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Old 29th July 2005, 10:01 AM   #1
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Default 1000V PSU questions...

Hello everybody.
I’m new on this forum as a writer (up to now I read it only).
First of all I apologize about my English, is not well but I hope you will forgive me .

I try to build +/- 1000 Vdc, +/- 10 mA PSU. In my short adventure with a switched PSU I’ve build several AC/DC converters (flyback), but output voltage finished at 48 Vdc. All PSU I build based on VIPer series from ST.
So first, I took my standard 230Vac / 24Vdc PSU (flyback) and I recalculated secondary transformer windings with control loop. Normally transformer had 100t/18t (primary/secondary) after modification 100t/400t (duty cycle has changed).
Power on … and don’t works as I would like to. Primary over-current protection acts all time. I’ve found secondary has too large leakage inductance. So I tried bigger core with 40t/160t.
Better but I forgot about reflected voltage… is too high (150/45)*1000 ~ 330V. Adding to mains voltage 310V + 330V = 640V on the drain of the VIPer. The snubber unsolders themselves … (transil 1,5KE200).
The question is, am I going the right way?
I try to build +/- 1kV (10mA) PSU in the flyback topology.
Maybe easiest way is to power it from 24V (no over voltage on transistors – no snubber).
Is it right way?
Maybe push-pull will be better to such task?
What is your opinion?
Have you any experiences with such voltages?

Best regards,
CodiJack
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Old 29th July 2005, 02:19 PM   #2
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Sounds like you need a better turns ratio and leakage inductance? 330V is more optimal than 24V for making 1kV, you just need some HOTs (horizontal output transistors).

Or maybe you should invest in a filament transformer and a 6KD6 tube. 7.5kV peak rating, and that's *within* ratings...

Tim
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Old 29th July 2005, 04:41 PM   #3
Eva is offline Eva  Spain
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The issue with flyback will be always too high reflected voltages, both on the main switch during flyback phase and on the output diodes during charging phase.

You should either use a 1000V primary switch with proper snubber and >2KV output diodes or try another topology like the two-switch forward converter or a half-bridge.

Also, as an alternative, I would consider a bipolar transistor as a switch in a self-oscillating topology, because some bipolars may whitstand even 1500V during flyback period provided a optimum snubber is employed. On the other hand, high voltage MOSFETs are clumsy devices but 1200V IGBTs may be an alternative.
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Old 29th July 2005, 06:22 PM   #4
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Eva
On the other hand, high voltage MOSFETs are clumsy devices but 1200V IGBTs may be an alternative.
Hitachi's IGBT modules are good example, 6500V and 600A. Sounds like serious power device.
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Old 29th July 2005, 07:36 PM   #5
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Hi,

flyback powered from 12V or 24V is very suitable technology for your requirements. You can get away by using 100 or 200V mosfet. The most difficult part to obtain is secondary HV diode. I once used BYV26G from Philips but is now discontinued. You might look at http://www.voltagemultipliers.com/.

But you can also split each secondary in half and use one diode between both halfs of the secondary and one diode at the end of the winding. This technique reduces secondary winding capacitance. I used it sucessfully in 24V to 25kV capacitor charging converter (about 20 secondary windings and diodes in series). Also be sure to use HV specified resistors for feedback divider. Ordinary metal oxide resistors tend to open in the presence of high E field.

Best regards,

Jaka Racman
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Old 29th July 2005, 08:35 PM   #6
mzzj is offline mzzj  Finland
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Microwave oven rectifiers are rated at several kV, but i am not sure of their suitability for higher frequencies.

Or you can try your luck with series connection, some diodes behave supprisingly nicely in series connection.
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Old 30th July 2005, 05:15 AM   #7
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Quote:
Originally posted by mzzj
Microwave oven rectifiers are rated at several kV, but i am not sure of their suitability for higher frequencies.
Probably just the equivalent of a few 1N5408's in series, made for 60Hz operation.

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Old 2nd August 2005, 09:03 AM   #8
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Quote:
Originally posted by Sch3mat1c
...
Or maybe you should invest in a filament transformer and a 6KD6 tube. 7.5kV peak rating, and that's *within* ratings...
Vacuum tubes… nice thing, but what I know about it is produce warm light (specially in the darkness) and about 10 exist in the old grandpa radio.

Quote:
Originally posted by Eva
...
... or try another topology like the two-switch forward converter or a half-bridge.
Yes, I thought about a half-bridge, it will be the "next victim" if I give up with flyback converter….

Quote:
Originally posted by mzzj

Hitachi's IGBT modules are good example, 6500V and 600A. Sounds like serious power device.
My 10mA of working current sounds like leakage current of 600A modules ...


To Jaka:

Quote:
Originally posted by Jaka Racman

... flyback powered from 12V or 24V is very suitable technology for your requirements. You can get away by using 100 or 200V mosfet. ...
But you can also split each secondary in half and use one diode between both halfs of the secondary and one diode at the end of the winding. This technique reduces secondary winding capacitance.
...
Hi,
I thought the same - to decrease primary voltage to eliminate high reflected voltage (wire cross section will increase – something for something).
To split secondary winding is a good idea (thanks, i'll test it) – also should solve reverse voltage of the diodes (which will be total output voltage divided by number of secondary sections – am I right?)
Would you tell me more details about your transformer (prim./sec. turns, frequency)?

Best regards,
CodiJack.
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Old 8th August 2005, 01:43 PM   #9
Kenshin is offline Kenshin  China
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low voltage supply are much safer for 1KV supply than AC mains.

battery+ black&white TV horizontal output transformer + almost any power transistor + feedback control , this will give +1000V.

for -1000V, you can use another black&white TV horizontal output transformer, wind another isolated primary around the core.

or wind a Xformer your self. diode...FR107 are 1000V fast recovery diode, use several in series, maybe with parallel resistor to equalize.
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