Technical Note
July 1996
Thermal Management for FC- and FW-Series
250 W—300 W 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.
Maintaining the operating case temperature (Tc)
within the specified range keeps internal component
temperatures within their specifications. This, in turn,
helps keep the expected mean time between failures
(MTBF) from falling below the specified rating.
Tyco's FC- and FW- Series 250 W to 300 W BMPMs
are designed with high efficiency as a primary goal.
The 5 V output units have typical full load efficiencies
of 83%, which result in less heat dissipation and
lower operating temperatures. Also, these modules
use temperature resistant components, such as
ceramic capacitors, that do not exhibit wearout
behavior during prolonged exposure to high tempera-
tures, as do aluminum electrolytic capacitors.
This application note provides the necessary infor-
mation to verify that adequate cooling is present in a
given operating environment. This information is
applicable to all Tyco 250 W to 300 W BMPMs in the
4.6 in. x 2.4 in. x 0.5 in. package.
Basic Thermal Management
Proper cooling can be verified by measuring the case
temperature of the module (Tc) at the location indi-
cated in Figure 1. Note that the view in Figure 1 is of
the metal surface of the module (the pin locations
shown are for reference). Tc must not exceed 100 °C
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 as close as possible to
the indicated position, taking into account the contact
resistance between the mounting surface and the
heat sink (see Heat Sink section).
V
I
(+)
V
I
(–)
ON/OFF
SYNC IN
1.20 (30.5)
SYNC OUT
CASE
MEASURE CASE
TEMPERATURE HERE
V
O
(+)
V
O
(–)
3.25 (82.6)
8-1303a
Figure 1. Case Temperature Measurement (Metal
Side)
While this is a valid method of checking for proper
thermal management, it it is only usable if the final
system configuration exists and can be used as a
test environment. The graphs on the accompanying
pages provide guidelines to predict the thermal per-
formance of the module for typical configurations that
include heat sinks in natural or forced airflow environ-
ments. However, due to differences between the test
setup and the final system environment, the module
case temperature must always be checked in the
final system configuration to verify proper operation.
Thermal Management for FC- and FW-Series
250 W—300 W Board-Mounted Power Modules
Technical Note
July 1996
Basic Thermal Management
(continued)
The goal of thermal management is to transfer the heat
dissipated by the module to the surrounding environ-
ment. The amount of power dissipated by the module
as heat (P
D
) is the difference between the input power
(P
I
) and the output power (Po) as shown by the equa-
tion below:
P
D
= P
I
– Po
Also, module efficiency (
η
) is defined as the ratio of
output power to input power as shown by the equation
below:
η
= Po / P
I
The input power term can be eliminated by the combi-
nation of these two equations to yield the equation
below:
P
D
= Po (1 –
η
) /
η
This equation can be used to calculate the module
power dissipation. However, efficiency is a nonlinear
function of the module input voltage (V
I
) and output
current (Io). Typically, a plot of power dissipation versus
output current over three different line voltages is given
in each module-specific data sheet. This is because
each module has a different power dissipation curve. A
typical curve of this type is shown below in Figure 2 for
a FW300A1 Power Module (5 V output voltage).
70
Module Derating
Experimental Setup
The derating curves in the following figures were
obtained from measurements obtained in an experi-
mental apparatus shown in Figure 3. Note that the
module and the printed-wiring board (PWB) onto which
it was mounted were vertically oriented. The passage
has a rectangular cross-section. The clearance
between the top of the module and the facing PWB was
kept constant at 0.5 in.
PWB
FACING
PWB
POWER DISSIPATION, P
D
(W)
60
50
40
30
20
10
0
0
10
20
30
40
50
60
OUTPUT CURRENT, I
O
(A)
8-1313
AIR VELOCITY
AND AMBIENT
TEMPERATURE
MEASURED
HERE
V = 72 V
V = 54 V
V = 36 V
AIRFLOW
8-690a
Figure 3. Experimental Test Setup
Figure 2. FW300A1 Power Dissipation vs. Output
Current
2
Tyco Electronics Corp.
Technical Note
July 1996
Thermal Management for FC- and FW-Series
250 W—300 W Board-Mounted Power Modules
Module Derating
(continued)
70
POWER DISSIPATION, P
D
(W)
Convection Without Heat Sinks
Increasing airflow over the module enhances heat
transfer via convection. Figures 4 and 5 show the maxi-
mum power that can be dissipated by the module with-
out exceeding the maximum case temperature versus
local ambient temperature (T
A
) for natural convection
through 800 ft./min. A natural convection condition is
produced when air is moved only through the buoyancy
effects produced by a temperature gradient between
the module and surrounding air. In the test setup used,
natural convection airflow was measured at 10 ft./min.
to 20 ft./min., whereas systems in which these power
modules may be used typically generate natural con-
vection airflow rates of 60 ft./min. due to other heat dis-
sipating components in the system. The 100 ft./min. to
800 ft./min. curves are for airflow added externally to
the test setup, usually through the use of fans. Note
that there is a thermal performance improvement when
the long axis of the module is perpendicular to the air-
flow direction (transverse orientation).
