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Driving (AC) mosfet switch directly from MPU using GDT
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beduino:
Hello,
I've already written a few lines of code

--- Code: --- 127                    .L6:
 128 003a C29A                  sbi 0x18,2
 129 003c C098                  cbi 0x18,0
 130 003e 00C0                  rjmp .
 131 0040 0000                  nop
 132 0042 C09A                  sbi 0x18,0
 133 0044 C298                  cbi 0x18,2
 134 0046 0000                  nop
 135 0048 00C0                  rjmp .L6

--- End code ---
to output on two Attiny85 8MHz MPU pins ~600kHz signals like below, thanks to optimizing _delay_us with taking into account delay in RJMP .L6 those output seams to be very symetrical, while I'd like to use them as inputs via resistors from MPU to small gate drive transformer primary, where GDT secondary turns will be connected directly to mosfet gate.



For the moment I've MPU connected to 3.7V Li-ion baterry without any voltage regulator, so we have 3.7Vcc and mosfet gate voltages in the range +/-20V in my case.
Do I need eg. zener diodes to protect mosfet gate against spikes above lets say 15V, when we chose GDT turn ratio to something like 1:3.75, so 5 turns on GDT primary vs 19 turns on GDT secondary connected to mosfet gate?

Idea behind this GDT is control power of resistive load (80W 230VAC red light bulb)  by managing MPU pulses frequency or maybe better pulses width, but at such low amount of turns on GDT primary probably better to keep this frequency high, however it will depend of course on GDT core material used.

I had some success of driving this way AC mosfets switches, but in on/off mode where GDT was used as transformer to step up voltage from 5V to 12V clamped by zener diode on mosfet gate with additional rectifier diodes and gate resistor around 100k.

Didn't tried this directly without rectifier diodes, so is it possible to  :-BROKE mosfet since its gate is itself short circuit with GDT secondary windings?  ::)

What do you think?

Simon:
I'm not an expert in pulse transformers but the idea of a zener sounds like a good one. It would mostly short circuit any negative outpu or you can put two back to back, or just rectify the pulse and then pun a zener in parallel. You should also put a resistor to discharge the gate quikly enough so that it turns off again particularly if you rectify the pulse.
T3sl4co1l:
0. Why use an MCU at all?  Especially bit-banged, where a CPU crash fries the driver (and probably the switches too)?
1. Why bit-bang instead of use the timer-counter?
2a. Why does the lamp/switch need to be driven at 600kHz?
2b. Or is this more like an isolated switch, why not use PV drivers instead?  Do you need bigger transistors than those are suitable for (because of leakage current and switching speed), so are considering a transformer coupled driver?  In that case, what kind of circuitry are you proposing to place around the transformer?

You didn't provide a schematic so it's hard to give any commentary on things like zener diodes or transformers.

Tim
beduino:

--- Quote from: Simon on October 22, 2019, 08:43:00 pm ---You should also put a resistor to discharge the gate quikly enough so that it turns off again particularly if you rectify the pulse.

--- End quote ---
Resistor not needed since as we can see below GDT secondary shorts mosfet gate  ;)



Those two zener diodes (12V) are really needed here since we have transformer secondary connected directly to mosfet gate, so when MPU do not sens pulses to GDT primary low resistance wire shorts mosfet gate?  ::)

Another thing is maybe we need also protect MPU pins against GDT primary few turns? We have 2k in series resistance with GDT primary,
but didn't tested if this will work.

I've already made such circuit with GDT 5 turn primary : 19 turns secondary, but without zenners on mosfest gate  :-/O
beduino:

--- Quote from: T3sl4co1l on October 22, 2019, 08:57:52 pm ---0. Why use an MCU at all?

--- End quote ---
Light bulb power should be adjusted among others based on temperature sensor or simply controled by radio communication etc, so MPU as control logic is a must.


--- Quote from: T3sl4co1l on October 22, 2019, 08:57:52 pm ---2a. Why does the lamp/switch need to be driven at 600kHz?

--- End quote ---
Because of we have only a few turns on GDT primary, so the higher switching frequency the better I think, since it works like DC-DC converter to step up voltage from VCC to something like 12V mosfet gate low RDSon gate voltage pulses-it will not be visible.
Another story is whether mosfets I've used in AC mosfet switch will turn on/off at such switching frequency by using this MPU driven GDT,
since so far I've used rectifier diodes in on/off switch, but I'd like to try control power without 100Hz mains blinking, so maybe AC mains will be rectified to 325DC with additional capacitor, so it desn't matter if it will be 10kHz or 600kHz.
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