PWM controllers for brushless DC motors are offered with application notes that typically suggest controlling the fan ground line by an N-MOSFET. The MOSFET is switched on and off by the PWM signal that is sent by the controller, hence controlling the fan speed. While this is the method to control a brush-type DC motor, it may cause a brushless DC motor to malfunction. Because there is electronic commutation circuit inside a brushless DC motor, some models use a microcontroller to perform the commutation function. Obviously, the microcontroller and its associated electronic components such as capacitors will not work normally when the ground line is switched at a frequency of 30 Hz or more. In figure 1, the fan speed signal (tachometer) is fed back to the controller for closed-loop control.
Figure 3 is a more elaborate way to control the power line of the fan. The motor commutation circuit still is being switched on and off by the PWM signal, and the tachometer signal is still likely to be invalid during the off cycle.
With closed-loop speed control, the tolerance of the fan PWM duty cycle vs. the speed curve can be wide. The controller can command the fan to achieve a desired speed (rpm) goal by adjusting the PWM duty cycle. If the speed is below the goal, the PWM duty cycle will be increased, and vice versa. The speed goal also will be maintained when there is voltage variation or load variation on the fan, working in the same manner that an automobile's cruise control system works. In figure 5, fan A and fan B can achieve the same temperature/rpm curve if the controller software is designed properly.
After a thermistor fan is installed in an enclosure, its speed is not controlled by the system. Usually, the fan sends a locked rotor signal to the system: Low indicates the fan is running and high indicates the fan has stopped. The fan can run out of rpm specification and the system will not know. Other types of thermistor fans can send tachometer signals but for monitoring purposes only. With PWM control loop, the fan self-adjusts to the required rpm.
One type of PWM fan uses the PWM signal to directly drive a MOSFET inside the fan. This results in 0 rpm when the duty cycle is below 5 percent to 8 percent, initial rotation when the duty cycle is more than 10 percent, and full speed when the duty cycle is at 100 percent. The PWM signal frequency must match the motor characteristics and normally is within the 30 to 80 Hz range.
Another type of PWM fan uses only the duty cycle information out of the PWM signal, which results in a certain percentage of its full speed at 0 percent duty cycle (figure 7).
PWM frequency is not important. PCE