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AN772

Title:
Speed Error in PWM Fan Control Systems (TC646, TC647, TC649)
Name:
AN772
Date:
07/27/2004
Author:
Don Alfano
Description:
Brushless DC fan speed can be varied using pulse-width modulation (PWM). The typical PWM control scheme inserts a power switch in series with the fan as shown in Figure 1. In such applications, an increase in the active duty cycle causes a corresponding increase in fan speed (i.e., fan speed is proportional to tON/(tON + tOFF). Conventional wisdom states that PWM duty cycle alone determines operating speed (for example, the fan speed runs at 50% of maximum when the PWM duty cycle is 50%). In reality, the fan operates at a higher percentage of full speed for any given duty cycle. The resulting difference between actual fan speed and the corresponding PWM duty cycle (herein referred to as Speed Error) can be problematic in open loop fan control systems.
Keywords:
PWM,fan,control,speed,error
 
Application Notes & Source Code
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  AN772
  07/27/2004
  176KB
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