Switching Characteristics of SCR Explained

Welcome to Power Electronics lecture series. Here in this session I'll be going to explain switching characteristic of SCR. So here in this lecture I'll be explaining a characteristic of SCR where SCR is getting turn on and SCR is getting turn off. So ultimately in switching characteristic I'll be explaining turn on and turn off characteristic of SCR. So before I explain how SCR is getting turn off and turn on. by the basic process let us try to understand what it requires to turn on SCR. So when we have a SCR, SCR is basically three layer four junction four layer three junction device where that is PN PN device so there are four layers and three junctions so this is what P1 N1 P2 4 layers are there. With this P1 there is anode and with this P2 there is cathode. And over here gate is connected. And there are 3 junctions. Junction J1, Junction J2 and Junction J3. Now basically when we want to turn on a CR. we need to provide SCR with forward bias condition. So to turn on SCR, we connect anode with positive terminal of battery and cathode with negative terminal of battery. So when we connect SCR in forward bias, we can turn on SCR but there are some basics that we need to understand how SCR is getting turned on. So if we see its forward bias characteristic then what happens is if we give higher forward breakover voltage to SCR, SCR is getting turned on and after getting turned on we will be finding SCR is conducting. So here after giving forward break over voltage we will be able to trigger SCR and if you want to turn on SCR with lower forward break over voltage all we can do is we can provide gate signal. So as if we provide gate over here if I say this is the characteristic with IG is equals to 0 then to have Lower forward breakdown we can increase gate current. So if I provide certain amount of gate current Let us say I give certain amount of Ig then forward break over voltage That will decrease let us say this is VF dash VF0 dash so here one can clearly see forward break over voltage that is lower as if we increase gate current so to increase gate current current will be connecting gate supply where gate is connected to positive terminal of battery and negative terminal of battery that is what we are applying it with respect to cathode. So by applying VG voltage we will be able to trigger SCR earlier and this is what anode voltage that is what we are providing it to keep SCR in forward bias where positive terminal of battery that is connected with. anode and negative terminal of battery that is connected with cathode. So ultimately here in this characteristic what will happen when we provide gate signal that is what we are dealing with to study with respect to time. So what happens with respect to time that is what I will be explaining in this session so let us try to understand this characteristic now as I have told earlier to turn on SCR we need to give gate signal so here I am giving gate voltage so this is what gate voltage that I am giving right now and that is to turn on SCR now as we give gate voltage gate current will increase you So gate current will increase like this and then it will get constant. So gate current that increases as we can see it in figure and then after it is getting constant. Now here one thing that we need to understand. SCR gate current in this duration it increases from 90 percentage to 100 percentage. In this duration SCR gate current that increases from 90 percentage to 100 percentage. So this is what delay time. There are multiple definitions of delay time that I'll explain like how there are multiple definitions of delay time. So here one should understand gate current that increases from 90 percentage. to 100 percentage and that is what delay time. Now how we can have some other definitions of delay time. So initially SCR's anode current that is very minimum. Let us say that current that is happening which is less right now and once gate current that reaches to 90 percentage SCR's anode current will start to increase. And once SCR's gate current that reaches to 100%, SCR's anode current that will reach to 10%. So this is what SCR's anode current, it is happening due to leakage current initially and as gate current increases from 90 to 100 percentage, SCR's anode current that will increase from leakage current to 10 percentage. So ultimately second definition that one can say if SCR anode current increases from leakage current to 10% then that time period is delay time. so here we are observing characteristic with respect to time where gate current that exceeds from 90% to 100% that is a delay time or one can say SCR's anode current that increases from leakage current to 10% that is even a delay time and one more definition that one can give and that is if SCR's voltage, anode voltage, initially SCR is off, so anode voltage is maximum and once gate current increases from 90 to 100 percentage, anode current will start, anode voltage will start to fall and that will fall by 100 percentage to 90 percentage. So this is what 90 percentage of anode voltage is. anode voltage so third definition that is if SCR's anode voltage falls from 100% to 90% that is delay time. One definition could be anode current increases from leakage current to 10% that is anode current. Now once gate current that reaches to its maximum value, anode current will start to increase. it will increase by 90 percentage and as anode current increases to 90 percentage anode voltage that will fall to 10 percentage so ultimately in this period one can clearly observe as anode current which is from 10% to 90% anode voltage that decreases from 90% to 10% and this period that is called as a rise time so this period this is rise time so there are two basic definitions of rise time one can say anode current increases from 10% to 90% that is rise time or there would be second definition that explains anode voltage decreases from 90% to 10% so that is even rise time and then after anode current will increase to its maximum value which is 100% and at the same time anode voltage will decrease and it will go to minimum voltage which is what? forward conducting voltage so forward conducting voltage that will be somewhat around 1.2 voltage if that SCR is made up of silicon so here this is the time where anode current increase from 90% to 100% and that is spread time. So this is spread time or one can say anode voltage that decreases from 10% to forward that is even a spread time. Now here this spread time that depends on cross section of cathode. So this spread time it depends on cross section of cathode. So anode time that