOSFET As we have discussed above, IGBT takes the best parts of both BJT and MOSFET. Therefore, it is superior in almost every way. Here is a chart of some of the characteristics showing the comparison between IGBT, BJT and MOSFET. we are comparing power devices in their max capabilities. Characteristic Power BJT Power MOSFET IGBT Voltage Rating High < 1kV High < 1kV Very High > 1kV Current Rating High < 500 A Low < 200 A Very High > 500 A Input Parameter Base Current, Ib Voltage, VGS Voltage, VGE Input Drive Current gain (hfe) 20-200 Voltage, VGS 3-10V Voltage, VGE 4-8V Input Drive Power High Low Low Input Drive Circuitry Complex Simple Simples Input Impedance Low High High Output Impedance Low Medium Low Switching Loss High Low Medium Switching Speed Low Fast Medium Cost Low Medium High IGBT as a whole has the advantages of both BJT and MOSFET. • It has higher voltage and current handling capabilities. • It has a very high input impedance. • It can switch very high currents using very low voltage. • It is voltage-controlled i.e. it has no input current and low input losses. • The gate drive circuitry is simple and cheap. • It can be easily switched ON by applying positive voltage and OFF by applying zero or slightly negative voltage. • It has very low ON-state resistance • It has a high current density, enabling it to have a smaller chip size. • It has a higher power gain than both BJT and MOSFET. • It has a higher switching speed than BJT. Disadvantages It has a lower switching speed than MOSFET. • It is unidirectional it cannot conduct in reverse. • It cannot block higher reverse voltage. • It is costlier than BJT and MOSFET. • It has latching problems due to the PNPN structure resembling thyristor. • Applications of IGBT • IGBTs have numerous applications used in AC as well as DC circuits. Here are some of the important 14 Applications of IGBT • It is used in SMPS (Switched Mode Power Supply) to supply power to sensitive medical equipment and computers. • It is used in UPS (Uninterruptible Power Supply) system. • It is used in AC and DC motor drives offering speed control. • It is used in chopper and inverters. • It is used in solar inverters. SCR - Silicon-controlled rectifier SCR is a four layer 3 junction p-n-p-n semiconductor device consisting of at least three p-n junctions, functioning as an electrical switch for high power operations. It has three basic terminals, namely the anode, cathode and the gate mounted on the semiconductor layers of the device. SCR Symbol The SCR symbol is very similar to the diode symbol, but it has an additional gate terminal. Fig : SCR Symbol SCR Construction SCR has four layers of extrinsic semiconductor materials. These four-layer form three PN junctions named J1, J2, and J3. The layers are either NPNP or PNPN. The anode and cathode terminals are placed at the end layers and where the gate terminal is placed with the third layer. The outer layers are heavily dopped and the inner two layers are lightly dopped. 15 Working and V-I Characteristics of SCR SCR has three basic mode of operation: Reverse Blocking Mode, Forward Blocking Mode and Forward Conduction Mode Reverse Blocking Mode :- If the anode terminal of the SCR connects to the negative and cathode terminal of SCR connects to the positive of battery terminals. The SCR is in reverse blocking mode. In this mode, J1 and J3 junctions are reverse biased. Where the middle junctionJ2 is forward bias. As two junctions are reverse bias, so there is no current flowing through it butonly a small leakage current due to the drift charge carrier. An SCR in reverse blocking mode behaves as if an open switch. Hence this mode is also known as OFF state of SCR. A small leakage current of the order of mili or micro ampere flows thorough the SCR in this mode. If the reverse voltage isincreased, then atsome critical voltage an avalanchebreakdown takes place at reverse biased junctions J1 and J3 which leads to sudden increase in reverse current. This critical reverse voltage is called Reverse Breakdown Voltage. VBR represents this reverse breakdown voltage in the V-I characteristics. It can be seen that, there is a sharp increase in reverse current at this voltage. This increased reverse current may result in more losses in the SCR which in turn may damage the SCR. Therefore the reverse voltage acrossthe SCR terminals should not exceed reverse breakdown voltage during its operation. Forward Blocking Mode :- Forward Blocking Mode is that operational mode of SCR in which it does not conduct even though it is forward biased. The term forward biased SCR implies that its anode terminal is positive with respect to cathode terminal with gate switch is open. In this mode, the junction J1 and J3 are forward biased but junction J2 is reverse biased. A small leakage current, called the forward leakage current, flows as shown by OM in the V-I characteristics of SCR in this mode. As the forward leakage current is small, SCR offers high impedance. Therefore an SCR can be treated as an open switch even in forward blocking mode. Forward Conduction Mode :- As we have seen that in Forward Blocking mode, even though the SCR is forward biased, it does not conduct. But the good thing is that, in forward blocking mode, junction J1 and J3 are forward biased and J2 is reversed biased. This means, there are two possibilities for making SCR to conduct in this mode: • Increase the anode to cathode voltage to such an extent which leads to avalanche breakdown of the reverse biased junction J2. • Apply positive gate pulse between gate and cathode terminal. When the forward biasing voltage is increased then at some critical voltage VBO, an avalanche breakdown take place at reverse biased junction J2. This critical voltage is knownas Forward Break over Voltage. Since junction J1 and J3 are already forward biased, an avalanche breakdown at J2 will result in sudden increase in anode current in forward direction. Hence the point M at V-I characteristics of SCR corresponding to Forward Break over Voltage VBO will at once shift to point N and then to any point in between N and K. Since the anode current in this mode will only be limited by the load, so based on the value of load the anode current will change and may lie at any point in between N 16 and K. Thus NK represents the forward conduction of SCR. Fig : V-I Characteristics of SCR The SCR can be turned ON at smaller applied voltage by the application of a small positive voltage at the gate terminal. When gate voltage is applied the junction J3 is forward biased and junction J2 is reverse biased. Thus, the electrons from n – type layer starts moving across the junction J3 toward p –type material and the holes from p –type material towards the n – type material. Due to the movement of holes and electrons across the junction J3 the gate current starts flowing. Because of gate current the anode current increases. The increased anode current makes the more electrons available at the junction J2. As a result of this process, in a small time, the junction J2 breaks down and the SCR is turn ON.A higher gate current can put SCR faster in the forwardconduction mode as in the graph Ig3>Ig2>Ig1. The SCR will remain in the forward conduction mode if the anode current is above the holding current. Latching current and holding current are both minimum currents required to maintain a thyristor in an on or off state: Latching current: The minimum anode current required to keep a thyristor on after it's been turned on and the gate signal is removed. Holding current: The minimum anode current required to keep a thyristor on. The latching current is always greater than the holding current: Explanation: The thyristor turns on when the current is greater than the latching current. Once the thyristor is on, the latching current is no longer relevant. The holding current is the minimum current that must flow through the thyristor to keep it on. SCR Applications The most common SCR application is the DC motor speed control. The DC motor has two 17 windings, where the armature winding is connected to the AC supply by two SCR. The SCRcontrols the amount of current to the motor and ultimately the speed of the DC motor. SCR working with two transistor model Woking of the SCR can be easily explained by two transistor model of SCR. As per figure you can see with supply voltage V and load resistance R is applied to SCR. Here first Assume the supply voltage V is less than break over voltage as is usually the case. When the gate is open (i.e. switch S open), there is base current Ib=0. For the base of the T2 is connected with the collector of The T1. Therefore, no current flows in the collector of T2 and hence that of T1. So for this condition, SCR is in OFF condition. Whenever switch S is closed, a small gate current will flow through the base of T2 which means its collector current will increase. The collector of the transistor T2 is connected with transistor T1. So, the collector current of T2 is the base current of T1. Therefore, the collector current of T1 increases. But collector current of T1 is the base current of T2. This action is accumulative since an increase of current in one transistor causes an increase of current in the other transistor. As a result of this action, both transistors are driven to saturation, and heavy current flows through the load RL. Under such conditions, the SCR closes. Switching characteristics during turn-on In the above section, we have discussed that a forward-biased thyristor conducts in the presence of positive gate potential at the gate cathode terminal. But you must keep in mind that there exists a transition time within the device between which the device transits from forward off state to forward on state. This time span is generally regarded as thyristor turn-on time and is classified into three intervals i.e., delay time denoted by td, rise time denoted by tr, and spread time, denoted by ts. Now, let us understand each one separately, Delay time (td): The delay time of the thyristor depends on the variation in the gate current in forward blocking mode as well as the temperature as it varies the crystal structure of the semiconductor. Delay time is defined as the time instant between the moment when gate current attains 90% of its final value while the anode current attains 10% of its initial value. During delay time, the thyristor remains in the forward blocking mode. Rise time (tr): Rise time is defined as the time taken by the anode current to rise from 10% of its value to 90% of the same. We have discussed in delay time that it corresponds to the time when anode current reaches 10% of its initial value. Thus, the rise time is started once the delay time gets over. Spread time (ts): Spread time of the thyristor exists between the time when the anode current approaches 100% from its 90% value. More simply, we can say, spread time is the time within which 18 the anode current rises from 90% to 100%. This is called so because, during