ECEN 45171Lecture 6ECEN 4517/5517Step-up dc-dc converterwith isolation (flyback)Feedback controller toregulate HVDCExperiment 4: inverter systemDC-AC inverter (H-bridge)12 VDCHVDC: 120 - 200 VDCAC load120 Vrms60 HzBatteryDC-ACinverterH-bridgeDC-DCconverterIsolatedflyback+–d(t)FeedbackcontrollerVrefDigitalcontrollerd(t)+vac(t)–ECEN 45172Due datesRight now:Prelab assignment for Exp. 3 Part 3 (one from every student)Due within five minutes of beginning of lectureThis week in lab (Feb. 19-21):Nothing due. Try to finish Exp. 3.Next week in lecture (Feb. 26):Prelab assignment for Exp. 4 Part 1 (one from every student)Next week in lab (Feb. 26-28):Definitely finish Exp. 3, and begin Exp. 4The following week in lab (Mar. 4-6):Exp. 3 final report dueECEN 45173Goals in upcoming weeksExp. 4: A three-week experimentExp. 4 Part 1:Design and fabrication offlyback transformerSnubber circuitDemonstrate flybackconverter power stageoperating open loopExp. 4 Part 2:Design feedback loopMeasure loop gain, compare with simulation and theoryDemonstrate closed-loop control of converter output voltagesnubberPWM Compensator+–VrefVbattvHVDCECEN 45174Exp. 4, Part 3H-bridge inverter, off gridIR3101 IR3101vHVDCAC load120 Vrms60 Hz+ vac(t)–Digitalcontrolleriac(t)• Filtering of ac output not explicitly shown• IR 3101 half-bridge modules withintegrated drivers• Grid-tied: control iac(t)• Off-grid: control vac(t)Exp. 4 Part 3: off-grid inverter• Demonstrate modified sine-wave inverter(required)• Demonstrate PWM inverter (extra credit)ECEN 45175“Modified Sine-Wave” Invertervac(t) has arectangularwaveformInverter transistorsswitch at 60 Hz,T = 8.33 msecT/2DT/2+ VHVDC– VHVDCvac(t)RMS value of vac(t) is:Vac,RMS=1Tvac2t dt0T= D VHVDC • Choose VHVDC larger thandesired Vac,RMS• Can regulate value ofVac,RMS by variation of D• Waveform is highlynonsinusoidal, withsignificant harmonicsECEN 45176PWM InverterAverage vac(t) has asinusoidal waveformInverter transistorsswitch at frequencysubstantially higherthan 60 Hz• Choose VHVDC larger than desired Vac,peak• Can regulate waveshape and value of Vac,RMS by variation of d(t)• Can achieve sinusoidal waveform, with negligible harmonics• Higher switching frequency leads to more switching loss andneed to filter high-frequency switching harmonics and common-mode currentstvac(t)ECEN 45177The buck-boost converterSubinterval 1 Subinterval 2+–+V–VgiL+–+V–VgiL+–+V–12VgiLVVg=–D1–DSwitch in position 1: Vg chargesinductorSwitch in position 2: energy stored ininductor is transferred to outputConversion ratio:ECEN 45178The flyback converter:A transformer-isolated buck-boost converterbuck-boost converter:construct inductorwinding using twoparallel wires:+–L–V+VgQ1D1+–L–V+VgQ1D11:1See also:supplementarynotes on Flybackconverter, Exp. 4web pageECEN 45179Derivation of flyback converter, cont.Isolate inductorwindings: the flybackconverterFlyback converterhaving a 1:n turnsratio and positiveoutput:+–LM–V+VgQ1D11:1+–LM+V–VgQ1D11:nCECEN 451710A simple transformer modelMultiple winding transformer Equivalent circuit modeln1 : n2: n3+v1(t)–+v2(t)–+v3(t)–i1(t) i2(t)i3(t)n1 : n2: