"Mike" wrote in message
...
On Mon, 10 Mar 2008 10:52:38 -0500, "amdx" wrote:

snip
Thanks Tim,

Let me start with your last sentence first, expensive is what I'm
trying
to prevent.

I found a UPS at a metal scrap yard, a salvaged the SCR, IGBTs,
capacitors
and several other parts from it. So right now I have $10.00 into the
project. I want to protect the IGBTs because I wouldn't spend the money
to
purchase these new, They're probably $100.00 each.

Now your first sentence, "why would you treat IGBTs like SCRs, of all
things?"

I don't understand the question, if it's because I'm using them as a
switch,
Well, it's

the high current semiconductor I have at hand.



The IGBTs do have internal diodes. Yes I agree I want a fast rise time.

You say, "perhaps a ZTX651 emitter follower per gate."



So you think I should drive each gate separately?



Then you said,

"(no 100 ohm to ground, I don't even know what made you think of that).

I started testing these IGBTs and the first few tested shorted, then
I
realized I was testing them with the gate floating. So I pulled it to
ground
with a resistor. Then they operated as expected. That's why I put the
resistor to ground.

Is your opinion it to small or not needed?



Now regarding "exercise in futility" it may be, I have since searched
schematics of can crushers, ring *******, etc, and see they use high
voltages across there inductors, and often a sparkgap as the switch.

More on the order of 10kv, not 450v.



Any thoughts on how to protect the IGBTs from ringing voltages.

Thanks for your input,

Mike


Unless you need to turn the IGBTs off before the pulse ends an SCR would
be a much better choice.
Perhaps you could scrounge around the scrapyard some more and find one for
cheap.

I suspect that Tim was refering to the way you thought of to drive the
gates by discharging a cap
into them. That is sometimes done to trigger an SCR and I've never seen it
done to turn IGBTs on.

If you must use the IGBTs then yes, you will have much better luck driving
each gate with it's own
high current driver.

The easiest way to protect from reverse voltages is to simply put a huge
diode across the the coil
and let it absorb the neg swings when it rings down. Not an elegant
solution, but it will work.
I know because I've done it.

Mike

Hi Mike,
I liked the cap driving the gates because once the current stops flowing
the SCR shuts off. I figured I could size the cap to set my
ON time.
Regarding the reverse diode, the IGBTs have a diode built in.
Is the internal diode a fast diode? Also I see mosfets and IGBTs have these
body diodes, are they purposely built in or are they part of the
manufacturing process?
Thanks, Mike