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

[SterianA@gmail.com]

I've seen numerous AC-to-DC flyback switching converter designs that
connect the isolated grounds on the primary and secondary side with a
2.2nF class Y capacitor, as in this figure:

http://claymore.engineer.gvsu.edu/~steriana/temp/isolationcap.jpg

Can anyone tell me why this cap is necessary and/or desirable?

I've also seen a 10MOhm resistor used instead of the cap, too. Any
thoughts on this?

Thanks....

 

[Ryan Weihl]

I've seen numerous AC-to-DC flyback switching converter designs that
connect the isolated grounds on the primary and secondary side with a
2.2nF class Y capacitor, as in this figure:

http://claymore.engineer.gvsu.edu/~steriana/temp/isolationcap.jpg

Can anyone tell me why this cap is necessary and/or desirable?

I've also seen a 10MOhm resistor used instead of the cap, too. Any
thoughts on this?

Thanks....

that is used to reduce noise.

--

 

[Joerg]

Hello Sterian,

I've seen numerous AC-to-DC flyback switching converter designs that
connect the isolated grounds on the primary and secondary side with a
2.2nF class Y capacitor, as in this figure:

http://claymore.engineer.gvsu.edu/~steriana/temp/isolationcap.jpg

Can anyone tell me why this cap is necessary and/or desirable?

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

I've also seen a 10MOhm resistor used instead of the cap, too. Any
thoughts on this?

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

Regards, Joerg

 

[Paul Mathews]

I've seen numerous AC-to-DC flyback switching converter designs that
connect the isolated grounds on the primary and secondary side with a
2.2nF class Y capacitor, as in this figure:

http://claymore.engineer.gvsu.edu/~steriana/temp/isolationcap.jpg

Can anyone tell me why this cap is necessary and/or desirable?

I've also seen a 10MOhm resistor used instead of the cap, too. Any
thoughts on this?

Thanks....

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

 

[SterianA@gmail.com]

Wow...excellent answer. Thanks!

 

[Pooh Bear]

I've seen numerous AC-to-DC flyback switching converter designs that
connect the isolated grounds on the primary and secondary side with a
2.2nF class Y capacitor, as in this figure:

http://claymore.engineer.gvsu.edu/~steriana/temp/isolationcap.jpg

Can anyone tell me why this cap is necessary and/or desirable?

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.

I've also seen a 10MOhm resistor used instead of the cap, too. Any
thoughts on this?

That's a new one on me.

Graham

 

[Pooh Bear]

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.

Or two Y1s in series ! ;-~)

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.

See IEC 60065 60950 etc.

Graham