Re: PWM Rectifiers


GTOs and IGBTs do block the negative. They are semicondutor devices and must be forward biased to work. The difference is that they are not line commutated like a thyristor or diode which turns off under its own volition when it becomes reverse biased. A thyrsitor cannot be turned off once ignited unless the supply across it goes negative (or current is otherwise interrupted). A GTO can be turned on by a pulse to one gate and off by a pulse to a second gate. This allows it to be turned off before the supply across it goes negative. Thus it can be turned on and off a number of times in any half cycle rather than once only as for the traditional thyristor. This is what allows it to be used for pulse width modulation.

The IGBT is a transistor and will only turn on when base current flows. Thus it can be turned on and off by turning on and off the base current. Through this it can also be turned on and off many times in a half cycle and thus be used for PWM applications.

GTOs and IGBTs are the building blocks of PWM controls. It is also worth noting that they are not only used for AC to DC conversion but also DC to AC conversion such as in VVVF drives and DC transmission links.

Whether you produce DC from AC or AC from DC depends on how you connect the IGBTs and what patterns of pulse witdth modulation you use.

Try getting hold of a manual of a VVVF drive from say Siemens or ABB and this will explain it all very well.