Samlexpower PST-1000-12 Owner's manual - Section 4 |
12 | SAMLEX AMERICA INC.
seCtIOn 4 |
Powering Direct / embedded switch
Mode Power supplies (sMPs)
SwitchModePowerSupplies(SMPS)areextensivelyusedtoconverttheincomingAC
powerintovariousvoltageslike3.3V,5V,12V,24Vetc.thatareusedtopowervarious
devicesandcircuitsusedinelectronicequipmentlikebatterychargers,computers,audio
andvideodevices,radiosetc.Thesepowersuppliesuselargecapacitorsintheirinput
sectionforfiltration.Whenthepowersupplyisfirstturnedon,thereisaverylarge
inrush current drawn by the power supply as the input capacitors are charged (The ca-
pacitorsactalmostlikeashortcircuitattheinstantthepoweristurnedon).Theinrush
current at turn-on is several to tens of times larger than the rated rMS input current
andlastsforafewmilliseconds.Anexampleoftheinputvoltageversusinputcurrent
waveforms is given in fig. 4.1. It will be seen that the initial input current pulse just after
turn-onis>15timeslargerthanthesteadystateRMScurrent.Theinrushdissipatesin
around2or3cyclesi.e.inaround33to50millisecondsfor60Hzsinewave.
further,duetothepresenceofhighvalueofinputfiltercapacitors,thecurrentdrawn
byanSMPS(WithnoPowerfactorcorrection)isnotsinusoidalbutnon-linearasshown
in fig 4.2 above. The steady state input current of SMPS is a train of non-linear pulses
instead of a sinusoidal wave. These pulses are two to four milliseconds duration each
whenon50Hzpower,withaveryhighCrestfactorcorrespondingtopeakvalues
around3timestheRMSvalueoftheinputcurrent:
(Crestfactor=Peakvalue÷RMSvalue).
ManySMPSunitsincorporate“InrushCurrentLimiting”.Themostcommonmethodis
theNTC(NegativeTemperatureCoefficient)resistor.TheNTCresistorhasahighresist-
ancewhencoldandalowresistancewhenhot.TheNTCresistorisplacedinserieswith
the input to the power supply. The cold resistance limits the input current as the input
capacitorschargeup.TheinputcurrentheatsuptheNTCandtheresistancedrops
duringnormaloperation.However,ifthepowersupplyisquicklyturnedoffandback
on,theNTCresistorwillbehotsoitslowresistancestatewillnotpreventaninrush
current event.
Theinvertershould,therefore,besizedadequatelytowithstandthehighinrushcurrent
andthehighCrestfactorofthecurrentdrawnbytheSMPS.Hence, it is recommended
that for purposes of sizing the inverter, the continuous power of the inverter should be
> 3 times the continuous rated power of the SMPS. For example, an SMPS rated at
100 Watts should be powered from an inverter that has continuous power of
> 300 Watts.