The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "Fritz Schlunder"
Subject: Re: 87% All that vector calculus paid off
Date: Sun, 22 Dec 2002 06:11:40 -0700
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
X-Comments: This message was posted through Newsfeeds.com
X-Comments2: IMPORTANT: Newsfeeds.com does not condone, nor support, spam or any illegal or copyrighted postings.
X-Comments3: IMPORTANT: Under NO circumstances will postings containing illegal or copyrighted material through this service be tolerated!!
X-Report: Please report illegal or inappropriate use to
X-Abuse-Info: Please be sure to forward a copy of ALL headers, INCLUDING the body (DO NOT SEND ATTACHMENTS)
Organization: Newsfeeds.com http://www.newsfeeds.com 80,000+ UNCENSORED Newsgroups.
> At the moment the thing is driving a 10k resistor with 351.5V, pulling
> 0.315A from a 45.1V supply, for 87.0% efficiency. I'm running at 150kHz
> with 40% duty. Not a bad efficiency for a flyback boost converter on
> a breadboard, huh? Sure better than the results that stupid toroid
> inductor gave. I'll update my www with this design once the whole thing
> is done.
> The core, coil, and FET are very slightly warm to the touch. The whole
> thing is built on a breadboard with moderate attention paid to
> minimizing parasitics, and providing ample bypassing reservoirs.
> Oh, my analog scope shows the drain spiking up to the 600-700V range
> before the HER105 diode turns on. Is this bad? The inductor only has
> about 65uJ per cycle, and the IRF730a is able to handle repetitive
> avalanche energy of 7.4mJ, so I can't imagine that the thing will break
> under these conditions. But I don't know much about this avalanche
> stuff. I'd appreciate some lectures on the subject. Should I try a
> faster diode?
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.
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?
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
Chances are there is something wrong with your measuring technique. My
guess is your scope probes/hookup wire to your circuit has significant
inductance and the high dv/dt MOSFET turn off is creating a small resonance
associated voltage overshoot with the scope input capacitance. What kind of
scope/scope probes/hookup wire to your breadboarded circuit/etc. are you
using? Either that or your measuring equipment may not be accurate.
Even if your MOSFET is avalanching I would not worry about it. Some people
may disagree with me on this, but it is perfectly acceptable to operate an
avalanche rated MOSFET in avalanche provided you never exceed the maximum
avalanche current rating (for any duration) or the maximum junction
temperature. For the circuit conditions you describe it doesn't sound
possible that you are ever exceeding either of those ratings so you are
-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 80,000 Newsgroups - 16 Different Servers! =-----
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup