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Speed error in PWM fan control systems

Posted: 15 Dec 2004     Print Version  Bookmark and Share


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2003 Microchip Technology Inc. DS00772B-page 1

M AN772


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 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.

Speed Error is attributed to both the mechanical char-

acteristics of the fan and the discontinuous nature of

PWM speed control. The fan's rotating hub has mass

and, therefore, stores rotational energy when the PWM

is in its ON state (i.e., when power is applied). The low

friction afforded by the fan's center bearings preserve a

portion of this stored energy when the PWM is in its

OFF state. This residual energy is carried forward into

the next PWM ON cycle. Fan speed increases until an

equilibrium is struck between mechanical frictional

losses and added rotational energy during PWM ON

periods. The resulting speed versus PWM duty cycle

error is a function of the mass and diameter of the fan

hub, the torque characteristics of the fan motor and the

losses in the center bearings. Air temperature and

humidity also contribute to speed error, but not


Figure 1 shows the measured speed of three different

sized fans that are controlled by a 30 Hz PWM. Fan 1

is a 1 1/2 inch, 12V, 80 mA (max) fan that is typical of

CPU chip-cooling fans. Fan 2 is a 3 1/2 inch, 12V,

200 mA (max) fan similar to those used in the typical

PC power supply. Fan 3 is also a 12V, 3 1/2 inch fan,

but has a 460 mA (max) current spec.

The "ideal" response is intuitively what one would

expect -- percentage of full speed exactly equal to

PWM duty cycle. Fan 1, the smallest of the three fans,

comes closest to matching this curve (due to its low

mass/low torque characteristics). Fan 2 has both

greater physical size and greater mass than Fan 1 and,

as a result, operates at a higher speed than Fan 1 for

every value of PWM duty cycle. Fan 3 is physically the

same size and mass as Fan 2, but has higher torque,

causing its operating speed to be slightly above that of

Fan 2. As shown, all curves converge when the PWM

duty cycle is 100%.

FIGURE 1: Measured Speed vs. PWM Duty Cycle.

Author: Microchip Technology Inc.



40 50 60 70 80 90 100





Fan 1 Actual

Fan 2 Actual

Fan 3 Actual

Fan Speed

(% of Full Speed)

PWM Duty Cycle (%)






Speed Error in PWM Fan Control Systems


DS00772B-page 2 2003 Microchip Technology Inc.




Speed error is not a problem when the fan control sys-

tem is closed loop in nature (such as a fan controller

referenced to a local temperature). In this case, a tem-

perature increase is answered by an offsetting increase

in fan speed (airflow) until temperature is returned to

normal. Another common closed loop fan control tech-

nique relies on fan speed feedback (usually a fan tach

signal). In either case, the control loop drives the fan

hard enough to satisfy the feedback loop.

To compensate for fan speed error in an open loop sys-

tem, it is first necessary to characterize the fan speed

versus PWM duty cycle under typical system condi-

tions. This can be accomplished using either an optical

(strobe) tachometer or by monitoring fan current (mea-

suring the time between the fan commutation pulses at

various speeds versus full speed). With this character-

istic known, the correct PWM duty cycle for a given fan

speed can be used in the open loop system. As seen

in Figure 1, the result of this typically indicates that the

use of a lower duty cycle in order to get the desired fan

speed is necessary.


When using PWM fan speed control, there will be a dif-

ference between the PWM duty cycle and the % of full

fan speed. This difference will vary based on the fan

being used. If the system is to be open loop, the fan

speed vs duty cycle relationship should be

characterized in order to obtain the desired results.

2003 Microchip Technology Inc. DS00772B - page 3

Information contained in this publication regarding device

applications and the like is intended through suggestion only

and may be superseded by updates. It is your responsibility to

ensure that your application meets with your specifications. No

representation or warranty is given and no liability is assumed

by Microchip Technology Incorporated with respect to the

accuracy or use of such information, or infringement of patents

or other intellectual property rights arising from such use or

otherwise. Use of Microchip's products as critical components in

life support systems is not authorized except with express

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The Microchip name and logo, the Microchip logo, KEELOQ,


PowerSmart are registered trademarks of Microchip Technology

Incorporated in the U.S.A. and other countries.


and The Embedded Control Solutions Company are registered

trademarks of Microchip Technology Incorporated in the U.S.A.


FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming,

ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB,


PowerCal, PowerInfo, PowerTool, rfPIC, Select Mode,

SmartSensor, SmartShunt, SmartTel and Total Endurance are

trademarks of Microchip Technology Incorporated in the U.S.A.

and other countries.

Serialized Quick Turn Programming (SQTP) is a service mark of

Microchip Technology Incorporated in the U.S.A.

All other trademarks mentioned herein are property of their

respective companies.

) 2003, Microchip Technology Incorporated, Printed in the

U.S.A., All Rights Reserved.

Printed on recycled paper.

Microchip received QS-9000 quality system

certification for its worldwide headquarters,

design and wafer fabrication facilities in

Chandler and Tempe, Arizona in July 1999

and Mountain View, California in March 2002.

The Company's quality system processes and

procedures are QS-9000 compliant for its


8-bit MCUs, KEELOQ.

code hopping

devices, Serial EEPROMs, microperipherals,

non-volatile memory and analog products. In

addition, Microchip's quality system for the

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Note the following details of the code protection feature on Microchip devices:

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intended manner and under normal conditions.

7 There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our

knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data

Sheets. Most likely, the person doing so is engaged in theft of intellectual property.

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mean that we are guaranteeing the product as "unbreakable."

Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our

products. Attempts to break microchip's code protection feature may be a violation of the Digital Millennium Copyright Act. If such

acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.

DS00772B-page 4 2003 Microchip Technology Inc.



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