cooling fan PWM function Introduction（2）

3, PWM control:

The most widely used method for fan speed control is PWM control.

In this method, a pan-PWM drive signal is applied to a field-effecttransistor (FET) connected to the high or low side of the fan.

The fan is switched on and off at a specific frequency k, and the fan speed is controlled by the duty cycle of the PWM signal, while the voltage applied to the fan is always full or zero.

The biggest advantages of this method are simple design, few external circuits, and low cost.

The disadvantage is that because the voltage is not always applied to the fan, the tachometer signal will be significantly reduced by the PWM drive signal.

In fact, the tachometer signal will follow the PWM signal.

To avoid this problem, a "pulse broadening" technique should be used to obtain tachometer information.

Figure 1 shows the tachometer signal after a significant reduction/change due to the PWM drive signal.

Figure 2 shows the reconstruction of the tachometer signal from the actual tachometer signal using the pulse broadening principle.