How to design a 7.5kV AC source?

[CristianM]

Hello! I need to design a high voltage AC source with the following specifications:
- 7.5kVpp max. output voltage;
- adjustable output voltage;
- isolated output;
- sine wave output;
- adjustable output (sine wave) frequency in the range 40Hz-2kHz;
- max. 1 mA output current.

Can someone recommend a possible solution? A basic transformer or flyback supply (maybe recommend some components, schematics) or even a commercial solution?

[profdc9]

I'll venture a solution...

Use a full-bridge or H-bridge of MOSFETS such as the IRF820, where each side is driven by two 50% duty cycle square wave signals of different frequencies with a difference between the frequencies that is the output frequency you want that would drive both sides of a small ferrite transformer primary.  For example, if you want 1 kHz, you would drive one side perhaps with 200 kHz, the other with 201 kHz.   You could rectify mains power for example to power the H-bridge, for example, from 220 VAC to 300 VDC, with a full-wave rectifier, and use a buck converter or even a small variac on the primary side to adjust the DC voltage on the bridge.  A ferrite transformer, for example, with a 10:350 turn ratio so that you could transform the 300 VDC voltage up to the 7.5 kVAC range.  A high voltage polypropylene capacitor (perhaps 1000 pF?) would need to be placed on the secondary output of the transformer to filter high frequency (200 kHz) and leave a clean sine wave, or you might have to build a pi lowpass filter if you want the high harmonics really attenuated.   You will probably also need a DC block capacitor as well in series with the primary winding.   What I've described is open loop and I think to close the loop you would need to use an optoisolator to feedback a signal to the buck converter to regulate the voltage, or you could just put a resistive load (like a 10 megaohm 10 watt resistor) to try to provide some stability to the output voltage and act as a bleeder resistor if you don't need super accurate output voltage.

Dan

[Mads Barnkob]

Hi Cristian and welcome to HVF

All your specifications would be reachable with a ferrite core with a HV winding and driven by a half-/full-bridge. The frequency is however what makes this less trivial, you would most likely need to make a voltage modulator to get such low frequencies.

I got no experience in the following suggestion, its just an idea I just had when thinking about how defibrillators work. You could form a high voltage pulse through a large output inductor to slow down the current and get a sine wave from a square input.

[CristianM]

Thanks for the suggestions. I'll try to implement them.