Transistor recommendation needed

Question

I'm looking for suggestions on how to control a DC motor with a transistor and a µC. The input voltage is 15 V DC, and the resistance across the two pins of the motor is 3.3R, so we'll call it 5 amps. The way it was originally controlled was that both pins on the motor had 15 V+ (float high), and when rotation was needed, one of the pins would be given a path to ground, presumably through a transistor, and I'd like to do the same.

I started with a TIP120 transistor, however it would get way too hot, and I've looked at an IRLZ44, however it appears that will also require heat management. Is there a better option (transistor or another way I haven't thought of) to control this motor without having to worry about heat management? The duty cycle should be minimal, but I'd prefer to overbuild so there is no risk of heat problems. I also do not believe a standard relay is a good alternative due to mechanical life cycle limits and the noise.

Answer 1

You want to look for a N-Channel MOSFET that has:

1. As low of R_ds-on as possible. This lowers the power dissipation in the MOSFET when it is switched on to as low as possible. Find something with a low number of milli-ohms rating.
2. Aim to achieve a fast switching time for the ON and OFF transitions of the MOSFET. The longer the switching time the more time the MOSFET spends in its active region dissipating power.
3. Choose a MOSFET that has a V_GS-ON rating that allows it to fully switch on at your worst case load current and is compatible with your microcontroller I/O pin control voltage range.
4. A MOSFET that as a V_GS voltage rating that is compatible with motor drive voltage you want to use. Good advice is to overrate this by at least 2 :: 1 so for 15V supply plan to use a rating of 30V or even 50V.
5. Study up on switching inductive loads like a motor and understand the need for a flyback diode and/or a snubber circuit to manage switching transients in your circuit.

Be aware that thermal management is something you have to face. There is no magic to eliminate the need. With careful design and component selection you can reduce this to the point for the type of load that you describe should be possible to achieve with a surface mount D-Pak type MOSFET that uses a large copper area of the circuit board to sink heat away from the component.