Understanding Buck Power Stages Mode Power …

1 UnderstandingBuckPowerStagesinSwitchmode PowerSuppliesMarch 1999 Mixed Signal ProductsApplicationReportSLVA057 IMPORTANT NOTICET exas 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 verify, before placing orders, that information being relied on is current and complete. All products are soldsubject to the terms and conditions of sale supplied at the time of order acknowledgement, including thosepertaining to warranty, patent infringement, and limitation of warrants performance of its semiconductor products to the specifications applicable at the time of sale inaccordance with TI s standard warranty. Testing and other quality control techniques are utilized to the extentTI deems necessary to support this warranty.

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4 buck Continuous Conduction Mode Small Signal Analysis13.. buck Discontinuous Conduction Mode Small-Signal Analysis17.. 4 Variations of the buck Power Stage20.. Synchronous- buck Power Stage20.. Forward Converter Power Stage21.. 5 Component Selection23.. Output Capacitance23.. Output Inductance24.. Power Switch25.. Catch Rectifier26.. 6 Example Designs28.. 7 Summary29.. 8 References31.. FiguresivSLVA057 List of Figures1 buck Power stage Schematic2.. 2 buck Power stage States4.. 3 Continuous-Mode buck Power stage Waveforms5.. 4 Boundary Between Continuous and Discontinuous Mode8.. 5 Discontinuous Current Mode8.. 6 Discontinuous-Mode buck Power stage Waveforms10.. 7 Power Supply Control Loop Components12.. 8 Boost Nonlinear Power stage Gain vs Duty Cycle13.

5 9 Averaged (Nonlinear) CCM PWM Switch Model14.. 10 DC and Small Signal CCM PWM Switch Model15.. 11 CCM buck Power stage Model16.. 12 Averaged (Nonlinear) DCM PWM Switch Model17.. 13 DCM buck Power stage DC Model18.. 14 Small Signal DCM PWM Switch Model19.. 15 Synchronous buck Power stage Schematic20.. 16 Forward Converter Power stage Schematic21.. 1 Understanding buck Power Stages in Switchmode PowerSuppliesEverett RogersABSTRACTA switching Power supply consists of the Power stage and the control circuit. The powerstage performs the basic Power conversion from the input voltage to the output voltageand includes switches and the output filter. This report addresses the buck Power stageonly and does not cover control circuits. Detailed steady-state and small-signal analysisof the buck Power stage operating in continuous and discontinuous mode are in the standard buck Power stage and a discussion of Power stage componentrequirements are IntroductionThe three basic switching Power supply topologies in common use are the buck ,boost, and buck -boost.

6 These topologies are nonisolated, that is, 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 properties 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 most common and probably the simplest Power stage topology is the buckpower stage , sometimes called a step-down Power stage . Power supplydesigners choose the buck Power stage because the output voltage is alwaysless than the input voltage in the same polarity and is not isolated from the input current for a buck Power stage is discontinuous or pulsating due to thepower switch (Q1) current that pulses from zero to IO every switching cycle.

7 Theoutput current for a buck Power stage is continuous or nonpulsating because theoutput current is supplied by the output inductor/capacitor combination; theoutput capacitor never supplies the entire load current (for continuous inductorcurrent mode operation, one of the two operating modes to be discussed in thenext section).This report describes the steady state operation of the buck Power stage 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 1 shows a simplified schematic of the buck Power stage with a drive circuitblock included. The Power switch, Q1, is an n-channel MOSFET. The diode, CR1,is usually called the catch diode, or freewheeling diode.

8 The inductor, L, andcapacitor, C, make up the output filter. The capacitor ESR, RC, (equivalent seriesresistance) and the inductor DC resistance, RL, are included in the analysis. Theresistor, R, represents the load seen by the Power stage +VIDriveCircuitpRLLIL = icCRCRVOF igure 1. buck Power stage SchematicDuring normal operation of the buck Power stage , Q1 is repeatedly switched onand off with the on and off times governed by the control circuit. This switchingaction causes a train of pulses at the junction of Q1, CR1, and L which is filteredby the L/C output filter to produce a dc output voltage, VO. A more detailedquantitative analysis is given in the following Power stage Steady-State Analysis3 Understanding buck Power Stages in Switchmode Power Supplies2 buck Power stage Steady-State AnalysisA Power stage can operate in continuous or discontinuous inductor current inductor current mode is characterized by current flowingcontinuously in the inductor during the entire switching cycle in steady stateoperation.

9 Discontinuous inductor current mode is characterized by the inductorcurrent being zero for a portion of the switching cycle. It starts at zero, reachesa peak value, 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 is is very desirable for a Power stage to stay in only one mode over its expectedoperating conditions, because the Power stage frequency response changessignificantly between the two modes of 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 circuitto 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.

10 Forthe same die size, a p-channel FET has a higher RDS(on) but usually does notrequire a floating drive transistor Q1 and diode CR1 are drawn inside a dashed-line box withterminals labeled a, p, and c. The inductor current IL is also labeled iC and refersto current flowing out of terminal c. These items are explained fully in the BuckPower stage Modeling buck Steady-State Continuous Conduction Mode AnalysisThe following is a description of steady-state operation in continuous conductionmode. The main result of this section is a derivation of the voltage conversionrelationship for the continuous conduction mode buck Power stage . This resultis important because it shows how the output voltage depends on duty cycle andinput voltage or, conversely, how the duty cycle can be calculated based on inputvoltage and output voltage.