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NNTP-Posting-Date: Fri, 10 Jan 2003 17:46:24 -0600
From: email@example.com (John Fields)
Subject: Re: Transient problem !
Date: Fri, 10 Jan 2003 23:30:55 GMT
Organization: Austin Instruments, Inc.
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On Thu, 9 Jan 2003 23:23:12 +0100, "Peter Larsson"
>I have this circuit:
> | | 10 ohm (0,1uH series inductance)
> D |
> Gate IRFP150
>The problem is this: I drive this test application with an 12V car battery.
>The gate is driven by a MAX627 mosfet driver(runs at 100kHz) with
>0.1uF(polyprop) and 680uF (low esr electrolytic) connected on the supply
>line and close to the MAX627 from the same battery.
>There is a high transient over the drain and source at turn off. The
>transient is about 25 volts between drain and source and very narrow.
>If I parallel another 10 ohm resistor the transient increases to 40 volts.
>When the GROUND line is measured, there are also an increase of the
>transient towards the electrolytic cap. This capacitor has been removed and
>the result is ringings over the squrewave but the transient is still there !
>My question is where does this come from?
>Source inductance:13 nH
>Drain inductance:5 nH
>How can I find the source of this narrow transient ?
Remembering that E = LdI/dT, I'd suspect stray inductance.
if this is what you have:
and your resistor looks like 100nH, then I'd suspect that the energy
stored in the resistor's inductance is the culprit.
Rearranging E = LdI/dT, dT = LdI/E = 100nH*1.2A/25V = 4.8nS, so if
you're turning off the MOSFET that fast it could happen. Seems
pretty fast, but depending on how you've got the circuit laid out
you could have a lot more inductance than that. Also, the voltage
increase when you double the current by parallelling the resistors
points to inductance doing it. Put a small cap from drain to ground
and see what happens...
Professional circuit designer
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