AN-7520| Application Note

Numerical Method for Evaluating IGBT Losses
Application Note January 2000 AN-7520

Authors: Alain Laprade and Ron H. Randall 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.

Introduction
An analysis is presented describing a numerical algorithm for determining IGBT losses. A math worksheet program such as MathCADTM 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

/Title AN75 0) Subect Nume ical etho for valuting GBT osse ) Autho Alexnder raig) Keyords Nume ical etho for valuting GBT osse , nteril orpoation, emionuctor,

Nomenclature
IL Eoff300(I,TJ) Eoff480(I,TJ) Eoff(V,I,TJ) Boost inductor peak to peak current. Turn-off loss energy at 300V as a function of current and junction temperature. Turn-off loss energy at 480V as a function of current and junction temperature. Turn-off loss as a function of peak clamp voltage, IGBT collector current and junction temperature. IGBT switching frequency. Curve fit vectors for switching loss function Eoff480(I,TJ). IGBT collector current. Average IGBT turn-off loss at a particular instant in time.

OPERATION CONDITION SELECTED VIN RANGE, POWER, FREQUENCY

HEAT SINK SELECTED OPERATING DUTY CYCLE CALCULATED

IGBT DEVICE DATA

fs ka(TJ), kb(TJ), kc(TJ), kd(TJ) I IgbtTurnOffLoss (Vac,Pout,t,TJ)

SWITCHING LOSSES CALCULATED

CONDUCTION LOSSES CALCULATED

OFF-STATE LOSSES CALCULATED

IGBT_TurnOffWatts Average IGBT turn-off loss as a function of Vac, output power and TJ. (Vac,Pout,TJ) Itoff(Vac,Pout,t) L Pout t T TJ Vac Vclamp VOFF RJA Collector current at IGBT turn-off. Boost inductor value. Power supply efficiency. Boost regulator output power. Time. Time period per AC mains cycle. IGBT junction temperature. Input mains RMS voltage. Maximum IGBT voltage at turn-off. IGBT voltage during off-state period. IGBT junction to ambient thermal impedance in oC/watt. AC mains radian frequency.

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.

2002 Fairchild Semiconductor Corporation

Application Note 7520 Rev. A1