Is the BSS138 capable of level shifting this voltage difference?

Question

The goal is to control the IRF4905 with the ESP32. From my understanding (which is limited) the IRF4905 will have the lowest RDS(on) with a -15 V VGS. To achieve this voltage difference I have a linear voltage regulator dropping my ~33 V (the power source is a 8s Li-ion, so the voltage range is 28.8 V - 33.6 V) to 15 V. Assuming the voltage regulator connects directly to the gate of the IRF4905 it would have a voltage difference between source and gate of 13.8 V - 18.6 V effectively turning on.

My question is, can I use the BSS138 as shown in the schematic below? From my understanding the BSS138 can have a drain-source voltage of 50 V and a gate source voltage of +/-20 V. I should be within spec on both of these but I don't have any experience with MOSFETs.

Note: I'm using the IRF4905 p-channel instead of a n-channel as I only want voltage at the motor connector when the MOSFET is on (to prevent a shock if someone touches the connector and ground in a n-channel configuration) so I believe that means I need to use a P-channel and switch before the load.

I believe there should be a 10k resistor pulling to ground from pin 1 of the BSS138 to prevent floating the gate.

Answer 1

It won't work because the output swing of 0/3.3V of the ESP32 is inadequate.

If you want to make a high-side switch you can do something like this:

^{simulate this circuit – Schematic created using CircuitLab}

However, the switching is relatively slow with this kind of simple gate driver and it would be unwise to feed the driver with a PWM. It's probably okay with on/off control, but more analysis would be required to be sure it won't kill M2.

For high frequency switching it would be better to use a proper gate driver and a low-side switch (use an N-channel MOSFET).

D1 is necessary, otherwise the large below-ground inductive spike when the motor turns off will avalanche and probably destroy M2.

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Since I have a model of the IRF4905, I did a simulation of a 8.3A average 50% PWM control of a 2\$\Omega\$ 500uH motor. Yours will likely be different, of course. I used 5kHz for the PWM frequency.

Average motor current: 8.27A RMS motor current: 8.33A (my fake motor has enough inductance to smooth the current fairly well)

Average dissipation of the IRF4905 1.91W, peak is a couple hundred W briefly while switching.

Increasing the PWM frequency to 25kHz increases typical dissipation in the transistor to 7.6W.

The problem will become more obvious if you go to less optimal duty cycles than 50%, I would not use this above a few kHz. If your motor is low inductance then the MOSFET may fail because the RMS current is too high, especially during startup.

Frankly, I would suggest getting a MOSFET driver chip unless you are prepared to run through a stack of IRF4905s.