SWITCHMODEâ¢ Power Supply Reference Manual

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SMPSRMThe Flyback–Mode ConverterThe basic flyback–mode converter uses the samecomponents as the basic forward–mode converter, but ina different configuration. Consequently, it operates in adifferent fashion from the forward–mode converter. Themost elementary flyback–mode converter, the boost orstep–up converter, is shown in Figure 2.LDC outV inIonSWI offI loadR loadV inSWITCH VOLTAGE(VOLTS)PowerSwitchONV satDiodeONV flbk(V out )PowerSwitchONDiodeONPowerSwitchONTIMEINDUCTOR CURRENT(AMPS)I pkI loadTIMEFigure 2. A Basic Boost–Mode Converter and Waveforms (Boost Converter Shown)Again, its operation is best understood by considering the“on” and “off” periods separately. When the powerswitch is turned on, the inductor is connected directlyacross the input voltage source. The inductor current thenrises from zero and is given by:iL(on) V int t L0on (eq. 4)Energy is stored within the flux in the core of the inductor.The peak current, i pk , occurs at the instant the powerswitch is turned off and is given by:ipk V in ton(eq. 5)LWhen the power switch turns off, the switched side ofthe inductor wants to fly–up in voltage, but is clamped bythe output rectifier when its voltage exceeds the outputvoltage. The energy within the core of the inductor is thenpassed to the output capacitor. The inductor currentduring the off period has a negative ramp whose slope isgiven by:iL(off) (V in Vout)L(eq. 6)The energy is then completely emptied into the outputcapacitor and the switched terminal of the inductor fallsback to the level of the input voltage. Some ringing isevident during this time due to residual energy flowingthrough parasitic elements such as the stray inductancesand capacitances in the circuit.http://onsemi.com7
SMPSRMWhen there is some residual energy permitted toremain within the inductor core, the operation is calledcontinuous– mode. This can be seen in Figure 3.Energy for the entire on and off time periods must bestored within the inductor. The stored energy is definedby:EL 0.5L ipk 2 (eq. 7)The boost–mode inductor must store enough energy tosupply the output load for the entire switching period (t on+ t off ). Also, boost–mode converters are typically limitedto a 50 percent duty cycle. There must be a time periodwhen the inductor is permitted to empty itself of itsenergy.The boost converter is used for board–level (i.e.,non–isolated) step–up applications and is limited to lessthan 100–150 watts due to high peak currents. Being anon–isolated converter, it is limited to input voltages ofless than 42.5 VDC. Replacing the inductor with atransformer results in a flyback converter, which may bestep–up or step–down. The transformer also providesdielectric isolation from input to output.SWITCH VOLTAGE(VOLTS)V inPowerSwitchONDiodeONV flbk(V out )PowerSwitchONDiodeONTIMEINDUCTOR CURRENT(AMPS)V satI pkTIMEFigure 3. Waveforms for a Continuous–Mode Boost ConverterCommon SwitchingPower Supply TopologiesA topology is the arrangement of the power devicesand their magnetic elements. Each topology has its ownmerits within certain applications. There are five majorfactors to consider when selecting a topology for aparticular application. These are:1. Is input–to–output dielectric isolation required forthe application? This is typically dictated by thesafety regulatory bodies in effect in the region.2. Are multiple outputs required?3. Does the prospective topology place a reasonablevoltage stress across the power semiconductors?4. Does the prospective topology place a reasonablecurrent stress upon the power semiconductors?5. How much of the input voltage is placed acrossthe primary transformer winding or inductor?Factor 1 is a safety–related issue. Input voltages above42.5 VDC are considered hazardous by the safetyregulatory agencies throughout the world. Therefore,only transformer–isolated topologies must be used abovethis voltage. These are the off–line applications where thepower supply is plugged into an AC source such as a wallsocket.Multiple outputs require a transformer–basedtopology. The input and output grounds may beconnected together if the input voltage is below42.5 VDC. Otherwise full dielectric isolation is required.http://onsemi.com8
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 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
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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
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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
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Page 42 and 43: http://onsemi.com41Part No. VRRM(V)
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SMPSRMLiterature Available from ON
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SMPSRMWeb Locations for Switching-M
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Analog ICs for SWITCHMODE Power Sup
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SMPSRMON SEMICONDUCTOR STANDARD DOC
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SWITCHMODEâ¢ Power Supply Reference Manual