the spread time, the conduction gets spread over the complete cathode region of the thyristor. Once the spread time is completed, the value of the anode current reaches a steady state Switching Characteristics during turn-off Turning the thyristor off means that the state of the thyristor is changed from on to off and it can be able to block forward voltage. We have discussed that, once the device gets on then even after the removal of the gate signal, current continues to flow through the device due to the fact that the charge carriers in the four layers favors conduction. To turn it off, a reverse potential must be necessarily applied for a specific time interval when anode current attains 0 value. So, the turn-off time of the thyristor is defined as the time being between the instants when anode current becomes 0 and the thyristor attains its forward blocking ability. This turn-off time corresponds to the duration within which all the excess carriers must be removed from the layers of the thyristor. This is nothing but the sweeping out of holes from the outer p region and holes from the outer n region. The turn of time of the thyristor is categorized into two intervals, namely • reverse recovery time trr, • gate recovery time tgr During the reverse recovery time, the anode current will flow in the reverse direction. Due to this reverse current charged carriers get removed from the pn junction. As the density of the n layer is more than that of the p region, the developed anode voltage leads to a reduction in the reverse recovery time. In the figure shown below, the time interval between t1 and t3 is the reverse recovery time. This forward voltage can be blocked when recombination of the charged carriers occurs at the junction. The instant where recombination is complete is denoted by t4. However, still, there is the possibility of a supply of forward voltage. This turn-off time generally ranges between 10 to 100 microseconds. 19 GTO (Gate Turn-off Thyristor) A Gate Turn off Thyristor or GTO is a three terminal, bipolar (current controlled minority carrier) semiconductor switching device. Similar to conventional thyristor, the terminalsare anode, cathode and gate as shown in figure below. GTO invented by General Electricals. It is a fully controllable switch that could be turn on or turn off by gate signals and it is an active semiconductor device. The symbol of GTO is shown below; the gate has double arrows on it which distinguish the GTO from normal thyristor. This indicates the bidirectional current flow through the gate terminal. Fig 5.1 Symbol A small positive gate current triggers the GTO into conduction mode and also by a negative pulse on the gate, it is capable of being turned off. GTO can be turned on with the positive gate current pulse and turned off with the negative gate current pulse. Its capability to turn off is due to the diversion of PNP collector current by the gate and thus breaking the regenerative feedback effect. Actually the design of GTO is made in such a way that the PNP current gain of GTO is reduced. Highly doped n spots in the anode p layer form a shorted emitter effect and ultimately decrease the current gain of GTO for lower current regeneration and also the reverse voltage blocking capability. 20 This reduction in reverse blocking capability can be improved by diffusing gold but this reduces the carrier lifetime. Construction Consider the below structure of GTO, which is almost similar to the thyristor. It is also a four layer, three junction P+ N P+ N+ device like a standard thyristor. In this, the n+ layer at the cathode end is highly doped to obtain high emitter efficiency. This result the breakdown voltage of the junction J3 is low which is typically in the range of 20 to 40 volts. The doping level of the p type gate is highly graded because the doping level should be low to maintain high emitter efficiency, whereas for having a good turn OFF properties, doping of this region should be high. In addition, gate and cathodes should be highly inter-digited with various geometric forms to optimize the current turn off capability. 1000A and above may have several thousand segments which are all connected to the common gate contact. Figure 5.2 :. Four Layers and Three Junctions of GTO The junction between the P+ anode and N base is called anode junction. A heavily doped P+ anode region is required to obtain the higher efficiency anode junction so that a good turn ON properties is achieved. However, the turn OFF capabilities are affected with such GTOs. This problem can be solved by introducing heavily doped N+ layers at regular intervals in P+ anode layer as shown in figure. So this N+ layer makes a direct contact with N layer at junction J1. This cause the electrons to travel from base N region directly to anode metal contact without causing hole injection from P+ anode. This is called as a anode shorted GTO structure. Due to these anode shorts, the reverse blocking capacity of the GTO is reduced to the reverse breakdown voltage of junction j3 and hence speeds up the turn OFF mechanism. However, with a large number of anode shorts, the efficiency of the anode junction reduces and hence the turn ON performance of the GTO degrades. Therefore, careful considerations have to be taken about the density of these anode shorts for a good turn ON and OFF performance. 21 Principle of Operation Fig : The equivalent circuit The turn ON operation of GTO is similar to a conventional thyristor. When the anode terminal is made positive with respect to cathode by applying a positive gate current, the hole current injection from gate forward bias the cathode p-base junction. This results in the emission of electrons from the cathode towards the anode terminal. This induces the hole injection from the anode terminal into the base region. This injection of holes and electrons continuous till the GTO comes into the conduction state. In case of thyristor, the conduction starts initially by turning ON the area of cathode adjacent to the gate terminal. And thus, by plasma spreading the remaining area comes into the conduction. Unlike a thyristor, GTO consists of narrow cathode elements which are heavily interdigitated with gate terminal, thereby initial turned ON area is very large and plasma spreading is small. Hence the GTO comes into the conduction state very quickly. To turn OFF a conducting GTO, a reverse bias is applied at the gate by making the gate negative with respect to cathode. A part of the holes from the P base layer is extracted through the gate which suppress the injection of electrons from the cathode. 22 In response to this, more hole current is extracted through the gate results more suppression of electrons from the cathode. Eventually, the voltage drop across the p basejunction causes to reverse bias the gate cathode junction and hence the GTO is turned OFF. During the hole extraction process, the p-base region is gradually depleted so that the conduction area squeezed. As this process continuous, the anode current flows through remote areas forming high current density filaments. This causes local hot spots which can damage the device unless these filaments are extinguished quickly. By the application of high negative gate voltage these filaments are extinguished rapidly. Due to the N base region stored charge, the anode to gate current continues to flow even though the cathode current is ceased. This is called a tail current which decays exponentially as the excess charge carriers are reduced by the recombination process. Once the tail current reduced toa leakage current level, the device retains itsforward blocking characteristics. V-I Characteristics During the turn ON, GTO is similar to thyristor in its operates. So the first quadrant characteristics are similar to the thyristor. When the anode is made positive with respect to cathode, the device operates in forward blocking mode. By the application of positive gate signaltriggers the GTO into conduction state. The latching current and forward leakage currents are considerably higher in GTO compared to the thyristor as shown in figure. The gate drive can be removed if the anode current is above the holding current level. But it is recommended not to remove the positive gate drive during conduction and to hold at value more than the maximum critical gate current. This is because the cathode is subdivided into small finger elements as discussed above to assist the turn OFF process. This causes the anode current dips below the holding current level transiently, which forces a high anode current at a high rate back into the GTO. This can be potentially destructive. Therefore, some manufacturers recommend the continuous gate signal during the conduction state. Fig 5.5 V-I Characteristics The GTO can be turned OFF by the application of reverse gate current which can be either step or ramp drive. The GTO can be turned OFF without reversing anode voltage. The dashed line in the figure shows i-v trajectory during the turn OFF for an inductive load. It should be noted that during the turn OFF, GTO can block a rated forward voltage only. 23 To avoid dv/dt triggering and protect the device during turn OFF, either a recommended value of resistance must be connected between the gate and cathode or a small reverse bias voltage (typically -2V) must be maintained on the gate terminal. This prevents the gate cathode junction to become forward biased and hence the GTO sustains during the turn OFF state. In reverse biased condition of GTO, the blocking capability is depends on the type of GTO. A symmetric GTO has a high reverse blocking capability while asymmetric GTO has a small reverse blocking capability as shown in figure. It is observed that, during reverse biased condition, after a small reverse voltage (20 to 30V) GTO starts conducting in reverse direction due to the anode short structure. This mode of operation does not destroy the device provided that the gate is negatively biased and the time of this operation should be small. Overall switching speed of GTO is faster than thyristor (SCR) but voltage drop of GTOis larger. The power range of GTO is better than BJT, IGBT or SCR. The static voltage current characteristics of GTO are similar to SCR except that the latching current of GTO is larger (about 2 A) as compared to SCR (around 100-500 mA). The switching characteristics is given Fig. Figure 5.6 . Turn-On and Turn-Off Characteristics of GTO Gate Turn-Off Thyristor Applications Due to the advantages like excellent switching characteristics, no need of commutation circuit, maintenance-free operation, etc makes the GTO usage predominant over thyristor in many applications. It is used as a main control device in choppers and inverters. Some of these applications are • AC drives • DC drives or DC choppers • AC stabilizing power supplies • DC circuit breakers • Induction heating • And other low power applications 24 Comparison of Power Semiconductor devices 25

