SWITCHMODEâ¢ Power Supply Reference Manual

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ÉÉ ÂÂÂÂÂÂÂÂÂÂÂÉÉÉÉÉÉÉÉSMPSRMThe grounds are extremely important to the properoperation of the switching power supply, since they formthe reference connections for the entire supply; eachground has its own unique set of signals which canadversely affect the operation of the supply if connectedimproperly.There are five distinct grounds within the typicalswitching power supply. Four of them form the returnpaths for the current loops described above. Theremaining ground is the low–level analog control groundwhich is critical for the proper operation of the supply.The grounds which are part of the major current loopsmust be connected together exactly as shown inFigure 21. Here again, the connecting point between thehigh–level AC grounds and the input or output groundsis at the negative terminal of the appropriate filtercapacitor (points A and B in Figures 21a and 21b). Noiseon the AC grounds can very easily escape into theenvironment if the grounds are not directly connected tothe negative terminal of the filter capacitor(s). Theanalog control ground must be connected to the pointwhere the control IC and associated circuitry mustmeasure key power parameters, such as AC or DCcurrent and the output voltage (point C in Figures 21a and21b). Here any noise introduced by large AC signalswithin the AC grounds will sum directly onto thelow–level control parameters and greatly affect theoperation of the supply. The purpose of connecting thecontrol ground to the lower side of the current sensingresistor or the output voltage resistor divider is to form a“Kelvin contact” where any common mode noise is notsensed by the control circuit. In short, follow the examplegiven by Figure 21 exactly as shown for best results.The last important factor in the PCB design is thelayout surrounding the AC voltage nodes. These are thedrain of the power MOSFET (or collector of a BJT) andthe anode of the output rectifier(s). These nodes cancapacitively couple into any trace on different layers ofthe PCB that run underneath the AC pad. In surfacemount designs, these nodes also need to be large enoughto provide heatsinking for the power switch or rectifier.This is at odds with the desire to keep the pad as small aspossible to discourage capacitive coupling to othertraces. One good compromise is to make all layers belowthe AC node identical to the AC node and connect themwith many vias (plated–through holes). This greatlyincreases the thermal mass of the pad for improvedheatsinking and locates any surrounding traces offlaterally where the coupling capacitance is much smaller.An example of this can be seen in Figure 22.Many times it is necessary to parallel filter capacitorsto reduce the amount of RMS ripple current eachcapacitor experiences. Close attention should be paid tothis layout. If the paralleled capacitors are in a line, thecapacitor closest to the source of the ripple current willoperate hotter than the others, shortening its operatinglife; the others will not see this level of AC current. Toensure that they will evenly share the ripple current,ideally, any paralleled capacitors should be laid out in aradially–symmetric manner around the current source,typically a rectifier or power switch.The PCB layout, if not done properly, can ruin a goodpaper design. It is important to follow these basicguidelines and monitor the layout every step of theprocess.Power DeviceViaPCB TopÉÉÉÉPlated–Thru HolePCB BottomÂÂÂÂÂÂÂÂÂÂÂ ÉÉFigure 22. Method for Minimizing AC Capacitive Coupling and Enhancing Heatsinkinghttp://onsemi.com23
