flyback power supply, capacitor between ground and isolation

[masoudk_81]

isolation capacitor flyback

Hi

I have designed a 75 watt flyback power supply with 4 outputs and i have
isolated two sides of transformer with 6N136 optocoupler. i have seen in
some designs that a capacitor is placed between the ground of primary
and secondary. the reason is for attenuating common mode current. but
i have a question that does this capacitor reduces the isolation and is it
necessary for the circuit or not?

masoud

 

[Moss]

Re: flyback power supply, capacitor between ground and isola

This capacitor close circuit for curent wich flows through transformer parasitic capacitance. If you miss this capacitor, supply will be noisy, even PWM controller can behave abnormal. In general it must have capacitance of 1 to several nF. Upper values are limited by isolation requirements. It must be so called "class Y1"