BZX83C10RL datasheet(22/42 Pages) MOTOROLA

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GENERAL DATA — 500 mW DO-35 GLASS

Motorola TVS/Zener Device Data

6-118

500 mW DO-35 Glass Data Sheet

APPLICATION NOTE

Since the actual voltage available from a given zener diode

is temperature dependent, it is necessary to determine junc-

tion temperature under any set of operating conditions in order

to calculate its value. The following procedure is recom-

mended:

Lead Temperature, TL, should be determined from:

TL = θLAPD + TA.

θLAisthelead-to-ambientthermalresistance(°C/W)andPDis

the power dissipation. The value for

θLA will vary and depends

on the device mounting method.

θLA is generally 30 to 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 mea-

sured value of TL, the junction temperature may be deter-

mined by:

TJ = TL + ∆TJL.

∆TJL is the increase in junction temperature above the lead

temperature and may be found as follows:

∆TJL = θJLPD.

θJL may be determined from Figure 3 for dc power condi-

tions. For worst-case design, using expected limits of IZ, limits

of PD and the extremes of TJ(∆TJ) may be estimated. Changes

in voltage, VZ, can then be found from:

∆V = θVZ ∆TJ.

θVZ, the zener voltage temperature coefficient, is found from

Figure 2.

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.

Surge limitations are given in Figure 5. They are lower than

would be expected by considering only junction temperature,

as current crowding effects cause temperatures to be ex-

tremely high in small spots, resulting in device degradation

should the limits of Figure 5 be exceeded.

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