Using the 1uF/25V tantalum I see a 12V square wave across the FET, you really think this is risky? I never see anything close to the cap's voltage rating.
My first attempt was an RC snubber across the FET, 1Ohms, and 470pF, I saw no impact then I tried larger capacitance and the ringing went away... I can experiment again with the small snubber
The risk is twofold:
1 Tantalums (especially solid types) cannot handle high peak charge or discharge currents. Look at tant cap datasheets you will often see a reccomended series resistance of one ohm per volt implying peak currents of no more than one amp. When your fet turns on the cap sees a very high discharge current and will sooner or later shatter/fail/explode/catch fire. Some tant makers even have inbuilt fuses for this reason. 2 Your fet will also experience this very high discharge current every time it switches on, less likely to cause damage to the fet but still an avoidable over-stress.I suggest you add some series resistance to the cap.
piglet
Yes. As noted the high AC current through the tantalum cap is not good for it and they fail in spectacular ways. A cap is a bad idea here, but a tantalum cap makes it many times worse. You're just asking for a fire. Any cap here should be a ceramic and it really needs the R to absorb the energy of the ringing. A cap alone will only change its frequency.
Your cap is probably too small.
Where did you take the measurements from, gate or drain?
It does makes a difference you know but I am guessing that you got these from the gate and a resistor in series to the gate should of solved that.
Jamie
I was measuring the Drain when I saw the ringing, I have a 1k resistor going to the gate now
So I take it you must have an inductive load?
jamie
Not necessarily very much. Wiring inductance (say, 12" leads to the TEC) will be sufficient to provide "ringing".
Tim
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