capacitor with common mode voltage change

[skywalker898]

Hi we are designing a totem pole pulsed power supply and a question has come up about overcurrent situations.

We have a floating power supply, 12VDC connected to a 10uF capacitor. This is isolated from ground and the power supply and capacitor float with respect to the high voltage output (going from -300V to +500V square wave pulses at 25us). We are getting overcurrent hiccup protection coming on after a certain number of pulses, and I'm wondering if it is due to the common mode voltage change on the 10uF capacitor. If both leads of a floating capacitor change voltage from -300V to 500V, doesn't charge have to be flowing in this capacitor so that effectively we are charging the capacitor up over and over again?

Basically, what current flows when a floating capacitor has common mode voltage changes?

Thanks,
Brandon

 

[barry]

I never really heard of 'common-mode voltage' with respect to a capacitor, but I think I get your idea. You don't really mean that "both leads change voltage from -300V to 500V", you mean that a cap with 12V across it rides on a reference that changes from -300 to +500. Current flows in a capacitor when the voltage ACROSS it changes; it doesn't care what the absolute values of the pin voltages are, only what the difference is.

i=C*dV/dt

If dV (voltage across the cap) is zero, zero current flows.

 

[FvM]

A reasonable question would be if the "floating" power is designed to tolerate 800 V common mode steps.

A complete schematic would generally help.