Electronic – IRF520 max frequency

frequencypwm

I'm having trouble supplying 300mA to an led COB using an IRF520 MOSFET as PMW, controlled with arduino nano.

What is the maximum Frequency, in which I can turn the led on and off in this setting?

In the IRF520 DATA sheet:
Turn on delay time: 8.8ns

Rise time: 30ns

Turn off delay time: 19ns

Fall time: 20ns

Is the maximum Frequency then 1/(sum of above numbers) for the IRF520?

And for the arduino timers, are they all 16MHz or does it change from pin to pin?

What is the correct calculation (please give details if different pins result in different frequencies) to determine my maximun possible operating frequency (MOSFET + arduino)?

Thanks

Best Answer

Nano output can only switch at 8 MHz max (except for the system clock), so the IFR520 could only change state every 125nS max. (62.5nS x 2 = 125nS, so one rising edge at time 0, followed by a falling edge 62.5nS later, then a rising edge at 125nS, and so on).

You should also look at logic level MOSFET, as th IRF520 is barely turned on with a logic level on the gate (Vgs(th) (Max) @ : 4V @ 250µA), and needs 10V to turn fully on; where as a logic level part such as AOD510/514/518 will turn fully on to conduct with a very low Rds (0.004 ohm for AOD510) at that level. (AOD514 and AOD518 are a few milliohm higher, check datasheets at Digikey.)

Related Solutions

Electronic – PWM driven LED array over a shift register

I believe you need to set PWM to off while you are changing content of the 595.

OCR0A = 255; // set PWM to no brightness (off)
_delay_a_little_more;
SPDR = 0b00000001; // select first LED
while(!(SPSR & (1<<SPIF))); // write to shift register
PORTB = 0; // latch outputs
PORTB = 1;
OCR0A = 0; // set PWM to full brightness
_delay_ms(1);

OCR0A = 255; // set PWM to no brightness (off)
_delay_a_little_more;
SPDR = 0b00000010; // select second LED
while(!(SPSR & (1<<SPIF)));
PORTB = 0;
PORTB = 1;
OCR0A = 255; // set PWM to no brightness (off)
_delay_ms(1);

On a side note - there is more efficient way of doing what you want (i.e. controlling brightness of multiple LEDs with 595 and EN pin). It's called Binary Code Modulation. http://www.batsocks.co.uk/readme/art_bcm_3.htm

High Side PWM motor driving circuit

Using R6 as the pull-up on a P channel mosfet is not great for efficiency of switching at higher frequencies. If the gate capacitance of the mosfet is 1nF then this and a 10k resistor form an RC circuit with a time constant of 10 micro seconds - generally, as a rule of thumb 5CR is a good estimate of how long an open-collector (ULN2803) digital signal takes to collapse when subject to an RC circuit and this looks like about 50 microseconds.

This on its own limits the highest frequency to 20kHz and in reality I wouldn't go above 5kHz with this. Maybe reduce R to 1kohm for operation at 25kHz.

Regarding the flyback diode, if the motor is taking 5 amps peak then a conservative approach suggests using a 5 amp diode but, given that a 1A diode will take peaks much in excess of 5A, you have to decide how much you are pushing the device to destruction. Every PWM cycle could push a peak current of 5A into the diode. Read the data sheets.

I wouldn't use a 1N400x because it sails just too close to its limits. You also don't need a diode that's rated at 1000 volts either in this configuration - 50% more than the supply voltage will suffice for this application.

Best Answer

Related Solutions

Electronic – PWM driven LED array over a shift register

High Side PWM motor driving circuit

Related Topic