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I want to design a very simple high side switch, using components already present in the bill of materials of a larger project. I do not want to choose special components, but just try to re-use what I got, to simplify production and reduce related costs. The switch needs to drive high-side a standard 3-wire Fan, one terminal being the (-), the other being (+) and the third one being tachometer signal. The Fan is rated 12V, 3.6W. The idea is to drive two fans in parallel with the same circuit. The output voltage to the Fan needs to be linearized, since I want to be able to read the tachometer without any difficulties. The output voltage needs to be controlled by a micro in PWM, using 62,5 KHz carrier frequency, 5V logic. Here's the simple circuit I have been working on:

schematic

simulate this circuit – Schematic created using CircuitLab

Do you see any flaws in this circuit?

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  • \$\begingroup\$ R3 is way too high. The turn off time might be very long (even longer than the period of your PWM). I would reduce it by a factor 10 or even 100. \$\endgroup\$
    – next-hack
    Commented Oct 12, 2017 at 9:39
  • \$\begingroup\$ How about using a diode in parallel of R3? In this way the gate of the M2 can get emptied out fast, while keep protecting M1 from high current while turning ON \$\endgroup\$
    – Francesco
    Commented Oct 12, 2017 at 10:23
  • \$\begingroup\$ The "trick" with the diode (with the cathode toward the 15V) works only when you remove the power (15V) to the circuit. Becase, otherwise it would be always OFF (the gate would always have a smaller voltage than 15V). If you put the diode with the anode toward VDD (That would speed-up the discharge of the gate), then you burn M1 when it's on. If you put a 1kOhm resistor (but I'd say 100 Ohm too. That would be just a waste of power, and R3 would need to be a power resistor), instead, you won't burn for sure M1. \$\endgroup\$
    – next-hack
    Commented Oct 12, 2017 at 10:30
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    \$\begingroup\$ Use a MOSFET driver then. \$\endgroup\$
    – next-hack
    Commented Oct 12, 2017 at 10:37
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    \$\begingroup\$ Better get an integrated high-side bootstrap MOSFET driver and an N-channel MOSFET. I've spent a lot of time on a similar problem trying to make the driver with discrete parts - level shifters, gate drivers, etc... and nothing was good enough for a reasonable price. If you manage to get it as nearly good as the IC driver, its cost will be higher ... not mentioning the design time... Discrete driver for 100Hz-5kHz frequency is achievable, but for 62kHz is not so simple. On the other hand - P-channel is risky, because it could turn on due to input voltage (MOSFET source) fluctuations. \$\endgroup\$
    – Todor Simeonov
    Commented Oct 12, 2017 at 12:53

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