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Application Notes/
Technical Brief
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AN772
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Title:
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Speed Error in PWM Fan Control Systems (TC646, TC647, TC649)
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Name:
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AN772
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Date:
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7/27/04
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Author:
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Don Alfano
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Description:
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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.
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Keywords:
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PWM,fan,control,speed,error
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Related Information
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App Notes and Source Code
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AN772
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