BCW61BLT1 BCW61CLT1 BCW61DLT1

6

Motorola Small–Signal Transistors, FETs and Diodes Device Data

TJ, JUNCTION TEMPERATURE (°C)

104

– 4

0

Figure 15. Typical Collector Leakage Current

DESIGN NOTE: USE OF THERMAL RESPONSE DATA

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

as shown in Figure 15. Using the model and the device thermal

response the normalized effective transient thermal resistance of

Figure 14 was calculated for various duty cycles.

To find Z

θJA(t), multiply the value obtained from Figure 14 by the

steady state value R

θJA.

Example:

The MPS3905 is dissipating 2.0 watts peak under the following

conditions:

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

Using Figure 14 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 V

ICEO

ICBO

AND

ICEX @ VBE(off) = 3.0 V