I'M trying to use MOSFET as a driver for my 10W LED with 12V supply. This LED will be used as a transmitter with signal coming from Arduino nano. I'm using IRFZ44N MOSFET with a Vgs(min)=2V and Vgs(max)=4V. The problem is whenever I give the transmitting signal to gate of the MOSFET it doesnt turn ON to the fullest. The voltage drop across LED would be 6V and across Vds its 6V. When I checked the drop across Vgs the DMM reads only 2V. As of what I read the DMM averages the value for such signals but that should mean the drop across Vgs is aprox. 5v and hence turning on the MOSFET but it doesnt turn ON. Therefore, I tried giving the blink program's 1 second ON and 1 second OFF signal to the gate. What I noticed was the MOSFET turned ON completely with very less Vds and max. voltage drop across the LED but the same isn't true for higher frequency. I would like to know how to drive the mosfet with very high frequency like PWM.
Summary of solution
- The 10 k series resistor will restrict the ability to drive sufficient gate voltage to most MOSFETs - this should be replaced by (say) 10 ohm or 100 ohm)
- This will also improve the high frequency activation of the MOSFET due to the low drive impedance charging gate capacitance up a lot quicker.
- The LEDs will look dimmer at a high frequency 50:50 drive compared to being switched on at a slow rate due to the persistence of the retina
- Using a DVM to measure 6 volts across the LEDs indicates that at a high frequency, the actual wave form is a fast on-off 12 volt signal that of course, averages to 6 volts. Ditto the voltage across the MOSFET.
Picture originally posted by OP: -
For a start the series 10 k resistor that connects to the Nano's output can be reduced to circa 100 ohms. At the moment it halves the drive voltage from the Nano and this will mean poor performance from the MOSFET in terms of switch on resistance.
If the Nano can produce 5 volts logic drive then you should be OK with the IRFZ44N: -
At higher frequencies, gate-source capacitance will degrade the signal seen at the gate if the driving current isn't sufficient. The gate source input capacitance is nearly 1.5 nF and with an effective 5 kohm source, this forms an RC low pass filter of cut-off 21.2 kHz. Significantly higher drive frequencies will turn into a mushy DC level at the gate. Try removing the series 10 kohm resistor and replacing it with 100 ohms for a start.
List of things that I got wrong and how I overcame.
So, I was wrong with circuit where the series 10k ohm resistor would act as voltage divider. So, removed the 10k ohm resistor connected to arduino but the pull down resistor is necessay. Thanks to @Andy aka.
High frequency signals wont affect the dimming of the signal but its duty cycle will.
The mosfet which I used was not a logical mosfet and hence it wouldn't turn ON properly. A logical mosfet like IRLZ44N did the trick.