Re: It works! Solid state driver

From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Thursday, December 11, 1997 3:40 PM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

Hi Roderick,
              The problem may well lie in the transformer 
construction rather than the switchmode controller (which is pretty 

> From:   RODERICK MAXWELL[SMTP:tank-at-mail.magnolia-dot-net]
> Reply To:   tank-at-mail.magnolia-dot-net
> Sent:   Wednesday, December 10, 1997 8:35 PM
> To:     Tesla List
> Subject:    Re: It works! Solid state driver
> Tesla List wrote:
> > 
> > From:   bmack[SMTP:bmack-at-frontiernet-dot-net]
> > Sent:   Tuesday, December 09, 1997 7:44 PM
> > To:     Tesla List
> > Subject:        Re: It works! Solid state driver
> > 
> > Max,
> > 
> > Your Pwm had an adjustable "dead time" delay which is supposed
> > to avoid simultaneous conduction of the power transistors.
> > 
> > Did you set this for the rise/fall spec's of your FET's?
> > 
> > This may be the problem.
> > 
> > Jim McVey
>    Yes the dead time is set per instructions. My problem seems to be
> "Flux Walking". When I monitor the current on the o'scope using a
> current transformer one half of the waveform clearly has much higher
> peaks. This indicates one transistor is pulling the majority of the
> current. Dead time is also plainly visable as a flat section in the
> waveform when neither transistor is conducting. My new power supply will
> be a split supply with the center-point of the series capacitors earth
> grounded. This will enable me to monitor the drain voltage/waveform
> directly. Before I put the push-pull driver back together I think I'll
> try my hand at a flyback. It will probably be a little easier. I have a
> HUGE npn bipolar transistor sitting on the shelf, gathering dust I've
> been meaning to use. It's made by Westinghouse, and is rated at
> 30A/450volts!
>                                 Frankensteins Helper
>                                         Max

It is *essential* in a pushpull design, all else being equal, to 
*bifilar* wind the primary. The imbalance is often the result of both 
less than best possible coupling between the primary halves and, more 
importantly, the difference in copper mass between the two halves if 
one is wound on top of the other.