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From: Don Pearce
Subject: Re: My flyback experiment is on the web
Date: Tue, 15 Oct 2002 16:32:11 +0430
References: <3DAA0563.9040507@BOGUS.earthlink.net> <firstname.lastname@example.org> <3DAB8ABA.4030602@BOGUS.earthlink.net> <email@example.com>
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On 15 Oct 2002 04:38:07 -0700, firstname.lastname@example.org (john
>Chris Carlen wrote in message news:<3DAB8ABA.4030602@BOGUS.earthlink.net>...
>> Frank Bemelman wrote:
>> > Just a question to thicken the plot: is your 1 ohm current sense resistor
>> > wirewound, and does it have an inductance too ?
>> I honestly don't know, but am inclined to think it isn't. It's a little
>> precision resistor, strange looking black cigar shaped thing, about 1W 1%.
>> But it has a very small diameter, so it can't have much inductance. I
>> suppose that could slow the FET turn-on by allowing the source to rise
>> up on the rising edge of the gate pulse.
>> But it seems to me there'd be some tell-tale signs of this, like a
>> segment of exponential rise in the current ramp before flattening out to
>> a linear ramp. But this isn't happening.
>> OTOH the little ring bursts coinciding with the gate drive edges could
>> be coming from this resistor, if it is a wirewound.
>> I am planning to get some for certain non-inductive current measuring
>> resistors in my next ordering cycle.
>> What I'd really love to have is a good high frequency and high voltage
>> differential voltage probe, so I could stick a resistor in the drain
>> instead of the source. Maybe I should learn how to use a current tranny?
>> Good day!
>I rarely design these things so I'm wondering what's the
>burst of 6, clean, decaying spikes?/echoes?/ringing? things that are
>turning up way after the the main pulse?. No one seems to have
>mentioned them, so I
>assume they are a normal expected artefact. What causes them?, what
I suspect they are layout-related. This method of breadboarding is
rather beyond its applicability in high speed circuits like this. Nice
short leads tied down to a ground plane are the order of the day. The
tiniest amount of common inductance in the ground would produce these
spikes. If the simulator says they shouldn't happen, then almost
certainly this is the cause.
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