Re: Richard Quick/flyback Xformers

To: tesla-at-pupman-dot-com
Subject: Re: Richard Quick/flyback Xformers
From: bigr-at-teleport-dot-com
Date: Sat, 08 Jun 1996 23:19:29 -0700
Flybacks from old tube TV's are best.  Find a junkyard or make friends 
with TV repair shop.  Some of these flybacks from old console type 
tv's are huge. 

An excellent driver for flybacks is a used computer 250W switching 
supply (half bridge only- don't use flyback type) with mosfets and 
SG3526 chip- Jamesco has these very cheap.  Cut away part of pc board 
containing low voltage and protection ckts. and put potentiometer in 
place for R-C oscillator circuit.  See a National Semi or Mot databook 
for chip connections.  Then connect power supply for full bridge 
config- thats the 220v jumper positions used with 120vac.  That gives 
160v on MOSFETS and much lower flux density on XFMR.  Make your own 
primary winding- try 50 turns to start- even that may make too high 
voltage- may need more.  Hook flyback primary to points on pcb where 
switching supply transformer primary used to be.

I've found alot of flybacks resonate around 45khz, which should be ok 
for the mosfet driver transformers used in these supplies.  Much  
lower frequencies will result in Mosfets not getting enough drive.  
Also, dead time capacitor can be increased to give more margin of 
safety that MOSFETS are not cross-conducting with source-drain diodes 
under any circumstances. 

You should probably also remove any metal parts to the flyback as they 
will heat up and lower Q drastically.  Also don't mount XFMR near any 
ferrous metal for the same reason.  Remember, flybacks contain gapped 
ferrite core and long magnetic path which creates large external 
field- causing problems when near metal.  

Have fun! Rob.