AN-2026 Datasheet, PDF (1/8 Pages) National Semiconductor (TI) – The Effect of PCB Design on the Thermal Performance of SIMPLE SWITCHER

The Effect of PCB Design on
the Thermal Performance of
SIMPLE SWITCHER® Power
Modules
Summary
The SIMPLE SWITCHER® Power Modules use a TO-PMOD
package similar to a TO-263. This application note focuses
on the low current modules which come in a 7 Lead, 10.16 x
4.57 x 9.81 mm package. This package has excellent thermal
performance enabled by an exposed pad, which can be sol-
dered to the PCB. The key thermal characteristics are:
• θ_JC = 1.9°C/W
• θ_JC 21.6°/W (On a 4-layer thermal board)
What Determines θ_JA
In order to understand how a PCB’s thermal performance de-
termines the thermal resistance (θ_JA) of a Power Module
mounted on the PCB, a brief analysis for θ_JA is given as fol-
lows. There are two heat dissipation paths, i.e., Junction-
PCB-Ambient and Junction-PKG surface-Ambient. Because
the two paths are in parallel, θ_JA can be expressed as
θ_JA = (θ_JCA x θ_JTA)/(θ_JCA + θ_JTA)
θ_JCA is the thermal resistance from junction to ambient
through the PCB and θ_JTA is the thermal resistance through
the package surface to ambient (mainly package top). For the
situation where no heat sink is applied on the package top,
95% or more of the power dissipates through the PCB, mean-
ing that θ_JA is dominated by θ_JCA (also meaning that θ_JTA is
much bigger than θ_JCA). As a result, θ_JA can be simply ex-
pressed as
θ_JA = θ_JCA - R_JTA = θ_JC + θ_CA - R_JTA
θ_CA is the thermal resistance from package bottom case to
ambient through the PCB. It is mainly dependent on the ther-
mal conductivity of the PCB and the thermal connection be-
tween the package and the PCB. R_JTA gives a small reduction
of θ_JA caused by the power dissipation through the package
top.
So, it is seen from the equation above that on any given board,
the small θ_JC and large exposed thermal pad should make the
National Semiconductor
Application Note 2026
Lianxi Shen
February 8, 2010
Power Module better in thermal performance than other pack-
age types. For example, LGA packages have a θ_JC of about
5C/W or larger for the similar package size, depending on the
copper and thermal vias in its substrate.
Parametric Study
In order to optimize the PCB design to get the best thermal
performance out of the SIMPLE SWITCHER® Power Module
and to understand the effect of environmental conditions, this
application note analyzes how some factors affect the thermal
performance of a PCB or the θ_JA of a package mounted on it.
These factors include:
1. Size of direct thermal attachment pad
2. Copper layers (2 or 4 layers)
3. PCB size
4. Air flow
5. Heat sink
Figure 1 shows these factors schematically.
For the parametric study, the above factors were varied as
follows:
1. The sizes of copper area on top and bottom layers
include:
Copper Area = DAP size (8.5x5.4mm)
Copper Area = Package body size (10x10mm)
Copper Area = 2 X package body size (20x20mm)
Copper Area = Full copper layer (4 solid copper layers)
2. 2 layer and 4 layer boards
3. The PCB size varies from 4”x3” (102x76mm) to
1.5”x1.5” (38x38mm)
4. The air flow includes Natural Convection, 200LFPM, and
400LFPM
5. The heat sink may be on the package top or on the PCB
bottom side
30112901
FIGURE 1. Thermal Management of the SIMPLE SWITCHER^® Power Module on a 4-Layer PCB
© 2010 National Semiconductor Corporation 301129
www.national.com