Electronics > Beginners

MC34063 high voltage dc-dc boost converter

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dazz:
Damn, of course. A cap is a short for AC signals. Electronics 101.  |O  :palm:

EDIT: The zener seems to be clamping the signal, but as anticipated by MagicSmoker, it's not doing a great job.
I'll try the RCD snubber next

MagicSmoker:
R2 in your circuit is a bit of kludge in that it provides some damping whenever the Zener/TVS conducts, but it has to be a realistic value given the expected peak current because the voltage drop across it adds to what the switch must withstand (keep in mind the leakage inductance acts like a current source).

dazz:

--- Quote from: MagicSmoker on January 03, 2020, 04:52:27 pm ---R2 in your circuit is a bit of kludge in that it provides some damping whenever the Zener/TVS conducts, but it has to be a realistic value given the expected peak current because the voltage drop across it adds to what the switch must withstand (keep in mind the leakage inductance acts like a current source).

--- End quote ---

I've tried anything from 200 up to 10k for R2 and a bunch of different zeners from 15V to 100V with no apparent luck, not sure if I'm doing something wrong, but no combination reduced the power dissipation in the mosfet significantly.  :-//

So D4 is a TVS? I was wondering what it's doing there. Picked a uF400x because I read somewhere that it needs to be a fast switching diode. But then again the MRA4007 that they're using in the schematic I posted above is a standard recovery diode according to the datasheet...  :-//

David Hess:
The diode has to turn on fast which is generally not a problem even with a standard recovery diode unless it is defective.  Diodes can do weird things sometimes.  The same of course applies to the zener or TVS.  Since high voltage zener and TVS diodes work in avalanche mode, I would expect them to turn on very quickly in reverse breakdown.

The alternative snubber uses a diode in series with a parallel RC load so the capacitor absorbs the spike and the resistor slowly bleeds off charge.  No zener or TVS is used.

dazz:

--- Quote from: David Hess on January 03, 2020, 08:20:48 pm ---The diode has to turn on fast which is generally not a problem even with a standard recovery diode unless it is defective.  Diodes can do weird things sometimes.  The same of course applies to the zener or TVS.  Since high voltage zener and TVS diodes work in avalanche mode, I would expect them to turn on very quickly in reverse breakdown.

The alternative snubber uses a diode in series with a parallel RC load so the capacitor absorbs the spike and the resistor slowly bleeds off charge.  No zener or TVS is used.

--- End quote ---

OK, so you mean only the Ton time matters here, because that's when the spikes happen, right?

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