Reply-To: "fred bartoli"
From: "fred bartoli"
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Subject: Re: My flyback experiment is on the web
Date: Wed, 16 Oct 2002 11:21:41 +0200
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NNTP-Posting-Date: 16 Oct 2002 11:20:10 MEST
John Larkin a écrit dans le
message : email@example.com...
> On Wed, 16 Oct 2002 00:07:58 GMT, John Popelish
> >John Larkin wrote:
> >> On Tue, 15 Oct 2002 19:12:16 GMT, John Popelish
> >> wrote:
> >> >I disagree with you and Fred. The ringing should be a damped
> >> >sinusoid, not a series of fractional sine pulses if it were just
> >> >ringing.
> >> It can't make a damped sine because of the fet substrate diode. This
> >> waveform is characteristic of drain circuit ringing.
> >I understand how the stray (inductor, wiring and drain) capacitance
> >charged up to output voltage will drain through supply voltage and
> >thus charge up the inductor with current which then drives the voltage
> >below zero (assuming that output voltage is more than twice supply
> >voltage). But that current is drained to zero through the diode, and
> >then the capacitance is charged again towards supply voltage which
> >runs the inductor current up the other way enough to drive the
> >capacitance up to twice the supply voltage. But then all subsequent
> >ringing should be centered on supply voltage, and not ever forward
> >bias the diode. The rest of the wave should be a damped sinusoid
> >centered around the supply voltage. This is not at all the waveform
> >shown. I see signs of repeated turn on of the fet.
> As I think about it, I'd be inclined to say you are right. But if you
> look at the Vg-Vd waveform, the positive gate spikes nearly align in
> time with the positive drain spikes. So during the 'low' parts of the
> ringing phase, the gate is biased off, and during the high spikes the
> gate is the most positive. This is backwards from what you'd get if
> the gate was causing the drain spikes. It looks more to me like the
> gate is being driven from the drain spikes through the D-G
> capacitance. Besides, the gate spikes look too small to turn on the
You're right, John.
On the first plot (Isrc & Vgs) one can see that the gate threshold voltage
is about 6 V (small plateau on the rising & falling edges), whilst the gate
"spikes" hardly come over 2V. This can't drive the mos on.
> So, where is the energy being stored during the 'low' periods? I've
> seen this weird spiking before, and never really understood it.
> Maybe the substrate diode is acting like a step-recovery diode, ie, a
> high-value capacitance when foreward biased, storing charge for some
> time after the current has turned around. The extended conductance
> holds the inductor's low side down long enough for a fresh load of
> current to be pushed into the inductor from the supply. This, if so,
> would clearly be very bad for efficiency.
Right again, except that mos reverse diode don't need to be SRD. Only long
reverse recovery time is needed.
Looking at the IRF730 datasheet (ST) the figures are really bad for this one
: 470ns typ. at 5.5A
Looking at the source current waveform, one can see that the reverse
recovery time is about 1us (small portion of the sawtooth that comes over
the baseline) for the first ringing, in accordance with the observed
> A test for this would be to plop a schottky diode in parallel with the
> substrate diode, and see if the spikes go away, and maybe efficiency
Schottky diodes for a 200V output will be "hard" to find. But 2 diodes, one
in series with the maos drain, the other in parallel with the "new" level
shifted drain will do the trick.
Oh, spice can provide you some of those nice multiKV ultra low forward drop
> Now that I think about it, I have seen this same waveshape in a
> step-recovery-diode pulser simulation.
BTW, SRDs becoming almost impossible to find, does somebody have a reference
for good "bad diode", exhibiting (really)good step recovery behaviour at
resonnable power levels ? I have some at SMPS power levels but that to much
power waste just for the purpose of a generator.
> Cool, if true.