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eBay Flyback High Voltage PSU

Flyback PSU
Flyback PSU

I have found myself needing some more in the way of High Voltage supplies of late, with the acquisition of the new He-Ne laser tubes, so I went trawling eBay for something that would be suitable to run these tubes. (I currently only have a single He-Ne laser PSU brick, and they’re notoriously hard to find & rather expensive).
This supply is rated at 1kV-10kV output, at 35W power level. Unfortunately this supply isn’t capable of sustaining the discharge in a large He-Ne tube, the impedance of the supply is far too high. Still, it’s useful for other experiments.
The flyback-type transformer clearly isn’t a surplus device from CRT manufacture, as there are very few pins on the bottom, and none of them connect to the primary side. The primary is separately wound on the open leg of the ferrite core.

Drive Electronics
Drive Electronics

The drive electronics are pretty simple, there’s a controller IC (with the number scrubbed off – guessing it’s either a 556 dual timer or a SMPS controller), a pair of FDP8N50NZ MOSFETs driving the centre-tapped primary winding.
The drive MOSFETs aren’t anything special in this case: they’re rated at 500v 8A, 850mĪ© on resistance. This high resistance does make them get rather hot even with no load on the output, so for high power use forced-air cooling from a fan would definitely be required.

Test Setup
Test Setup

Here’s the supply on test, I’ve got the scope probes connected to the gate resistors of the drive MOSFETs.

Waveforms
Waveforms

On the scope the primary switching waveforms can be seen. The FETs operate in push-pull mode, there’s a bit of a ring on the waveform, but they’re pretty nice square waves otherwise.

Arc
Arc

At maximum power on 12v input, about 25mm of gap is possible with an arc.