PR8275
High Voltage Start-Up PWM Controller
Application Information
Operation Overview
Asꢀ longꢀ asꢀ theꢀ greenꢀ powerꢀ requirementꢀ
requirements.ꢀ Furthermore,ꢀ theꢀ costꢀ andꢀ
sizeꢀ limitationꢀ forceꢀ theꢀ PWMꢀ controllersꢀ
needꢀ toꢀ beꢀ powerfulꢀ toꢀ integrateꢀ moreꢀ
functionsꢀ toꢀ reduceꢀ theꢀ externalꢀ partꢀ
counts.
becomesꢀaꢀtrendꢀandꢀtheꢀpowerꢀsavingꢀisꢀ
gettingꢀ moreꢀ andꢀ moreꢀ importantꢀ forꢀ theꢀ
switchingꢀ powerꢀ suppliesꢀ andꢀ switchingꢀ
adaptors,ꢀtheꢀtraditionalꢀPWMꢀcontrollers.ꢀ
areꢀ notꢀ ableꢀ toꢀ supportꢀ suchꢀ newꢀ
ꢀ
Internal High Voltage start up and UVLO
currentꢀ withꢀ 1mA.ꢀ Meanwhile,ꢀ theꢀ Vccꢀ
supplyꢀcurrentꢀisꢀasꢀlowꢀas100ꢂAꢀthusꢀmostꢀ
ofꢀ theꢀ HVꢀ currentꢀ isꢀ utilizedꢀ toꢀ chargeꢀ theꢀ
Vccꢀcapacitor.ꢀByꢀusingꢀsuchꢀconfiguration,ꢀ
theꢀturnꢁonꢀdelayꢀtimeꢀwillꢀbeꢀalmostꢀsameꢀ
noꢀ matterꢀ underꢀ lowꢁlineꢀ orꢀ highꢁlineꢀ
conditions.ꢀ Wheneverꢀ theꢀ Vccꢀ voltageꢀ isꢀ
higherꢀ thanꢀ UVLO(on)ꢀ toꢀ powerꢀ onꢀ theꢀ
PR8275ꢀandꢀfurtherꢀtoꢀdeliverꢀtheꢀgateꢀdriveꢀ
signal,ꢀtheꢀhighꢁvoltageꢀcurrentꢀsourceꢀisꢀoffꢀ
andꢀtheꢀsupplyꢀcurrentꢀisꢀprovidedꢀfromꢀtheꢀ
auxiliaryꢀ windingꢀ ofꢀ theꢀ transformer.ꢀ
Therefore,ꢀtheꢀpowerꢀlossesꢀonꢀtheꢀstartupꢀ
circuitꢀ canꢀ beꢀ eliminatedꢀ andꢀ theꢀ powerꢀ
savingꢀcanꢀbeꢀeasilyꢀachieved.
Fig.3ꢀ
Traditionalꢀ circuitꢀ powersꢀ upꢀ theꢀ PWMꢀ
controllerꢀ throughꢀ aꢀ startupꢀ resistorꢀ toꢀ
provideꢀ theꢀ startupꢀ current.ꢀ However,ꢀ theꢀ
startupꢀresistorꢀconsumesꢀsignificantꢀpowerꢀ
whichꢀ isꢀ moreꢀ andꢀ moreꢀ criticalꢀ wheneverꢀ
theꢀ powerꢀ savingꢀ requirementꢀ isꢀ comingꢀ
tight.ꢀTheoretically,ꢀthisꢀstartupꢀresistorꢀcanꢀ
beꢀ veryꢀ highꢀ resistanceꢀ value.ꢀ However,ꢀ
higherꢀ resistorꢀ valueꢀ willꢀ causeꢀ longerꢀ
startupꢀ time.ꢀ Toꢀ achieveꢀ anꢀ optimizedꢀ
topology,ꢀ asꢀ shownꢀ inꢀ figureꢀ 3,ꢀ PR8275ꢀ
implementsꢀaꢀhighꢁvoltageꢀstartupꢀcircuitꢀforꢀ
suchꢀ requirement.ꢀ Duringꢀ theꢀ startup,ꢀ aꢀ
highꢁvoltageꢀ currentꢀ sourceꢀ sinksꢀ currentꢀ
fromꢀ theꢀ bulkꢀ capacitorꢀ toꢀ provideꢀ theꢀ
startupꢀ currentꢀ asꢀ wellꢀ asꢀ chargeꢀ theꢀ Vccꢀ
capacitorꢀ C1.ꢀ Duringꢀ theꢀ startupꢀ transient,ꢀ
theꢀ Vccꢀ isꢀ lowerꢀ thanꢀ theꢀ UVLOꢀ thresholdꢀ
thusꢀ theꢀ currentꢀ sourceꢀ isꢀ onꢀ toꢀ supplyꢀ aꢀ
Fig.4
Anꢀ UVLOꢀ comparatorꢀisꢀincludedꢀtoꢀ detectꢀ
theꢀ voltageꢀ onꢀ theꢀ Vccꢀ pinꢀ toꢀ ensureꢀ theꢀ
V1.0
5/10
ETC [ ETC ]
