Reply-To: "fred bartoli"
From: "fred bartoli"
References: <3DAA0563.9040507@BOGUS.earthlink.net> <firstname.lastname@example.org> <3DAB8ABA.4030602@BOGUS.earthlink.net> <email@example.com>
Subject: Re: My flyback experiment is on the web
Date: Tue, 15 Oct 2002 13:55:34 +0200
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NNTP-Posting-Date: 15 Oct 2002 13:54:12 MEST
john jardine a écrit dans le message :
> Chris Carlen wrote in message
> > Frank Bemelman wrote:
> > >
> > > Just a question to thicken the plot: is your 1 ohm current sense
> > > 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
> > 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
> > 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
> are they?.
Yes, they are perfectly normal.
The flyback is working under discontinuous mode, what you can see from the
MOS vds, isrc plots :
source current start from zero at each cycle.
What happens is that, after the mos is opened, you have a short positive
pulse clamped to the ouput voltage (200V 1+us). After that, the inductor
stored energy being discharged (partly in the parasitics caps) the inductor
begins to ring with these parasitics. As the starting voltage of the ringing
is 200V-18V (output-input supplies) then the VDS should ring between 18V +/-
182V, but there's the drain-source mos diode that clamps the negative part
of the waveform, hence the ringing you can see on VDS *and* source current.