AN4043 datasheet(47/60 Pages) STMICROELECTRONICS

Part #	AN4043

Description	In recent years the variable speed motor control market has required high performance

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

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47 / 60 page

AN4043

Power losses and dissipation

Doc ID 022726 Rev 2

47/60

Figure 29 shows an equivalent circuit of the thermal resistance between junction and

ambient Rth(j-a).

Figure 29.

Rth(j-a) equivalent thermal circuit

As the power loss Ptot is cyclic, also the transient thermal impedance must be considered. It

is defined as the ratio between the time dependent temperature increase above the

reference,

ΔT(t), and the relevant heat flow:

Equation 31

Contrary to that already seen regarding the thermal resistance, the thermal impedance is

typically represented by an RC equivalent circuit. For pulsed power loss, the thermal

capacitance effect delays the rise in junction temperature and therefore the advantage of

this behavior is the short-term overload capability of the SLLIMM-nano.

For example,

Figure 30 shows thermal impedance from a junction to ambient curve for a

single IGBT of the SLLIMM-nano.

Figure 30.

Thermal impedance Zth(j-a) curve for a single IGBT

AM11817v1

)

(

)

(

Zth

AM11818v1

SLLIMM-nano Zth(j-a)

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

time (sec)

AN4043 Datasheet(PDF) 47 Page - STMicroelectronics