Fig 1.2 : Symbol
1

Power Diode 
Power diode having three layers the P+ layer, n– layer and n+ layer. 
The upper layer is the P+ layer, it is heavily doped and acts as an anode, the thickness of this layer is  10 μm & the doping level is 1019 cm-3. 
The lower layer is n+ layer, it is also heavily doped and acts as a cathode, the thickness of this layer is  250-300 μm & the doping level is 1019 cm-3. 
The middle layer is n– layer, it is lightly doped. It acts as a drift layer, the doping level is 1014 cm-3 and  the thickness of this layer mainly depends up on the breakdown voltage. 
The heavily doped upper and lower (P+ layer and n+ layer) is called terminal layers and terminal  layers is always heavily doped to increase the conductivity of the device. 
Once drift layer width increases then breakdown voltage will be increased. Due to its light doping  concentration of drift layer with increased thickness (the thickness of the depletion region increases  with a decrease in doping concentration) helps the diode to block larger reverse- biased voltage and  hence have a greater breakdown voltage. 
  
VI characteristics of power diode 
The V-I Characteristics of Power Diode is shown in figure it is just similar to signal diode. When we  increase applied forward voltage the forward current increases linearly. In forward biased condition  when anode is positive w.r.t. cathode the forward current increase linearly with an increase in  forward voltage.
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Reverse recovery time of power diode 
If a diode is initially driven in forward bias, and the polarity suddenly switches to reverse bias, the  diode will still remain conducting for some time. The time required for conduction to settle into the  reverse bias state is the diode's reverse recovery time. 
Reverse recovery time (trr) : After the forward diode current decays to zero, the continue to conduct  in the reverse direction because of the presence of the stored charges in the two layers . The reverse  flow for a time called reverse recovery time (trr). The diode retains its blocking capability until  reverse recovery current decays to zero. 
The reverse recovery time is defined as the time between the instance forward diode current  become zero and the instant reverse recovery current decays to 25% of its reverse peak value Irm. The reverse recovery time is the combination of two segment of time ta and tb i.e. trr = ta + tb where time ta is the time between zero crossing of forward current and peak reverse  current Irm. During the time ta, charged stored in depletion region is removed. Time tb is measured from the instant of Irm to the instant where 0.25 Irm is reached. During tb charge in two semiconductor area is removed. 
Softness factor or S-factor :- The ratio of ta/tb is called softness factor or S-factor. The diode with  softness equal to one is called soft recovery diode and diode with softness factor less than one is  called snappy recovery diode or fast recovery diode. 
Where 
• Time ta : Charges stored in the depletion layer removed. 
• Time tb : Charges from the semiconductor layer is removed. 
S-factor: measure of the voltage transient that occurs during the time the diode recovers.  S-factor = 1 ⇒ low oscillatory reverse-recovery process. (Soft –recovery diode) S-factor <1 ⇒ large oscillatory over voltage (snappy-recovery diode or fast-recovery diode). Power diodes now exist with forward current rating of 1A to several thousand amperes with reverse recovery voltage ratings of 50V to 5000V or more. 
  
