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Re: It works! Solid state driver


From: 	Harri Suomalainen[SMTP:haba-at-cc.hut.fi]
Sent: 	Friday, December 12, 1997 1:09 PM
To: 	Tesla List
Subject: 	Re: It works! Solid state driver

On Fri, 12 Dec 1997, Tesla List wrote:
> 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.

There are other things as well. PWM driver output is not perfectly
symmetrical. Output paths (layout) are not symmetrical always. Most
important one is dissimilar switching devices: turn-on or turn-off
delays etc are usually not equal even for two same brand mosfets. This
results in dissimilar conduction period which means dc flux saturating
possibly the core eventually.

Bifilar winding is essential, Malcom is right on that one. However,
switching dissimilarities should be considered as well. Usual solution
(for push-pull) is to use current mode controll which switches the
devices off as soon as certain current level is reached. That will be
below huge saturation currents and will therefore protect system.

For tesla systems this approach may not be too easy to implement if near
full duty cycle is wanted. The coil tends to draw sinuoidal current and
using peak current limiting system would switch off usually at around
peak current of sinuoid that is duty cycle might be reduced to half due
to current limiting.

Gapping the core to make it able to handle enough dc component as well
is probably the best solution to try out with push-pull.

Bridge topologies on the other hand are easier in this problem: just put
a dc-blocking cap in series with the transformer and the problem is
solved.
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