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Old 07-10-2010, 03:34 PM   #17 (permalink)
Mark Weisheimer
Join Date: Aug 2009
Location: Columbus Ohio
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Originally Posted by electrowizard View Post
Thanks for the reply Mark;

I've read enough about IGBTs to understand what you're talking about, however my controller is using MOSFETs (after three weeks of reading and deciding)

The TVS is a unidirectional 15.2V unit so it should be quite similar to a 15V zener. My max Vgs is 16V so I should be safe there.

I know a lot of people use the 10k pulldown resistors.
Maybe my driver connection came loose and my mosfet went into triode... I don't believe they are as sensitive to that as you say IGBTs are. I suppose I will include it also just to be safe.

Six mosfets down, four to go! Good thing I bought lots of spares :P

Unrelated question: mosfet gate resistors (not pulldown)- absolutely necessary? I have looked high and low; it seems they are chosen empirically rather than actually calculated. My circuit seems to want a very low gate resistance. Maybe I should have chosen a more powerful gate driver? I don't really understand the tradeoffs here.

I had to laugh. I was just so sure you were using IGBT's!

I pulled up the datasheet. They show using gate drive resistors in diagram 21.
The resistors are usually chosen for "turn on time"
(dv/dt, or delta voltage / delta time)

It can be calculated as an RC time constant using C(ISS), drive voltage,
and gate drive resistance value. You are shooting for a fast turn on
without creating severe ringing.
That device is made to use "logic level drive" so you should be able to drive
it with 5vdc.

I would still use gate pull down resistors, and 10K is fine.
Your TVS value is fine for voltage protection too.

I wonder if the power supplies are as isolated as you are led to believe.
I have seen supposed isolated supplies that had resistors or caps
tied to ground or a supply line and were NOT isolated.
Do you have any surplus switching supplies lying around,
such as are commonly used for laptops or LCD TV's and etc?
I have found them to be good sources for test supplies
and I am very careful when using transformer based wall warts.

I was trying to draw a relationship to the device blowing when
you pulled your probe, and did not even think of the common ground
issue. I tend to use a battery powered portable scope and
ignore common/grounding issues as a result.

Good Luck!

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