SCR Thyristor Circuit Symbol and Terminal Identification ECE

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

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

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 3

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

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

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 IC 2=IB 1 IC 1 = IB 2 IF ECE 441 6

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

Apply a Gate Current For 0 < VAK < VBR(F), Turn Q 2 ON by applying a current into the Gate IF IC 2 = IB 1 This causes Q 1 to turn ON, and eventually both transistors SATURATE IB 2 VAK = VCEsat + VBEsat If the Gate pulse is removed, Q 1 and Q 2 still stay ON! VG ECE 441 IF 8

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 9

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 10

Input / Output Voltages ECE 441 11

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

Average Load Current ECE 441 13