An experimental study of a soft-switched isolated AC/DC converter without auxiliary circuit

An isolated AC/DC converter topology incorporates a capacitively snubbered voltage source converter (VSC) and a cycloconverter, coupled by a medium frequency transformer. The topology offers the possibility of bilateral power flow as well as three-level pulse width modulation on the AC side. It is shown that by alternately commutating the VSC and the cycloconverter it is possible to achieve either zero-voltage or zero-current switching conditions for all of the semiconductor devices in all points of operation. This is the case without any need for auxiliary semiconductor devices. At low load the transformer current may be insufficient for recharging the VSC snubber capacitors. In this case however it is possible to utilize the cycloconverter for providing a current path by which a quasiresonant commutation can be effected. The design and operation of a 40 kVA prototype converter system is described. It is shown how the rather complex switching logic required to implement the chosen algorithm for commutation and modulation can be realized by using modern programmable logic devices (FPGA). Measurement results from the prototype converter are presented and analysed. The measurements indicate that the studied commutation algorithm works well in practice.