I would like to drive a 180W, 24VDC motor with a PWM signal using mosfets such as follows:
The motor is inside a control loop with negative feedback which regulates the speed of rotation by acting on the PWM output. Given a mechanical load, the speed of the motor can be regulated by varying the applied voltage. The variation in the voltage supplied to the motor is achieved by varying the duty cycle of the pwm output. So far all my efforts have been focused on designing the control system with the analogue PID controller.
Suppose the motor is rotating at a speed w which requires a 10V voltage and a current of around 2A. In this case I can see at least two problems with this approach:
- The mosfet is dissipating 14 * 2 = 28 W. Not acceptable unless using a heat sink
- The diode is dissipating 1.8 * 2 = 3.6 W.
As for the first problem, my solution would be use more mosfets in parallel in order to share the motor current. The number of parallel mosfets would be designed to handle the worst case scenario. However, I have never used mosfets in parallel and since I know that no two transistors are the same, I am afraid the current will not be shared equally (as in a resistor current divider) resulting in a possible overload of one/two mosfets. What are the critical points to consider while paralleling mosfets for this particular application? Furthermore, in case the "current sharing dilemma" is solved, should I put a resistor in series to each gate when sending the PWM signal to each mosfet?
As far as the diode is concerned, I would look for a diode that can withstand more current within the worst case scenario current for the motor which should be around 10 A. Is there a better way to do this?
If you think there is a better way to achieve the same goal, please do provide your solution. This is a hobby project and I need some constructive feedback/critics.
PS: when I talk about worst case scenario current I mean the nominal current under (a heavy) load. In case of high currents due to blocked rotor or short circuit, I plan to add a fuse for protection.