[TCML] Gate drive transformer help

[Scott Bogard]

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 at gmail.com>  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 at case.edu>  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 at gmail.com>
>> 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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