Power Electronics Lecture Series: Switching Characteristic of SCR

Introduction

• The session focuses on the switching characteristics of SCR (Silicon Controlled Rectifier).
• Key aspects covered: Turning ON and OFF characteristics of SCR.

Turning ON SCR

SCR Structure

• SCR is a PNPN device with:
  • 4 layers (P1, N1, P2)
  • 3 junctions (J1, J2, J3)
  • Anode connected to P1, Cathode to P2, and a Gate.

Conditions for Turning ON

• SCR must be in forward bias:
  • Anode connected to the positive terminal of the battery.
  • Cathode connected to the negative terminal.

Forward Breakdown Voltage

• SCR turns ON at higher forward breakover voltage.
• To trigger at a lower voltage, a gate signal can be applied.
• Increasing gate current (Ig) reduces the required forward breakover voltage.

Gate Current and Delay Time

• Delay time occurs as gate current increases from 90% to 100%.
• Several definitions of delay time:
  1. SCR anode current rises from leakage current to 10% after gate current reaches 90%.
  2. Anode voltage drops from 100% to 90% during the same period.

Rise Time

• Rise time occurs as:
  • Anode current rises from 10% to 90%.
  • Anode voltage drops from 90% to 10%.

Spread Time

• Spread time occurs as:
  • Anode current rises from 90% to 100%.
  • Anode voltage falls from 10% to forward conducting voltage (approximately 1.2V for silicon).
• Spread time depends on the cross-section of the cathode.

Summary of Turn ON Characteristic

• Combination of:
  • Delay Time
  • Rise Time
  • Spread Time

Turning OFF SCR

Conditions for Turning OFF

• To turn OFF, the anode current must be decreased below the holding current.
• All charge carriers in junctions J1, J2, and J3 must be removed.

Turn OFF Characteristic Components

1. Reverse Recovery Time (TrR)
   • Time to remove charge carriers from junctions J1 and J3.
2. Gate Recovery Time (TGR)
   • Time to remove charge carriers from junction J2.

Current and Voltage Behavior During Turn OFF

• As the anode current is decreased:
  • It reduces at a rate (di/dt).
  • May briefly go negative, leading to a voltage spike.
  • The current eventually stabilizes to zero.
• Anode voltage drops to zero after anode current reaches zero indicating the SCR is off.

Circuit Turn OFF Time

• Defined as the time from when anode current reaches zero to when anode voltage reaches zero.
• After circuit turn off time, SCR is completely OFF.

Summary of Turn OFF Characteristic

• Combination of:
  • Reverse Recovery Time
  • Gate Recovery Time

Conclusion

• Understanding the switching characteristics (turn ON and OFF) of SCR is crucial for power electronics applications.
• For questions or suggestions regarding future topics, comments are welcomed.