GENERAL DATA — 500 mW DO-35 GLASS
APPLICATION NOTE — ZENER VOLTAGE
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-
Lead Temperature, TL, should be determined from:
TL = θLAPD + TA.
θLA is the lead-to-ambient thermal resistance (°C/W) and PD is
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-
TJ = TL + ∆TJL.
∆TJL is the increase in junction temperature above the lead
temperature and may be found from Figure 2 for dc power:
∆TJL = θJLPD.
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 = θVZTJ.
θVZ, the zener voltage temperature coefficient, is found from
Figures 4 and 5.
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 7. 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 7 be exceeded.
L, LEAD LENGTH TO HEAT SINK (INCH)
Figure 2. Typical Thermal Resistance
TYPICAL LEAKAGE CURRENT
AT 80% OF NOMINAL
4 5 6 7 8 9 10 11 12 13 14 15
VZ, NOMINAL ZENER VOLTAGE (VOLTS)
Figure 3. Typical Leakage Current
Motorola TVS/Zener Device Data
500 mW DO-35 Glass Data Sheet