70
POWER DISSIPATION, P
D
(W)
60
50
40
30
20
10
20 ft./min. (NAT. CONV.)
0
0
10
20
30
40
50
60
800 ft./min.
700 ft./min.
600 ft./min.
500 ft./min.
400 ft./min.
300 ft./min.
200 ft./min.
100 ft./min.
70
80
90 100
LOCAL AMBIENT TEMPERATURE, T
A
(°C)
8-1315
Figure 5. Convection Power Derating with No
Heat Sink; Airflow Along Width (Trans-
verse)
Figures 4 and 5 can be used to determine the appropri-
ate airflow for a given set of operating conditions as
shown in the following examples.
Example 1: Airflow Required to Maintain Tc
What is the minimum airflow necessary for a FW300A1
in the transverse orientation, operating at 54 V input, an
output current of 50 A, and a maximum ambient tem-
perature of 35 °C?
Solution:
Given: V
I
= 54 V, Io = 50 A, T
A
= 35 °C
Determine P
D
(Figure 2): P
D
= 46 W
Determine Airflow (Figure 5): v = 800 ft./min.
60
50
40
30
20
10
20 ft./min. (NAT. CONV.)
0
0
10
20
30
40
50
60
800 ft./min.
700 ft./min.
600 ft./min.
500 ft./min.
400 ft./min.
300 ft./min.
200 ft./min.
100 ft./min.
70
80
90 100
Example 2: Maximum Power Output
What is the maximum power output for a FW300A1 in
the longitudinal orientation, operating at 54 V input, in
an environment that provides 600 ft./min. with a maxi-
mum ambient temperature of 40 °C?
Solution:
Given: V
I
= 54 V, v = 600 ft./min., T
A
= 40 °C
Determine P
D
(Figure 4): P
D
= 34 W
Determine Io (Figure 2): Io = 40 A
Calculate Po = (Vo) * (Io) = 5 x 40 = 200 W
Although the above two examples use 100 °C as the
operating case temperature, for extremely high reliabil-
ity applications, one may design to a lower case tem-
perature as shown later in Example 4.
LOCAL AMBIENT TEMPERATURE, T
A
(°C)
8-1314
Figure 4. Convection Power Derating with No
Heat Sink; Airflow Along Length (Lon-
gitudinal)
Tyco Electronics Corp.
3
Thermal Management for FC- and FW-Series
250 W—300 W Board-Mounted Power Modules
Technical Note
July 1996
Module Derating
(continued)
Heat Sink Configuration
Several standard heat sinks are available for the FC- and FW-Series 250 W—300 W BMPMs, as shown in Figures
6 and 7. The heat sinks mount to the top surface of the module with M3 x 0.5 screws torqued to 5 in.-lb. (0.56 N-m).
Placing a thermally conductive dry pad or thermal grease between the case and the heat sink minimizes contact
resistance (typically 0.1 °C/W to 0.3 °C/W) and temperature drop. All heat sink curve data taken had such a dry
pad present.
1/4 IN. (MHSL02555)
1/2 IN. (MHSL05055)
1 IN. (MHSL10055)
4.56
1 1/2 IN. (MHSL15055)
2.36
8-1316
Figure 6. Heat Sinks with Longitudinal Fins
4
Tyco Electronics Corp.
Technical Note
July 1996
Thermal Management for FC- and FW-Series
250 W—300 W Board-Mounted Power Modules
Module Derating
(continued)
Heat Sink Configuration
(continued)
1/4 IN. (MHST02555)
2.36
1/2 IN. (MHST05065)
4.56
1 IN. (MHST10055)
1 1/2 IN. (MHST15055)
8-1317
Figure 7. Heat Sinks with Transverse Fins
Nomenclature for this family of heat sinks is as follows:
MHSxyyy55
where:
x = fin orientation; longitudinal (L) or transverse (T)
yyy = heat sink height (in 100ths of inch)
For example, MHST10055 is a heat sink that is transverse mounted (see Figure 7) for a 4.6 in. x 2.4 in. module
with a heat sink height of 1 in. The “M” prefix represents a heat sink kit with metric hardware.
Tyco Electronics Corp.
5
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