AN4043 datasheet(48/60 Pages) STMICROELECTRONICS

48 / 60 page

Power losses and dissipation

AN4043

48/60

Doc ID 022726 Rev 2

More generally, in the case of the device, power is time dependent too. The device

temperature can be calculated by using the convolution integral method applied to

Equation

31Equation 31, as follows:

Equation 32

An alternative method, very useful for the simulator tools, is the transient thermal impedance

model, which provides a simple method to estimate the junction temperature rise under a

transient condition.

By using the thermo-electrical analogy, the transient thermal impedance Zth(t) can be

transformed into an electrical equivalent RC network. The number of RC sections increases

the model details, therefore a twelfth order model, for Zth(j-a), based on the Cauer and Foster

networks, has been used in order to improve the accuracy of both models.

Figure 31 and Figure 32 show the general Cauer and Foster RC equivalent circuit used for

the thermal impedance model.

Figure 31.

Cauer RC equivalent circuit

Figure 32.

Foster RC equivalent circuit

Temperatures inside the electrical RC network represent voltages, power flows represent

currents, electrical resistances and capacitances represent thermal resistances and

capacitances respectively. The case temperature is represented with a DC voltage source

and it can be interpreted as the initial junction temperature.

Transient thermal impedance models are derived by curve fitting an equation to the

measured data. Values for the individual resistors and capacitors are the variables from that

equation and are defined in

Table 11, for both Zth(j-a) Cauer and Foster thermal impedance

models.

⋅

−

∫

)

(

)

(

)

(

AM11819v1

AM11820v1

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