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Understanding SCR Switching Characteristics

Sep 13, 2024

Power Electronics Lecture Series: Switching Characteristic of SCR

Overview

  • Focus on the switching characteristic of SCR (Silicon Controlled Rectifier).
  • Explanation of SCR turn-on and turn-off characteristics.

SCR Basics

  • Structure: SCR has 3 layers and 4 junctions (PNPN).
    • Layers: P1, N1, P2 (Anode and Cathode).
    • Junctions: J1, J2, J3.

Turning On SCR

  • Forward Bias Condition: Connect anode to positive and cathode to negative terminal of the battery.
  • Forward Breakover Voltage: SCR turns on when forward breakover voltage is applied.
  • Gate Signal: To reduce forward breakover voltage, apply gate signal.
    • Increasing gate current (Ig) results in lower forward breakover voltage.

Key Characteristics

  • Delay Time: Time taken for gate current to increase from 90% to 100%.

    • Definitions:
      • Anode current increases from leakage current to 10%.
      • Anode voltage decreases from 100% to 90%.

  • Rise Time: Time taken for anode current to rise from 10% to 90%, or voltage to decrease from 90% to 10%.

  • Spread Time: Time taken for anode current to rise from 90% to 100% or voltage to fall from 10% to forward conducting voltage (approx. 1.2V for silicon).

Complete Turn-On Characteristic

  • Combination: Turn-on characteristic combines:
    • Delay time
    • Rise time
    • Spread time

Turning Off SCR

  • Charge Carrier Removal: To turn off SCR, all charge carriers in junctions must be removed.
  • Turn-Off Characteristics: Combination of:
    • Reverse Recovery Time (Trr): Time to remove charge carriers from junctions J1 and J3.
    • Gate Recovery Time (Tgr): Time to remove charge carriers from junction J2.

Key Points in Turn-Off Process

  • Anode Current: Must fall below holding current to turn off SCR.
  • Current Behavior:
    • Initial steady state, then decrease at rate di/dt.
    • Current will go negative before returning to zero.
  • Voltage Spike: A negative voltage spike occurs during charge removal.

Circuit Turn Off Time

  • Time taken from when anode current reaches zero to when anode voltage reaches zero.

Summary of Time Periods

  • T_on: Delay time + Rise time + Spread time.
  • T_off: Reverse recovery time + Gate recovery time.

Conclusion

  • Understanding SCR switching characteristics is crucial for power electronics.
  • Open for questions and suggestions for future video topics.

Full transcript