voltage across the snubber capacitor greatly exceeds what one would expect (>1kV).
I think it will help if you make C larger, and R smaller. Notice if R is too small, then it tends to cause continued current flow around a small loop which includes the transformer primary. You may need to bump up the resistor's W rating lest it burn up.
I should mention that I'm using a 7W wirewound, enamelled snubber resistor and it's burning up as it is. Will making it smaller really help? Won't it just burn even more?
More importantly where is this power coming from? I short-circuited the secondary winding(s) and measured the inductance on the primary with my crappy LCR meter to get an approximate reading of the leakage inductance. It gave an approximate reading of 3.5uH -- 3.5% of Lmag -- which is what the web says it would roughly be. I measured the primary current under load and it got to a maximum peak of 4A. This suggests 28uJ of leakage energy to dissipate. Multiplied by 65kHz this suggests 1.8W. I don't think the resistor should be so hot considering it's using less than half its capacity.
My knowledge is insufficient to disagree with your formula, but seeing the schematic it appears you have upwards of 110W going through the transformer (output 24v x 4.5A). This indicates the amount of energy that must be absorbed briefly by the capacitor at turn-off. It may not be 110W continuous, but it is momentary. It still charges the capacitor to a few hundred volts. The goal is to keep the capacitor voltage from getting so high that it exceeds the diode rating, or mosfet rating, etc.
You must discharge the capacitor sufficiently during each cycle. The spikes of current from the transformer are invincible. No matter how high the capacitor voltage is, another spike arriving automatically generates sufficient voltage to push it even higher. More than 1000V appeared at terminal 1 of your transformer. That's how it charged your capacitor to more than 1kV. The solution is that you have to keep that node voltage below your mosfet rating (650V). Therefore your resistor needs to be low enough ohms so that it discharges the capacitor almost completely during each cycle.
Several W is the power rating of your resistor, and agrees with the schematic. That is probably adequate. But I think the schematic's value of 20k is too high ohm value.
This DCR snubber appears to be the optimum configuration of snubber, to do the job of dissipating single polarity spikes emerging from the transformer. There is a tradeoff of RC values which ought to work best, and it requires testing with several combinations of values, to find the proper tradeoff.
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There is another way to look at RC values. C can be a large value, so that its voltage goes up and down only a few V during a cycle. This means it hovers around a certain voltage. It does this because R discharges the capacitor at a more or less constant current. In effect, R sets the 'hovering' voltage level. Picture how R does this. High ohm, high V. Low ohm, low V. You can try to get away with a high ohm value, but the voltage eventually rises to dangerous levels. This is where the proper tradeoff must be found.
you'd better post your circuit as else all is guess work...
there appears to be A LOT of leakage inductance in your transformer, causing the massive turn off voltage and subsequent ring to gnd...!
so your spec is
vin = 120-240vac
vout = 24vdc
iout = 4.5A
...THIS WOULD ONLY BE POSSIBLE WITH A SINGLE SWITCH FLYBACK
So you need to tell us you lleak value. And also the peak pri current, and f(sw)
also, you really need pfc for your smps because its over 75w...unless its just at 24*4.5 Watts for very short intervals?
for PFC you could use a "boundary conduction mode" boost PFC chip.
I think ti.com do them.
Infineon might do some
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if you do use a pfc stage then that eases your flyback design, because your primary voltage will be around 400v and your primary current will be very low.
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here is an ltspce sim of your flyback, also the excel design file.
Its quite a few watts in the rcd clamp at min vin. ..but not impossible
You can go ALT+LEFT CLICK to see dissipations, which of course, are just guideline.
the turns ratio is 1:1 and I made it like that to get the right flyback CCM waveforms.
You need a low primary conduction time, and longer secondary conduction time.
If you have changed the turns ratio and are getting subharmonic oscillations, then that is because you have not changed it correctly, why did you want to change it?......subharmonic instability occurs at duties of 0.5 plus and this flyback should never be operating with 0.5 pus duty cycle.
I tried lots of different combinations but could not get the snubber resistor to a reasonable size! :bang:
When I'm ramping the voltage up to ~120VDC (minimum mains) the controller is going flat-out, i.e. 50% duty (because the FB pin is still high), the primary current gets quite high, and the voltage across the snubber capacitor greatly exceeds what one would expect (>1kV).
coates claimed 3.5 µH leakage throughout this thread, unfortunately the number doesn't fit the observations of burned snubber resistor.
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