Electronics > Projects, Designs, and Technical Stuff
Driving (AC) mosfet switch directly from MPU using GDT
beduino:
--- Quote from: T3sl4co1l on October 23, 2019, 11:05:25 am ---There's no 50Hz flicker, it's 100Hz, and you can't get rid of that without energy storage.
--- End quote ---
Yep, of course it is 100Hz flicker - I've light sensor to test quality of the light around me ;)
Anyway, flickering is not a problem, because of we could rectify mains and use step down converter with inductor and yep AC mosfet switch instead of single mosfet, so (AC) marked this way, because of I want drive those mosfets using GDT.
I think, it is time to make some real experiments with 100k resistor removed in my AC mosfet switch shown a few posts above :-/O
beduino:
--- Quote from: T3sl4co1l on October 23, 2019, 03:17:12 am ---I think you will discover something interesting if you dig into the gate charge.
--- End quote ---
Yep, probably now it is time to look closer into this, since I've quite nice shiny leds in antiparallel configuration with 1k resistor on my GDT secondary >:D
GDT is quite "bulky" but it does the job and @ 600kHz with 4Vcc Attiny85 MPU 8MHz current on GDT primary is now limited for pin safety by two 100 Ohm resistors 8)
As we can see test LEDs are powered directly from MPU with galvanic insulation by GDT :popcorn:
Plenty of charge to turn on/off AC mosfet switch, but now question is what switching frequency of this AC mosfet switch based on two IRF840 we can get?
Since, I do not know turns ratio in GDT (randomly chosen from old project) lets rectify its secondary and see maximum voltage we can charge capacitor to have some idea what turns ratio could be? ???
T3sl4co1l:
--- Quote from: beduino on October 23, 2019, 09:22:45 pm ---Plenty of charge
--- End quote ---
Citation needed. ;)
--- Quote ---to turn on/off AC mosfet switch, but now question is what switching frequency of this AC mosfet switch based on two IRF840 we can get?
--- End quote ---
And how low can the transformer go, say if in some strange reality it can't supply enough charge after all?
Say, did you have a go with any of those equations? Like, C = Qg / Vgs(on) and t = RC? How does t compare to 1/(600kHz)?
--- Quote ---Since, I do not know turns ratio in GDT (randomly chosen from old project) lets rectify its secondary and see maximum voltage we can charge capacitor to have some idea what turns ratio could be? ???
--- End quote ---
Mind that, if the secondary waveform exhibits ringing, you'll be measuring that, in addition to the turns ratio.
I suspect with ~280 ohms in series with the primary, it's going to be underdamped so this won't be an issue after all, but with that windup, you may have the opposite problem that you're running out of bandwidth, i.e., the output will be very soft and very not square at all.
Tim
beduino:
--- Quote from: T3sl4co1l on October 23, 2019, 10:06:17 pm ---Say, did you have a go with any of those equations? Like, C = Qg / Vgs(on) and t = RC? How does t compare to 1/(600kHz)?
--- End quote ---
IRF840 datasheet says Ciss=832pF Input Capacitance @ VDS25V f =1MHz VGS=0 , 1664pF capacitance for two mosfets in AC switch.
It should be at least +/-10Vmax/min on GDT secondary according to quick test I've made right now,
so we have something like
Q: 1.664e-8
However, there is no current limiting resistor on GDT secondary, only turns of copper wire, so its resistance should be quite low - cheap YATO multimeter says: R: 1.3 Ohm ::)
This gives: t: R*C: 2.1632e-9
beduino:
Now calulated something like average current needed to provide charge for 10Vgs of those IRF840 @ 600kHz and it looks liek this if I do not missed something-I've assumed half of the time charging, second half discharging gate by GDT secondary:
--- Code: ---(%i11) F: 600000.0;
(%o11) 600000.0
(%i12) T: 1.0/F;
(%o12) 1.666666666666667e-6
(%i13) T2: T/2;
(%o13) 8.333333333333334e-7
(%i14) t: R*C;
(%o14) 2.1632e-9
(%i15) T2/t;
(%o15) 385.2317554240631
(%i16) Q/T2;
(%o16) 0.019968
--- End code ---
Looks like something about ~20mA average current.
Probably by lowering down this switching frequency a few times will help get enougth current for full open/close AC mosfet switch? :-\
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