For the ROV project I need to drive six DC motors (waterproof motors from bilge pumps). They are 12 V DC brushed motors running a 6 amps at full speed and I want to control them using PWM (for variable speed) from an LPC1347 MCU.
A popular way of controlling DC motors is by using an H bridge. However, since I only need to drive the motor in one direction I can use a simpler configuration: a single N-channel MOSFET as a low-side switch.
First I need to find a MOSFET that can handle the 6 amps continously and not get too hot. And since I will switching the MOSFET on and off using PWM at a frequency of 20 kHz I also need to consider switching power loss.
Here are a couple of useful links: choosing a MOSFET, MOSFETs and drivers, MOSFET power dissipation.
Gate current and MOSFET driver
Even though the MOSFET doesn't require a high voltage to fully switch on, the MOSFET gate does have a quite high capacitance (typically, the higher current a MOSFET can handle the higher gate capacitance it has) and in order to switch the MOSFET on and off quickly enough we need a high current (of the order of a few amps).
If the gate current is low it takes longer to switch the MOSFET and the resistance through the MOSFET is much higher when the MOSFET is switching. And higher resistance means higher power loss and higher temperatures.
Since the MCU cannot provide or sink more than 4 mA we'll use a MOSFET gate driver to boost the current from the MCU and to handle the higher sink current.
Flywheel diode and capacitor
I won't go into details since you can read about it in great detail elsewhere (Wikipedia for example) but you do need a diode and a capacitor on the motor circuit. I tried without diode and capacitor with a scope attached. With no flywheel diode there were some really large voltage spikes every time the motor stopped (about 130 V). And without the capacitor (but with diode) there was a nasty "ringing" (starting out a about 20 V) every time the motor stopped.
So the final circuit is: