High Voltage Forum

Tesla coils => Solid State Tesla Coils (SSTC) => Topic started by: costas_p on December 01, 2020, 11:17:16 AM

Title: Half-brigde with pot on mosfet gates question
Post by: costas_p on December 01, 2020, 11:17:16 AM
General design practice is to use around 5-10 Ohm resistor to mosfet gates after the GDT in order to minimize ringing.

While I was experimenting on my half bridge sstc I accidentally put two 5k resistors on the mosfet gates, thus I had very little output from my secondary.

This made me speculate that maybe we can have the 5-10 ohm resistor for non-ringing, and maybe put a Dual lineal pot connecting pins 2 and 3 in series with the gate resistors, so that way
we can not only vary the frequency and pulse width from the driver, but also the power of the output without the need of a variac? ( of course if primary design is perfect)
When pot is all way on the left we have Gate + Pot resistors and min output, while all way to the right we only have Gate resistor and max output.

Any thoughts about this working or not?

I am attaching schematic
Title: Re: Half-brigde with pot on mosfet gates question
Post by: TMaxElectronics on December 01, 2020, 12:08:56 PM
I would highly recommend not doing that, as it will put a lot of stress on the transistors.
The reason why you got little output with the large resistors is because the gates didn't get fully charged and discharged every cycle.
That will result in the mosfets staying in their linear region, where the drain to source resistance is much higher than usual, which limits the current they drive into the primary.

The issue is that because their resistance is higher they will have much higher losses too. If you are lucky they might only get warmer, if you aren't they'll blow themselves to pieces :P

Another thing that I'd be concerned about is the 300V or so (and direct mains reference) you'd have between the two sliders on the potentiometer and earth. AFAIK they are only intended for touch safe voltages and might not meet any of the safety requirements for direct mains use, and don't have enough separation between the two channels.
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