Isolation capacitor in an isolated DC DC converter

[peatear]

Hello,

I'm laying out an isolated DC DC converter and I have a question.

Do I need an isolation capacitor between the two ground planes? Will it make a difference if I am using an optocoupler or a transformer as my isolation feedback?

Thank you!

 

[FvM]

The capacitor across the isolation barrier (also called Y-capacitor) has the purpose of reducing high frequent interferences. It's not functionally required.
The feedback signal need to be DC coupled, so it can be easily transmitted with an optocoupler. A transformer needs a modulation circuit.

 

[Ice-Tea]

Keep in mind you can't just use any cap, look up X and Y ratings for mains and safety related caps