IGBT Modules - Technologies, Driver and Application (Second Edition) - page 390

Fig. 11.8
Examples of diode (B6) and thyristor (B6C) rectifierswith IGBTbrake chopper
11.2.1ActiveFront End (AFE)
The disadvantage of using a brake chopper, especially for high power applications, is
the high temperature created in the brake resistor. A better solutionwould be to feed the
regenerated energy directly back into the supplyingmains. This can be achieved with a
second thyristor bridge for example as shown in
which operates anti-parallel
to the rectifier.
Fig. 11.9
Exampleof a rectifier equippedwith thyristorswith regenerating unit
A further stage of this topology is the Active Front End (AFE), which is usually equipped
with IGBTs. The advantages of an AFE with IGBTs compared with a rectifier
regenerating unit based on thyristors are as follows:
Avoidance of commutation errors in regenerative operation with the line-
commutated thyristors, which is a problem that can occur when the voltage
drops temporarily or fails completely. The use of IGBTs makes it possible to
commutate independently of themains.
When IGBTs are used, with an appropriate design the DC-bus voltage (boost
mode) can be increased to a level above the voltage achievablewith thyristors.
This is done with the aid of the mains impedances, the impedance of an input
transformer and/or additional input inductors.
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