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6

REV.02 20120403

BCX70G-J-K

T

Figure 19A.

V

–4

–2

0

+20

+40

+60

+80

+100

+120

+140

+160

I

CBO

AND

I

I

CEO

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 device

thermal response the normalized effective transient thermal re-

sistance of Figure 19 was calculated for various duty cycles.

To find Z

the steady state value R

Example:

The MPS3904 is dissipating 2.0 watts peak under the follow-

ing conditions:

t

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

∆T = r(t) x P

For more information, see AN–569.

t, TIME (ms)

Figure 19. Thermal Response

D = 0.5

0.02

0.05

0.1

0.2

0.01

SINGLE PULSE

DUTY CYCLE, D = t

D CURVES APPLY FOR POWER

PULSE TRAIN SHOWN

READ TIME AT t

Z

T

θJA(t)

FIGURE 19A

P

(pk)

t

2

t

1

1.0

0.7

0.5

0.3

0.2

0.1

0.07

0.05

0.03

0.02

0.01

0.01

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

V

Figure 20.

CURRENT LIMIT

THERMAL LIMIT

SECOND BREAKDOWN LIMIT

2.0

4.0

6.0

8.0

10

20

40

The safe operating area curves indicate I

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.

The data of Figure 20 is based upon T

T

valid for duty cycles to 10% provided T

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.

T

T

T

100

µs

1.0 ms

1.0 s

10

µs

dc

dc

400

200

100

60

40

20

10

6.0

4.0

DEVICE CHARACTERISTICS