AN3338 datasheet(53/72 Pages) STMICROELECTRONICS

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AN3338

Power losses and dissipation

The total power losses in an inverter are comprised of conduction losses, switching losses,

and off-state losses and they are essentially generated by the power devices of the inverter

stage, such as the IGBTs and the freewheeling diodes. The conduction losses (Pcond) are

the on-state losses during the conduction phase. The switching losses (Psw) are the

dynamic losses encountered during the turn-on and the turn-off. The off-state losses, due to

the blocking voltage and leakage current, can be neglected.

Finally, the total power losses are given by:

Equation 17

Figure 34 shows a typical waveform of an inductive hard switching application such as a

motor drive, where the major sources of power losses are specified.

4.1

Conduction power losses

The conduction losses are caused by IGBT and freewheeling diode forward voltage drop at

rated current. They can be calculated using a linear approximation of the forward

characteristics for both IGBT and diode, having a series connection of DC voltage source

representing the threshold voltage, VTO for IGBT, (and VFO for diode) and a collector emitter

on-state resistance, RCE, (and anode cathode on-state resistance, RAK), as shown in

Figure 35, for reference.

cond

tot

≈

Figure 34. Typical IGBT power losses

AM09357v1

10% I

10% V

10% I

sw(on)

sw(off)

CE(sat)

c(on)

c(off)

conduction

AN3338 Datasheet(PDF) 53 Page - STMicroelectronics