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From: Chris Carlen
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.1) Gecko/20020826
X-Accept-Language: en-us, en
Subject: Re: 87% All that vector calculus paid off
Date: Mon, 23 Dec 2002 03:53:12 GMT
NNTP-Posting-Date: Sun, 22 Dec 2002 19:53:12 PST
Fritz Schlunder wrote:
> I rather doubt you are actually getting real spikes to 600-700V at the drain
> at MOSFET switch off. If we look at figure 12d from the IRF730a datasheet
> (http://www.irf.com/product-info/datasheets/data/irf730a.pdf), we can very
> roughly estimate that the anticipated avalanche voltage would be around 550V
> at an avalanche current of around 1.5A (whatever the maximum inductor
> current at switchoff is, but in your case in the vicinity of 1.5A).
> Assuming you are reallying using an IRF730a (by International Rectifier),
> and assuming you aren't dealing with a major fluke on your hands, it is most
> unlikely that you are actually getting 600-700V spikes.
Perhaps you are right, since I wasn't measuring this spike with proper
treatment of the high frequencies involved. It was a quick and dirty
look at the drain with a 20MHz B&K scope probe, with the long ground
I was afraid to hook up my Tek DPO to the thing. I need to build or get
my hands on some probes rated to more than 300V so I can use the DSO.
> BTW, for clarity this isn't a transformer isolated flyback converter is it?
> This is a simple discontinuous conduction mode boost converter (which you
> refer to as a boost flyback) topology, yes?
Yes. There is only one winding, only an inductor.
> I also find it most unlikely that a HER105 could possibly have a forward
> recovery voltage anywhere near 700V-351V = 349V. Unforuntately diode
> datasheets virtually never publish information about forward recovery
> characteristics, but in my experience forward recovery on an ultrafast diode
> doesn't normally exceed a few volts maximum (even with very high dv/dt and
Hmm. This is interesting. Ok, you got motivatied me to measure it better.
I just measured the drain with my 100MHz scope probe, with the RF tip
(no long ground wire, just the barrel touching the ground bus near the
FET source, and the pin touching the drain lead.)
Now I see a totally different picture. The flyback swings from 0V to
about 380V in 40ns. Then it settles to 325V in another 20ns or so. So
it seems the diode is behaving consistent with its 50ns reverse recovery
The forward recovery voltage is a new concept for me. I'm glad you have
made me aware of it. I hope I understand it better in time.
Christopher R. Carlen
Suse 8.1 Linux 2.4.19
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