On 2009-01-11 12:14:37 -0800, "BobW"
said:


"Admin" wrote in message
news:2009011111185316807-sowhati@comcastnet...

Hi all,

I have this circuit that I am hope to use for HV electrolysis. It has
damaged every Mosfet and IGBT I have tried. It even damaged an IGBT
that is rated 600V and 58 amps. The circuit works without a hitch when
the load is resistive like a light bulb.

But as soon as I hook up a transformer the IGBT fails in 10 seconds. It
has trouble with inductive loads.

I have tried clamping the IGBT with a diode to no avail.

Please, somebody take a look and tell me what I am doing wrong. My
power source is and auto inverter.


What is the purpose of the top (120V) zener? You could remove and count on
the diode to clamp the drain to the plus supply, right? That zener might not
be fast enough to start conducting as soon as the fet is turned off and the
drain voltage wants to fly way above the plus supply.

It's possible that the whole transformer/Water cap circuit is ringing and
the drain is trying to go below the source, too.

Have you looked at the drain on a scope with a low capacitance probe?

If you just want to try stuff, try shorting out the top zener and also
replace the bottom zener with another fast diode. That should keep the drain
in bounds with reactive loads.

Bob

Thanks for taking the time to reply.
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