MZP4729A Series
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5
APPLICATION NOTE
Since the actual voltage available from a given zener
diode is temperature dependent, it is necessary to determine
junction temperature under any set of operating conditions
in order to calculate its value. The following procedure is
recommended:
Lead Temperature, T
L
, should be determined from:
T
L
= θ
LA
P
D
+ T
A
θ
LA
is the lead-to-ambient thermal resistance (°C/W) and P
D
is the power dissipation. The value for θ
LA
will vary and
depends on the device mounting method. θ
LA
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 T
L
, the junction temperature
may be determined by:
T
J
= T
L
+ ΔT
JL
ΔT
JL
is the increase in junction temperature above the lead
temperature and may be found from Figure 2 for a train of
power pulses (L = 3/8 inch) or from Figure 10 for dc power.
ΔT
JL
= θ
JL
P
D
For worst-case design, using expected limits of I
Z
, limits
of P
D
and the extremes of T
J
(ΔT
J
) may be estimated.
Changes in voltage, V
Z
, can then be found from:
ΔV = θ
VZ
ΔT
J
θ
VZ
, 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
capability. 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.
