A bi-directional AC-DC power converter for fixed polarity DC loads

Semiconductor power modules which include both transistors and diodes are extensively used for variable frequency inverters feeding motors, and they can also be used for the supply-side converter, providing rectification via the diodes and inversion via the transistors. The drawbacks are their cost, and supply waveform disturbances caused by transistor switching which is essential to the control of the inversion process. The use of an autotransformer (operating at supply frequency) to correct the voltage mismatch is possible but unattractive in terms of cost, and especially bulk. Other solutions, involving the isolation of the diode bridge during inversion by means of series transistors or thyristors, are possible, but involve transitional difficulties when power flow reverses. The authors describe a circuit which aims to solve these problems in an economical way involving minimum bulk and maximum controllability both in steady state and during transient operation