If your MOSFET (or other chip) does not behave well AND you bought from not-really-known seller (even if his/her positive feedback ratio is over 99%) for a very competitive price, MAYBE you got a "factory FAKE" item (means: intentionally produced as FAKE in factory).
This realization took about 3-4 nights to me, because I just started using MOSFETs.... very VERY VERY big eye opening experience.
I checked "MOSFET does not turn off", or even "does not turn of totally" related topics. There the solution was usually or other (parasitic way of power), or MOSFET driving problem.
I thought I understand MOSFET (voltage driven and which voltage counts), however I faced the same problem with 2 different simple circuits, so likely I missed something regarding MOSFET basics.
What I wanted to achieve: driving a 12V LED strip (draws about 2A).
1st I used P channel MOSFET - as switch. It was working, BUT very often the LED strip lightly dimmed and vibrated (MOSFET ON was OK, OFF was not perfect). I read somewhere P channel MOSFET gate needs little higher voltage than source to surely turn off..
In both circuits I created them in simulator (drawings came from there), so V1 (dataIN pin) is coming from a 3.3V microcontroller (which I realized only in 2nd circuit).
So I discarded that circuit and made a simpler one with N channel MOSFET: (after taking apart, I checked the P channel MOSFET and .... just now I saw a Youtube video regarding real and false MOSFETs: Youtube regarding REAL and FALSE MOSFETs )
BUT This 2nd circuit does exactly the same. I moved the pin to a PWM capable pin, so now I can dim the LED strip nicely, I can switch on the LED strip as well, but when I switch it off it "starts" to dim very lightly and "blinks" randomly. Again, I can make steady 5%-100% LED power and works nicely, but with 0-1% (I am interested in 0%) it does not switch off firmly.
In 2nd N channel MOSFET circuit I also checked the voltages:
logic high = 3 V (MOSFET gate), MOSFET DS = 0,2V - works as expected (however MOSFET datasheet states 2-4V is needed to turn on fully).
Logic low = 0,2V (MOSFET gate), MOSFET DS = 5V (7V drops in LED). I shorted R3 (150 Ohms) to make things even simpler. When LED strip started "very dim blinking", I even shorted gate to source (with multimeter) - blink continued and I measured about 1mA <- THIS when I decided to ask help, because this ruined everything I imagined.
I have to admit, that my power source is far from perfect, 2A drain in 12V made it 11,5V, but obviously does not make few volts spikes etc.
Very rarely I achieved the "total off" state (without modifying everything), but within few mins - few hours I got back the low light & blinking LED strip. I was thinking to make some capacitor to eliminate any ripple (if any).
The dataIN is about 50 cm wire, LED strip is 2-2,5 meters, wire to LED strip is about 1 meter. Again, I shorted (in 2nd circuit during logical low dataIN) gate and source - without reaching the desired TOTAL OFF.
- P channel MOSFET is: IRF9540N
- N channel MOSFET is: IRFZ44N
I know both has a diode between source and drain (connected as reverse bias), but as I heard it is a byproduct of the MOSFET production and should not make any trouble here.
If anybody could help me:
- Why MOSFETS does not turn totally off in the above 2 circuits.
- How can I modify them (currently I have the lower circuit only, but I can recreate the 1st circuit if the solution is there).
- I added this question finally (after realizing that very likely I inserted a fake MOSFET from China to my circuit) - could be this my problem is? Therefore my 2nd circuit should not able to turn ON a real MOSFET or at least it is very borderline (as stated in datasheet).