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
=
LA
LA
P
D
+ T
A
is the lead-to-ambient thermal resistance ( /W)
and P
D
is the power dissipation.
Junction Temperature, T
J
, may be found from:
T
J
= T
L
+ T
JL
T
JL
is the increase in junction temperature above the
lead temperature and may be found from Figure 3 for a
train of power pulses or from Figure 4 for dc power.
T
JL
=
JL
P
D
For worst-case design, using expected limits of Iz, limits
of P
D
and the extremes of T
J
( T
J
) may be estimated.
Changes in voltage, Vz, can then be found from:
V=
VZ
,
VZ
T
J
the zener voltage temperature coefficient, is fount
from Figures 2.
Under high power-pulse operation, the zener voltage will
vary with time and may also be affected significantly be
the zener resistance. For best regulation, keep current
excursions as low as possible.
Data of Figure 3 should not be used to compute surge
capability. 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 extremely high in small spots
resulting in device degradation should the limits of
Figure. 5 be exceeded.
TRSYS [ TRANSYS Electronics Limited ]
