Q.What are opto-couplers?

Every discrete controlled power device has a control terminal (the gate, with one main terminal) that must be driven correctly to switch the device. A gate drive circuit satisfies these control-terminal requirements so as to turn each device of a converter reliably ON and OFF.

Because the devices in a converter sit at different points and voltage levels in the power circuit but are commanded by one common controller, electrical isolation is essential — it protects the low-voltage controller from the high-voltage power side, and each device usually needs its own isolated drive supply. There are two functional types of isolation:

Pulse-transformer isolation is used for thyristors (SCR and TRIAC). A pulse transformer — usually a 1:1 two-winding type with low winding and leakage resistance — couples the gate signal from the control circuit on the primary to the gate and cathode of the device on the secondary, with no direct electrical connection between the two sides, and needs no separate supply on the secondary. …