On Electromagnetic Processes in HV Transformers of Switching-Mode Power Supplies at No-Load Conditions

When applying some kind of waveform to the primary of a transformer, one expects to see a more or less resembling waveform at the secondary. This is not the case for high-frequency HV transformers. The basic reason is that the parasitic parameters (PP) (leakage inductance and numerous parasitic capacitances) have a considerable influence on the transformer performance. HV transformer equivalent circuits are discussed, and the choice of one for further analysis is theoretically and experimentally justified. It is shown that HV rectifier/multiplier adds considerably to the parasitic capacitance of the secondary winding at no-load conditions. Further analysis concentrates on the electromagnetic processes in a transformer fed by a square-wave voltage, which is characteristic for no-load operation. Overvoltages (OV), i.e., ratio of the maximum voltage at the secondary to that calculated by the transformation ratio, generated by high harmonics are analyzed using Fourier transform. For special cases of asymmetrical waveshapes, closed-form expressions are obtained. The influence of the primary voltage risetime on the OV is also analyzed. It is shown that the characteristic OV is close to three for steep risetimes. Main conclusions were upheld by PSpice simulations and experimental investigation