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I'm trying to understand what is a typical conductive noise emission of a flyback converter.

I made a simplified model (I skip the transformer replaced with a single coil - I think that this simplification does nothing to noise emission, 300 pF capacitors and 70 mH coils (having 700 mOhms serial resistance) represents normal 100 m (or something) long power line.

After I run the simulation I was surprised to see such HUGE ringing on the power line:

enter image description here

The amplitude is start with 170 Volts and frequency about 500 kHz:

enter image description here

I've never see anything like this in real world circumstances.

I suspect that my simulation model is wrong in some way. But my colleague I was able to consult with told me that it looks OK.

Any thoughts?

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The MOSFET has capacitance between drain and source when it switches off. It forms a resonant circuit with the inductor and you get a sustained but decaying sine wave produced. This happens on all fly-back circuits.

So, if your inductor is 1 mH and your capacitor is 70 pF you will get it ringing at 600 kHz. Your MOSFET (STP8NM60) is specified as having an output capacitance of 100 pF so, not a million miles off.

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  • \$\begingroup\$ What is the best practice to suppress this ringing? \$\endgroup\$ – Roman Matveev Nov 21 '16 at 11:27
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    \$\begingroup\$ I had a brief look at the data sheet and that's the number I saw but all MOSFETs are round about this figure so it shouldn't be a big surprise because all MOSFETs will do this. \$\endgroup\$ – Andy aka Nov 21 '16 at 11:28
  • \$\begingroup\$ Why do you feel you need to suppress it? It has to be "managed" and one way is a snubber. The snubber will prevent excessive voltages damaging the MOSFET but, still allow the ringing. \$\endgroup\$ – Andy aka Nov 21 '16 at 11:29
  • \$\begingroup\$ I concerned that the AC line has this kind of a noise. I didn't conduct FFT analysis yet but I suspect that this is far from being acceptable (from EMC point of view) \$\endgroup\$ – Roman Matveev Nov 21 '16 at 11:32
  • \$\begingroup\$ Some of this will get onto the AC line so increase the 100 nF capacitor to several tens or hundreds of micro farad - that would be a more normal value for this components. \$\endgroup\$ – Andy aka Nov 21 '16 at 11:39
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  • your filter is missing a bridge input shunt cap, so your filter is resonating in series from the output pulse. (move C11 X-cap right) then you have a symmetrical diff. pi filter. CLC but still missing CM Y-shunt caps to gnd.

  • often each diode must shunted too to suppress parallel resonance for conducted EMI regulations. >=100pF

  • its not the FETs

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