ECEN 45171Lecture 4ECEN 4517/5517Step-up dc-dc converter with isolation (flyback)Feedback controller to regulate HVDCExperiment 3 weeks 2 and 3: interleaved flyback and feedback loopParallel two flybacks with phase-shifted gate drive signals12 VDCHVDC: 120 - 200 VDCAC load120 Vrms60 HzBatteryDC-ACinverterH-bridgeDC-DCconverterIsolatedflyback+–d(t)FeedbackcontrollerVrefDigitalcontrollerd(t)+vac(t)–ECEN 45172Due dates and goalsRight now:Prelab assignment for Exp. 4 Part 1 (one from every student)Due within five minutes of beginning of lectureThis week in lab (Feb. 3-5):Final reports for Exps. 1 and 2 dueBegin Exp. 3: construct and debug basic flyback power stageNext week in lab (Feb. 10-13):Get parallel flyback power stages working at 85 WBegin simulation of ac transfer functions and feedback loop designECEN 45173Goals in upcoming weeksExp. 3: Flyback step-up dc-dc converterExp. 3 Part 1:Design and fabrication of flyback transformerSnubber circuitDemonstrate flyback converter power stage operating open loopExp. 3 Part 2:Construct, debug, and demonstrateparalleled flyback converters producing 85 WExp. 3 Part 3:Design feedback loopMeasure loop gain, compare with simulation and theoryDemonstrate closed-loop control of converter output voltagesnubberPWM Compensator+–VrefVbattvHVDCECEN 45174Layout of power stageIdentify loops having high di/dt (pulsating currents). Since v = L di/di, stray inductance in these loops leads to voltage spikes and ringing on components (usually the MOSFET) that can exceed their peak voltage ratings.Minimize the inductance of the critical loops: keep area of loop small, use twisted pairs, add bypass capacitors.ECEN 45174Effect of transformer leakage inductance+–LM+v–VgQ1D11:nCTransformer modeliigRLl+ vl –+vT(t)–• Leakage inductance Ll is caused by imperfect coupling of primary and secondary windings• Leakage inductance is effectively in series with transistor Q1• When MOSFET switches off, it interrupts the current in Ll•Ll induces a voltage spike across Q1tVg + v/nvT(t)iRonDTs{Voltage spikecaused byleakageinductance If the peak magnitude of the voltage spike exceeds the voltage rating of the MOSFET, then the MOSFET will fail.ECEN 45175Protection of Q1using a voltage-clamp snubber+–+v–VgQ1D11:nCFlyback transformerigR+vT(t)–CsRs–vs+Snubber{• Snubber provides a place for current in leakage inductance to flow after Q1 has turned off• Peak transistor voltage is clamped to Vg + vs• vs > V/n•Energy stored in leakage inductance (plus more) is transferred to capacitor Cs, then dissipated in RsUsually, Cs is largeDecreasing Rs decreases the peak transistor voltage but increases the snubber power lossSee supplementary flyback notes for an example of estimating Cs and RsECEN 45176Overvoltage on output diode+–LM+v–VgQ1D11:nCTransformer modeliigRLl1+ vl –+vT(t)–Ll2Diode turn-off (reverse recovery) transition:Transformer leakage inductance causes voltage ringing and overshoot on secondary diodeLeakage inductance plus diode output capacitance form resonant circuit:tArea– Qrtt3t1t2vB(t)iL(t)–V200+–LiL(t)vL(t)+–+–Silicondiodevi(t)CvB(t)iB(t)diode capacitanceleakage inductancesecondary induced voltageECEN 45177Diode snubber+–LM+v–VgQ1D11:nCTransformer modeliigRLl1+ vl1 –+vT(t)–Ll2Diode snubber– vl2 +Damp the ringing with R-C snubber networkSnubber capacitance similar in value to diode capacitanceSnubber resistance similar in value to resonant circuit characteristic impedanceMore capacitance and/or smaller resistance lower peak voltage, larger snubber lossECEN 45178Limits on maximum output powerWeek 1 circuit•Wiring inductance causes ac component of iflyback to flow through capacitor C, while the dc component flows from the battery •Capacitor rms current must not exceed the rating of 4.42 A •Decreasing converter efficiency caused by snubber and other losses, along with capacitor current rating, limit the maximum output power •How much output power can you produce?ECEN 45179Increasing the output powerWeek 2 circuitInterleaving of parallel-connected flyback converters: •AC components of phase-shifted input current waveforms partially cancel out •Less rms capacitor current per unit of output power Produce 85 W output power by end of week 2ECEN 451710Exp. 3 Part 3Regulation of output voltage via feedbacksnubberPWM Compensator+–VrefVbattvHVDC• Model and measure control-to-output transfer function Gvd(s) • Design and build feedback loop • Measure loop gain to verify phase margin and crossover frequency •Demonstrate closed-loop regulation of vHVDCECEN 451711Negative feedback:a switching regulator system+–+v–vgSwitching converterPowerinputLoad–+CompensatorvrefReferenceinputHvPulse-widthmodulatorvcTransistorgate driverGc(s)H(s)veErrorsignalSensorgainiloadECEN 451712Transfer functions ofsome basic CCM convertersTable 8.2. S alient features of the small-signal CCM transfer functions of some basic dc-dc convertersConverterGg0Gd00Qzbuck D VD 1LC RCLboost 1D' VD' D'LC D'RCL D'2RLbuck-boost –DD' VDD'2 D'LC D'RCL D'2RDLwhere the transfer functions are written in the standard formsGvd(s)=Gd01–sz1+sQ0+s02Gvg(s)=Gg011+sQ0+s02Flyback: push L and C to same side of transformer, then use buck-boost equations. DC gains Gg0 and Gd0 have additional factors of n (turns ratio).ECEN 451713Bode plot: control-to-output transfer functionbuck-boost or flyback converter examplef0˚–90˚–180˚–270˚ Gvd Gd0 = 187 V 45.5 dBV Gvd Gvd0 dBV–20 dBV–40 dBV20 dBV40 dBV60 dBV80 dBVQ = 4 12 dBfz2.6 kHzRHP Gvd10-1/2Qf0101/2Qf00˚300 Hz533 Hz–20 dB/decade–40 dB/decade–270˚fz /10260 Hz10fz26 kHz1 MHz10 Hz 100 Hz 1 kHz 10 kHz 100 kHzf0400 HzECEN 451714The loop gain T(s)+–+v–vgSwitching converterPowerinputLoad–+CompensatorvrefReferenceinputHvPulse-widthmodulatorvcTransistorgate driverGc(s)H(s)veErrorsignalSensorgainiloadLoop gain T(s) = product of gains around the feedback loopMore loop gain ||T|| leads to better regulation of output
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