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TNY255P Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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TNY255P Datasheet(HTML) 4 Page - Power Integrations, Inc. |
4 / 18 page Rev E 02/12 TNY253/254/255 4 Figure 4. TinySwitch Operation at Heavy Load. Figure 5. TinySwitch Operation at Medium Load. V DRAIN V EN CLOCK DC DRAIN I MAX PI-2255-061298 V DRAIN V EN CLOCK DC DRAIN I MAX PI-2259-061298 TheresponsetimeofTinySwitchON/OFFcontrolschemeis veryfastcomparedtonormalPWMcontrol.Thisprovideshigh lineripplerejectionandexcellenttransientresponse. Power Up/Down TinySwitchrequiresonlya0.1µFcapacitorontheBYPASS pin.Becauseofthesmallsizeofthiscapacitor,thepower-up delayiskepttoanabsoluteminimum,typically0.3ms(Fig- ure7).DuetothefastnatureoftheON/OFFfeedback,thereis noovershootatthepowersupplyoutput.Duringpower-down, thepowerMOSFETwillswitchuntiltherectifiedlinevoltage dropstoapproximately12V.ThepowerMOSFETwillthen remainoffwithoutanyglitches(Figure8). Bias Winding Eliminated TinySwitchdoesnotrequireabiaswindingtoprovidepower to the chip.Instead it draws the power directly from the DRAINpin(seeFunctionalDescriptionabove).Thishastwo mainbenefits.Firstforanominalapplication,thiseliminates thecostofanextrabiaswindingandassociatedcomponents. Secondly,forchargerapplications,thecurrent-voltagechar- acteristicoftenallowstheoutputvoltagetofalltolowvalues whilestilldeliveringpower.Thistypeofapplicationnormally requiresaforward-biaswindingwhichhasmanymoreassoci- atedcomponents,noneofwhicharenecessarywithTinySwitch. Current Limit Operation EachswitchingcycleisterminatedwhentheDRAIN current reachesthecurrentlimitoftheTinySwitch.Foragivenprimary inductanceandinputvoltage,thedutycycleisconstant.How- ever,dutycycledoeschangeinverselywiththeinputvoltage providing“voltagefeed-forward”advantages:goodlineripple rejectionandrelativelyconstantpowerdeliveryindependent oftheinputvoltage. 44 kHz Switching Frequency (TNY253/254) Switchingfrequency(withnocycleskipping)issetat44kHz. Thisprovidesseveraladvantages.Athigherswitchingfrequen- cies,thecapacitiveswitchinglossesareasignificantproportion ofthepowerlossesinapowersupply.Athigherfrequencies, thepreferredsnubbingschemesareRCDordiode-Zenerclamps. However,duetothelowerswitchingfrequencyofTinySwitch, itispossibletouseasimpleRCsnubber(andevenjustacapaci- toralonein115VACapplicationsatpowerslevelsbelow4W). Secondly,alowswitchingfrequencyalsoreducesEMIfiltering requirements.At44kHz,thefirst,secondandthirdharmon- icsareallbelow150kHzwheretheEMIlimitsarenotvery restrictive.Forpowerlevelsbelow4Witispossibletomeet worldwideEMIrequirementswithonlyresistiveandcapaci- tivefilterelements(noinductorsorchokes).Thissignificantly reducesEMIfiltercosts. Finally,iftheapplicationrequiresstringentnoiseemissions (suchasvideoapplications),thentheTNY253/254willallow more effective use of diode snubbing (and other secondary snubbingtechniques).Thelowerswitchingfrequencyallows RCsnubberstobeusedtoreducenoise,withoutsignificantly impactingtheefficiencyofthesupply. 130 kHz Switching Frequency (TNY255) The switching frequency (with no cycle skipping) is set at 130kHz.ThisallowstheTNY255todeliver10Wwhilestill usingthesamesize,lowcosttransformer(EE16)asusedby theTNY253/254forlowerpowerapplications. |
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