BCW33LT1

6

Motorola Small–Signal Transistors, FETs and Diodes Device Data

Figure 16A.

TJ, JUNCTION TEMPERATURE (°C)

104

– 4

0

DESIGN NOTE: USE OF THERMAL RESPONSE DATA

A train of periodical power pulses can be represented by the model

as shown in Figure 16A. Using the model and the device thermal

response the normalized effective transient thermal resistance of

Figure 16 was calculated for various duty cycles.

To find Z

θJA(t), multiply the value obtained from Figure 16 by the

steady state value R

θJA.

Example:

The MPS3904 is dissipating 2.0 watts peak under the following

conditions:

t1 = 1.0 ms, t2 = 5.0 ms. (D = 0.2)

Using Figure 16 at a pulse width of 1.0 ms and D = 0.2, the reading of

r(t) is 0.22.

The peak rise in junction temperature is therefore

∆T = r(t) x P(pk) x RθJA = 0.22 x 2.0 x 200 = 88°C.

For more information, see AN–569.

10–2

10–1

100

101

102

103

– 2

0

0

+ 20

+ 40

+ 60

+ 80 + 100 + 120 + 140 + 160

VCC = 30 Vdc

ICEO

ICBO

AND

ICEX @ VBE(off) = 3.0 Vdc