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I am using this circuit to control a 24V fan at a frequency of 31kHz (MCU - 72 MHz, Prescaler - 9, Counter period - 255): enter image description here

The scheme works. But I want to add an optocoupler to the circuit, I did it like this: enter image description here

It's work to. But the fan speed is maximum when duty cycle more than 30%. If I set duty cycle less than 30%, the fan speed decreases. I tried lowering the frequency to 15kHz, 4kHz, 1kHz but it's not help. Perhaps I made the diagram incorrectly?

IRF7319 Datasheet

Oscillogram of signal at pin 2 of Q1: enter image description here

Power supply: enter image description here

Sem
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    Why the optocoupler in the first place? Is the fan power supply completely separate (including ground) from the MCU power supply? Were you seeing interference effects before using the opto? – vir Dec 17 '23 at 20:00
  • @vir Power supplies of MCU and Fan are different, ground are connected between themselves. I want to isolate a microcontroller pin. Do you think the optocoupler is may not be used? – Sem Dec 17 '23 at 20:20
  • Can you show a picture (oscilloscope) of what the signal looks like at pin 2 of Q1 for the two schematics you posted? – Rodo Dec 18 '23 at 02:47
  • @Rodo I add pictures – Sem Dec 18 '23 at 16:34
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    ”ground are connected between themselves” Please clarify with a schematic. As drawn, they are the same and the optocoupler only adds cost and complexity. – winny Dec 18 '23 at 16:47
  • @winny Hello, I add picture – Sem Dec 18 '23 at 17:12
  • Ok, you only have one ground. Forget the optocoupler. – winny Dec 18 '23 at 17:37
  • @winny Why? please explain. If I separate them, then what next? – Sem Dec 18 '23 at 18:02
  • The P-FET receives 24 V at the gate in both circuits. This is beyond the specs. – Jens Dec 18 '23 at 18:17
  • @Jens Thank you – Sem Dec 18 '23 at 18:22
  • Adds nothing of value. If your fan however does not need to be tied to ground, you can cheat with a low side N-MOS buck with high side fan and save cost and complexity. – winny Dec 18 '23 at 18:37
  • @winny Thank you – Sem Dec 18 '23 at 18:39
  • Get yourself a bog-standard low Vgsth MOSFET and do it like this: https://electronics.stackexchange.com/questions/330471/low-side-n-mosfet-buck-converter – winny Dec 18 '23 at 19:17
  • @winny Thank you very much for the tip. – Sem Dec 18 '23 at 19:44

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