AN4544 datasheet(17/35 Pages) STMICROELECTRONICS

17 / 35 page

DocID026535 Rev 1

17/35

AN4544

Datasheet explanation

Figure 13. Maximum normalized ZthJC function of pulse duty factor (δ) and loading

time (tp)

Figure 13 shows the variation of the normalized thermal impedance for the specified pulse

duty factor

δ=tp/t as a function of the loading time tp (pulse width).

The dissipated heat has to pass through several different layers with their thermal

resistances and thermal capacitances. Therefore, according to the pulse length, either the

thermal resistance or the thermal capacitance handle with the device’s behavior. The

increase of the junction temperature can be calculated as follows: TJstart = TC:

Equation 7

-5

-4

-3

-2

-1

tp(s)

-2

-1

0.2

0.05

0.02

0.01

0.1

Zth=k Rthj-c

δ=tp/t

Single pulse

δ=0.5

ZthTO2T_B

Jstart

ΔT

Jstart

thJc tp,δ

() P

tot

Jstart

p,δ

() R

thJc

tot

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