Types of Power Diodes  
Standard Diodes or General Purpose Diodes: 
Standard or general purpose diodes have a comparatively high reverse recovery time, when  compared to other diodes. Due to this reason they are used in applications which are not time  sensitive and generally run on low speeds. Usually the reverse recovery time for general purpose  diodes varies between 20 micro seconds to 30 micro seconds which is quite a lot. Typical low speed 
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applications for general purpose diodes include the power diode being used as a rectifier or in a  converter, where the frequency input is quite low. 
Fast Recovery Diodes: 
As their name suggests, these are the type of power diodes which have a relatively faster reverse  recovery time, which usually varies from 2 micro seconds to 5 micro seconds. With such a fast  recovery time, they can be easily used in high speed switching applications where the time is of great  importance. Due to their property of fast reverse recovery, they are also comparatively expensive as  compared to the general purpose diodes. 
Schottky Diodes: 
Schottky Diode: It has an aluminum-silicon junction where the silicon is an n-type. As the metal has  no holes, there is no stored charge and no reverse-recovery time. Therefore, there is only the  movement of the majority carriers (electrons) and the turn-off delay caused by recombination  process is avoided. It can also switch off much faster than a p-n junction diode. As compared to the  p-n junction diode it has: 
(a) Lower cut-in voltage 
(b) Higher reverse leakage current 
(c) Higher operating frequency 
Application: high-frequency instrumentation and switching power supplies. 
Advantages and Disadvantages of Power Diode 
The advantages and disadvantages of power diode include the following. 
• The PN-junction region of this diode is large & can supply huge current, however, the  capacitance of this junction can also be large, which works at a lower frequency & it is generally used  for rectification only. 
• It will resolve AC at high current and a high voltage. 
• The main disadvantage is its size & probably needs to be fixed to a heat sink while  conducting a high current. 
• It needs specialized hardware for installing and insulating from the metal frames which are  available in the surrounding. 
Applications :- 
The applications of power diode include the following. 
• This diode provides uncontrolled power rectification 
• It is used in different applications like DC power supplies, for charging the battery, inverters  and AC rectifiers. 
• These are used like snubber networks and free-wheeling diodes due to their characteristics  like voltage & high-current. 
• These diodes are used as feedback, freewheeling diodes, and high-voltage rectifier. POWER Transistors 
Power transistors are three terminal devices which are composed of semiconductor materials. it can  handle large voltage and current. The structure and construction of a power transistor is entirely  different from that of a signal transistor but their characteristics and operation in almost same.  Power transistors are available in different types with different power and switching speed ratings.
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Types of Power transistor 
Power transistors are classified into the following types: 
1. Bipolar Junction Transistors (BJTs) 
2. Metal Oxide Semiconductor Field-Effect Transistor (MOSFETs) 
3. Insulated Gate Bipolar Transistor (IGBTs) 
BIPOLAR JUNCTION TRANSISTORS (BJTS) 
Bipolar Junction Transistors can either be used as an amplifier or switch. These are mainly used to  control the current, the process of controlling includes the act of amplification, switch-on, and  switch-off. Power BJT has 3 terminals — collector, base and emitter. It is a current controlled device.  It is the base current that controls the device. Bipolar Junction Transistors are of two types NPN  transistor and PNP transistor.

Symbol of Power BJT (a) NPN BJT (b) PNP BJT

Power BJT NPN Structure 
The construction of the Power Transistor is different from the signal transistor as shown in the figure.  It has three terminals labelled as Collector, Base, and Emitter. A Power BJT has a vertically oriented  four layer structure of alternating P and N type doping as shown in above NPN transistor. This is  maximising the cross-section area results in current rating of BJT, minimize the on-state resistance, and  thus reduce the power losses 
The doping of emitter layer and collector layer is quite large typically 1019 cm-3. A thin p-layer with  doping level of 1016 cm 3is sandwiched between two n-layers as shown in figure. A special layer called  the collector drift region (n-) has a light doping level of 1014 cm-3. The thickness of the drift region  determines the breakdown voltage of the transistor. The n layer is added in the power BJT which is  known as drift region. 
The characteristics of the device are determined by the doping level in each of the layers and the  thickness of the layers. The base thickness is made as small as possible in order to have good 
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amplification capabilities, however if the base thickness is small the breakdown voltage capability of  the transistor is compromised. 
V-I Output Characteristics of a Power Transistor 
• The below graph represents various regions like the cut-off region, active region, hard  saturation region, quasi saturation region. 
• For different values of VBE, there are different current values IB0, IB1, IB2, IB3, IB4, IB5, IB6. • Whenever there is no current flow, it means the transistor is off. But few current flows which  are ICEO. 
• For increased value of IB = 0, 1,2, 3, 4, 5. Where IB0 is the minimum value and IB6 is the  maximum value. When VCE increases ICE also increases slightly. Where IC = ßIB, hence the  device is known as a current control device. Which means the device is in active region,  which exists for a particular period. 
• Once the IC has reached to maximum the transistor switches to the saturation region. • Where it has two saturation regions quasi saturation region and hard saturation region. • A transistor is said to be in a quasi saturation region if and only if the switching speed from  
on to off or off to on is fast. This type of saturation is observed in the medium-frequency  application. 
• Whereas in a hard saturation region the transistor requires a certain amount of time to  switch from on to off or off to on state. This type of saturation is observed in the low frequency applications. 
  
Switching characteristics of power BJT 
Switching characteristics of power BJT is shown in Figure. Due to internal capacitances, the transistor  does not turn on instantly. VB rises from zero to V1 and the base current rises to IB1, the collector  current does not respond immediately. The delay is due to the time required to charge up the base– emitter junction capacitance to the forward bias voltage VBE(0.7V). This time is known as the delay  time(td) . 
For t > td, collector current starts rising and VCE starts to drop with the magnitude of 9/10th of its peak  value and the collector current rises to the steady value of ICS and this time is called rise time, tr,  required to turn on the transistor. The transistor remains on so long as the collector current is at least  of this value. 
When the input voltage is reversed from V1 to -V2, the reverse current –IB2 helps to discharge the  base. The base current required during the steady-state operation is more than that required to 
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saturate the transistor. Thus, excess minority carrier charges are stored in the base region which needs  to be removed during the turn-off process. The time required to nullify this charge is the storage time,  ts. Without –IB2 the saturating charge has to be removed entirely due to recombination and the  storage time ts would be longer. 
Collector current remains at the same value for this time. After this, collector current starts decreasing  and base-to-emitter junction charges to the negative polarity; base current also get reduced. Once the  extra charge is removed, base–emitter junction charges to the input voltage  
–V2 and the base current falls to zero. That time is called fall time, tf . tf depends on the time constant  which is determined by the reverse biased base–emitter junction capacitance   
Turn-On and Turn-Off Characteristics of BJT 
ADVANTAGES OF BJT’S 
• BJT’s have high switching frequencies since their turn-on and turn-off time is low. • The turn-on losses of a BJT are small. 
• BJT has controlled turn-on and turn-off characteristics since base drive control is possible. • BJT does not require commutation circuits. 
DEMERITS OF BJT 
• Drive circuit of BJT is complex. 
• It has the problem of charge storage which sets a limit on switching frequencies. • It cannot be used in parallel operation due to problems of negative temperature coefficient 
Power Bipolar Junction Transistors has the following characteristics: 
• Bipolar Junction Transistors are large in size and hence allow maximum current to flow. • Bipolar Junction Transistors have high breakdown voltage. 
• BJTs have high handling capability as well as current carrying capacity. 
• Mainly seen in high power applications. 
APPLICATIONS OF POWER BJT
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• SMPS(Switch mode power supply) commonly used in computers. 
• Final audio amplifier in stereo systems. 
• Power amplifiers. 
• DC to AC inverters. 
• Relay and display drivers. 
• AC motor speed controllers. 
• Power control circuits. 
MOSFET- Metal Oxide Semiconductor Field-Effect Transistor 
MOSFET which handles high levels of power is known as Power MOSFET. As compared to normal  MOSFETs in the less voltage range, these MOSFETS works much better by exhibiting high speed of  switching. Its operating principle is the same as general MOSFETs. 
The power MOSFET frequency is high like up to 100 kiloHertz. The power MOSFET symbol is shown  below.

