Favorite MosFet for fast PWM

For many years I’ve used the Infineon IRLL2705 HexFet as a general MosFet. It handles quite a bit of power/amps, has low resistance and triggers fully at 2V. Great part for 3V3 designs. The main drawback is that it is slow to turn on/off (6ns turn on, 12ns rise time, 35ns turn off). Up until now, that’s not been a problem.

For a new design, I need something that can turn on/off faster than 20hz to drive a PWM output 24V/2A. Does anyone have suggestions for parts to look at?

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20Hz is very slow relative to the rise and fall times you mention. I don’t think you will have a lot of switching losses at 20Hz. Be careful when use low gate drive mosfets, they got normally higher gate capacitance than normal mosfets. You mention 3.3V which I assume you driver the gate straight from some CPU. If switching loss is an issue and you drive via a CPU pin, consider a gate driver first.

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I doubt you’ll have any problems with that mosfet at even 100hz, but for pwm outputs I’m a fan of using class d audio amps. I use them for driving irrigation solenoids at 24v ac. If there is a need for 100% duty cycle it can get a bit tricky to find one that fits the application though.

Hey Jensa! Thanks for posting this question, I studied power electronics in grad school and this question touches upon some things I learned in my first power electronics lab class. First, I checked out the IRLL2705 datasheet available here: https://www.infineon.com/dgdl/irll2705pbf.pdf?fileId=5546d462533600a401535664653425db

This is a nice general purpose switching MOSFET. Good find, I may use it myself in the future! :slight_smile:

Regarding your question about the PWM switching application:

  1. The total turn-on time (td-on + tr = 6.2ns + 12ns ~= 18ns) and turn-off time (td-off + tf = 35ns + 22ns ~= 57ns) is totally in line with industry standards for a silicon MOSFET of this switching class. It’s going to be hard to find something a lot better without moving to something exotic like GaN or SiC, and frankly why on earth bother? I’m not sure how you came up with “… something that can turn on/off faster than 20hz”. If you take the switching times from the datasheet and add them up (18ns + 57ns = 75ns), then compute the maximum theoretical frequency if the device were only switching on and off again as 1/(total switching time) you get 1/75ns = 13.33E6 or 13.33MHz. Realistically, in a generic PWM application you’re looking at switching at about 20kHz to maybe a few hundred kHz max. So assuming a switching frequency of 100kHz, the total PWM period is 1/100kHz = 10µs. So in this 10µs period, what percentage is spent switching on and off with the IRLL2705? 75ns / 10µs = 0.75%. So less than 1% of your cycle is spent switching. Perfect, there’s nothing wrong with this!

  2. Your IRLL2705 will not switch this fast when connected to a 3.3V microcontroller, not anywhere near this fast actually. In fact, this may be why you’re seeing only 20Hz maximum switching if you tested this out. Why? Take a look at the the datasheet, figure 10a. This is the circuit used to test the switching times. Notice that the square wave driving the gate is an ideal voltage source of 10V amplitude, driving the gate through a very small (6.2Ω) resistor. So how much current is flowing out of this ideal source during a typical turn-on transition (td-on + tr = 6.2ns + 12ns ~= 18ns)? An approximation of the average current required over the entire 18ns can be easily computed. Look up Qg in the datasheet, it’s up to 48nC. Recall that 1C = 1A * 1s. So 48nC in 18ns = 48nC / 18ns = 2.667A of current! There’s no way that a microcontroller can drive this MOSFET directly if you’re wanting anywhere near the switching performance specified in the datasheet. While the MOSFET gate is separated by a very good insulating layer from the drain and source, the gate still has capacitance relative to these, and in power MOSFETs this capacitance is significant if you’re wanting to quickly charge and discharge it to turn the MOSFET on and off. I recommend driving the MOSFET with a gate driver IC like the the MCP1416 (even though the MCP1416 is only rated for 1.5A drive, it will still work fine for a lower switching frequency application).
    Don’t forget to put a (~100kΩ) pull-down resistor on the input to the '1416 so that the MOSFET is off while your µC comes out of reset and your outputs are tri-stated!

One other tip: Make sure you have a schottky diode in your PWM circuit if your load has any kind of inductance or even any stray inductance to carry the current while the MOSFET is off. Have fun!

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In order to minimize EMI generation, you’re generally going to want to use the lowest PWM carrier frequency you can get away with, especially if you’re switching 2A. For the same reason, you also want the slowest rise and fall times that your heat dissipation limitations allow. 20kHz with fast rise and fall times can easily create serious emissions (both radiated and conducted) issues into hundreds of MHz that will require careful attention to keep within statutory limits.

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Thanks all. Lots of good advice here!
I found the 20Hz by looking at the output with an oscilloscope. Anything faster than this and it didn’t look remotely like a square wave. I’ll look into Gate Drivers and see if they will make the output look better.

EMI is a real problem, so I’ll avoid using the PWM output if possible. This board is primarily for motor control, so there is bound to be some EMI :wink: