在PWM风扇控制系统的速度误差

BrushlessDCfan(BDC)speedcanbevariedusing

pulse-widthmodulation(PWM).ThetypicalPWMcon-

trolschemeinsertsapowerswitchinserieswiththe

fan,asshowninFigure1.Insuchapplications,an

increaseintheactivedutycyclecausesacorrespond-

ingincreaseinfanspeed(i.e.,fanspeedisproportional

tot

ON

/(t

ON

+t

OFF

).Conventionalwisdomstatesthat

PWMdutycyclealonedeterminesoperatingspeed(for

example,thefanspeedrunsat50%ofmaximumwhen

thePWMdutycycleis50%).Inreality,thefanoperates

atahigherpercentageoffullspeedforanygivenduty

cycle.Theresultingdifferencebetweenactualfan

speedandthecorrespondingPWMdutycycle(herein

referredtoasSpeedError)canbeproblematicinopen

loopfancontrolsystems.MAN772SpeedErrorinPWMFanControlSystemsperiods.TheresultingspeedversusPWMdutycycleAuthor:MicrochipTechnologyInc.errorisafunctionofthemassanddiameterofthefanhub,thetorquecharacteristicsofthefanmotorandtheINTRODUCTIONlossesinthecenterbearings.AirtemperatureandBrushlessDCfan(BDC)speedcanbevariedusinghumidityalsocontributetospeederror,butnotp