Lecture 7: MOSFET, IGBT, and Switching dcostine/ECE481/fall2013/lectures/L7_ 7: MOSFET, IGBT, and...

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Transcript of Lecture 7: MOSFET, IGBT, and Switching dcostine/ECE481/fall2013/lectures/L7_ 7: MOSFET, IGBT, and...

  • Lecture 7: MOSFET, IGBT, and Switching Loss

    ECE481:PowerElectronicsProf.DanielCostinett

    DepartmentofElectricalEngineeringandComputerScienceUniversityofTennesseeKnoxville

    Fall2013

  • IRLHM620EPC2015

    20V40V

    40A33A

    0.00250.004

    52nC10.5nC

    IPW60R099CPIPW90R340C3TPH3006PDCMF20120

    600V900V600V1200V

    31A15A17A24A

    0.10.340.150.098

    60nC93nC6nC49nC

  • Inclusion of Switching Loss in theAveraged Equivalent Circuit Model

    The methods of Chapter 3 can be extended to include switching loss in the converter equivalent circuit model

    Include switching transitions in the converter waveforms

    Model effects of diode reverse recovery, etc.

    To obtain tractable results, the waveforms during the switching transitions must usually be approximated

    Things that can substantially change the results:

    Ringing caused by parasitic tank circuits

    Snubber circuits

  • The Modeling ApproachExtension of Chapter 3 Methods

    Sketchtheconverterwaveforms Includingtheswitchingtransitions(idealizingassumptionsaremadetoleadtotractableresults)

    Inparticular,sketchinductorvoltage,capacitorcurrent,andinputcurrentwaveforms

    Theusualsteadystaterelationships: vL = 0, iC = 0, ig = Ig

    Usetheresultingequationstoconstructanequivalentcircuitmodel,asusual

    Buck Converter Example

    IdealMOSFET,pn diodewithreverserecovery Neglectsemiconductordevicecapacitances,MOSFETswitchingtimes,etc. Neglectconductionlosses Neglectrippleininductorcurrentandcapacitorvoltage

  • Assumed waveforms

    Diode recovered charge Qr,reverse recovery time trThese waveforms assume that the diode voltage changes at the end of the reverse recovery transient

    a snappy diode

    Voltage of soft-recovery diodes changes sooner

    Leads to a pessimistic estimate of induced switching loss

    Inductor volt-second balanceand capacitor charge balance

    Asusual: vL = 0 = DVg V

    Alsoasusual: iC = 0 = IL V/R

  • Average input current

    ig = Ig = (area under curve)/Ts= (DTsIL + trIL + Qr)/Ts= DIL + trIL /Ts + Qr /Ts

    Construction of Equivalent Circuit Model

    Frominductorvoltsecondbalance: vL = 0 = DVg V

    Fromcapacitorchargebalance: iC = 0 = IL V/R

  • Input port of model

    ig = Ig = DIL + trIL /Ts + Qr /Ts

    Combine for complete model

    Thetwoindependentcurrentsourcesconsumepower

    Vg (trIL /Ts + Qr /Ts)equaltotheswitchinglossinducedbydiodereverserecovery

  • Solution of model

    Output:

    V = DVg

    Efficiency: = Pout / PinPout = VIL Pin = Vg (DIL + trIL /Ts + Qr /Ts)

    Combineandsimplify:

    = 1 / [1 + fs (tr /D + Qr R /D2Vg )]

    Predicted Efficiency vs Duty Cycle

    Switching frequency 100 kHz Input voltage 24 V Load resistance 15 Recovered charge 0.75 Coul Reverse recovery time 75 nsec

    (no attempt is made here to model how the reverse recovery process varies with inductor current)

    Substantialdegradationofefficiency

    Poorefficiencyatlowdutycycle

    Buck converter with diode reverse recovery

    0.00%

    10.00%

    20.00%

    30.00%

    40.00%

    50.00%

    60.00%

    70.00%

    80.00%

    90.00%

    100.00%

    0 0.2 0.4 0.6 0.8 1

    Duty cycle

  • Boost Converter Example

    Modelsameeffectsasinpreviousbuckconverterexample: IdealMOSFET,pn diodewithreverserecovery Neglectsemiconductordevicecapacitances,MOSFETswitchingtimes,

    etc. Neglectconductionlosses Neglectrippleininductorcurrentandcapacitorvoltage

    Boost converter

    Transistoranddiodewaveformshavesameshapesasinbuckexample,butdependondifferentquantities

  • Inductor volt-second balanceand average input current

    Asusual: vL = 0 = Vg DV

    Alsoasusual: ig = IL

    Capacitor charge balance

    iC = id V/R = 0

    = V/R + IL(DTs tr)/Ts Qr /TsCollectterms: V/R = IL(DTs tr)/Ts Qr /Ts

  • Construct model

    Thetwoindependentcurrentsourcesconsumepower

    V (trIL /Ts + Qr /Ts)equaltotheswitchinglossinducedbydiodereverserecovery

    Theresultis:

    Predicted V/Vg vs duty cycle

    Boost converter with diode reverse recovery

    0

    1

    2

    3

    4

    5

    6

    7

    8

    0 0.2 0.4 0.6 0.8 1

    Duty cycle

    Switching frequency 100 kHz Input voltage 24 V Load resistance 60 Recovered charge 5 Coul Reverse recovery time 100

    nsec Inductor resistance RL = 0.3 (inductor resistance also

    inserted into averaged model here)

    With RL only

    With RL and diode reverse recovery

  • Summary

    Theaveragedmodelingapproachcanbeextendedtoincludeeffectsofswitchingloss

    Transistoranddiodewaveformsareconstructed,includingtheswitchingtransitions.Theeffectsoftheswitchingtransitionsontheinductor,capacitor,andinputcurrentwaveformscanthenbedetermined

    Inductorvoltsecondbalanceandcapacitorchargebalanceareapplied

    Converterinputcurrentisaveraged Equivalentcircuitcorrespondingtothetheaveragedequationsisconstructed