MMBTA20LT1

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6

Figure 19. 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 1.0k 2.0k

5.0k 10k 20k 50k 100k

D = 0.5

0.2

0.1

0.05

0.02

0.01

SINGLE PULSE

D CURVES APPLY FOR POWER

PULSE TRAIN SHOWN

FIGURE 19A

Figure 19A.

-40

Figure 20.

400

2.0

DESIGN NOTE: USE OF THERMAL RESPONSE DATA

A train of periodical power pulses can be represented by the

model as shown in Figure 19A. Using the model and the de-

vice thermal response the normalized effective transient ther-

mal resistance of Figure 19 was calculated for various duty

cycles.

Example:

The MPS3904 is dissipating 2.0 watts peak under the follow-

ing conditions:

Using Figure 19 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

For more information, see AN–569.

the transistor that must be observed for reliable operation.

Collector load lines for specific circuits must fall below the

limits indicated by the applicable curve.

may be calculated from the data in Figure 19. At high case or

ambient temperatures, thermal limitations will reduce the

power that can be handled to values less than the limitations

imposed by second breakdown.

-20

0

+20 +40 +60 +80 +100 +120 +140 +160

AND

CURRENT LIMIT

THERMAL LIMIT

SECOND BREAKDOWN LIMIT

1.0 ms

10 µs

1.0 s

dc

dc

4.0

6.0

10

20

40

60

100

200

4.0

6.0 8.0 10

20

40

100 µs