Power MOSFET Symbols 
Operating Principle 
Similar to normal MOSFETs, these types of MOSFETs will switch & control the flow of current in  between the two terminals like source & drain through changing the voltage on the gate terminal.  Once the voltage is applied to the gate terminal, then a channel can be formed in between the  source & the gate terminals which allows the flow of current.
  
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Initially there is no path for any current to flow between the source and the drain terminals.  However, application of a positive voltage at the gate terminal with respect to the source will convert  the silicon surface beneath the gate oxide into an n type layer or “channel”, thus connecting the  Source to the Drain. The operation of MOSFET divides into two parts formation of the depletion layer  and Creation of Inversion Layer 
The gate region of a MOSFET which is composed of the gate metallization, the gate (silicon) oxide  layer and the p-body silicon forms a high quality capacitor. When a small voltage is application to this  capacitor structure with gate terminal positive with respect to the source (note that body and source  are shorted) a depletion region forms at the interface between the . SiO2and the silicon Further  increase in VGS causes the depletion layer to grow in thickness. 
MOSFET Switching characteristics:

Fig 3.12 Switching characteristics of power MOSFET 
The switching characteristics of a power MOSFET are influenced to a large  extent by the internal capacitance of the device and the internal impedance of the  gate drive circuit. Switching waveforms for a power MOSFET are shown in Fig . At turn-on, there is an initial 
delay tdn during which input capacitance charges to gate threshold voltage VGST .  Here tdn is called turn-on delay time. 
There is further delay tr , called rise time, during which gate voltage rises 
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to VGSP , a voltage sufficient to drive the MOSFET into on state. During tr , drain  current rises from zero to full on current ID . Thus, the total turn-on time is  ton=tdn+tr . The turn-on time can be reduced by using low-impedance gate drive source. 
As MOSFET is a majority carrier device, turn-off process is initiated soon  after removal of gate voltage at time t1 . The turn-off delay time, tdf is the time during which input capacitance discharges from overdrive gate voltage V1 to VGSP . 
The fall time, tf is the time during which input capacitance discharges from  VGSP to threshold voltage. During tf, drain current falls from ID to zero. toff=tdf+tf . 
So when VGS≤VGST, MOSFET turn-off is complete. 
Advantages of a power MOSFET 
1. The GATE driving circuit of MOSFET is simple. 
2. It can operate at the high switching frequency. 
3. It has better thermal stability because the temperature coefficient of MOSFET is positive. 
4. Easy to turn ON and OFF. 
5. On-state resistance is low. 
6. The second breakdown does not take place. 
Demerits of a power MOSFET 
1. The on-state voltage across the MOSFET is very high. So, on-state power dissipation is high. 
2. It has the asymmetric blocking capacity. They can block a high forward voltage but they cannotblock high reverse voltage. Therefore, we need to connect a diode to protect the MOSFET. 
Application of Power MOSFET 
1. Uninterrupted Power Supplies (UPS) 
2. Switch Mode Power Supplies (SMPS) 
3. High-frequency inverter 
4. Motor control application 
5. Display driver 
6. In power amplifier 
7. Industrial applications 
8. Relay driver
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IGBT- Insulated Gate Bipolar Transistor 
The IGBT or Insulated Gate Bipolar Transistor is the combination of BJT and MOSFET. Its name also  implies the fusion between them. “Insulated Gate” refers to the input part of MOSFET having very high  input impedance. It does not draw any input current rather it operates on the voltage at its gate  terminal. “Bipolar” refers to the output part of the BJT having bipolar nature where the current flow  is due to both types of charge carriers. It allows it to handle very large currents and voltages using  small voltage signals. This hybrid combination makes the IGBT a voltage-controlled device.

It is a four-layer PNPN device having three PN junctions. It has three terminals Gate (G), Collector(C)  and Emitter (E). The terminal’s name also implies being taken from both transistors. Gate terminal as  it is the input part, taken from MOSFET while the collector and emitter as they are the output, taken  from the BJT. 
Construction of IGBT 
IGBT is made of four layers of semiconductor to form a PNPN structure. The collector (C) electrode is  attached to P layer while the emitter (E) is attached between the P and N layers. A P+ substrate is used  for the construction of IGBT. An N- layer is placed on top of it to form PN junction J1. Two P regions are  fabricated on top of N- layer to form PN junction J2. The P region is designed in such a way to leave a  path in the middle for the gate (G) electrode. N+ regions are diffused over the P region as shown in the  figure.

The emitter and gate are metal electrodes. The emitter is directly attached to the N+ region while the  gate is insulated using a silicon dioxide layer. 
Working of IGBT (https://www.youtube.com/watch?v=h9qMQFB8_iQ) 
The two terminals of IGBT collector (C) and emitter (E) are used for the conduction of current while  the gate (G) is used for controlling the IGBT. Its working is based on the biasing between Gate-Emitter  terminals and Collector-Emitter terminals.
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The collector-emitter is connected to Vcc such that the collector is kept at a positive voltage than the  emitter. The junction j1 becomes forward biased and j2 becomes reverse biased. At this point, there is  no voltage at the gate. Due to reverse j2, the IGBT remains switched off and no current will flow  between collector and emitter. 
Applying a gate voltage VG positive than the emitter, negative charges will accumulate right beneath  the SiO2- layer due to capacitance. Increasing the VG increases the number of charges which  eventually form a layer when the VG exceeds the threshold voltage, in the upper P-region. This layer  form N-channel that shorts N- drift region and N+ region. 
The electrons from the emitter flow from N+ region into N- drift region. While the holes from the  collector are injected from the P+ region into the N- drift region. Due to the excess of both electrons  and holes in the drift region, its conductivity increase and starts the conduction of current. Hence the  IGBT switches ON. 
V-I Characteristics of IGBT 
Unlike BJT, IGBT is a voltage-controlled device that requires only a small voltage at its gate to control  the collector current. However, the gate-emitter voltage VGE needs to be greater than the threshold  voltage. 
Transfer characteristics of the IGBT show the relation of input voltage VGE to output collector current  IC. When the VGE is 0v, there is no IC and the device remains switched off. When the VGE is slightly  increased but remains below threshold voltage VGET, the device remains switched off but there is a  leakage current. When the VGE exceeds the threshold limit, the I-C starts to increase and the device  switches ON. Since it is a unidirectional device, the current only flows in one direction.