n3+v1(t)–+v2(t)–+v3(t)–i1(t) i2(t)i3(t)Idealtransformeri1'(t)LMiM(t)v1(t)n1=v2(t)n2=v3(t)n3= ...0=n1i1'(t)+n2i2(t)+n3i3(t) + ...ECEN 451711The magnetizing inductance LMTransformer core B-H characteristic• Models magnetization oftransformer core material• Appears effectively in parallel withwindings• If all secondary windings aredisconnected, then primary windingbehaves as an inductor, equal to themagnetizing inductance• At dc: magnetizing inductance tendsto short-circuit. Transformers cannotpass dc voltages• Transformer saturates whenmagnetizing current iM is too largeB(t) ∝ v1(t) dtH(t) ∝ iM(t)slope ∝ LMsaturationECEN 451712Volt-second balance in LMThe magnetizing inductance is a real inductor,obeyingintegrate:Magnetizing current is determined by integral ofthe applied winding voltage. The magnetizingcurrent and the winding currents are independentquantities. Volt-second balance applies: insteady-state, iM(Ts) = iM(0), and hencen1 : n2: n3+v1(t)–+v2(t)–+v3(t)–i1(t) i2(t)i3(t)Idealtransformeri1'(t)LMiM(t)v1(t)=LMdiM(t)dtiM(t)–iM(0) =1LMv1(τ)dτ0t0=1Tsv1(t)dt0TsECEN 451713The “flyback transformer” A two-winding inductor Symbol is same astransformer, but functiondiffers significantly fromideal transformer Energy is stored inmagnetizing inductance Magnetizing inductance isrelatively small Current does not simultaneously flow in primary and secondary windings Instantaneous winding voltages follow turns ratio Instantaneous (and rms) winding currents do not follow turns ratio Model as (small) magnetizing inductance in parallel with ideal transformer+–LM+v–VgQ1D11:nCTransformer modeliigRiC+vL–ECEN 451714Subinterval 1CCM: small rippleapproximation leads to+–LM+v–Vg1:nCTransformer modeliigRiC+vL–vL= VgiC=–vRig= ivL= VgiC=–VRig= IQ1 on, D1 offECEN 451715Subinterval 2CCM: small rippleapproximation leads tovL=–vniC=in–vRig=0vL=–VniC=In–VRig=0+–+v–Vg1:nCTransformer modeliRiCi/n–v/n++vL–ig= 0Q1 off, D1 onECEN 451716CCM Flyback waveforms and solutionVolt-second balance:Conversion ratio isCharge balance:Dc component of magnetizingcurrent isDc component of source current isvLiCigtVg0DTsD'TsTsQ1D1Conductingdevices:–V/n–V/RI/n – V/RIvL= DVg+ D'–Vn=0M(D)=VVg= nDD'iC= D –VR+ D'In–VR=0I=nVD'RIg= ig= DI + D'0ECEN 451717Equivalent circuit model: CCM Flyback+–+–R+V–VgD'InD'Vn+–DVgDIIIg+–R+V–VgIIg1 : D D' : nvL= DVg+ D'–Vn=0iC= D –VR+ D'In–VR=0Ig= ig= DI + D'0ECEN 451718Step-up DC-DC flyback converterNeed to step up the 12 V battery voltage to HVDC (120-200 V)How much power can you get using the parts in your kit?Key limitations:MOSFET on-resistance (90 m)Input capacitor rms current rating: 25 V 2200 μF: 2.88 A 35 V 2200 μF: 3.45 ASnubber lossNeed to choose turns ratio, as well as D, fs, to minimize peak currentsPossible project for expo: build a better (and more complex) step-up dc-dcconverterECEN 451719Design of CCM flyback transformer+–LM+V–VgQ1D1n1 : n2CTransformer modeliMi1R+vM–i2vM(t)0VgDTsiM(t)IM0ΔiMi1(t)IM0i2(t)IM0n1n2ECEN 451720ApproachUse your PQ 32/20 coreChoose turns ratio n2/n1, LM, D, and fs (choose your ownvalues, dont use values in supplementary notes)Select primary turns n1 so that total loss Ptot in flybacktransformer
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