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From: email@example.com (john jardine)
Subject: Re: My flyback experiment is on the web
Date: 15 Oct 2002 16:07:00 -0700
References: <3DAA0563.9040507@BOGUS.earthlink.net> <firstname.lastname@example.org> <3DAB8ABA.4030602@BOGUS.earthlink.net> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 15 Oct 2002 23:07:00 GMT
"fred bartoli" wrote in message news:<email@example.com>...
> 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?.
> > Thanks
> > john
> 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.
Thanks guys. Spot on!.
It made clear sense from your descriptions and a casual simulation
gave results nearly identical to the supplied pics. (needed 50k loss R
across the L to mimic the amount of ringing damping).
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