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Small capacitor's in Power switch loop?

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cupoftea

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Hi,
We are doing 400W Two transistor forward. Vin 390vdc, vout 40v, fsw=100kHZ.
Is it considered a good idea to put small ceramic caps in tight to the primary and secondary power switch current loops? (to reduce the area of those current loops)...or is this just a recipe for increased RF emissions from the stray L and these capacitors resonating?
 

Basically a good idea. You can expect less overshoot and lower Q of parasitic oscillations. In some cases shift of oscillation frequency may cause EMI problems, usually not. You should use all options to reduce commutation loop inductance.
 
The point of adding such a capacitor is to provide a return path for high frequency currents. In theory, if this current path radiated/coupled to the outside world, then it's possible that adding the capacitor could actually create an even worse EMC scenario. But by making the return path very short/small, you both encourage the high frequency currents through that path and simultaneously reduce how much they radiate/couple to nearby systems. In practically all real scenarios, the end result is less RFI.
 
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