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Fundamentals of Power Electronics Chapter 6: Converter circuits1Chapter 6. Converter Circuits6.1. Circuit manipulations6.2. A short list ofconverters6.3. Transformer isolation6.4. Converter evaluationand design6.5. Summary of keypoints•Where do the boost,buck-boost, and otherconverters originate?• How can we obtain aconverter having givendesired properties?•What converters arepossible?• How can we obtaintransformer isolation in aconverter?•For a given application,which converter is best?Fundamentals of Power Electronics Chapter 6: Converter circuits26.1. Circuit ManipulationsBegin with buck converter: derived in Chapter 1 from first principles•Switch changes dc component, low-pass filter removesswitching harmonics• Conversion ratio is M = D+–LCR+V–12VgFundamentals of Power Electronics Chapter 6: Converter circuits36.1.1. Inversion of source and loadInterchange power input and output ports of a converterBuck converter exampleV2= DV1Port 1 Port 2+–L12+V1–+V2–Power flowFundamentals of Power Electronics Chapter 6: Converter circuits4Inversion of source and loadInterchange power source and load:Port 1 Port 2+–L12+V1–+V2–Power flowV2= DV1V1=1DV2Fundamentals of Power Electronics Chapter 6: Converter circuits5Realization of switchesas in Chapter 4•Reversal of powerflow requires newrealization ofswitches•Transistor conductswhen switch is inposition 2•Interchange of Dand D’Inversion of buck converter yields boost converterV1=1D'V2Port 1 Port 2+–L+V1–+V2–Power flowFundamentals of Power Electronics Chapter 6: Converter circuits66.1.2. Cascade connection of converters+–Converter 2Converter 1Vg+V1–+V–DV1Vg= M1(D)VV1= M2(D)V1= M1(D)VgV = M2(D)V1VVg= M(D)=M1(D)M2(D)Fundamentals of Power Electronics Chapter 6: Converter circuits7Example: buck cascaded by boost+–12L1C1+V1–R+V–12L2C2{{Buck converter Boost converterVgV1Vg=DVV1=11–DVVg=D1–DFundamentals of Power Electronics Chapter 6: Converter circuits8Buck cascaded by boost:simplification of internal filterRemove capacitor C1Combine inductors L1 and L2Noninvertingbuck-boostconverter+–12L1R+V–12L2C2Vg+V–12L+–12iLVgFundamentals of Power Electronics Chapter 6: Converter circuits9Noninverting buck-boost convertersubinterval 1 subinterval 2+V–12L+–12iLVg+–+V–VgiL+–+V–VgiLFundamentals of Power Electronics Chapter 6: Converter circuits10Reversal of output voltage polaritysubinterval 1 subinterval 2noninvertingbuck-boostinvertingbuck-boost+–+V–VgiL+–+V–VgiL+–+V–VgiL+–+V–VgiLFundamentals of Power Electronics Chapter 6: Converter circuits11Reduction of number of switches:inverting buck-boostSubinterval 1 Subinterval 2One side of inductor always connected to ground— hence, only one SPDT switch needed:+–+V–VgiL+–+V–VgiL+–+V–12VgiLVVg=–D1–DFundamentals of Power Electronics Chapter 6: Converter circuits12Discussion: cascade connections•Properties of buck-boost converter follow from its derivationas buck cascaded by boostEquivalent circuit model: buck 1:D transformer cascaded by boostD’:1 transformerPulsating input current of buck converterPulsating output current of boost converter•Other cascade connections are possibleCuk converter: boost cascaded by buckFundamentals of Power Electronics Chapter 6: Converter circuits136.1.3. Rotation of three-terminal cellTreat inductor andSPDT switch as three-terminal cell:Three-terminal cell can be connected between source and load in threenontrivial distinct ways:a-A b-B c-C buck convertera-C b-A c-B boost convertera-A b-C c-B buck-boost converter+–+v–12VgThree-terminalcellaAbBcCFundamentals of Power Electronics Chapter 6: Converter circuits14Rotation of a dual three-terminal networkA capacitor and SPDTswitch as a three-terminal cell:Three-terminal cell can be connected between source and load in threenontrivial distinct ways:a-A b-B c-C buck converter with L-C input filtera-C b-A c-B boost converter with L-C output filtera-A b-C c-B Cuk converter+–+v–12Three-terminalcellAa bBcCVgFundamentals of Power Electronics Chapter 6: Converter circuits156.1.4. Differential connection of loadto obtain bipolar output voltageDifferential loadvoltage isThe outputs V1 and V2may both be positive,but the differentialoutput voltage V can bepositive or negative.Converter 1+V1–+V–DConverter 2+–Vg+V2–D'loaddc sourceV1= M(D) VgV2= M(D') VgV = V1– V2Fundamentals of Power Electronics Chapter 6: Converter circuits16Differential connection using two buck convertersConverter #1 transistordriven with duty cycle DConverter #2 transistordriven with duty cyclecomplement D’Differential load voltageisSimplify:+V1–+V–+–Vg+V2–1212Buck converter 1}Buck converter 2{V = DVg– D'VgV =(2D –1)VgFundamentals of Power Electronics Chapter 6: Converter circuits17Conversion ratio M(D),differentially-connected buck convertersV =(2D –1)VgDM(D)10.510– 1Fundamentals of Power Electronics Chapter 6: Converter circuits18Simplification of filter circuit,differentially-connected buck convertersOriginal circuit Bypass load directly with capacitor+V1–+V–+–Vg+V2–1212Buck converter 1}Buck converter 2{+–Vg1212+V–Fundamentals of Power Electronics Chapter 6: Converter circuits19Simplification of filter circuit,differentially-connected buck convertersCombine series-connectedinductorsRe-draw for clarityH-bridge, or bridge inverterCommonly used in single-phaseinverter applications and in servoamplifier applications+–Vg1212+V–+–LCR+ V –21iLVg12Fundamentals of Power Electronics Chapter 6: Converter circuits20Differential connection to obtain 3ø inverterWith balanced 3ø load,neutral voltage isPhase voltages areControl converters such thattheir output voltages containthe same dc biases. This dcbias will appear at theneutral point Vn. It thencancels out, so phasevoltages contain no dc bias.+V1–+–Vg+V2–3øac loaddc source+V3–D2D3Vn+ vbn –– van +– vcn +V2= M(D2) VgV3= M(D3) VgConverter 1D1V1= M(D1) VgConverter 2Converter 3Vn=13V1+ V2+ V3Van= V1– VnVbn= V2– VnVcn= V3– VnFundamentals of Power Electronics Chapter 6: Converter circuits213ø differential connection of three buck converters+–Vg+V2–3φac loaddc source+V3–Vn+ vbn –– van +– vcn ++V1–Fundamentals of Power Electronics Chapter 6: Converter circuits223ø differential connection of three buck convertersRe-draw for clarity:“Voltage-source inverter” or buck-derived three-phase inverter3φac loaddc


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CU-Boulder ECEN 5797 - Converter Circuits

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