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From: "Klaus Vestergaard Kragelund"
References: <firstname.lastname@example.org> <email@example.com>
Subject: Re: Two diodes in series to double PIV rating - is that a "no-no"?
Date: Thu, 21 Nov 2002 19:12:49 -0500
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Organization: TDC Internet
"John Larkin" wrote in
> On Wed, 20 Nov 2002 22:29:28 -0500, "Klaus Vestergaard Kragelund"
> >I have designed a low power flyback supply and I'm using a BAV74 diode
> >one of the secondary outputs.
> >Trouble is that the PIV (Peak Inverse Voltage) on the diode is very close
> >the allowable rating for the diode. Is it possible to connect two in
> >(like the BAV99 series- two diodes on one die) to increase the PIV
> >Will the applied voltage split approximately equally between the diodes -
> >even at fast switching?
> If it's 'very close' but still below the rated PIV, it's probably OK.
> Nearly all diodes have lots of margin on this spec.
We employ 80% derating on the PIV voltage specified by the manufactor. By
the way anyone got any experience using semiconductors just on the limit -
that is to say the impacts on predicted lifetime. I haven't found this info
anywhere - unless we are talking electrolytic capacitors where they often
spec the lifetime to double every 10 degrees drop in ambient temperature
> >I would suspect that if one of the diodes has a higher reverse leakage
> >current from the "get go" - then the bulk of the voltage would be
> >on the other diode. But when the voltage on the other diode rises - so
> >the leakage and possibly a equlibrium would exist. Anyone?
> The nearly-equal diode capacitances will tend to equalize the drops
> for fast reverse pulse situations - well into the microseconds. If the
> diodes zener nicely, you'll get your equilibrium so it's safe to stack
> them. If they have a foldback breakdown behavior, two diodes may not
> be safe at 2x PIV... maybe somewhat less would be OK.
> Take a look at the MMBD2004/CMSD2004... dual 240 volt diode in SOT-23.
> I think BAV21 is similar.
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