Analysis of capacitor-related mid-voltage point shift problems in high-voltage half-bridge DC/DC converters

Half-bridge topology is an attractive topology for many power electronics applications. It is simple and more cost-effective than other topologies. The new high-voltage IGBTs on the market enlarge the possibilities and add to the attractiveness of that topology, in particular in high voltage applications. However, despite the overall design simplicity, the designer must guard against a number of problems with this type of a converter. A major problem is in the staircase saturation of the isolation transformer core. One of the possible reasons is the mid-voltage point shift, which can be caused by the input capacitor mismatch. This paper analyses in detail how input capacitors influence the voltage unbalance in a high voltage half-bridge converter. In this project polypropylene capacitors were used. Thus special attention will be paid to polypropylene capacitors. Through analysis the maximal possible asymmetry between the input capacitors can be estimated. From the asymmetry the resulting mid-voltage point shift will be calculated and finally verified with simulations and measurements.