BCW29LT1 BCW30LT1

6

Motorola Small–Signal Transistors, FETs and Diodes Device Data

Figure 17. Thermal Response

t, TIME (ms)

1.0

0.01

0.01

0.02

0.03

0.05

0.07

0.1

0.2

0.3

0.5

0.7

0.02

0.05

0.1

0.2

0.5

1.0

2.0

5.0

10

20

50

100

200

500

1.0 k 2.0 k

5.0 k 10 k

20 k

50 k 100 k

D = 0.5

0.2

0.1

0.05

0.02

0.01

SINGLE PULSE

DUTY CYCLE, D = t1/t2

D CURVES APPLY FOR POWER

PULSE TRAIN SHOWN

READ TIME AT t1 (SEE AN–569)

Z

θJA(t) = r(t) • RθJA

TJ(pk) – TA = P(pk) Z

θJA(t)

t1

t2

P(pk)

FIGURE 19

TJ, JUNCTION TEMPERATURE (°C)

104

– 4

0

Figure 18. 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 19. Using the model and the device thermal

response the normalized effective transient thermal resistance of

Figure 17 was calculated for various duty cycles.

To find Z

θJA(t), multiply the value obtained from Figure 17 by the

steady state value R

θJA.

Example:

The BCW29LT1 is dissipating 2.0 watts peak under the following

conditions:

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

Using Figure 17 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