i finished the high voltage power supply

This article borrows from previous ones. In particular, this one goes into detail on the design of this subsystem. I'll summarize briefly:

HVPS design

Block diagram

110V from the low voltage supply is used as the main power source. It enters the HVPS and proceeds to an "upside-down" buck converter, which is the point where the output voltage is regulated. This intermediate rail, at about 50 to 60V, is then made into a square wave at 25 kHz by an inverter, which then drives the two transformers in parallel. This design affords maximum flexibility in the transformer design, as they can be separate (one for high voltage, one for filament) and gapless.


Building the transformer

Back in March, I did the initial HV transformer build. This consisted of a couple 3D-printed bobbins, one with isolation fins, placed onto a square ferrite core and potted in a 3D-printed box:

Transformer before potting

The bobbins are dipped twice in Glyptal enamel. The transformer is build and placed in the box, with a shim of fishpaper to keep a tight fit. Fiberglass sleeves protect the silicone wires from nicks. Then, the whole thing is potted in MG Chemicals 832TC epoxy potting compound.

Transformer after potting

Following potting, brass sheet (just basic shit from the hardware store) is formed around it into a shield. This turned out to be necessary — the characteristics of the potted transformer without shielding made the control loop severely unstable for reasons I don't really have a good guess about (before potting, this did not happen). Interestingly, the full potted and shielded assembly results in a significantly more efficient power supply than unpotted, by a factor of about half the idle current.

Transformer with shielding

Testing the system

Assembled system

Now that the whole system is together, it needs to be tested again. I built a setup with a separate off-board rectifier for the filament supply (moving it from a -2kV float back up to ground) so the filament supply could be loaded down with a basic e-load — I am not silly enough to try to float an e-load.

Setup not pictured here as I probably wasn't safe enough and don't want to set any bad examples.

Plotting the startup shows it's still more or less stable after the transformer potting. It's slightly underdamped, but I'm not worried about that. The deliberately loose coupling across the HV transformer makes stabilization really tricky, and I am lazy.

Channel 1 (yellow) is the PWM controller compensation node; channel 3 (pink) is the -2kV output:

Startup waveform

Injecting a 50kHz reference signal confirms synchronization works as intended:


The system seems a bit less stable without sync, so I'm considering revisiting loop compensation after all. But I don't want to fuck with 2000V on the bench any more than I need to. Maybe if I can get the power stability analysis feature on this Rigol MSO5 working here I'll do a quick writeup on that — cute feature. With sync it's quite stable so I'm not too worried. (Without sync, it runs at a slightly lower frequency.)

Some more pics

Top of PCB layout

Another view

Closing thoughts

Design files are here

Last modified: Wed Jun 8 20:49:32 MDT 2022. Converted from blog/20220608-hvps.md.
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