Why does a capacitor connect isolated grounds in a flyback converter?

that is used to reduce noise.

Hello Sterian,

Often this is done to pass the EMC test. Or at least scrape by on the test...

Probably for bleeding off a charge. Should be done any time there is a cap, to avoid getting shocked.

Regards, Joerg

Due to high dv/dt present in switching circuits, currents WILL flow in any parasitic capacitances. There are parasitic capacitances between transformer primary and secondary. Unless there is a a deliberate path for return currents (the completion of the circuit), the parasitic currents will flow in unintended paths. If the unintended paths have a large loop area (as they generally will), the loop will radiate electromagnetic interference. The capacitor provides a short path, with a small loop area, for parasitic currents. Since it bridges the isolation barrier between primary and secondary, it must be a Y2 safety capacitor. It's also worth noting that parasitic currents flowing through resistive and reactive paths can cause potential differences that interfere with sensitive circuits, including, for example, the feedback circuitry of the power supply itself. The capacitance of the bridging cap must be small enough so that leakage currents flowing across the insulation barrier are under a safe limit, typically 0.75 mA. This is why the capacitance is usually just a few nF. Paul Mathews

Wow...excellent answer. Thanks!

It dumps rfi back to the bulk storage cap. Hence you'll find it on Class II supplies too. It confuses a lot of ppl including me originally.

That's a new one on me.

Graham

Or two Y1s in series ! ;-~)

See IEC 60065 60950 etc.

Graham