Choice of Diode (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 03 Apr 2004 20:50:44 +0930
From: Matthew Smith <matt@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Choice of Diode

Hi All

Further to the discussion on the Tesla list regarding pulsing car (automobile)
ignition coils with an HV supply capacitor and gap, I have discovered that I am
lacking a power supply in the 800-1500V range.

Looking at the parts that I have lying around, I have decided to build an SMPS
(TL494 or similar) with full voltage feedback using a 1:58 flyback transformer,
similar to a TV line-out transformer.  This is rated at 1500V output, which
would suggest a convenient 25V drive.  (I have several matrix printer SMPSs to
hand with 5V/24V output.)

I'll run at somewhere around 40kHz, as I don't know if the ferrite core is up to
being driven at the 100kHz of some of the other switching devices I have lying
around.  Switching will be done by an IRFZ48V, which has a very low 'on'
resistance.  A suitable snubber will be provided since this device is only rated
to 60V.

On the secondary side, I am unsure what to do.  Do I make a string of, say, four
standard 1000V Schottky diodes as normally used in SMPS duty, or would a
microwave oven diode be suitable for operation at this frequency?  Also, would a
microwave oven capacitor be appropriate after the diode?  Peak voltage, if run
at 25V in, would be just over 2100V, so well within range of a cap like this.
But what about frequency?

Cheers

M

-- 
Matthew Smith
Kadina Business Consultancy
South Australia
http://www.kbc.net.au