1、C0085C0110C0100C0101C0114C0115C0116C0097C0110C0100C0105C0110C0103 C0066C0117C0099C0107C0262C0066C0111C0111C0115C0116C0080C0111C0119C0101C0114 C0083C0116C0097C0103C0101C0115 C0105C0110C0083C0119C0105C0116C0099C0104C0109C0111C0100C0101 C0080C0111C0119C0101C0114 C0083C0117C0112C0112C0108C0105C0101C0115
2、March 1999 Mixed Signal ProductsApplicationReportSLVA059IMPORTANT NOTICETexas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinueany product or service without notice, and advise customers to obtain the latest version of relevant informationto
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9、anty or endorsement thereof.Copyright 1999, Texas Instruments IncorporatediiiUnderstanding Buck-Boost Power Stages in Switchmode Power SuppliesContents1 Introduction 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10、 . . . . . . . . . . . . . . . . 2 Buck-Boost Stage Steady-State Analysis 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Buck-Boost Steady-State Continuous Conduction Mode Analysis 3. . . . . . . . . . . . . . . . . . . . . . . .
11、. . . . . . . 2.2 Buck-Boost Steady-State Discontinuous Conduction Mode Analysis 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Critical Inductance 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12、. . . . 3 Buck-Boost Power Stage Small Signal Modeling 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Buck-Boost Continuous Conduction Mode Small-Signal Analysis 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Buck-Boost Discon
13、tinuous Conduction Mode Small-Signal Analysis 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Variations of the Buck-Boost Power Stage 22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Flyback Power Stage 22. . . . . . . . .
14、 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Component Selection 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Outpu
15、t Capacitance 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Output Inductance 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16、. . . . . . . . . . . 5.3 Power Switch 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Output Diode 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17、. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Summary 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 References 32. . . . . . . . . . . . . . . . . . . . .
18、 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FiguresivSLVA059List of Figures1 Buck-Boost Power Stage Schematic 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19、. . . . . . . . . . 2 Buck-Boost Power Stage States 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Continuous Mode Buck-Boost Power Stage Waveforms 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20、 . . . . . . . . . . . . . . . . 4 Boundary Between Continuous and Discontinuous Mode 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Discontinuous Current Mode 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21、 . . . . . . . . . . . . . . . . . . . . . . . . 6 Discontinuous Mode Buck-Boost Converter Waveforms 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Power Supply Control Loop Components 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22、. . . . . . . . . . . . . . . . . . . . . . . . . . 8 Buck-Boost Power Stage Gain vs Duty Cycle 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 DC and Small Signal CCM PWM Switch Model 15. . . . . . . . . . . . . . . . . . . . . . . .
23、 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 CCM Buck-Boost Power Stage Model 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 DCM PWM Switch Model 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24、 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 DCM Buck-Boost Converter Model 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Small Signal DCM Buck-Boost Power Stage Model 21.
25、. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Flyback Power Stage Schematic 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Discontinuous Mode Flyback Waveforms
26、 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Understanding Buck-Boost Power Stages in Switchmode PowerSuppliesEverett Rogers ABSTRACTA switching power supply consists of the power stage and the control circuit. The powerstage p
27、erforms the basic power conversion from the input voltage to the output voltageand includes switches and the output filter. This report addresses the buck-boost powerstage only and does not cover control circuits. Detailed steady-state and small-signalanalysis of the buck-boost power stage operating
28、 in continuous and discontinuous modeis presented. Variations in the standard buck-boost power stage and a discussion ofpower stage component requirements are included.1 IntroductionThe three basic switching power supply topologies in common use are the buck,boost, and buck-boost. These topologies a
29、re nonisolated, i.e., the input andoutput voltages share a common ground. There are, however, isolatedderivations of these nonisolated topologies. The power supply topology refers tohow the switches, output inductor, and output capacitor are connected. Eachtopology has unique properties. These prope
30、rties include the steady-statevoltage conversion ratios, the nature of the input and output currents, and thecharacter of the output voltage ripple. Another important property is the frequencyresponse of the duty-cycle-to-output-voltage transfer function.The buck-boost is a popular nonisolated, inve
31、rting power stage topology,sometimes called a step-up/down power stage. Power supply designers choosethe buck-boost power stage because the output voltage is inverted from the inputvoltage, and the output voltage can be either higher or lower than the inputvoltage. The topology gets its name from pr
