"220V Cooling Fan" pulse width modulation (PWM) is the fan power switching frequency is set at a fixed value, by changing the duty cycle to regulate the fan speed approach. The larger the duty cycle, the faster the fan. The key to this control method is to select the appropriate switching frequency, if the frequency is too low, "220V Cooling Fan" will speed with PWM cycle oscillation. Conversely, if the frequency is too high, the fan inside the commutation rectifier circuit will disconnect the fan acceleration / deceleration of the power supply circuit and cause the fan to stop. General frequency range is 20Hz ~ 160Hz. In addition, PWM rise and fall times to be slow enough to ensure that the "220V Cooling Fan" long-term stability.