Electronics > Projects, Designs, and Technical Stuff
Driving (AC) mosfet switch directly from MPU using GDT
Simon:
Oh, all you wanted to do was dim a light? so just use a conventional dimmer setup. For 50Hz an opto coupler can give you any isolation desired.
beduino:
--- Quote from: Simon on October 23, 2019, 06:46:10 am ---Oh, all you wanted to do was dim a light?
--- End quote ---
Nope, I want control power of this light bulb using GDT (gate driver transformer) directly from MPU and additionally eliminate 50Hz flickering.
However, according do IRF840 datasheet its gate capacitance 832pF in prallel double of that, realized now now chance to get 600kHz switching speeds at such limited MPU GDT primary current ~2mA.
I've also no specs of random E core used for testing, so it require more sophisticated calculations and more tests.
Yansi:
Jeeeez... And how do you eliminate that flicker with this, as the bulb still operates at AC?
Simon:
If it's incandescent the time constant for the filament cooling is longer than the 100 Hz pulse period. You may get some flicker at very low brightness. I found i had to run LED's at a few hundred Hz to get rid of flicker. You don't need 100's of kHz, even 1 kHz is ample which again is well within the limits of an opto and other more expensive isolators exist if you can power both sides.
If you are paranoid about flicker why not rectify the supply and use a very small smoothing capacitor to not ruin power factor and then just use 1 MOSFET in a classic setup?
T3sl4co1l:
Brian: would be easier to just FWB rectify the gate signal. :P
There's no 50Hz flicker, it's 100Hz, and you can't get rid of that without energy storage. You could make a VFD where AC comes in, gets rectified to DC, then is output as either infrequently changing DC (i.e., swapping polarities to reduce sputtering of the filament; low frequency AC, full duty cycle square wave), or high frequency AC (a few kHz will not be visible to the eye under any condition). The DC supply could be varied (with a buck converter, or since efficiency really doesn't matter here given the incandescent light and apparently thermal application, a linear regulator), varying output. In the higher frequency AC case, duty cycle could also be varied.
I once did this for strings of LEDs:
https://www.seventransistorlabs.com/tmoranwms/Elec_LEDs.gif
Tim
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