32、oducing an output voltage that canbe higher (like a boost power stage) or lower (like a buck power stage) inmagnitude than the input voltage. However, the output voltage is opposite inpolarity from the input voltage. The input current for a buck-boost power stage isdiscontinuous or pulsating due to
33、the power switch (Q1) current that pulses fromzero to ILevery switching cycle. The output current for a buck-boost power stageis also discontinuous or pulsating. This is because the output diode only conductsduring a portion of the switching cycle. The output capacitor supplies the entireload curren
34、t for the rest of the switching cycle.This report describes steady state operation of the buck-boost converter incontinuous-mode and discontinuous-mode operation with ideal waveformsgiven. The duty-cycle-to-output-voltage transfer function is given after anintroduction of the PWM switch model.Introd
35、uction2 SLVA059Figure 1 shows a simplified schematic of the buck-boost power stage with a drivecircuit block included. The power switch, Q1, is an n-channel MOSFET. Theoutput diode is CR1. The inductor, L, and capacitor, C, make up the effectiveoutput filter. The capacitor ESR, RC, (equivalent serie
36、s resistance) and theinductor DC resistance, RL, are included in the analysis. The resistor, R,represents the load seen by the power stage output.CR1Q1acia+VIDriveCircuitpRLIL= icCRCRVOLFigure 1. Buck-Boost Power Stage SchematicDuring normal operation of the buck-boost power stage, Q1 is repeatedlys
37、witched on and off with the on- and off-times governed by the control circuit. Thisswitching action gives rise to a train of pulses at the junction of Q1, CR1, and L.Although the inductor, L, is connected to the output capacitor, C, only when CR1conducts, an effective L/C output filter is formed. It
38、 filters the train of pulses toproduce a DC output voltage.Buck-Boost Stage Steady-State Analysis3 Understanding Buck-Boost Power Stages in Switchmode Power Supplies2 Buck-Boost Stage Steady-State AnalysisA power stage can operate in continuous or discontinuous inductor current mode.Continuous induc
39、tor current mode is characterized by current flowingcontinuously in the inductor during the entire switching cycle in steady-stateoperation. Discontinuous inductor current mode is characterized by the inductorcurrent being zero for a portion of the switching cycle. It starts at zero, reachesa peak v
40、alue, and returns to zero during each switching cycle. The two differentmodes are discussed in greater detail later and design guidelines for the inductorvalue to maintain a chosen mode of operation as a function of rated load aregiven. It is very desirable for a converter to stay in one mode only o
41、ver itsexpected operating conditions because the power stage frequency responsechanges significantly between the two different modes of operation.For this analysis, an n-channel power MOSFET is used and a positive voltage,VGS(ON), is applied from the Gate to the Source terminals of Q1 by the drive c
42、ircuitto turn ON the FET. The advantage of using an n-channel FET is its lower RDS(on)but the drive circuit is more complicated because a floating drive is required. Forthe same die size, a p-channel FET has a higher RDS(on)but usually does notrequire a floating drive circuit.The transistor Q1 and d
43、iode CR1 are drawn inside a dashed-line box withterminals labeled a, p, and c. This is explained fully in the Buck-Boost PowerStage Modeling section.2.1 Buck-Boost Steady-State Continuous Conduction Mode AnalysisThe following is a description of steady-state operation in continuous conductionmode. T
44、he main goal of this section is to provide a derivation of the voltageconversion relationship for the continuous conduction mode buck-boost powerstage. This is important because it shows how the output voltage depends on dutycycle and input voltage or conversely, how the duty cycle can be calculated
45、 basedon input voltage and output voltage. Steady-state implies that the input voltage,output voltage, output load current, and duty-cycle are fixed and not varying.Capital letters are generally given to variable names to indicate a steady-statequantity.In continuous conduction mode, the buck-boost
46、converter assumes two statesper switching cycle. The ON State is when Q1 is ON and CR1 is OFF. The OFFState is when Q1 is OFF and CR1 is ON. A simple linear circuit can representeach of the two states where the switches in the circuit are replaced by theirequivalent circuit during each state. The ci
47、rcuit diagram for each of the two statesis shown in Figure 2.Buck-Boost Stage Steady-State Analysis4 SLVA059acia+VIpRLLIL= icCRCRRDS(on)VOONStateacia+VIpCRCRVOOFFStateVdLIL= icRLIOIOFigure 2. Buck-Boost Power Stage StatesThe duration of the ON state is D TS= TONwhere D is the duty cycle, set by thec
48、ontrol circuit, expressed as a ratio of the switch ON time to the time of onecomplete switching cycle, Ts. The duration of the OFF state is called TOFF. Sincethere are only two states per switching cycle for continuous conduction mode,TOFFis equal to (1D) TS. The quantity (1D) is sometimes called D.
49、 Thesetimes are shown along with the waveforms in Figure 3.Buck-Boost Stage Steady-State Analysis5 Understanding Buck-Boost Power Stages in Switchmode Power SuppliesILTONTOFFTSIQ1ICR1ILSolidIODashedVc-pSolidVODashed00Figure 3. Continuous Mode Buck-Boost Power Stage WaveformsReferring to Figure 2, during the ON state, Q1 presents a low resistance, RDS(on),from its drain to source and exhibits a small voltage drop of VDS=IL RDS(on).There is also a small voltage drop across the dc resistance of the inductor equal toIL RL. Thus, the input voltage, VI, minus losses, (VDS+ IL RL),
