Re: switched mode power supply

To: tesla-at-poodle.pupman-dot-com
Subject: Re: switched mode power supply
From: Tesla List <tesla-at-stic-dot-net>
Date: Sat, 22 Mar 1997 15:19:11 -0600
Approved: tesla-at-stic-dot-net
Subject: 
        Re: switched mode power supply
  Date: 
        22 Mar 97 13:56:19 EST
  From: 
        Alan Sharp <100624.504-at-CompuServe.COM>
    To: 
        "INTERNET:tesla-at-pupman-dot-com" <tesla-at-pupman-dot-com>


Kenneth wrote:

>> I have had great difficulty in obtaining a cheap 2kw transformer,
>> so my plan is to create a SMPS and step up the voltage. the idea i
>> have in mind is a design that operates the switcher at the self
>> resonant frequency of the module so i get max efficiency. has
>> anyone tried this before ? How do the spark gaps and caps handle
>> the 30..80khz input to them ? is anyone interested in the schematics
>> of the SMPS ?

Malcolm replied:
>What module? You can run a switcher and feed the secondary like a 
>magnifier. A number of people have done it. You don't need a gap.
>Harri and Alan are experienced operators and Mark Barton has done it 
>to about 8kW CW and around 40kW pulsed carrier. A low frequency 
>resonator <<100kHz is the key to efficiency in the switcher.

I'm with Malcolm on this. It would be possible to build a SMPS to supply
say 6Kv at 60ma and then to full wave rectify this, smooth it and use
the
DC supply to run a traditional spark gap.
To do what you propose there would be no need for a sparkgap or a
capacitor
- you adjust the frequency of the SMPS to the resonance frequency and
then
You could put this into a primary coil but there are a lot of problems.

1) Energy is transfered from the primary into the secondary but it also
comes
back down again - at a much higher voltage and high currents.
I tried a very simple rig:

PWM chip ----- FET ---OOOprimaryOOO---- +ive
I found that while say 1A was being fed into the circuit, 4A say
(average) was
being switched by the FET and 3A was fed back onto the +ve rail via the
protection diodes. (with out these diodes overvoltage killed the FET)

2) Mark Barton ran this at 8 - 10kW using a full bridge. The secondary
was
an pvc oil drum, the primary was (from memory) 18" high 3' diameter.
But he had massive problems with strikes to the primary. He blew a lot
of FET's.

I now use Duane Bylands magnifier arrangement:

ground----OOOSMPS secondaryOOOOO------OOmagnifier coilOOO-----toriodal

the SMPS supplies 3kV. The magnifier coil is a large polythene bucket -
sold
for homebrew beer. about 1000 turns of 0.4mm copper wire. (80kHz)

I would be very interested in seeing your schematic for a SMPS.

Have fun,

Alan Sharp (UK)