Slow diode used in primary clamp of Offline Flyback SMPS...Why so popular?

Hello,
Thanks, actually we wouldn’t need a toroid etc to measure the current in the slow diode, since we have a 51R resistor in series with it (as attached) , and so could just get a diff probe across that.
I do wish I’d had chance to get a thermocouple on that 51R. It was certainly mad hot to the touch when at room temp/max load and “open air” PCB.

you should look at the current in the diode (small current clamp or toroid) alongside the Vds waveform - and zoom in - you will find it very interesting - then put in a fast diode...!

Click to expand...

I am not sure if you are referring to the ringing current in the slow diode just after FET_OFF, or are you referring to the spike of current in the slow diode at FET_ON?

So I hear what you are saying, and in our schematic, shown in post 16 above, it can be seen that there is a 51R resistor in series with the slow diode (trr = 500ns), ….so that resistor will quench the ringing and as long as that resistor is rated for the power, then things should be ok from that point of view, surely.
The slow diode does have the good effect of meaning that the clamp capacitor does not carry so much voltage, and so there is a reduction in dissipation. The situation of the prolonged ringing due to the slow diode is good from the point of view that the leakage inductance current goes into the clamp capacitor, but, advantageously, comes back out again due to the slow diode allowing this…but….
My worry would be that a 500ns diode would simply not have reverse recovered as well as would a 50ns diode, and that when the FET switches back on again (whether in DCM or CCM), there would be a spike of current flowing through the slow diode which would be bigger than it would be with a 50ns diode.
The typical RCD clamp with a 50ns diode avoids the slow diode problem, but does mean that the clamp capacitor carries relatively more voltage and thus there is more dissipation in the clamp resistance.
So, all in all, I do agree that the 50ns diode is best, but for me, the main problem with the 500ns diode is in the reverse current spike at FET switch ON.