Fan Speed Control Circuit Using Back-EMF Feedback

The patent implements a feedback mechanism by sensing the actual fan rotary speed and comparing it with the target speed. The sensing circuit detects the back electromotive force (EMF) of the motor to obtain actual speed information, which is then fed back to the control circuit. The control circuit adjusts the PWM duty cycle based on the difference between actual and target speeds, forming a closed-loop control system that maintains stable rotary speed despite external voltage changes or air resistance variations.

The patent replaces direct voltage control with a feedback-based PWM control system. Instead of directly adjusting the input voltage to control speed, the system uses PWM signals with adjusted duty cycles based on feedback information. This substitution of control mechanism allows for more precise and stable speed control by continuously monitoring and adjusting the motor operation based on actual conditions.

The control circuit uses feedback from the speed sensing circuit to precisely adjust the PWM duty cycle. The sensing circuit measures the actual motor speed by detecting back EMF, and the control circuit compares this measurement with the target speed to make precise adjustments. This feedback mechanism ensures high speed control precision even when external voltage fluctuates, as the system continuously corrects for voltage variations.

The patent changes the control parameter from direct voltage adjustment to PWM duty cycle adjustment. By controlling the duty cycle of PWM signals rather than directly changing voltage, the system achieves more precise speed control. The duty cycle can be finely adjusted in digital steps, providing better control resolution and precision compared to analog voltage control, which is susceptible to voltage instability.