Re: Frustrating Problems with Gate Drive Transformers on SSTC - Ahhghhghghg! !! ! !

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 20:26 27/05/03 -0600, you wrote:
>Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><dhmccauley-at-spacecatlighting-dot-com>
>
>I'm having some seriously frustrating problems with the gate drive circuit
>on my full-bridge SSTC.

snip

>Rewired my gate transformer so that now Output A of TL494 controls upper
>left and lower right quadrant and Output B of TL494 controls lower left, and
>upper right quadrant of full-bridge.
>Each control via their own gate drive transformer.

That's probably not going to work, because transformers can't pass a signal 
with a DC component. One popular way of driving a full-bridge is to use a 
full-bridge of gate driver chips and a single gate drive transformer, thusly:

oOo

Use each output from your TL494 to drive one TC442x chip (both chips to be 
the non-inverting kind whatever that is)

Connect the primary of the GDT between the outputs of the driver chips, 
with a parallel combo of (1uF plastic film and 470uF electrolytic) in 
series with it to block DC. If you have two GDTs each with two secondaries 
then use both GDTs with the primaries in parallel.

Connect the 4 secondaries of the GDT to your MOSFETs. The top left and 
bottom right ones with normal polarity, and the top right/bottom left ones 
with inverse polarity.

oOo

In this way, the waveform seen by the GDT is symmetrical whatever the 
deadtime happens to be, so there are no hassles with DC shifts and 
saturation. The MOSFET gates are driven with +/-15v instead of +15v and 0v 
but this doesn't really matter. There are a few variations on the theme eg 
some folk use two GDTs and four driver chips.

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Steve C.