A modified version of the PCB that I discussed on this post was implemented. It is still 2-layer with all components on top and the ground on the bottom layer. The transformer windings were directly soldered to holes on the PCB, but the input and output used connecteors. Here is a screenshot of the output voltage waveform (from experiment), before adding any common-mode chokes.
The switching frequency is around 140kHz. My initial questions are:
It is expected that the transformer leakage inductance will resonate with the parasitic capacitance of doide (and MOSFET too?). Is this usually the cause for the two ringings per switching period? Does the EMI play a role here as well. One thing I noticed was that when I press with my hand on the big (variable) resistor load, the peak of the riniging dropped by almost half (which was the moment at which the screenshot was recorded) - so it seems the an alternate impedance path was provided for the noise. Another thing is that the load was connected through really long cables (which we can also fix).
Why is the ringing in the output different at diode turn-on than at didoe turn-off? Mainly, how come the freuqencies seem to be different?
There is a lot of room for improvement here, but at this stage it is just trying to understand what causes what...