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Changing Diode DC Blocking Voltage
bdunham7:
--- Quote from: b2theory on September 28, 2021, 12:48:33 pm ---It's a 200V schottky diode used in a buck regulator. Without getting too deep into the weeds about the end use, the diode is substantially over spec'd for the circuit. We are noticing anomalies in the regulator output on a few return units. During our investigation we put the diode on a curve tracer and found that it was breaking over at 300V. We also know if you touch the "malfunctioning" diode in circuit the regulator output changes.
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What sort of voltages are you seeing in the circuit? How about Vf ? Does your circuit actually depend in some way--intentional or not--that the Schottky diode in question exhibit a certain amount of leakage at high reverse voltages? If 'touching' the diode changes the operation of the device, perhaps capacitance is an issue. Have you measured the capacitance of a diode from a failed unit vs a new one?
I strongly suspect your issue is elsewhere and that even if a small or random change in the characteristics of the diode have an effect, that is because of instability elsewhere due to either design or component malfunction. No reasonable design would fail just because a diode wasn't precisely to spec, and certainly not because the reverse breakdown voltage was over spec. If your device isn't top-secret, feel free to post a lot more details and I'd sure you'll get better advice. Don't worry about getting deep in the weeds--you are probably already there.
T3sl4co1l:
Bogey parts can have ultimate ratings far in excess of their datasheet values, for example early BJTs that fit a 60V spec, say, might actually break down anywhere from 80 to 200V+. This is not a change thing, it's a characteristic of the part; just that no one might've tested the part before (at least after initial testing, which is a simple go/no-go test, not a characterization).
I think modern parts tend to be tighter grouped, but it wouldn't surprise me if there are schottkys out there rated for say 100V and breaking down at up to 200.
There's also the possibility that it's completely the wrong part, whether by accident or counterfeit.
--- Quote from: b2theory on September 28, 2021, 12:48:33 pm ---We also know if you touch the "malfunctioning" diode in circuit the regulator output changes.
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Presumably it has a ton of AC applied to one or both sides? Without further information, this is easily explained as the result of the EMI going up your finger, affecting the control circuit.
Tim
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