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I'm driving a DC motor (12 V, approx. 4 A) and I need to control the speed by PWM but I need one direction only and braking is not required.

My question is: is there any advantage to using a half-H bridge over using a simple low-side N-channel MOSFET (e.g. a IRLS3034-7PPbF)?

I'll be using a MOSFET driver (IXDN404 or similar) in order to switch fast enough and provide enough current to the gate and the PWM will be generated by an MCU.

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If this is a simple brushed DC motor with just two connections, then yes, you can tie one lead to power and use a low side switching element on the other. Don't forget to put a reverse Schottky diode accross the motor else the switch will get destroyed from the high voltage pulses.

At only 12 V, you can find N channel FETs that will switch well enough directly from a digital signal. These are often called "logicl level" FETs. In that case you don't need a FET driver.

Another possibility is a NPN as the switch. That is easy to drive from low voltage logic, but turning off a saturated bipolar is always a bit tricky. It depends on how fast you want to go. For a normal motor drive PWM frequency of about 25 kHz, it should be fine. However, unless you are using unusually low logic levels or the micro has a weak output drive, I'd look at logical level N channel FETs first.

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