SMPSRMLosses and Stresses in SwitchingPower SuppliesMuch of the designer’s time during a switching powersupply design is spent in identifying and minimizing thelosses within the supply. Most of the losses occur in thepower components within the switching power supply.Some of these losses can also present stresses to thepower semiconductors which may affect the long termreliability of the power supply, so knowing where theyarise and how to control them is important.Whenever there is a simultaneous voltage drop acrossa component with a current flowing through, there is aloss. Some of these losses are controllable by modifyingthe circuitry, and some are controlled by simply selectinga different part. Identifying the major sources for loss canbe as easy as placing a finger on each of the componentsin search of heat, or measuring the currents and voltagesassociated with each power component using anoscilloscope, AC current probe and voltage probe.Semiconductor losses fall into two categories:conduction losses and switching losses. The conductionloss is the product of the terminal voltage and currentduring the power device’s on period. Examples ofconduction losses are the saturation voltage of a bipolarpower transistor and the “on” loss of a power MOSFETshown in Figure 23 and Figure 24 respectively.COLLECTOR-TO-EMITTER(VOLTS)RISETIMEDYNAMICSATURATIONSATURATIONVOLTAGEV PEAKSTORAGETIMEFALLTIMEDRAIN-TO-SOURCE VOLTAGE(VOLTS)RISETIMEON VOLTAGEV PEAKFALLTIMECOLLECTOR CURRENT(AMPS)PINCHING OFF INDUCTIVECHARACTERISTICS OF THETRANSFORMERTURN-ONCURRENTCLEARINGRECTIFIERSSATURATIONCURRENTTURN-OFFCURRENTI PEAKCURRENTTAILDRAIN CURRENT(AMPS)TURN-ONCURRENTCLEARINGRECTIFIERSPINCHING OFF INDUCTIVECHARACTERISTICS OF THETRANSFORMERON CURRENTI PEAKTURN-OFFCURRENTINSTANTANEOUS ENERGYLOSS (JOULES)TURN-ONLOSSSECONDBREAKDOWNPERIODSATURATIONLOSSTURN-OFF LOSSSWITCHING LOSSCURRENTCROWDINGPERIODINSTANTANEOUS ENERGYLOSS (JOULES)TURN-ONLOSSON LOSSTURN-OFF LOSSSWITCHING LOSSFigure 23. Stresses and Losseswithin a Bipolar Power TransistorFigure 24. Stresses and Losseswithin a Power MOSFEThttp://onsemi.com24
Page 1 and 2: SMPSRM/DRev. 3, Jul-2002SWITCHMODE
Page 3 and 4: SMPSRMON Semiconductor and are regi
Page 6 and 7: SMPSRMIntroductionThe never-ending
Page 8 and 9: SMPSRMThe Flyback-Mode ConverterThe
Page 10 and 11: SMPSRMFactors 3, 4 and 5 have a dir
Page 12 and 13: SMPSRM+N1 N2C inV inControlTSWDL OC
Page 14 and 15: SMPSRMInterleaved Multiphase Conver
Page 16 and 17: SMPSRMV CCOSCChargeClock RampV erro
Page 18 and 19: SMPSRMThe Power MOSFETPower MOSFETs
Page 22 and 23: SMPSRMThe Magnetic ComponentsThe ma
Page 26 and 27: SMPSRMThe forward conduction loss o
Page 28 and 29: SMPSRMSnubbers and ClampsSnubbers a
Page 30 and 31: SMPSRMThe Active ClampAn active cla
Page 32 and 33: SMPSRMIn this design, both the powe
Page 34 and 35: SMPSRMA Power Factor Correction (PF
Page 36 and 37: SWITCHMODE Power Supply ExamplesSMP
Page 38 and 39: V_SyncH_SyncMonitorMCUSYNC PROCESSO
Page 40 and 41: SMPSRMACLineRectifier+Start-upSwitc
Page 42 and 43: http://onsemi.com41Part No. VRRM(V)
Page 44 and 45: SMPSRMApplication: Compact Power Fa
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Page 48 and 49: SMPSRMFeatures:70 W output power fr
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Page 52 and 53: SMPSRMApplication: 110 W Output Fly
Page 54 and 55: SMPSRMApplication: Efficient Safety
Page 56 and 57: SMPSRMApplication: AC-DC Battery Ch
Page 58 and 59: SMPSRMLiterature Available from ON
Page 60 and 61: SMPSRMWeb Locations for Switching-M
Page 62 and 63: Analog ICs for SWITCHMODE Power Sup
Page 64 and 65: SMPSRMON SEMICONDUCTOR STANDARD DOC

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SWITCHMODEâ¢ Power Supply Reference Manual