Re: solid state coil problem

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: solid state coil problem
From: Tesla List <tesla-at-stic-dot-net>
Date: Tue, 11 Nov 1997 00:48:21 -0600
Approved: tesla-at-stic-dot-net


From:	bmack [SMTP:bmack-at-frontiernet-dot-net]
Sent:	Monday, November 10, 1997 10:29 PM
To:	Tesla List
Subject:	Re: solid state coil problem



 
> From: 	RODERICK MAXWELL[SMTP:tank-at-mail.magnolia-dot-net]
> Reply To: 	tank-at-mail.magnolia-dot-net
> Sent: 	Sunday, November 09, 1997 10:04 PM
> To: 	tesla-at-pupman-dot-com
> Subject: 	solid state coil problem
> 
> "I have finaly gotten my solid state coil driver up and running. But I,m
> running into a problem when it comes to the amount of current the
> transistors are drawing. When power is first applied with the variac the
> current surges up to about 30 amps then drops down to about 12. Thats
> okay, "

???? Is it?  You need to check your bias and/or drive scheme.


"but the voltage applied at this point is only 24 volts!"

What voltage parameters did you design it t work within?

" I,ve tried
> resistors, decreasing the duty cycle, and increasing the freq. but
> nothing seems to help. The reistors eat up all the power and generate a
> lot of heat."

Resistance has a nasty way of doing that!  Thats why tou try to minimize
the ac/dc resistance in the coils as well as the associated cicuitry.


" Decreasing the duty cycle only helps a little."

Again, sounds like too much quiescent bias- but only a guess since
I don't know what config you're using.

" And
> increasing the freq. in hopes of getting more inductive reactance only
> generates more heat in the switching transistors, and increases current
> draw."

This is a conceptual problem.  I would check out some books on RF
design and try again.  If this is an RF stage, it shouidn't be reactive.
Also switching large currents into large inductances will result in huge
voltage spikes, causing the transistors to produce smoke!
 
> " The output transfomer has been wound in push-pull configuration. "

Did you use magnetic design technics on this?

"The
> two switching transistors are IRF450. The freq. range is from 135khz to
> 222khz. The inductance from each leg to the center tap is 135uH. The
> signal generator is a SG3525A pulse width modulator. And the driver is
> an SG3626R dual high speed driver. 
>   Does any one know how to limit the current to the output transformer?"

Your PWM controls the power and a properly designed transformer for the
application will limit the current. Can the core you're using function at
the
frequency above?


>" I was thinking that maybe you could use another inductor of much higher
> value in series with the transformer to limit current?" 
> 
 Not a real eloquent solution, but it will work, better in some topolgies
than others.


 Keep up the midnight work in the ol' lab, and say hello to Igor for me.

Jim M.      
  
> 
>