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Re: [TCML] Gate drive transformer help

Hi Tyler,
Thanks for the input, I don't think I can get rid of the blocking cap entirely because I will be playing with duty cycle quite a bit, but I will try the gate resistor. I don't know if it makes a difference but when I was probing the leads the transformer was not actually hooked to anything but the power supply, and the probes on the scope, I imagine it will perform better when properly grounded and hooked to a capacitive gate load on the output, so I'll have to try that once I have some time later this week. 

Scott Bogard.

On 10/31/2011 12:30 PM, Tyler 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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