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I am trying to switch a IRL520N logic-level MOSFET using a 0-5V PWM signal from my microcontroller (PyCom FiPy).

I tested just the MOSFET with a 10 Ohm series gate resistor and my load is a 9-15V brushless DC pump (TD-3LS from this manufacturer). I had my load rail at +12V and a power supply driving the gate at around 2.3V-4V. The pump ran just fine.

Then I hooked it up to a function generator to test the PWM signal at the gate. I tested a variety of amplitudes (0-5V, 0-10V), frequencies (500mHz to 5kHz), and duty cycles (0-100%). None of them turned on the MOSFET and ran the pump.

I've attached a schematic of the final pump drive circuit that this MOSFET will be going into and the original designer said that he got it to work with a PWM signal of 5V, 2.4kHz, and 0-100%, so I'm not sure why I can't even get the MOSFET to work on it's own (note: I've also tested this circuit with a variety of PWM signals and steady drive voltages, and could not get it to switch on the pump).

C35: UFW1H470MED1TD L2: 1536Y Pump DCR: 9kOhm

Pump Drive Circuit

EDIT: I was able to get the pump running off the IRL520N MOSFET. It needs to run for a few seconds at 100% duty cycle first before switching to a pulse (duty cycle was 50-75% at a frequency of 500 Hz from the Arduino). However, still not able to get it running with the driver circuit below, so open to any suggestions about what could be wrong with that. I thought maybe I wasn't giving enough time for the capacitor to charge up so I ran the pump at 100% duty for 20s before switching to the PWM, but still nothing.

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  • \$\begingroup\$ Your schematic has several mistakes that would stop it cold. Where did you get this from? \$\endgroup\$ – Sparky256 Mar 21 at 3:24
  • \$\begingroup\$ I'm adapting an existing design. This circuit has been built and tested by the original designer and it worked for him. But regardless of this circuit, I can't even get the MOSFET on it's own to run on PWM. \$\endgroup\$ – YNGVV Mar 21 at 3:28
  • \$\begingroup\$ Repeat 1st test "12V and a power supply driving the gate at around 2.3V-4V." if that works then you have a schematic and/or wiring errors. If it fails congratulations on your 1st ESD failures \$\endgroup\$ – Sunnyskyguy EE75 Mar 21 at 3:40
  • \$\begingroup\$ @SunnyskyguyEE75 thank you for your comments but that does not address the PWM issue. Please disregard the schematic for now. I care first about getting the MOSFET alone to work. As mentioned, it works with a constant drive voltage, but NOT with PWM. I hasn't been destroyed by ESD (yet) because I alternate between testing constant voltage and PWM and it works with the constant voltage and not with the PWM. As for the schematic, I do not see what is wrong with it but I'd be grateful if you pointed out any errors. I tested the very simple two-layer board for wiring continuity and it is fine. \$\endgroup\$ – YNGVV Mar 21 at 4:09
  • \$\begingroup\$ The pump seems to be a two wire brushless pump which would have it's own breaker inside ...ie you apply 12V and it pumps. This is unsuitable for a PWM drive unless it was at very very low frame rates ….perhaps only 5-10Hz or less. \$\endgroup\$ – Jack Creasey Mar 21 at 4:37

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