depends on cross section of cathode. So ultimately all together this is SCR's turn on characteristic which is the combination of delay time, rise time and settling time. So ultimately turn on characteristic. That is a combination of delay time, rise time and settling time. So delay time that is a time required for gate current to increase from 90% to 100%. And at the same time one can say second definition could be anode current requires to increase it from leakage current to 10% or one can say it could be anode voltage falls from 100% to 90% and then after rise time that is what time required for anode current to increase from 10% to 90% or for voltage anode voltage that decreases from 90% to 10% and then after that is time which is a time required to increase anode current from 90% to 100% or for voltage to fall from 10% to forward conducting voltage. So minimum voltage during forward conducting. induction that is a time for spread time and spread time depends on cross section and this is what turn on characteristic which is a combination of three time period delay time rise time and settling time so once SCR is getting turned on there are charge carriers which is there in junction j1 j2 and j3 now so i will be explaining turn off characteristic where a ton of characteristic requires to turn off scr and to turn off scr we need to remove complete charge carrier so when we talk about turn off characteristic it is a combination turn off characteristic so turn off characteristic that is a combination of reverse recovery time trr and gate recovery time tgr. So reverse recovery time that is a time required to remove charge carrier from junction j1 and j3. So this is what a time required to remove charge carrier from j1 and j3. And TGR that is what gate recovery time and that is a time required to remove charge carriers from junction J2. And when we talk about turn off characteristic, we need to give anode current below holding current to turn it off. So to turn off SCR it is very compulsory to decrease anode current below holding current to turn it off and before it gets completely turned off we need to remove all those charge carriers which is present in junction J1, J2 and J3. So I'll explain how that happens. So here we go. anode current is constant it is steady here anode voltage that is constant that is forward conducting voltage now once we start to decrease anode current it will decrease and then after it decrease with a rate di by dt let us say it decreases with the rate di by dt once it is reaching to zero it will further decrease with the same rate and it will decrease up to certain value and then that will be decreasing negatively after it reaches to zero and then after once it attains certain maximum value again it will start to increase positively and then after it will get steady and then it will get to zero. That will be the case which will be happening. Now here as we start to decrease anode current with certain amount. of rate di by dt. It will reach to zero and it will again further go below zero means it will go to negative and that happens because of charge carrier which is there in junction j1 j2 and j3. Now, here once it reaches to maximum negative value, again it will start to decrease and it will go towards 0 and it will attain certain value over here. So, if I say here it is getting 0 and here it is getting steady somewhat steady so during this period it will be removing complete charge carriers which is there in junction j1 and j3 so this is what reverse recovery time where it will be removing all those charge carrier which is there in junction J1 and J3. Now what will happen with anode voltage that is quite interesting. So as that voltage that attain sorry as that current that attains maximum negative value there will be a voltage spike which will happen. and once that J1 J3 charge carriers are getting removed that voltage even will decrease and it will decrease like this and then it will get to zero. So ultimately at this instant where current attains its maximum negative amount at that time there will be a voltage spike that one can see at anode voltage that will be a negative voltage spike. And from current zero to certain negative value, it will be having reverse recovery of charge carrier which is there in junction J1 and J3. Still SCR is off but there are some charge carrier which is there in junction J2. so it will never attain that forward blocking state as we have it it completely off so to make it completely off we need to remove charge carriers from junction j2 so ultimately those charge carrier which is there in junction j2 and that is what getting removed in this period so this is what gate recovery time. So after having reverse recovery time and gate recovery time there are no charge carriers which is there in junction j1 j2 and j3. So now one can say SCR is off and one more simple basic definition that is circuit turn off time. So circuit turn off time that is a time required from anode current to reaches to zero to anode voltage reaches to zero. So here one can see anode voltage that reaches to zero over here and anode current that is what reaching to zero earlier over here and then after anode voltage reaches to zero at this instant. So this is what circuit turn off time. So after circuit turn off time SCR is completely off. So ultimately this is what the complete time period from 4T on and this is what circuit time period off. Means one can say it is what T off time period. So T on is a combination of delay time, rise time and spread time. And T off that is what a combination of reverse recovery time. and gate recovery time. Reverse recovery time that is associated with charge removal from junction J1 and J3 and gate recovery time that is associated with charge removal of junction J2. So this is what switching characters do. stick of SCR. Thank you so much for watching this video. If you have any query regarding this just place that in comment so that one can have discussion regarding this and if you have any suggestions regarding videos to be there in this power electronics series you just add it in comment so that I will be able to make out those videos which is required to study to understand power electronics. Thank you so much for watching this video.

by the basic process let us try to understand what it requires to turn on SCR. So when we have a SCR, SCR is basically three layer four junction four layer three junction device where that is PN PN device so there are four layers and three junctions so this is what P1 N1 P2 4 layers are there. With this P1 there is anode and with this P2 there is cathode.

And over here gate is connected. And there are 3 junctions. Junction J1, Junction J2 and Junction J3.

Now basically when we want to turn on a CR. we need to provide SCR with forward bias condition. So to turn on SCR, we connect anode with positive terminal of battery and cathode with negative terminal of battery.

So when we connect SCR in forward bias, we can turn on SCR but there are some basics that we need to understand how SCR is getting turned on. So if we see its forward bias characteristic then what happens is if we give higher forward breakover voltage to SCR, SCR is getting turned on and after getting turned on we will be finding SCR is conducting. So here after giving forward break over voltage we will be able to trigger SCR and if you want to turn on SCR with lower forward break over voltage all we can do is we can provide gate signal.

So as if we provide gate over here if I say this is the characteristic with IG is equals to 0 then to have Lower forward breakdown we can increase gate current. So if I provide certain amount of gate current Let us say I give certain amount of Ig then forward break over voltage That will decrease let us say this is VF dash VF0 dash so here one can clearly see forward break over voltage that is lower as if we increase gate current so to increase gate current current will be connecting gate supply where gate is connected to positive terminal of battery and negative terminal of battery that is what we are applying it with respect to cathode. So by applying VG voltage we will be able to trigger SCR earlier and this is what anode voltage that is what we are providing it to keep SCR in forward bias where positive terminal of battery that is connected with.

anode and negative terminal of battery that is connected with cathode. So ultimately here in this characteristic what will happen when we provide gate signal that is what we are dealing with to study with respect to time. So what happens with respect to time that is what I will be explaining in this session so let us try to understand this characteristic now as I have told earlier to turn on SCR we need to give gate signal so here I am giving gate voltage so this is what gate voltage that I am giving right now and that is to turn on SCR now as we give gate voltage gate current will increase you So gate current will increase like this and then it will get constant. So gate current that increases as we can see it in figure and then after it is getting constant.

Now here one thing that we need to understand. SCR gate current in this duration it increases from 90 percentage to 100 percentage. In this duration SCR gate current that increases from 90 percentage to 100 percentage.

So this is what delay time. There are multiple definitions of delay time that I'll explain like how there are multiple definitions of delay time. So here one should understand gate current that increases from 90 percentage. to 100 percentage and that is what delay time. Now how we can have some other definitions of delay time.

So initially SCR's anode current that is very minimum. Let us say that current that is happening which is less right now and once gate current that reaches to 90 percentage SCR's anode current will start to increase. And once SCR's gate current that reaches to 100%, SCR's anode current that will reach to 10%.

So this is what SCR's anode current, it is happening due to leakage current initially and as gate current increases from 90 to 100 percentage, SCR's anode current that will increase from leakage current to 10 percentage. So ultimately second definition that one can say if SCR anode current increases from leakage current to 10% then that time period is delay time. so here we are observing characteristic with respect to time where gate current that exceeds from 90% to 100% that is a delay time or one can say SCR's anode current that increases from leakage current to 10% that is even a delay time and one more definition that one can give and that is if SCR's voltage, anode voltage, initially SCR is off, so anode voltage is maximum and once gate current increases from 90 to 100 percentage, anode current will start, anode voltage will start to fall and that will fall by 100 percentage to 90 percentage.

So this is what 90 percentage of anode voltage is. anode voltage so third definition that is if SCR's anode voltage falls from 100% to 90% that is delay time. One definition could be anode current increases from leakage current to 10% that is anode current. Now once gate current that reaches to its maximum value, anode current will start to increase.

it will increase by 90 percentage and as anode current increases to 90 percentage anode voltage that will fall to 10 percentage so ultimately in this period one can clearly observe as anode current which is from 10% to 90% anode voltage that decreases from 90% to 10% and this period that is called as a rise time so this period this is rise time so there are two basic definitions of rise time one can say anode current increases from 10% to 90% that is rise time or there would be second definition that explains anode voltage decreases from 90% to 10% so that is even rise time and then after anode current will increase to its maximum value which is 100% and at the same time anode voltage will decrease and it will go to minimum voltage which is what? forward conducting voltage so forward conducting voltage that will be somewhat around 1.2 voltage if that SCR is made up of silicon so here this is the time where anode current increase from 90% to 100% and that is spread time. So this is spread time or one can say anode voltage that decreases from 10% to forward that is even a spread time. Now here this spread time that depends on cross section of cathode. So this spread time it depends on cross section of cathode.

So anode time that depends on cross section of cathode. So ultimately all together this is SCR's turn on characteristic which is the combination of delay time, rise time and settling time. So ultimately turn on characteristic.

That is a combination of delay time, rise time and settling time. So delay time that is a time required for gate current to increase from 90% to 100%. And at the same time one can say second definition could be anode current requires to increase it from leakage current to 10% or one can say it could be anode voltage falls from 100% to 90% and then after rise time that is what time required for anode current to increase from 10% to 90% or for voltage anode voltage that decreases from 90% to 10% and then after that is time which is a time required to increase anode current from 90% to 100% or for voltage to fall from 10% to forward conducting voltage. So minimum voltage during forward conducting. induction that is a time for spread time and spread time depends on cross section and this is what turn on characteristic which is a combination of three time period delay time rise time and settling time so once SCR is getting turned on there are charge carriers which is there in junction j1 j2 and j3 now so i will be explaining turn off characteristic where a ton of characteristic requires to turn off scr and to turn off scr we need to remove complete charge carrier so when we talk about turn off characteristic it is a combination turn off characteristic so turn off characteristic that is a combination of reverse recovery time trr and gate recovery time tgr.

So reverse recovery time that is a time required to remove charge carrier from junction j1 and j3. So this is what a time required to remove charge carrier from j1 and j3. And TGR that is what gate recovery time and that is a time required to remove charge carriers from junction J2.

And when we talk about turn off characteristic, we need to give anode current below holding current to turn it off. So to turn off SCR it is very compulsory to decrease anode current below holding current to turn it off and before it gets completely turned off we need to remove all those charge carriers which is present in junction J1, J2 and J3. So I'll explain how that happens.

So here we go. anode current is constant it is steady here anode voltage that is constant that is forward conducting voltage now once we start to decrease anode current it will decrease and then after it decrease with a rate di by dt let us say it decreases with the rate di by dt once it is reaching to zero it will further decrease with the same rate and it will decrease up to certain value and then that will be decreasing negatively after it reaches to zero and then after once it attains certain maximum value again it will start to increase positively and then after it will get steady and then it will get to zero. That will be the case which will be happening. Now here as we start to decrease anode current with certain amount.

of rate di by dt. It will reach to zero and it will again further go below zero means it will go to negative and that happens because of charge carrier which is there in junction j1 j2 and j3. Now, here once it reaches to maximum negative value, again it will start to decrease and it will go towards 0 and it will attain certain value over here.

So, if I say here it is getting 0 and here it is getting steady somewhat steady so during this period it will be removing complete charge carriers which is there in junction j1 and j3 so this is what reverse recovery time where it will be removing all those charge carrier which is there in junction J1 and J3. Now what will happen with anode voltage that is quite interesting. So as that voltage that attain sorry as that current that attains maximum negative value there will be a voltage spike which will happen.

and once that J1 J3 charge carriers are getting removed that voltage even will decrease and it will decrease like this and then it will get to zero. So ultimately at this instant where current attains its maximum negative amount at that time there will be a voltage spike that one can see at anode voltage that will be a negative voltage spike. And from current zero to certain negative value, it will be having reverse recovery of charge carrier which is there in junction J1 and J3.

Still SCR is off but there are some charge carrier which is there in junction J2. so it will never attain that forward blocking state as we have it it completely off so to make it completely off we need to remove charge carriers from junction j2 so ultimately those charge carrier which is there in junction j2 and that is what getting removed in this period so this is what gate recovery time. So after having reverse recovery time and gate recovery time there are no charge carriers which is there in junction j1 j2 and j3.

So now one can say SCR is off and one more simple basic definition that is circuit turn off time. So circuit turn off time that is a time required from anode current to reaches to zero to anode voltage reaches to zero. So here one can see anode voltage that reaches to zero over here and anode current that is what reaching to zero earlier over here and then after anode voltage reaches to zero at this instant. So this is what circuit turn off time.

So after circuit turn off time SCR is completely off. So ultimately this is what the complete time period from 4T on and this is what circuit time period off. Means one can say it is what T off time period.

So T on is a combination of delay time, rise time and spread time. And T off that is what a combination of reverse recovery time. and gate recovery time. Reverse recovery time that is associated with charge removal from junction J1 and J3 and gate recovery time that is associated with charge removal of junction J2.

So this is what switching characters do. stick of SCR. Thank you so much for watching this video. If you have any query regarding this just place that in comment so that one can have discussion regarding this and if you have any suggestions regarding videos to be there in this power electronics series you just add it in comment so that I will be able to make out those videos which is required to study to understand power electronics.

Thank you so much for watching this video.

Transcript for:Switching Characteristics of SCR Explained

Transcript for:
Switching Characteristics of SCR Explained