What is the cause of the speed regulation cooling fan startup latency

In recent years, due to the equipment, low noise and low power consumption demand is increasing, and PWM speed regulation cooling fan just meets the requirements of these two points, so the demand of the PWM speed regulation cooling fan has also been gradually rise; Demand is bigger, the question will slowly emerge, so today we will learn about the PWM speed regulation cooling fan electricity after what was the reason for the delay, and put forward Suggestions for improvement;

phenomenon: installed on the device speed regulation cooling fan, fan delay for a few seconds after the power supply turns, rotational speed control function is normal, after the cooling fan PWM control line disconnected, electric cooling fan rotation.

analysis: the cooling fan can speed the PWM control function is normal, but the fan does not turn there are two possible:

1, the PWM is low level.

2, duty ratio is too small, the fan can't start

detection: two oscilloscope probe, fan of the red and blue line, respectively, were to meet fans black line, power to the fan start within a few seconds, the PWM and whether to maintain a few seconds between low level, or the duty ratio is not small,

advice: fan PWM control hardware circuit design, the MCU PWM is not involved, give to the fans of PWM is a high level, the benefits is abnormal, PWM control of the fan is running, does not affect the cooling equipment.

the figure below: PWM is not involved, R201 is not short circuit;