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STAD-MAR.25.2009

1

1SMB3EZ6.8~1SMB3EZ51

APPLICATION NOTE:

Since the actual voltage available from a given zener diode is temperature dependent, it is necessary to determinejunction

temperature under any set of operating conditions in order to calculate its value. The following procedure is recommended:

Lead Temperature, T , should be determined from:

T= LA P + T

L is the lead-to-ambient thermal resistance ( C/W) and Pd is the power dissipation. The value for L will vary and depends

on the device mounting method.

L is generally 30-40 C/W for the various clips and tie points in common use and for printed

circuit board wiring.

The temperature of the lead can also be measured using a thermocouple placed on the lead as close as possible to the tie point.

The thermal mass connected to the tie point is normally large enough so that it will not significantly respond to heat surges

generated in the diode as a result of pulsed operation once steady-state conditions are achieved. Using the measured value of

TL, the junction temperature may be determined by:

T= T + T

T

is the increase in junction temperature above the lead temperature and may be found from Figure 2 for a train of power pulses

or from Figure 10 for dc power.

T= J P

For worst-case design, using expected limits of I , limits of P and the extremes of T ( T ) may be estimated. Changes in voltage,

V , can then be found from:

V= V

T

V , the zener voltage temperature coefficient, is found from Figures 5 and 6.

Under high power-pulse operation, the zener voltage will vary with time and may also be affected significantly by the zener resistance.

For best regulation, keep current excursions as low as possible.

Data of Figure 2 should not be used to compute surge capa-bility. Surge limitations are given in Figure 3. They are lower than would

be expected by considering only junction temperature, as current crowding effects cause temperatures to be extremely high in small

spots resulting in device degradation should the limits of Figure 3 be exceeded.

L

LD

A

A

A

A

JL

JL

JL

Z

q

q

q

q

D

D

Dq

D

Dq

D

q

O

O

JL

L

D

ZD

J

J

Z

ZJ

1

2

3

1000

500

300

200

100

50

30

20

10