JBW050A Power Modules: dc-dc Converter;
36 to 75 Vdc Input, 5 Vdc Output; 50 W
Data Sheet
March 27, 2008
Use Figure 20 to determine air velocity for the 1/2 inch
heat sink.
Thermal Considerations (continued)
Heat Transfer With Heat Sinks (continued)
The minimum airflow necessary for the JBW050A
module is 1.52 m/s (300 ft./min.).
8
Custom Heat Sinks
7
NO HEAT SINK
A more detailed model can be used to determine the
required thermal resistance of a heat sink to provide
necessary cooling. The total module resistance can be
separated into a resistance from case-to-sink (θcs) and
sink-to-ambient (θsa) shown below (Figure 21).
1/4 in. HEAT SINK
1/2 in. HEAT SINK
1 in. HEAT SINK
1 1/2 in. HEAT SINK
6
5
4
3
2
TC
TS
TA
PD
θcs
θsa
1
0
8-1304
Figure 21. Resistance from Case-to-Sink and
Sink-to-Ambient
0
0.25 0.51 0.76 1.02 1.27
1.52 1.78 2.03
(50) (100) (150) (200) (250) (300) (350) (400)
AIR VELOCITY, ms-1 (ft./min.)
For a managed interface using thermal grease or foils,
a value of θcs = 0.1 °C/W to 0.3 °C/W is typical. The
solution for heat sink resistance is:
8-1052.a
Figure 20. Case-to-Ambient Thermal Resistance
Curves; Either Orientation
(TC – TA)
θsa = ------------------------ – θcs
PD
These measured resistances are from heat transfer
from the sides and bottom of the module as well as the
top side with the attached heat sink; therefore, the
case-to-ambient thermal resistances shown are gener-
ally lower than the resistance of the heat sink by itself.
The module used to collect the data in Figure 20 had a
thermal-conductive dry pad between the case and the
heat sink to minimize contact resistance. The use of
Figure 20 is shown in the following example.
This equation assumes that all dissipated power must
be shed by the heat sink. Depending on the user-
defined application environment, a more accurate
model, including heat transfer from the sides and bot-
tom of the module, can be used. This equation pro-
vides a conservative estimate for such instances.
Solder, Cleaning, and Drying
Considerations
Example
If an 85 °C case temperature is desired, what is the
minimum airflow necessary? Assume the JBW050A
module is operating at VI = 54 V and an output current
of 10 A, maximum ambient air temperature of 70 °C,
and the heat sink is 1/2 inch.
Post solder cleaning is usually the final circuit-board
assembly process prior to electrical testing. The result
of inadequate circuit-board cleaning and drying can
affect both the reliability of a power module and the
testability of the finished circuit-board assembly. For
guidance on appropriate soldering, cleaning, and dry-
ing procedures, refer to the Board-Mounted Power
Modules Soldering and Cleaning Application Note
(AP01-056EPS).
Solution
Given: VI = 54 V
IO = 10 A
TA = 70 °C
TC = 85 °C
Heat sink = 1/2 in.
EMC Considerations
For assistance with designing for EMC compliance,
please refer to the FLTR100V10 data sheet
(FDS01-043EPS).
Determine PD by using Figure 18:
PD = 9.5 W
Then solve the following equation:
Layout Considerations
T
C – TA
θca = -------------------
PD
Copper paths must not be routed beneath the power
module mounting inserts. For additional layout guide-
lines, refer to the FLTR100V10 data sheet
(FDS01-043EPS).
85 – 70
θca = -----------------
9.5
·
θca = 1.58 °C/W
12
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