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MC34063 high voltage dc-dc boost converter

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dazz:

--- Quote from: MagicSmoker on January 03, 2020, 11:41:34 pm ---There's no hard-and-fast rule for sizing the resistor - in fact, it can be eliminated if an RC damper is also present - so don't get too hung up on the voltage drop; I just ballparked something that looked like it would be about right based on experience.

--- End quote ---

The RC dumper is in my things-to-google list, thanks again, MS

magic:
Actually, I have a dumb question ;)

What if we take the RCD capacitor and return it to the negative supply, in parallel with the switch, rather than to the positive supply?
This would seem to absorb the energy of the whole loop, including capacitor and PCB, not just the transformer. Energy which otherwise rings with the switch and/or avalanches it, I suppose.
The only drawback I see is that now the process of discharging the clamp needs to deal with supply ESL, but that's relatively slow and resistively damped so meh (?):-//

T3sl4co1l:
Yes, it is electrically preferable to put the clamp across the switch.

It is done to the supply, when the supply inductance is low enough not to mind; the advantage is easier layout.

Instead of an RCD clamp snubber, an RCD rate snubber is also often seen, which allows some savings in turn-off losses.  The RC can also be dimensioned to dampen the transformer's free ringdown waveform.  (This tends to be impractical when leakage is high, as the rate snubber will not significantly reduce the peak voltage.)

Tim

dazz:
I found what looks like a great series of videos about flyback design.

https://www.youtube.com/user/rbola35618/search?query=analysis+and+design+of+a+flyback

dazz:
OK guys, I'm really confused about the results I'm getting with different snubber configurations. I attached screenshots of the drain voltage with no snubber, with a zener clamp, with a RCD snubber, and a combination of zener+RCD. First thing I don't understand is the zener clamp doesn't seem to be clamping at all. The zener in the pic has a 20V reverse voltage, yet most of the signal while the mosfet is on is above 20V. It does seem to do a good job at eliminating the ringing while the during the off time of the mosfet though (pic #2). The RCD filter, OTOH, does the oposite apparently. It filters out the ringing while the mosfet is on (pic 3). So I tried a combination of both, which seems to be working pretty well (pic 4).

One thing I noticed is that the voltage in the drain while the mosfet is off, is not 24V like in the input, but 48V. It must have something to do with the transformer, because if I decrease the inductance in the secondary, off drain voltage increases... but shouldn't it be the other way around?

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