High Voltage Switch across Capacitor

[joker12]

Does anyone know how to switch a high voltage across a capacitor extremely fast?

Given:
500V DC Supply
90pF Capacitor (custom)

Problem:
Switch polarity of voltage across cap, activated by digital input.
The switch should be complete in about 2us.
It can switch back and forth, controlled by digital input.
The voltage across the capacitor ideally looks like a unit step.

Thanks for the help in advance.

 

[IanP]

Try one of the horizontal deflection transistors rated for >1000dc ..
Example: BU204, BU205 ..
https://pdf1.alldatasheet.com/datasheet-pdf/view/56447/BOCA/BU204.html

Rgds,
IanP

 

[joker12]

Thank you for suggesting that device.
I'm still an undergraduate in EE. Can you point me a to good deflection circuit that would make use of such a transistor? The university doesn't teach BJT's in circuits classes anymore.

 

[IanP]

I wouldn't point you to deflection circuits, but rather to real idea on how to drive a power HV BJT working as a switch .. see attached picture ..
These transistors have quite low Hfe (or β - gain factor) so in practice there should be another transistor between the optocoupler and the power BJT, just to boost the Base current ..

Rgds,
IanP

 

[joker12]

Would Vcc be my 500 volt supply?
Is the optocoupler just for isolation safety?