[Boatanchors] Parallel Operation of HV Power Upplies

[J. Forster]

The problem with just paralleling the transformer secondary leads is that
you will get circulating currents in the secondary windings, if the
transformers are not exactly identical. This increases their internal
dissipation and heating. The amount depends on how unequal the units are.
It essentially shorted turns.

Possible solutions:

General Radio made special "balancing chokes" for use when paralleling
Variacs. They were essentially a choke with a center tap. The outer legs
went to the transformers, the center leg was the output. The cleverness of
this is that it has a high impedance to circulating currents, but a low
impedance to the wanted power (because the magnetic fields buck in the
choke). You could easily use an old trandformer this way.

A second solution is to use two series chokes, one per transformer, but
this is lossier than the mothod suggested above.

A third way is to use separate diodes for each transformer (two per, if
it's center tapped, four per if you're using a FW bridge) and connect the
unfiltered DC outputs in parallel. While this will not guarantee complete
equal load sharing, it will prevent circulating currents.

A fourth way is to parallel the filtered DC outputs. I see no real
advantage to that, unless you have two fully built, unregulated, power
supplies.

If the supplies have active regulation, all bets are off.

FWIW,
-John




> A Quick Question-
>         I need about 800 mA at 2KV. I do not have a single transformer to
> supply this. I do have two identical transformers that will supply
> 400 mA at 2 KV.
>         I have been studying parallel operation of transformers, but had
> these questions:
>                 Would just paralleling the transformers be sufficient, or
> should there be low value resistors isolating the HV
> windings?
>                 Would it be better to use two identical power supplies
> (transformers, rectifiers, filters) and connect the HV DC
> leads together and parallel them in this fashion? (I have
> full power supplies already containg the 400 mA at 2 KV
> transformers)
>                 If I went the complete power supply paralleling route
> would there be any benefit in passing each HV DC lead
> through a HV diode before actually tying leads together?
>                                                                 Thanks,
> Byron
> WA5THJ
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