Reflected Power

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: Reflected Power
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 30 Jan 1998 08:44:51 -0600
Approved: tesla-at-stic-dot-net

----------
From:  Alan Sharp [SMTP:100624.504-at-compuserve-dot-com]
Sent:  Friday, January 30, 1998 6:48 AM
To:  INTERNET:tesla-at-pupman-dot-com
Subject:  Reflected Power

Scott wrote:
>I've been troubled by the same problem lately. If I make my primary
>resonant, and try to operate at near maximum voltage, and don't dissipate
>all my power in my load, VSWR (the reflected wave) will add and destroy my
>transistors.

>But I have no load until a spark occurs. And after a spark, I have to
charge
>up my coil again, resulting in a heavy load. So the requirement is realy
for
>a variable primary.

>Solutions:

>Directional coupler and pulse-width/rate modulation. The directional
coupler
>can redirect the reflected power into a dummy load. Otherwise my
>snubber/transorbs will die. The modulation can prevent overload of the
>transistors.

Scott,

I had great fun with driving a convenional primary/ secondary arrangement
with a flyback. Lots of blown FETs due to overvoltage - from the reflected
amplified voltage. The other problem is that the current going through the
FETs can be double the meter reading as reflected power goes via the
reverse diodes into the power supply caps and is recycled.

my solutions would be,
1) design a machine gun mechanism to eject the blown FETs and feed in
the replacements from a belt - 1000 rounds per second would be about
right :)

2) bottom feed from a transformer.

3) use a halfbridge or a full bridge to drive the primary with reversed
output diodes to clamp the output to the supply rails.

I'm not sure about transorbs - I think these can only take a limited
number of hits before they blow.

Have fun,

Alan Sharp (UK)