资料介绍
Brushless DC fan (BDC) speed can be varied using
pulse-width modulation (PWM). The typical PWM con-
trol 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 correspond-
ing increase in fan speed (i.e., fan speed is proportional
to t
ON
/(t
ON
+ t
OFF
). 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.M AN772
Speed Error in PWM Fan Control Systems
periods. The resulting speed versus PWM duty cycle
Author: Microchip Technology Inc.
error is a function of the mass and diameter of the fan
hub, the torque characteristics of the fan motor and the
INTRODUCTION losses in the center bearings. Air temperature and
Brushless DC fan (BDC) speed can be varied using humidity also contribute to speed error, but not
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