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Switching Characteristics of IGBT 
Switching Characteristics of IGBT is basically the graphical representation of behavior of IGBT during  its turn-on & turn-off process. 
The turn-on time is defined as the time between the instant of forward blocking to forward conduction  mode. Here, forward conduction means the device conducts in forward direction. Turn-on time (ton)  is basically composed of two different times: Delay time (tdn) and Rise time (tr). Therefore, we can say  that ton = tdn + tr. 
The delay time is defined as the time for the collector-emitter voltage (VCE) to fall from VCE to 0.9VCE.  This simply means that, the collector-emitter voltage drops to 90% in delay time and hence the  collector current rises from initial leakage current to 0.1IC (10%). Thus, delay time may also be defined  as the time period during which collector current rises from zero (in fact a small leakage current) to  10% of the final value of collector current IC. 
The rise time tr is the time during which collector-emitter voltage falls from 0.9VCE to 0.1 VCE. This  means, during rise time collector-emitter voltage falls to 10% from 90%. Therefore, the collector  current builds up to final value of collector current IC from 10%. After time ton, the collector current  becomes IC and the collector-emitter voltage drops to very small value called conduction drop (VCES). 
A typical Switching Characteristics of an IGBT is shown below. You may corelate the delay time, rise  time and turn-on time.

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General Comparison with BJT and MOSFET 
As we have discussed above, IGBT takes the best parts of both BJT and MOSFET. Therefore, it is superior  in almost every way. Here is a chart of some of the characteristics showing the comparison between  IGBT, BJT and MOSFET. we are comparing power devices in their max capabilities. 
Characteristic 
	Power BJT 
	Power MOSFET 
	IGBT
	Voltage Rating 
	High < 1kV 
	High < 1kV 
	Very High > 1kV
	Current Rating 
	High < 500 A 
	Low < 200 A 
	Very High > 500 A
	Input Parameter 
	Base Current, Ib 
	Voltage, VGS 
	Voltage, VGE
	Input Drive
	Current gain (hfe) 
20-200
	Voltage, VGS 
3-10V
	Voltage, VGE 
4-8V
	Input Drive Power 
	High 
	Low 
	Low
	Input Drive Circuitry 
	Complex 
	Simple 
	Simples
	Input Impedance 
	Low 
	High 
	High
	Output Impedance 
	Low 
	Medium 
	Low
	Switching Loss 
	High 
	Low 
	Medium
	Switching Speed 
	Low 
	Fast 
	Medium
	Cost 
	Low 
	Medium 
	High

IGBT as a whole has the advantages of both BJT and MOSFET. 
• It has higher voltage and current handling capabilities. 
• It has a very high input impedance. 
• It can switch very high currents using very low voltage. 
• It is voltage-controlled i.e. it has no input current and low input losses. 
• The gate drive circuitry is simple and cheap. 
• It can be easily switched ON by applying positive voltage and OFF by applying zero or slightly  negative voltage. 
• It has very low ON-state resistance 
• It has a high current density, enabling it to have a smaller chip size. 
• It has a higher power gain than both BJT and MOSFET. 
• It has a higher switching speed than BJT. 
Disadvantages 
It has a lower switching speed than MOSFET. 
• It is unidirectional it cannot conduct in reverse. 
• It cannot block higher reverse voltage. 
• It is costlier than BJT and MOSFET. 
• It has latching problems due to the PNPN structure resembling thyristor. 
• Applications of IGBT 
• IGBTs have numerous applications used in AC as well as DC circuits. Here are some of the  important 
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Applications of IGBT 
• It is used in SMPS (Switched Mode Power Supply) to supply power to sensitive medical  equipment and computers. 
• It is used in UPS (Uninterruptible Power Supply) system. 
• It is used in AC and DC motor drives offering speed control. 
• It is used in chopper and inverters. 
• It is used in solar inverters. 
SCR - Silicon-controlled rectifier 
SCR is a four layer 3 junction p-n-p-n semiconductor device consisting of at least three p-n  junctions, functioning as an electrical switch for high power operations. It has three basic terminals,  namely the anode, cathode and the gate mounted on the semiconductor layers of the device. 
SCR Symbol 
The SCR symbol is very similar to the diode symbol, but it has an additional gate terminal. 
Fig : SCR Symbol 
SCR Construction 
SCR has four layers of extrinsic semiconductor materials. These four-layer form three PN junctions named J1, J2, and J3. The layers are either NPNP or PNPN. The anode and cathode terminals  are placed at the end layers and where the gate terminal is placed with the third layer. The outer layers are heavily dopped and the inner two layers are lightly dopped.

