25kW 3-Phase Unity Power Factor Buck Boost Rectifier with Wide Input and Output Range for Pulse Load Applications

Solid state pulsed power systems containing IGBT switches are mainly operating with input voltages between 400 V and 3 kV depending on the maximum allowable blocking voltage of the IGBTs. Nevertheless, high output voltages of several kilovolt can be achieved for example by the use of a pulse transformer, a Marx generator configuration or a combination of both. The pulse power (e.g. 20MW) is normally provided from a capacitor bank whereas the average power (here 25kW) is supplied by a converter connected to the mains, whose line to line voltage can vary from 177 V to 528 V world wide. Depending on the ratio of the mains to the capacitor bank voltage the converter has to work in a buck or boost mode. Additionally, due to the possible high repetition rate of pulse modulators (500-1000Hz) the capacitor bank has to be recharged before the next pulse is generated, which would demand a high dynamic voltage control. In the 3-phase buck boost rectifier a constant current through the buck boost inductor is needed to provide a unity power factor. But the pulse load in combination with the high dynamic voltage control would result in periodic peak currents at the converter input/mains and therefore a unity power factor at the input of the converter can not be guaranteed and the mains current would be distorted. Therefore, a new control for pulse load applications has to be implemented which achieves a unity power factor and an accurate regulation of the output voltage. First, in the paper the structure/operation principle of a standard buck boost converter is described and the problems resulting from the pulse load are discussed. Thereafter, a new control is derived which avoids the mentioned problems. Finally, the paper is concluded with measurement results for the input currents and the DC voltage during pulse operation.