flyback capacitor selection

[zappedagain]

This is continuing the discussion started in thread240-309895

Are there any guidelines for the value of the bypass capacitor from the primary to secondary of a flyback transformer to eliminate (reduce) the common mode currents that can cause EMI issues?

Other than stating that the capacitor voltage needs to be at least as high as the transformer isolation voltage I haven't seen any other specifications for this part. A few articles give a typical range (2.2-4.7 nF). I'd think the transformer manufacturers would define it, but I haven't seen that in the datasheets of several or the larger flyback transformer manufacturer's datasheets. I'm disappointed in how many application notes I see that don't even show the bypass capacitor; is this a trick of the EMI/EMC trade and not power supply design?

Z

 

[sreid]

Isn't the usual recomendation to have a small inductor in series with the secondary rectifying diode to stop the Winding Capacitance current spike?

 

[zappedagain]

Interesting. I ran into this problem with a regulator that has a FET switching the primary and a FET switching the secondary so it doesn't have a rectifying diode. While one can say the secondary FET acts as the secondary rectifier, I haven't seen any notes on an inductor in series.

As I understand it, the flyback power supply dumps a lot of leakage current across the parasitic capacitance between the primary and secondary windings. This current needs the bypass cap to provide a defined return path from the secondary 'ground' to the primary 'ground'. Otherwise the current takes a much larger path and generates EMI issues. I discovered this (i.e. learned this the hard way) with a current probe on a shorting jumper between the two grounds.

Z