BZX85C3V3RL Series

http://onsemi.com

7

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:

is the power dissipation. The value for

depends on the device mounting method.

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.

may be determined by:

temperature and may be found as follows:

conditions. For worst-case design, using expected limits of

from Figure 2.

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 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.