Tyco Electronics Corp.

5

Technical Note

August 1997

High-Power Board-Mounted Power Modules

Thermal Mangement for

If an airflow of only 0.75 m/s (150 ft./min.) is available

for the same arrangement as in Example A, determine

length.

From Figure 11,

Detailed Thermal Model

Thermal resistance in the previous section includes

heat transfer by conduction, convection, and radiation

from the entire module to the surrounding environment.

Typically, the power module is soldered to a vertically

oriented PWB as shown in Figure 4. Although most of

the heat is transferred by convection and radiation from

the top mounting surface of the module, significant

amounts of heat are also removed by convection from

the sides of the module, and by conduction to the PWB

and then convection off of the opposite side of the

PWB. Heat flow paths in this configuration are shown in

Figure 12.

8-698(C)

Figure 12. Boundaries Contributing to Thermal

Resistance

ambient

module bottom to ambient

ambient

When a heat sink is mounted to the top surface of a

module,

Several types of heat transfer cool the module. In a

fan-cooled environment, convection is the predominant

mode of heat transfer. Radiation is also important,

especially in natural convection. Conduction is

important when using unusual heat sinks or cold-plate

methods of cooling.

Convection Heat Transfer

Convection heat transfer is a function of the surface

area,

approach to convection cooling over a flat plate leads

to the following relationship:

where:

θ = thermal resistance

c = surface-dependent constant

v = airflow in m/s

Radiation Heat Transfer

Radiation is not dependent upon the airflow over the

module, but on the temperature difference between the

module and the surrounding environment. For a partic-

ular module type,

θ due to radiation can be determined

experimentally. For example, Tyco’s high-power mod-

BMPM WITH

HEAT SINK

R

BLACK BODY

H

I

B

PWB

AIRFLOW

E

θ

c

v

0.5

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