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From: John Larkin
Subject: Re: Two diodes in series to double PIV rating - is that a "no-no"?
Date: Wed, 20 Nov 2002 16:59:50 -0800
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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 for
>one of the secondary outputs.
>Trouble is that the PIV (Peak Inverse Voltage) on the diode is very close to
>the allowable rating for the diode. Is it possible to connect two in series
>(like the BAV99 series- two diodes on one die) to increase the PIV rating?
>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.
>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 impressed
>on the other diode. But when the voltage on the other diode rises - so does
>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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