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MC34063 high voltage dc-dc boost converter

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T3sl4co1l:
24V zener, 24V supply, what's 24+24V? ;D

RCD, try more like 1k and 100nF.

Tim

magic:
The peaks are higher than 24+24V. That's because primary current at turn-off is presumably several amps. This means significant voltage drop across the 10Ω resistor and trouble for the tiny 0.5W zener.

BTW, it seems that off-time primary voltage approaches zener breakdown voltage. That's not a good idea, you may end up dissipating all flyback transformer energy in the zener rather than the secondary ;)

MagicSmoker:

--- Quote from: dazz on January 04, 2020, 11:34:22 pm ---OK guys, I'm really confused about the results I'm getting with different snubber configurations.
...
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?

--- End quote ---

Well, someone's not paying attention. Go back and (re-)read posts #44, 46 and 56 as I already covered all of the above questions!

The voltage across the switch when it turns off is the sum of the supply voltage and the "reflected" output voltage (ie - the output voltage transformed by the sec:pri turns ratio [note the order]). If the output is 300V and the pri:sec turns ratio is 1:12, then the ratio going in the other direction is 12:1, so the secondary voltage will be divided by 12 when seen at the primary, or 25V in this example. The reflected 25V from the secondary adds to the 24V supply to give a minimum voltage the switch must withstand at the instant of turn-off of 49V (not including ringing, leakage spikes, etc.). If the Zener clamp across the primary breaks down at a voltage lower than 25V then energy stored in the transformer during the switch on time will preferentially exit through the clamp during the switch off time (ie - the flyback period, hence the name), rather than from the secondary. To use a classic military abbreviation, that's NFG, hence why I suggested in one of the aforementioned posts to set the Zener voltage to between 36V and 51V.

dazz:
Yes, I need to take a break from the sim and read more instead. The reason I used such a low voltage zener is simply because it seemed to work. The higher voltage ones weren't clamping at all, perhaps because of the high frequency of the ringing, not sure. It's also obviously true that the zener I picked was dissipating far too much power, some 15W on average. I probably should also swap the mosfet for one with lower Cds, I think that will reduce the ringing a bit, correct? Is the output capacitance in the datasheet the same as Cds?

ETA: One question, please. Is the snubber supposed to improve overall efficiency? Or it simply lowers the dissipation in the mosfet by the same amount the snubber dissipates?

dazz:

--- Quote from: MagicSmoker on January 03, 2020, 12:09:19 pm ---You can return the clamp to ground - ie, wire it across the switch - but then it has to withstand the sum of the input voltage + the reflected voltage (ie - same as the switch). The transformer (and switch) don't really care one way or the other. The same applies to the RC damper, if used. BTW - a good rule of thumb for the RC damper is to make C about 3x the output capacitance of the switch and R somewhere around 1x to 3x the characteristic impedance of the LC network formed between the total capacitance of switch and damper and the leakage inductance. For example, if there is 2.4uH of leakage and the switch output capacitance is 50pF then a damper comprised of 150pF and 100-330R will likely clean up the highest frequency ringing (the lower frequency ringing in the flyback is between the magnetizing inductance and the lumped capacitance and can't really be suppressed as it is invariably too close to the switching frequency).

One other thing is that 1% leakage is a more realistic minimum for a typical E-core design (and even that requires considerable care in winding geometry), so set the coupling coefficient, K1 (2, 3, etc.) to 0.995 (leakage factor is 1-K2).

--- End quote ---

OK, just reread this and I see now you answered many of my questions there. Yep, I'm a bit slow, sorry about that

ETA: This looks much better, right? (Added screenshot)

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