GENERAL DATA — 500 mW DO-35 GLASS
APPLICATION NOTE — ZENER VOLTAGE
500
400
300
200
100
0
Since the actual voltage available from a given zener diode
is temperature dependent, it is necessary to determine junc-
tiontemperatureunderanysetofoperatingconditionsinorder
to calculate its value. The following procedure is recom-
mended:
L
L
Lead Temperature, T , should be determined from:
L
T = θ
L
P
LA D
+ T .
A
2.4–60 V
θ
isthelead-to-ambientthermalresistance(°C/W)andP is
LA
D
the power dissipation. The value forθ willvaryanddepends
onthedevicemountingmethod.θ isgenerally30to40°C/W
for the various clips and tie points in common use and for
printed circuit board wiring.
LA
62–200 V
0.6
LA
0
0.2
0.4
0.8
1
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-
L, LEAD LENGTH TO HEAT SINK (INCH)
Figure 2. Typical Thermal Resistance
1000
7000
5000
sured value of T , the junction temperature may be deter-
L
TYPICAL LEAKAGE CURRENT
AT 80% OF NOMINAL
BREAKDOWN VOLTAGE
mined by:
T = T + ∆T .
JL
2000
1000
J
L
∆T is the increase in junction temperature above the lead
JL
700
temperature and may be found from Figure 2 for dc power:
500
∆T = θ P .
JL JL D
200
For worst-case design, using expected limits of I , limits of
Z
100
70
P andtheextremesofT (∆T )maybeestimated.Changesin
D
J
J
50
voltage, V , can then be found from:
Z
∆V = θ T .
VZ J
20
θ
, the zener voltage temperature coefficient, is found from
VZ
10
7
Figures 4 and 5.
5
Under high power-pulse operation, the zener voltage will
vary with time and may also be affected significantly by the
zenerresistance. Forbestregulation, keepcurrentexcursions
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.
2
1
0.7
0.5
+125°C
0.2
0.1
0.07
0.05
0.02
0.01
0.007
0.005
+25°C
0.002
0.001
3
4
5
6
7
8
9
10
11
12 13 14 15
V , NOMINAL ZENER VOLTAGE (VOLTS)
Z
Figure 3. Typical Leakage Current
Motorola TVS/Zener Device Data
500 mW DO-35 Glass Data Sheet
6-4
