AN-7520 Datasheet, PDF (1/6 Pages) Fairchild Semiconductor – Numerical Method for Evaluating IGBT Losses

Numerical Method for Evaluating IGBT Losses
Application Note
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AN75
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Sub-
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Nume
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Autho
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Key-
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Nume
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An analysis is presented describing a numerical algorithm
that develops loss prediction techniques for IGBTs operating
in switched mode power circuits. A 600W zero-current
switching boost PFC (Power Factor Correction) circuit is
analyzed as a design example. Predicted losses are
validated by test data measured from an operating circuit.
Nomenclature
∆I_L
Eoff₃₀₀(I,T_J)
Boost inductor peak to peak current.
Turn-off loss energy at 300V as a function of
current and junction temperature.
Eoff₄₈₀(I,T_J)
Turn-off loss energy at 480V as a function of
current and junction temperature.
Eoff(V,I,T_J)
Turn-off loss as a function of peak clamp
voltage, IGBT collector current and junction
temperature.
f_s
ka(T_J), kb(T_J),
kc(T_J), kd(T_J)
I
IGBT switching frequency.
Curve fit vectors for switching loss function
Eoff480(I,T_J).
IGBT collector current.
IgbtTurnOffLoss
(Vac,Pout,t,T_J)
Average IGBT turn-off loss at a particular
instant in time.
IGBT_TurnOffWatts Average IGBT turn-off loss as a function of
(Vac,Pout,T_J)
Vac, output power and T_J.
I_toff(Vac,Pout,t)
Collector current at IGBT turn-off.
L
Boost inductor value.
η
Power supply efficiency.
Pout
Boost regulator output power.
t
Time.
T
Time period per AC mains cycle.
T_J
IGBT junction temperature.
Vac
Input mains RMS voltage.
Vclamp
Maximum IGBT voltage at turn-off.
V_OFF
Rθ_JA
IGBT voltage during off-state period.
IGBT junction to ambient thermal impedance
in ^oC/watt.
ω
AC mains radian frequency.
©2002 Fairchild Semiconductor Corporation
January 2000
AN-7520
Authors: Alain Laprade and Ron H. Randall
Introduction
An analysis is presented describing a numerical algorithm
for determining IGBT losses. A math worksheet program
such as MathCAD™ may be used for this application. The
algorithm flow chart is shown in Figure 1. The required IGBT
parametric test data is obtained from basic device test
circuits used by semiconductor manufacturers.
TOPOLOGY SELECTED
OPERATION CONDITION SELECTED
VIN RANGE, POWER, FREQUENCY
HEAT SINK SELECTED
OPERATING DUTY CYCLE
CALCULATED
IGBT DEVICE DATA
SWITCHING LOSSES
CALCULATED
CONDUCTION LOSSES
CALCULATED
OFF-STATE LOSSES
CALCULATED
TOTAL LOSSES DETERMINED
AS A FUNCTION OF TJ AND VIN
TJ OPERATING POINT FOR
SELECTED HEAT SINK DETERMINED
FIGURE 1. LOSS CALCULATION ALGORITHM
Determining switching device losses in power circuits such
as active power factor correction (PFC) circuits, AC output
UPS systems and solid state AC motor drives that utilize
IGBTs as the switching device is extremely complex. The
switching device conduction duty cycle and switch current
are continually changing as a function of the instantaneous
magnitude of the AC mains input or AC output voltage. The
problem is further exacerbated by the fact that the IGBT
losses are a complex function of turn-off clamp voltage,
collector current and junction temperature. The relationship
between turn-off energy, collector current and junction
temperature is illustrated for a single turn-off clamp voltage
of 480V in the surface plot of Figure 2.
Conventional time domain SPICE analysis requires lengthy
simulations that generate massive output files. SPICE models
representing IGBT switching characteristics may only be run
for preset junction temperatures. In addition, IGBT
manufacturer data sheets do not provide sufficient information
to analyze a device’s losses under all switching conditions.
Application Note 7520 Rev. A1