AN-2020 Datasheet, PDF (1/10 Pages) National Semiconductor (TI) – Thermal Design By Insight, Not Hindsight

Thermal Design By Insight,
Not Hindsight
National Semiconductor
Application Note 2020
Marc Davis-Marsh
June 14, 2010
Introduction
All electronics contain; semiconductor devices, capacitors
and other components that are vulnerable to thermally accel-
erated failure mechanisms. Thermal design becomes vital to
improving the reliability of any design. Unfortunately, thermal
design can be very difficult because of the mathematical anal-
ysis of fluid dynamics for complex geometries. Although this
will remain true for the foreseeable future, this application
note will cover the basics of thermal design for DC-DC con-
verters using a simplified resistor model of heat transfer. We
will focus on the thermal design for the semiconductor de-
vices, but all of these techniques can be applied to other
components. The resistor model is very useful for quickly es-
timating your design requirements, such as the PCB size and
whether airflow is required. Finite element analysis software
can then be used to analyze the design in more detail. The
listed reference material is home to additional data and many
useful thermal calculators, covering material that is beyond
the scope of this application note.
Our discussion of thermal design will begin with the definition
of parameters used in datasheets such as θ_JA and θ_JC, and
end with some rules of thumb for the thermal design of a DC-
DC converter, including their derivation. An accompanying
spreadsheet (see References) uses these derivations to
quickly provide a ballpark figure for the thermal performance
of your design.
Definitions
Description of Thermal Terms
Parameters of interest : θ_JA, θ_JC, θ_CA, θ_JT
FIGURE 1. IC Mounted On A Four-Layer Printed Circuit Board
30110901
The most commonly specified parameter, in datasheets, for
thermal performance is θ_JA. θ_JA is defined as the thermal
impedance from the Junction, of the integrated circuit under
test, to the Ambient environment. If we describe it using a
resistor model, it is the parallel combination of all the paths
that heat can take to move from the IC junction to the ambient
air. The equation for this thermal resistance is:
In our resistor model the heat transfer, measured in watts,
takes the place of charge transfer measured in amps, and the
temperature potential between the junction and ambient tem-
peratures replaces the voltage potential. The heat that needs
to be transferred away from the junction is the power dissi-
pated by the IC.
© 2010 National Semiconductor Corporation 301109
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