BCX70GLT1 BCX70JLT1 BCX70KLT1

6

Motorola Small–Signal Transistors, FETs and Diodes Device Data

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

1.0 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 19A

Figure 19A.

TJ, JUNCTION TEMPERATURE (°C)

104

– 4

0

Figure 20.

VCE, COLLECTOR–EMITTER VOLTAGE (VOLTS)

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 device thermal

response the normalized effective transient thermal resistance of

Figure 19 was calculated for various duty cycles.

To find Z

θJA(t), multiply the value obtained from Figure 19 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 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(pk) x RθJA = 0.22 x 2.0 x 200 = 88°C.

For more information, see AN–569.

The safe operating area curves indicate IC–VCE limits of 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 TJ(pk) = 150°C; TC or TA is

variable depending upon conditions. Pulse curves are valid for duty

cycles to 10% provided TJ(pk) ≤ 150°C. TJ(pk) 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.

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

TA = 25°C

CURRENT LIMIT

THERMAL LIMIT

SECOND BREAKDOWN LIMIT

1.0 ms

10

µs

TC = 25°C

1.0 s

dc

dc

4.0

6.0

10

20

40

60

100

200

4.0

6.0

8.0

10

20

40

TJ = 150°C

100

µs