Re: [TCML] Gate drive transformer help

[Michael Twieg <mdt24@xxxxxxxx>]

I never use blocking capacitors on my GDT.  The average DC voltage applied
to the primary is generally in the tens of millivolts, and the total
resistance is around an ohm or two, so saturation isn't a problem (though I
use a pretty big core...).

-Mike

On Mon, Oct 31, 2011 at 12:30 PM, Tyler <tyler.c.c@xxxxxxxxx> wrote:

> The spike you describe at the beginning of the switching may be ringing of
> leakage inductance in the secondary with the gate capacitance. Try adding a
> few ohms of gate resistance if you haven't, this usually damps that out.
> Also, I was working on some GDT's this weekend and also experienced the
> height falling, and was using an 0.47uF blocking capacitor. I removed the
> blocking capacitor entirely (don't do this unless you really understand
> your driver and what the blocking cap is for!) and now I get a perfect
> square waveform at the gate with zero ringing. A blocking cap is not needed
> in a DRSSTC in principle, assuming proper operation and a perfectly tuned
> duty cycle. But it is a good safety margin to have it there, especially for
> initial testing.
>
> Tyler
>
>
> On Mon, Oct 31, 2011 at 12:17 PM, Scott Bogard <sdbogard@xxxxxxxxx> wrote:
>
> > Hi Mike,
> >     What I am seeing on the scope is the wave form is hardly square, it
> > starts and bumps up high, then comes down; at higher frequencies this is
> > where it ends, but at lower frequencies it then cuts out, then comes back
> > up at about 3/4 height and ramps down, breaking a single pulse into two.
>  I
> > have a lot of off axis inductance, the thing was gator clipped together,
> > and subtracting 3 out of the 17 turns made no difference...  When I turn
> > down the duty cycle, the two bumps merge, and form one distorted but
> stable
> > bump, I can run really low frequencies at less than 10 percent duty
> > cycle...  I need a gate drive transformer because I am going to be
> running
> > this thing off of 300V DC once all is said and done (always though using
> a
> > step down transformer to drive a step up transformer was ludicrous...)
>  I'm
> > not using a resistor from the driver output to the transformer, and my dc
> > blocking cap is .68uf I believe...  Not even sure if it is necessary for
> > proper operation, tried doubling this and halving it, no appreciable
> > difference.  Thanks for your input guys!  Oh and my turns ratio for the
> > xformer is 1:1:1...
> >
> > Scott Bogard.
> >
> > On Mon, Oct 31, 2011 at 8:34 AM, Michael Twieg <mdt24@xxxxxxxx> wrote:
> >
> > > If you're seeing "distortion" only at lower frequencies then that
> > probably
> > > means you don't have enough magnetizing inductance.  Your driver may be
> > > drooping due to excessive magnetizing current, or the core is
> saturating.
> > > Though without a better description of the "distortion" it's hard to
> say.
> > > Just try adding a few more turns and see if it helps.
> > >
> > > -Mike
> > >
> > > On Sun, Oct 30, 2011 at 6:26 PM, Scott Bogard <sdbogard@xxxxxxxxx>
> > wrote:
> > >
> > > > Greetings all.
> > > >     Built my gate drive transformer for my flyback driver, oddly
> enough
> > > > when scoping it it works great at higher frequencies (30+kHz) but
> > > distorts
> > > > below that, except at low duty cycles.  So my questions is do I need
> a
> > > > different core material, possibly a larger core, fewer turns?  I have
> > one
> > > > larger core, and also several smaller ones, the larger core is
> already
> > > > wound but seems to only work at much higher frequencies in that
> > > > configuration.  Also tried doubling and halving the dc blocking cap,
> > > didn't
> > > > change much...  Any advice would be greatly welcomed.  Also the
> > waveforms
> > > > are a bit distorted even when they are stable, up until very high
> > > > frequencies (100kHz ish) I still think I could use them this way but
> > > seems
> > > > to me changing parameters should fix that as well, at least my my
> > > intended
> > > > operating range of 10-100 kHz...  Thanks.
> > > >
> > > > Scott Bogard.
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