GENERAL DATA — 1–1.3 WATT DO-41 GLASS
APPLICATION NOTE
Since the actual voltage available from a given zener diode
temperature and may be found as follows:
is temperature dependent, it is necessary to determine junc-
tiontemperatureunderanysetofoperatingconditionsinorder
to calculate its value. The following procedure is recom-
mended:
?T = θ P .
JL JL D
θ
may be determined from Figure 3 for dc power condi-
JL
tions. For worst-case design, using expected limits of I , limits
Z
ofP andtheextremesofT (?T )maybeestimated.Changes
D
J
J
Lead Temperature, T , should be determined from:
L
in voltage, V , can then be found from:
Z
T = θ
L
P
LA D
+ T .
A
?V = θ
?T .
J
VZ
, the zener voltage temperature coefficient, is found from
θ
isthelead-to-ambientthermalresistance(°C/W)andP is
LA
D
θ
VZ
Figure 2.
Under high power-pulse operation, the zener voltage will
thepower dissipation. The value for θ willvaryanddepends
onthedevicemountingmethod.θ isgenerally30to40°C/W
for the various clips and tie points in common use and for
printed circuit board wiring.
LA
LA
vary with time and may also be affected significantly by the
zenerresistance. Forbestregulation, keepcurrentexcursions
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.
The temperature of the lead can also be measured using a
thermocoupleplacedontheleadascloseaspossibletothetie
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 T , the junction temperature may be deter-
L
mined by:
T = T + ?T .
JL
J
L
?T is the increase in junction temperature above the lead
JL
Motorola TVS/Zener Device Data
1–1.3 Watt DO-41 Glass Data Sheet
6-23