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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
If you were to make a 200kHz, 100W, ~10-20kV HV transformer how would you do it? Things I've tried: -Coil from a TV flyback transformer - gets hot really fast -Air core transformer, like a non-resonant Tesla coil (though with a resonant primary to get rid of reactive currents) - gets very hot too, but not nearly as fast
I know I could make a big air cored transformer but I'd like to keep this compact... Any ideas?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
First you need to know the load,
the major problem I have found is dealing with the enormous dv/dt e.g. peak dv/dt = 2.pi.F.Vpk = 2xpix200E3x20E3x1.414 = 35.5E9 V/s = 35.5 V/ns so even 1pF capacitance draws 35.5 mA pk, 25 mA rms = 500 VAR ! you will probably have to run at the natural resonant frequency of the secondary//load.
so low-loss dielectric is needed for inter-layer insulation etc. and of course there is the corona problem which means exclusion of air. so far I have not had much luck with hv hf insulation !
dropping the frequency to 20 kHz means 10x less reactive current with still a reasonable ferrite cross section area requirement.
if the application allows, 10-0-10 kV is much easier than 0-20 kV also. and diodes for 10-0-10 kVrms are much cheaper than for 20 kV rms and hv diodes for 200 kHz are not easy to find, for a multiplier etc. (I have tried eBay Chinese 30 kV 30 mA 300 ns diodes but they are 'fragile' :) I think you will need oil and a vacuum pump to initially remove air at least.
like I said, I have not had any reliable success, so these are just some ideas.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
It is for a "deluxe" plasma globe driver . After all it looks like 150kHz and 10kV might be enough, but this is probably impossible to do with a small air core transformer, my current 250kHz air core transformer already has a Q of the primary circuit over 20. The unloaded secondary draws 100mA of base current, and when I connect the bulb socket the current increases to 180mA, this already means ~13W of just resistive losses in the secondary.
I think I will try a ferrite transformer and wind a layered secondary, with just a few mm layer width to keep the capacitance down, and put a primary under the secondary. But this will be a problem when I need to change the primary specs, so I'll need to find a way to make the primary removable.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Too much leakage inductance. :P
Wolfram (Anders M) has some experience with switched mode X-ray heads running in the 100s of kHz, and I helped him reverse engineer a driver for one particular model.
The transformer in these used a 2-chamber bobbin with primary in one chamber and secondary in the other. As far as we could tell, the transformer was resonant (reflected secondary self-capacitance and leakage inductance, plus a small additional inductor on the driver board) near to the operating frequency, and power control was by detuning away from the resonant frequency. The transformer was under oil along with all the other HV stuff and the tube.
I think resonating the secondary self-capacitance is the way to go, as you'll never get it small enough to ignore at 200kHz. You can also use resonant rise to get a larger output voltage than the turns ratio would suggest, which means fewer secondary turns for a given output voltage, hence less self-capacitance. So you can probably calculate some optimal amount of resonant rise.
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~200kHz 100W HV transformer
. After all it looks like 150kHz and 10kV might be enough, but this is probably impossible to do with a small air core transformer, my current 250kHz air core transformer already has a Q of the primary circuit over 20. The unloaded secondary draws 100mA of base current, and when I connect the bulb socket the current increases to 180mA, this already means ~13W of just resistive losses in the secondary.
