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I am about to design a flyback converter.

As all flybacks with no extra measures it has some ringing.

You can see that ringing in this example from -1µs to 4µs:

enter image description here

This ringing can be surpressed by a snubber (common measure to cancel this type of ringing.) The drawback might be a bit less efficiency of the flyback converter.

Here would be an example of the snubber which I would implement, if the ringing should be canceled:

enter image description here

-Is it necessary to cancel it due to EMI emissions? -Do some of you have experience with bad side effects of such a ringing, when not canceled?

Edit (clarification): The green circle marks the ringing.

enter image description here

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    \$\begingroup\$ You cannot avoid this oscillations as it's part Vds waveform on fly back QR in discontinuous mode due to Lmag and Coss .You will need an input filter stage in you want to pass the EMI/EMC. I will be more concerned about the overshoot (high dv/dt) and the ringing due to the leakage inductance. \$\endgroup\$
    – Delphesk
    Commented Aug 14, 2020 at 13:24
  • \$\begingroup\$ Which capacitance controls the 0.3MHz ringing? LC C load when diode blocks is it C on the snubber that attenuates the 2.5MHz initial resonance?? or is it change in inductance? \$\endgroup\$ Commented Aug 14, 2020 at 14:15
  • \$\begingroup\$ have you got the secondary the right way round? \$\endgroup\$ Commented Aug 14, 2020 at 19:54

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Note that this answer was provided before the OP made an adjustment to the question.

Now my question is: Is it necessary to cancel it due to EMI emissions?

No, it's necessary to cancel the ringing to prevent an excessive back-emf damaging the MOSFET due to over-voltage. It's caused by leakage inductance in the primary and, any energy it stores during the charge cycle, cannot be passed to the secondary hence, it should be suppressed if too excessive. As a side-issue it can reduce some EMI emissions.

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  • \$\begingroup\$ I think you are talking about the ringing at -8µs in the phase from closed switch to open switch. I have already taken measures to surpress this type of ringing. I think more of the ringing between -1µs and 4µs. I updated my question to clarify. I think this type of ringing cannot exeed the maximum voltage of the switch \$\endgroup\$ Commented Aug 14, 2020 at 12:43
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    \$\begingroup\$ You don't need to suppress that ringing because (a) it's generally sinusoidal and doesn't contain high frequency components, (b) it has a small energy value. To suppress it also means you will suppress forward energy that normally gets passed to the output. Hence, why I concentrated on the suppression of back emf energy as it can be higher in energy, can have many more harmonics, is usually much higher in frequency and is not energy that can be useful to the output stage. \$\endgroup\$
    – Andy aka
    Commented Aug 14, 2020 at 12:50

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