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Working and V-I Characteristics of SCR 
SCR has three basic mode of operation: Reverse Blocking Mode, Forward Blocking Mode  and Forward Conduction Mode 
Reverse Blocking Mode :- If the anode terminal of the SCR connects to the negative and cathode  terminal of SCR connects to the positive of battery terminals. The SCR is in reverse blocking mode. In this mode, J1 and J3 junctions are reverse biased. Where the middle junctionJ2 is forward bias. As two junctions are reverse bias, so there is no current flowing through it butonly a small leakage  current due to the drift charge carrier. An SCR in reverse blocking mode behaves as if an open switch. Hence this mode is also known as OFF state of SCR. 
A small leakage current of the order of mili or micro ampere flows thorough the SCR in this mode. If the reverse voltage isincreased, then atsome critical voltage an avalanchebreakdown takes  place at reverse biased junctions J1 and J3 which leads to sudden increase in reverse current. This critical reverse voltage is called Reverse Breakdown Voltage. VBR represents this reverse breakdown  
voltage in the V-I characteristics. It can be seen that, there is a sharp increase in reverse current at  this voltage. This increased reverse current may result in more losses in the SCR which in turn may  damage the SCR. Therefore the reverse voltage acrossthe SCR terminals should not exceed reverse breakdown voltage during its operation. 
Forward Blocking Mode :- Forward Blocking Mode is that operational mode of SCR in which it does  not conduct even though it is forward biased. The term forward biased SCR implies that its anode  terminal is positive with respect to cathode terminal with gate switch is open. In this mode, the  junction J1 and J3 are forward biased but junction J2 is reverse biased. A small leakage current, called the forward leakage current, flows as shown by OM in the V-I characteristics of SCR in this mode. As  the forward leakage current is small, SCR offers high impedance. Therefore an SCR can be treated as 
an open switch even in forward blocking mode. 
Forward Conduction Mode :- As we have seen that in Forward Blocking mode, even though the SCR  is forward biased, it does not conduct. But the good thing is that, in forward blocking mode, junction  J1 and J3 are forward biased and J2 is reversed biased. This means, there are two possibilities for making SCR to conduct in this mode: 
• Increase the anode to cathode voltage to such an extent which leads to avalanche breakdown of the reverse biased junction J2. 
• Apply positive gate pulse between gate and cathode terminal. 
When the forward biasing voltage is increased then at some critical voltage VBO, an avalanche breakdown take place at reverse biased junction J2. This critical voltage is knownas Forward Break over Voltage. 
Since junction J1 and J3 are already forward biased, an avalanche breakdown at J2 will result in sudden increase in anode current in forward direction. Hence the point M at V-I characteristics of  SCR corresponding to Forward Break over Voltage VBO will at once shift to point N and then to any point in between N and K. Since the anode current in this mode will only be limited by the load,  
so based on the value of load the anode current will change and may lie at any point in between N
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and K. Thus NK represents the forward conduction of SCR. 
Fig : V-I Characteristics of SCR 
The SCR can be turned ON at smaller applied voltage by the application of a small positive  voltage at the gate terminal. When gate voltage is applied the junction J3 is forward biased and  junction J2 is reverse biased. Thus, the electrons from n – type layer starts moving across the junction  J3 toward p –type material and the holes from p –type material towards the n – type material. Due  to the movement of holes and electrons across the junction J3 the gate current starts flowing.  Because of gate current the anode current increases. The increased anode current makes the more  electrons available at the junction J2. As a result of this process, in a small time, the junction J2 breaks  down and the SCR is turn ON.A higher gate current can put SCR faster in the forwardconduction mode  as in the graph Ig3>Ig2>Ig1. The SCR will remain in the forward conduction mode if the anode current is above the holding current. 
Latching current and holding current are both minimum currents required to maintain a  thyristor in an on or off state:  
Latching current: The minimum anode current required to keep a thyristor on after it's been  turned on and the gate signal is removed. 
Holding current: The minimum anode current required to keep a thyristor on. The latching current is always greater than the holding current:  
Explanation: The thyristor turns on when the current is greater than the latching current.  Once the thyristor is on, the latching current is no longer relevant. The holding current is the  minimum current that must flow through the thyristor to keep it on. 
SCR Applications 
The most common SCR application is the DC motor speed control. The DC motor has two 
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windings, where the armature winding is connected to the AC supply by two SCR. The SCRcontrols the amount of current to the motor and ultimately the speed of the DC motor. 
SCR working with two transistor model 
Woking of the SCR can be easily explained by two transistor model of SCR. As per figure you can see  with supply voltage V and load resistance R is applied to SCR. Here first Assume the supply voltage V  is less than break over voltage as is usually the case. When the gate is open (i.e. switch S open),  there is base current Ib=0. For the base of the T2 is connected with the collector of The T1.  Therefore, no current flows in the collector of T2 and hence that of T1. So for this condition, SCR is in  OFF condition. 
Whenever switch S is closed, a small gate current will flow through the base of T2 which means its  collector current will increase. The collector of the transistor T2 is connected with transistor T1. So,  the collector current of T2 is the base current of T1. Therefore, the collector current of T1 increases.  But collector current of T1 is the base current of T2. This action is accumulative since an increase of  current in one transistor causes an increase of current in the other transistor. As a result of this  action, both transistors are driven to saturation, and heavy current flows through the load RL. Under  such conditions, the SCR closes.

Switching characteristics during turn-on 
In the above section, we have discussed that a forward-biased thyristor conducts in the presence of  positive gate potential at the gate cathode terminal. But you must keep in mind that there exists a  transition time within the device between which the device transits from forward off state to forward  on state. This time span is generally regarded as thyristor turn-on time and is classified into three  intervals i.e., delay time denoted by td, rise time denoted by tr, and spread time, denoted by ts. 
Now, let us understand each one separately, 
Delay time (td): The delay time of the thyristor depends on the variation in the gate current in forward  blocking mode as well as the temperature as it varies the crystal structure of the semiconductor. Delay  time is defined as the time instant between the moment when gate current attains 90% of its final  value while the anode current attains 10% of its initial value. During delay time, the thyristor remains  in the forward blocking mode. 
Rise time (tr): Rise time is defined as the time taken by the anode current to rise from 10% of its value  to 90% of the same. We have discussed in delay time that it corresponds to the time when anode  current reaches 10% of its initial value. Thus, the rise time is started once the delay time gets over. Spread time (ts): Spread time of the thyristor exists between the time when the anode current  approaches 100% from its 90% value. More simply, we can say, spread time is the time within which 
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the anode current rises from 90% to 100%. This is called so because, during the spread time, the  conduction gets spread over the complete cathode region of the thyristor. Once the spread time is  completed, the value of the anode current reaches a steady state 
Switching Characteristics during turn-off 
Turning the thyristor off means that the state of the thyristor is changed from on to off and it can be  able to block forward voltage. 
We have discussed that, once the device gets on then even after the removal of the gate signal, current  continues to flow through the device due to the fact that the charge carriers in the four layers favors  conduction. To turn it off, a reverse potential must be necessarily applied for a specific time interval  when anode current attains 0 value. 
So, the turn-off time of the thyristor is defined as the time being between the instants when anode  current becomes 0 and the thyristor attains its forward blocking ability. This turn-off time corresponds  to the duration within which all the excess carriers must be removed from the layers of the thyristor.  This is nothing but the sweeping out of holes from the outer p region and holes from the outer n  region. 
The turn of time of the thyristor is categorized into two intervals, namely 
• reverse recovery time trr, 
• gate recovery time tgr

During the reverse recovery time, the anode current will flow in the reverse direction. Due to this  reverse current charged carriers get removed from the pn junction. As the density of the n layer is  more than that of the p region, the developed anode voltage leads to a reduction in the reverse  recovery time. In the figure shown below, the time interval between t1 and t3 is the reverse recovery  time. 
This forward voltage can be blocked when recombination of the charged carriers occurs at the junction.  The instant where recombination is complete is denoted by t4. However, still, there is the possibility of  a supply of forward voltage. This turn-off time generally ranges between 10 to 100 microseconds.
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GTO (Gate Turn-off Thyristor) 
A Gate Turn off Thyristor or GTO is a three terminal, bipolar (current controlled minority  carrier) semiconductor switching device. Similar to conventional thyristor, the terminalsare anode,  cathode and gate as shown in figure below. GTO invented by General Electricals. It is a fully  controllable switch that could be turn on or turn off by gate signals and it is an active semiconductor device. 
The symbol of GTO is shown below; the gate has double arrows on it which distinguish the GTO from normal thyristor. This indicates the bidirectional current flow through the gate terminal. Fig 5.1 Symbol

