371Thermal Analysis and Design
This is for the typical PC board-mounted application where only the PC board traces are
used to conduct the heat a
way from the diode. The typical value range of the lead-to-air
thermal resistance is between 30 to 40 C/W and is a variable which is dependent on the
lead length.
There are some heatsinks for radially-leaded diodes that solder to one of the leads. These
are also available from the transistor heatsink manufacturers. In this situation the thermal
equation (see Figure 12.6b ) becomes
These heatsinks will help a marginal heat situation. The alternative is to use a rectifi er in a
power transistor package such as a TO-220, TO-218, etc., and place it on a heatsink or to
investigate a different technology of diode that exhibits a lower forward voltage drop such
as a Schottky.
12.5 Surface Mount Parts
The use of surface mount parts is widespread. Surface mount parts can rid themselves of
heat only through their leads which are soldered to a printed circuit board. The thickness
and surface area of the copper island become the heatsink system. The thermal resistances
in surface mount devices are much higher, therefore their designs have much less margin
and room for error. Table 12.3 has nominal values for thermal resistances of common
surface mount packages. Please refer to the individual part data sheet for the exact value.
It is very important to select the package that is appropriate for the function being
performed. For switching signals, which are signal currents of less than 50 mA, the
SOT23, SOD123, and other simple packages with gull-wing, J-leaded, and solder bump
Table 12.3 : Typical surface mount
package thermal resistances
Package J-A
SOD123 340 150
SOT23 556 75
SOT223 159 7.5
SO-8 63 21
SMB 13
SMC 11
DPAK 80 6
D2PAK 50 2
Thermal resistance for a reference pad size.
Thermal resistance for a very large pad size.

Get Power Sources and Supplies: World Class Designs now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.