Its purpose is to damp parasitic ringing of the "DC blocking capacitor"
with the GDT leakage inductance. Without the damping resistance, the GDT
output will ring at the end of the driving cycle, sometimes enough to turn
IGBTs back ON when they should be OFF. You can inspect the waveforms
without the resistance, i found it to be unacceptable with ringing up to 5V
or more after shut down.
The resistor lets some DC through, but its limited to less current than
would ever saturate a proper GDT. Leaving the drivers in a driving state
too long (one high, one low) could burn out the resistor, but i have never
had problems with it.
Steve
On Mon, Jul 21, 2014 at 7:54 AM, Derek, Extreme Electronics <
tesla@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> Dorian,
>
> Another thought
>
> If the driver isnt tri-state, then the resistor is there to limit the
> drivers DC steady state current, and the capacitor is used to give a high
> current drive for the RF during a burst
>
>
> Derek
>
>
> On 21/07/2014 10:06, Dorian Black wrote:
>
>> Hi,
>>
>> Looking at Steve Ward's UD driver, there's a capacitor shunted with a
>> small resistance at the output of the MOSFET driver pair leading to the
>> GDT. Any idea what purpose this resistor serves? The capacitor is to
>> decouple DC as I'm presuming, but how does that work with this parallel
>> resistor in place?
>>
>> Many thanks.
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>
>
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