Technical Note

August 1997

Thermal Management for

High-Power Board-Mounted Power Modules

Introduction

Board-mounted power modules (BMPMs) enhance

the capabilities of advanced computer and communi-

cations systems by providing flexible power architec-

tures; however, proper cooling of the power modules is

required for reliable and consistent operation. Main-

specified range keeps internal-component tempera-

tures within their specifications, which, in turn, helps

keep the expected mean time between failures

(MTBF) from falling below the specified rating.

Tyco's FC-, FE-, and FW-Series 50 W to 200 W

BMPMs are designed with high efficiency as a pri-

mary goal. The 5 V output units have typical full-load

efficiencies greater than 80%, which result in less

heat dissipation and lower module case tempera-

tures. Furthermore, these modules use temperature-

resistant components, such as ceramic capacitors,

that do not degrade during prolonged exposure to

high temperatures, as do aluminum electrolytic

capacitors.

This application note provides the information to ver-

ify that adequate cooling is present in a given operat-

ing environment. A thermal model is included that

can be used to design heat sinks and other means

for meeting the cooling and reliability requirements

for a given application. The information can be

applied to all Tyco high-power BMPMs in the

121.9 mm x 63.5 mm x 12.7 mm (4.8 in. x 2.5 in. x

0.5 in.) package.

Basic Thermal Management

must not exceed 95 °C (85 °C for 200 W modules)

while operating in the final system configuration.

After the module has reached thermal equilibrium,

the measurement can be made with a thermocouple

or surface probe. If a heat sink is mounted to the

case, make the measurement on the base of the heat

sink as close as possible to the indicated position,

taking into account the contact resistance between

the mounting surface and the heat sink (see Detailed

Thermal Model section).

8-582(C).c

Note: Top view, pin locations are for reference.

Dimensions shown in millimeters and (inches).

Figure 1. Case Temperature Measurement

While this is a valid method to check for proper ther-

mal management, it makes the assumption that the

final system configuration exists and can be used for

a test environment or can be modeled. The following

graphs provide guidelines to predict the thermal

performance of the module for typical configurations

that include heat sinks in natural or forced-airflow

environments.

The goal of thermal management is to transfer the

heat dissipated by the module to the surrounding

environment. The amount of power dissipated by the

the equation below:

Also, module efficiency (

η) is defined as the ratio of

output power to input power, shown by the following

equation:

The input power term can be eliminated by combin-

ing these two equations. They yield the equation

below, which can be used to calculate module power

dissipation:

PARALLEL

+

SENSE

–

+

OUT

–

CASE

ON/OFF

+

–

IN

MEASURE CASE

TEMPERATURE HERE

18 (0.7)

76 (3.0)

FE150A

DC-DC Power Module

IN:DC 48V, 3.7A OUT:DC 5V, 30A

150W

MADE IN USA

Lucent

Protected by U.S. Patents: 5,036,452 5,179,365

TUV Rheinland

6238

η

-------

=

η

–

()

η

-------------------------

=