[Boatanchors] Parallel Operation of HV Power Upplies

[Brian Clarke]

Hi Byron,

The internal impedances looking back from the secondaries of the two transformers will determine the power / load sharing. If you don't know how to measure this, ask.

If both impedances are the same - not the same as the DC resistance - use diodes off the windings and parallel the diode outputs into a common filter. Now, if you wish to save power, you can turn off one of the primaries and nothing untoward will happen as long as the diodes hold up.

If however, you are using strings of dodes in series in order to meet the maximum reverse repetitive Voltage rating of the diodes, then you should use small capacitors in parallel with each diode - say 10 to 100 nF - to equalise the reverse bias. In that case, there will be some energy lost to the non-powered transformer via the capacitors - it's the core mag loss.

If the transformer impedances are not the same, then you are in a more risky area - there may not be equal power / load sharing.

Another little thought - how are you measuring the 2 kV @ 400 mA capacity of the transformers? Is this what the label says, or have you measured it? How? Don't believe the label - every transformer is slightly different. If you use full-wave rectification into a capacitive-input filter, then the Voltage will be 2.8 kV and the current available will be 280 mA. If you are using full-wave rectification into a choke-input filter, the the output will be 1.8 kV at approx 400 mA.

73 de Brian, VK2GCE.
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