A small positive gate current triggers the GTO into conduction mode and also by a negative pulse on the gate, it is capable of being turned off. 
GTO can be turned on with the positive gate current pulse and turned off with the negative  gate current pulse. Its capability to turn off is due to the diversion of PNP collector current by the  gate and thus breaking the regenerative feedback effect. Actually the design of GTO is made in such a way that the PNP current gain of GTO is reduced. 
Highly doped n spots in the anode p layer form a shorted emitter effect and ultimately decrease the current gain of GTO for lower current regeneration and also the reverse voltage blocking capability.
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This reduction in reverse blocking capability can be improved by diffusing gold but this reduces the carrier lifetime. 
Construction 
Consider the below structure of GTO, which is almost similar to the thyristor. It is also a four  layer, three junction P+ N P+ N+ device like a standard thyristor. In this, the n+ layer at the cathode  end is highly doped to obtain high emitter efficiency. This result the breakdown voltage of the junction J3 is low which is typically in the range of 20 to 40 volts. 
The doping level of the p type gate is highly graded because the doping level should be low to  maintain high emitter efficiency, whereas for having a good turn OFF properties, doping of this region should be high. In addition, gate and cathodes should be highly inter-digited with 
various geometric forms to optimize the current turn off capability. 1000A and above may have several thousand segments which are all connected to the common gate contact.

Figure 5.2 :. Four Layers and Three Junctions of GTO 
The junction between the P+ anode and N base is called anode junction. A heavily doped P+ anode region is required to obtain the higher efficiency anode junction so that a good turn ON properties is achieved. However, the turn OFF capabilities are affected with such GTOs. This problem can be solved by introducing heavily doped N+ layers at regular intervals in P+ anode  layer as shown in figure. So this N+ layer makes a direct contact with N layer at junction J1. This cause  the electrons to travel from base N region directly to anode metal contact without causing hole injection from P+ anode. This is called as a anode shorted GTO structure. 
Due to these anode shorts, the reverse blocking capacity of the GTO is reduced to the reverse breakdown voltage of junction j3 and hence speeds up the turn OFF mechanism. However, with a large number of anode shorts, the efficiency of the anode junction reduces and hence the turn ON performance of the GTO degrades. Therefore, careful considerations have to be  taken about the density of these anode shorts for a good turn ON and OFF performance.
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Principle of Operation 
Fig : The equivalent circuit

The turn ON operation of GTO is similar to a conventional thyristor. When the anode terminal  is made positive with respect to cathode by applying a positive gate current, the hole current injection from gate forward bias the cathode p-base junction. 
This results in the emission of electrons from the cathode towards the anode terminal. This  induces the hole injection from the anode terminal into the base region. This injection of holes and electrons continuous till the GTO comes into the conduction state. 
In case of thyristor, the conduction starts initially by turning ON the area of cathode adjacent  to the gate terminal. And thus, by plasma spreading the remaining area comes into the conduction. Unlike a thyristor, GTO consists of narrow cathode elements which are heavily interdigitated with gate terminal, thereby initial turned ON area is very large and plasma spreading is small. Hence the GTO comes into the conduction state very quickly.

To turn OFF a conducting GTO, a reverse bias is applied at the gate by making the gate negative with respect to cathode. A part of the holes from the P base layer is extracted through the  gate which suppress the injection of electrons from the cathode.
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In response to this, more hole current is extracted through the gate results more suppression of electrons from the cathode. Eventually, the voltage drop across the p basejunction causes to reverse bias the gate cathode junction and hence the GTO is turned OFF. 
During the hole extraction process, the p-base region is gradually depleted so that the conduction area squeezed. As this process continuous, the anode current flows through remote areas  forming high current density filaments. This causes local hot spots which can damage the device unless these filaments are extinguished quickly. 
By the application of high negative gate voltage these filaments are extinguished rapidly. Due  to the N base region stored charge, the anode to gate current continues to flow even though the  cathode current is ceased. This is called a tail current which decays exponentially as the excess charge  carriers are reduced by the recombination process. Once the tail current reduced toa leakage current level, the device retains itsforward blocking characteristics. 
V-I Characteristics 
During the turn ON, GTO is similar to thyristor in its operates. So the first quadrant characteristics are similar to the thyristor. When the anode is made positive with respect to cathode,  the device operates in forward blocking mode. By the application of positive gate signaltriggers the GTO into conduction state. 
The latching current and forward leakage currents are considerably higher in GTO compared  to the thyristor as shown in figure. The gate drive can be removed if the anode current is above the holding current level. 
But it is recommended not to remove the positive gate drive during conduction and to hold  at value more than the maximum critical gate current. This is because the cathode is subdivided into small finger elements as discussed above to assist the turn OFF process. 
This causes the anode current dips below the holding current level transiently, which forces  a high anode current at a high rate back into the GTO. This can be potentially destructive. Therefore,  some manufacturers recommend the continuous gate signal during the conduction state. 
Fig 5.5 V-I Characteristics 
The GTO can be turned OFF by the application of reverse gate current which can be either  step or ramp drive. The GTO can be turned OFF without reversing anode voltage. The dashed line in  the figure shows i-v trajectory during the turn OFF for an inductive load. It should be noted that during the turn OFF, GTO can block a rated forward voltage only.
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To avoid dv/dt triggering and protect the device during turn OFF, either a recommended value of resistance must be connected between the gate and cathode or a small reverse bias voltage  (typically -2V) must be maintained on the gate terminal. This prevents the gate cathode junction to become forward biased and hence the GTO sustains during the turn OFF state. 
In reverse biased condition of GTO, the blocking capability is depends on the type of GTO. A symmetric GTO has a high reverse blocking capability while asymmetric GTO has a small reverse blocking capability as shown in figure. 
It is observed that, during reverse biased condition, after a small reverse voltage (20 to 30V)  GTO starts conducting in reverse direction due to the anode short structure. This mode of operation  does not destroy the device provided that the gate is negatively biased and the time of this operation should be small. 
Overall switching speed of GTO is faster than thyristor (SCR) but voltage drop of GTOis larger. The power range of GTO is better than BJT, IGBT or SCR. 
The static voltage current characteristics of GTO are similar to SCR except that the latching current  of GTO is larger (about 2 A) as compared to SCR (around 100-500 mA). 
The switching characteristics is given Fig. 
Figure 5.6 . Turn-On and Turn-Off Characteristics of GTO 
Gate Turn-Off Thyristor Applications 
Due to the advantages like excellent switching characteristics, no need of commutation circuit,  maintenance-free operation, etc makes the GTO usage predominant over thyristor in many  applications. It is used as a main control device in choppers and inverters. Some of these  applications are 
• AC drives 
• DC drives or DC choppers 
• AC stabilizing power supplies 
• DC circuit breakers 
• Induction heating 
• And other low power applications
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Comparison of Power Semiconductor devices
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Transcript for:Overview of Power Electronics Devices

Transcript for:
Overview of Power Electronics Devices