Schottky diodes with 0.2V Vf?

[jogri]

I am currently trying to repair the switch mode power supply of an oscilloscope and i stumbled across three diodes on the secondary side with suspiciously low forward voltages:

-two Schottky rectifiers (B20H100G) that are supposed to have a Vf of .4V but barely manage .2V at room temperature
-one silicon diode (BYV29-400) that has a Vf of .4V instead of the .9V that the datasheet suggests.

They didn't show a reverse voltage when i tested them with my multimeter (the one schottky that i tested with my lab power supply had a Vref above 60V, so i guess they are fine in that regard). Am i correct in the assumption that those diodes are toast when they deviate that much from their datasheets?

[oPossum]

What is your test current and what is the current specified in the data sheet? Diodes will have lower V_f at lower current.

[jogri]

Thanks for the tip with the current, that solved it: The current vs voltage drop graphs on the datasheet goes down to 0.1A, so i just assumed that the reading on my multimeter should be at roughly the same voltage. I just tested them with 2A, they behave completely normal at that current (10A are specified in the datasheet, but i see no point in going that high if the Vf is within the specified range at 2A).

[jackthomson41]

yup 2A battery source will work for that Schottky diode i.e. 1n5819 diode.

[fourfathom]

For what it's worth, diodes usually fail shorted or open.  I don't think I've ever seen one where the failure mode was a lower than normal forward voltage drop -- I don't think semiconductor physics permit it.

[Kleinstein]

I expect the aging in amorphous silicon solar cells (and they do age significanty) to cause the forward to drop. More defects (dangling bonds) cause lower carrier life time and thus more current at a given voltage.
Still it needs quite a lot of change in the device to get a significant lower forward voltage, as the current about doubles every 20 mV of higher V_f.

The normal failure mode is a short, rarely open.

The usually DMMs test at some 1 mA. So expect a low reading for larger power devices.

[fourfathom]

The normal failure mode is a short, rarely open.

Technically true, but when it shorts it gets hot, and then burns up (sometimes).  When I see a crunchy diode body burned in half with an air-gap between the leads then I consider that "open".

[magic]

If there existed a failure mode which reduces forward voltage while still maintaining 60V reverse withstand, diode manufacturers would be producing this failure right in the factory

[jogri]

If there existed a failure mode which reduces forward voltage while still maintaining 60V reverse withstand, diode manufacturers would be producing this failure right in the factory

That's what made it so suspicious... Aging diodes that actually got better? I even threw them in the refrigerator to make sure the junction wasn't somehow still hot after desoldering them. And after seeing that the forward voltage still correlated with the temperature i was pretty much out of ideas. Their behaviour didn't seem to make any sense, the only "explanation" i could think of was that they entered a failure mode where their forward voltage was reduced under no load but where they wouldn't behave like diodes under load.