Author Topic: Single mosfet driving with GDT  (Read 949 times)

Offline costas_p

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Single mosfet driving with GDT
« on: November 20, 2020, 05:19:06 PM »
Hello,

I am trying to operate a single mosfet sstc, with GDT for isolation, I am looking the web about driving 1 mosfet with a GDT, but in various schematics, there is a resistor across Gate and Source of the mosfet as a pull-down, along with the Gate resistor to avoid ringing.

So my questions are:

- Is the Gate-SourceResistor necessary? If yes, when how can I calculate it?
- Is the Source-Drain diode necessary for just 1 mosfet?
- Will this work? Or a need a half-bridge in order to start oscillations?

I am attaching a self made schematic for the topic.
V1 logic voltage
V2 primary coil voltage
Logic and primary ground are not connected
Papadakis Costas

Offline davekni

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Re: Single mosfet driving with GDT
« Reply #1 on: November 21, 2020, 04:01:13 AM »
Why do you want to do 90% of the work of a half-bridge, then settle for all the disadvantages of a single FET?
David Knierim

Offline costas_p

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Re: Single mosfet driving with GDT
« Reply #2 on: November 21, 2020, 11:11:03 AM »
Hello davekni,

I have some pcbs left from my 'unintentional drsst', so I have the ucc3732x driver ready.

Also I have some counterfeit mosfets from eBay, that work good on lower voltages.

I would like to experiment on a musical interrupter, with low voltage,  and don't want to have the hassle of blowing 2 mosfets every time.

If this experiment gives good results , then I will try implement the musical interrupter to my main project of half bridge sstc
Papadakis Costas

Offline davekni

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Re: Single mosfet driving with GDT
« Reply #3 on: November 21, 2020, 07:53:00 PM »
I suspect that a half-bridge running on lower voltage is less likely to fry FETs.  However, experimenting with a single-FET option is a great way to learn and may work.  The biggest issue is likely the voltage spike on the FET drain as it turns off, as there's no diode to clamp the voltage.  There's also nothing to limit current if feedback doesn't kick in and the FET stays on a long time.  So start with very-short enable pulses.

For oscillation to start, the feedback needs to be biased SLIGHTLY low.  That way the FET gate starts high at the beginning of an enable pulse.  Feedback needs to be strong enough so that the second half-cycle turns the FET off.

No need for G-S resistor.  Likely no need for D-S diode.  (The D-S diode isn't needed in most half-bridges either.)  FETs have body diodes.  External diodes are useful only when the FET's internal diode is too slow, and when the external diode has a lower forward voltage drop so that it takes most of the current.
David Knierim

Offline costas_p

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Re: Single mosfet driving with GDT
« Reply #4 on: November 27, 2020, 08:42:35 PM »
I made the test circuit, and I test it with an older secondary coil I had, 50mm diameter 250mm height 32AWG magnet wire, javatc says that its resonance frequency is around 1,5Mhz. (i strongly doubt it) and primary coil is 100mm diameter 6 turns 14AWG.

The control circuit seems that is maybe working and be able to operate the single mosfet with only gate resistor: I use a bench power supply 30V 5A, at 12 volts amp draw starts to rise, but around 20v the power supply goes into OC condition and turns its output off, at those voltage there is not a spark, also the mosfet get tested with multimeter every time and is fully functional.

Unfortunately in Greece we have a lockdown due to Covid19 and markets still operating don't sell CFL lamp anymore (in order to visually confirm that there are oscillations).

I am thinking that mosfet just stays on, that explains the OC protections of the power supply.

I still don't have a scope.

Any thoughts?

I am attaching javatc picture.
Papadakis Costas

Offline Magneticitist

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Re: Single mosfet driving with GDT
« Reply #5 on: November 27, 2020, 10:57:31 PM »
Unless I'm missing something about the gate drivers I'd get rid of one of them or rewind the GDT with another output to drive a half bridge. I've tried this setup by just sort of making a hybrid of two Steve Ward circuits and it works ok but I'm having trouble adding RC snubber resistors that don't get too hot, although I've pushed it to 230VDC without them. I'm actually messing around with it right now just floating around the net for ideas etc.
Basically just used the antenna into the hex inverter then to the single gate driver and out to the GDT. I don't use any pull down resistor and the UCC273x chip I have is always-enabled so I used the AND gating to interrupt it. I'm using IRFP460's and I've blown a few of them. To be honest I feel like I'm more likely to blow through more on this single mosfet setup.

Offline costas_p

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Re: Single mosfet driving with GDT
« Reply #6 on: December 03, 2020, 10:55:08 AM »
Indeed I used a 10k resistor between gate and source to make circuit oscillate, without it the mosfet just stayed on.
I used a cfl lamp to verify the output since the variable psu I am using can supply insufficient voltage, in order to have a breakout, but when exceeding 7-8 volts primary voltage, the circuit stops running.

Also I implemented kaizer music interrupter, and be able to play some faintly volume music (again 7-8 volts primary) but sound comes from the counterpoise instead of breakout point.

I tried reversing primary, i tried increasing voltage, i tried various mkp value caps, so far no success.
Papadakis Costas

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Re: Single mosfet driving with GDT
« Reply #6 on: December 03, 2020, 10:55:08 AM »

 


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