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Subject: Diode Reverse dV/dT Problem

Date: 06/22/02 at 9:59 AM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

I am experiencing a reverse breakdown problem with the rectifier diodes of a forward converter. There is a Motorola application note at https://www.onsemi.com/pub/Collateral/AN1108-D.PDF which shows a forward converter much like the one that I'm working on, in which an RC snubber network is placed across each of two Schottky rectifiers.

In spite of the extremely fast Trr ratings of Schottky diodes due to very, very little stored charge, they still have another and separate breakdown mechanism related to the maximum dV/dT that they can support in the reverse direction. For the diodes in the application note, they talk about devices rated for up to 5000 volts per microsecond. Go faster than that and the diodes will fail!

I remember how that limitation on Schottkys was an unpleasant surprise to the power supply industry when those diodes were first introduced!

In my case, instead of a +5V output, I am making a +27V output which, I now think, puts a LOT of reverse dV/dT stress on my rectifiers, more than they seem able to take. They are NOT Schottkys, though.

The diode folks I've spoken to about this don't seem to understand. They don't even recall the Schottky situation, maybe because it was roughly thirty years ago and they're too young.

Does anybody have more up to date information on this? I think that I need a diode type with adequate, SPECIFIED dV/dT capabilitiy and that seems to be a problem!

Thank you.

John Dunn - President
Ambertec, Inc.
ambertec@ieee.org


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