Page 252 - Power Electronics Handbook

Chapter 11
Forced commutation techniques

11.1 Introduction

Some power semiconductors, such as transistors and gate turn-off switches
(GTO), can be turned off by means of a signal on their gate terminal, whilst
in others the gate is only able to turn the device on and it will turn off when
the current through it has decayed to zero. The turn-off process is known as
commutation, and if the power semiconductor is operating from an a.c.
supply, then this will occur when the supply reverses, the process being
called natural commutation. If the power supply is d.c., or if the conducting
semiconductor is to be turned off at the non-zero part of an a.c. cycle, then
it must be commutated by forcing the current through the device to zero, and
this is called forced commutation.
Choppers and inverters have already been introduced in Chapter 6 and if
thyristors are used as the power switches for these circuits, as is usual, then
they need to be forced commutated. Figure 11.1 shows the principle

F

By-pass circuit

+vd

Flpre 11.1 A generic forced commutated system
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