AN4209 datasheet(9/19 Pages) STMICROELECTRONICS

9 / 19 page

DocID023949 Rev 2

9/19

AN4209

STRVS parameters

Figure 6. Thermal impedance variation of SMB package versus the pulse duration

When the STRVS works under repetitive mode operation, the peak junction temperature

Tjpeak must be below absolute rating Tjmax specified in the datasheet.

For an infinite pulse train case, Tjpeak is expressed as shown in Equation 10:

Equation 10

Note:

The background average power PD has been subtracted, to avoid counting this effect twice

in calculating temperature rise.

In power conversion applications, two main cases occur. The time constants of the transient

thermal impedance



thermal must be considered regarding the switching period tSW in the

application. Figure 7 illustrates the impact of operating frequency on the ripple temperature.

The time constant of a thermal system is dependent on its transient thermal impedance

curve. A sufficient estimation of thermal constant can be easily made from the simple RC

thermal model representing the Zth(j-Ref) curve.

Equation 11

Equation 12

Equation 12 applied to Figure 6 for an application working switching period tsw, around

10 µs, gives tau much greater than tsw due to the low value of Zth(@ 10 µs) before the Rth

value. In this case the junction temperature rises and falls with a negligible dynamic

temperature

T

j near Tjavg as shown in Figure 7.

Since Tjpeak  Tjavg, only the average junction temperature evaluation is needed using

Equation 6. Notice that this case represents 90% of switch mode power supply applications

(FSW > 20 kHz). Under these conditions, pulse widths are generally in the range of some

tens to hundreds of nanoseconds and pulses may be viewed as a short block of constant

power PSTEP, sustaining the junction temperature.

0.001

0.01

0.1

100

1000

0.000001 0.00001 0.0001

0.001

0.01

0.1

100

1000

10000

°C/W

tp(s)

ZTH(j-a)

ZTH(j-l)

Recommended pad layout

Printed circuit board FR4,

copper thickness = 35 µm

Step

peak

T+

)

(

)

(

)

(

)

(

)

(

Zth(j-Ref)(t) = Rth(j-Ref) · (1 - e

(-t/tau))

tau = -t(ln(1-Zth(t)/Rth)

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