Selecting diodes for HV full bridge rectifier

Question

I'm using a current-fed Royer self resonant circuit to generate 1kV DC output voltage. The transformer is producing a sine wave of 100kHz/2kVpp (Cap-Royer = 100nF). At the output of the transformer I added a full bridge rectifier consisting of four BY203 HV diodes (2kV, <300ns), now I see that after adding the bridge rectifier the idle supply current of the Royer converter doubled from 25mA to 50mA, without adding a load!

By coincidence I found that adding a 20pF capacitor in parallel to one of the diodes is reducing the idle current to 27~30mA, but adding capacitors to two diodes (U$15, U$17) brings it back to the high current.

Is the problem here maybe that the diodes are not fast enough? I couldn't find diodes with lower recovery time for the high voltage >1.5kV.

What effect is causing the parallel capacitor to reduce the idle current?

Answer 1

It's possible that with the secondary open circuit, its natural resonant frequency (due to secondary leakage inductance and parasitic capacitance between windings) is somewhat higher than 100kHz (the switching frequency). When you add the diodes you are also adding capacitance to the secondary and lowering its resonant frequency, possibly to closer coincide with 100kHz. This can mean that you now find it easier to to generate higher output voltages and in fact your AC output voltage may have doubled (take care on this of course because you may destroy the diodes and final output capacitor). What you ought to do is try and determine the secondary AC voltage before and after adding the diodes (which are OK speed wise and reverse recovery wise).

Adding yet another capacitor possibly re-tunes things differently and current drops.