SCR / Thyristor

1. SCR / Thyristor • Circuit Symbol and Terminal Identification ECE 441

2. SCR / Thyristor • Anode and Cathode terminals as conventional pn junction diode • Gate terminal for a controlling input signal ECE 441

3. SCR/ Thyristor • An SCR (Thyristor) is a “controlled” rectifier (diode) • Control the conduction under forward bias by applying a current into the Gate terminal • Under reverse bias, looks like conventional pn junction diode ECE 441

4. SCR / Thyristor Anode • 4-layer (pnpn) device • Anode, Cathode as for a conventional pn junction diode • Cathode Gate brought out for controlling input P N Gate P N Cathode ECE 441

5. Equivalent Circuit ANODE P N N GATE P P N CATHODE ECE 441

6. Apply Biasing With the Gate terminal OPEN, both transistors are OFF. As the applied voltage increases, there will be a “breakdown” that causes both transistors to conduct (saturate) making IF > 0 and VAK = 0. VBreakdown = VBR(F) IF IC2=IB1 IC1 = IB2 IF ECE 441

7. Volt-Ampere Characteristic IF Holding Current IH VBR(F) VAK Breakdown Voltage ECE 441

8. Apply a Gate Current For 0 < VAK < VBR(F), Turn Q2 ON by applying a current into the Gate This causes Q1 to turn ON, and eventually both transistors SATURATE VAK = VCEsat + VBEsat If the Gate pulse is removed, Q1 and Q2 still stay ON! IF IC2 = IB1 IB2 VG IF ECE 441

9. How do you turn it OFF? • Cause the forward current to fall below the value if the “holding” current, IH • Reverse bias the device ECE 441

10. SCR Application – Power Control When the voltage across the capacitor reaches the “trigger-point” voltage of the device, the SCR turns ON, current flows in the Load for the remainder of the positive half-cycle. Current flow stops when the applied voltage goes negative. ECE 441

11. Input / Output Voltages ECE 441

12. Look at the LOAD Current Conduction time  Conduction Angle = 180 -  ECE 441 “Firing” time  Firing Angle ()

13. Average Load Current ECE 441