JW075M Lineage Power, JW075M Datasheet - Page 11

• Manufacturer Part Number: JW075M
• Manufacturer: Lineage Power

April 2008
Thermal Considerations
Heat Transfer with Heat Sinks
Figure 17. Case-to-Ambient Thermal Resistance
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 17 had a
thermal-conductive dry pad between the case and the
heat sink to minimize contact resistance. The use of
Figure 17 is shown in the following example.
Example
If an 85 °C case temperature is desired, what is the
minimum airflow necessary? Assume the JW100M
module is operating at V
of 20 A, maximum ambient air temperature of 40 °C,
and the heat sink is 1/2 inch.
Solution
Given: V
Determine P
Lineage Power
8
7
6
5
4
3
2
1
0
I
T
T
Heat sink = 1/2 inch
P
0
O
A
C
I
D
= 20 A
= 54 V
= 40 °C
= 85 °C
Curves; Either Orientation
= 14.3 W
D
(100)
by using Figure 16:
0.5
AIR VELOCITY, m/s (ft./min.)
(200)
1.0
I
= 54 V and an output current
dc-dc Converters; 36 to 75 Vdc Input, 1.5 Vdc Output; 15 W to 45 W
(300)
1.5
1 1/2 IN. HEAT SINK
1 IN. HEAT SINK
1/2 IN. HEAT SINK
1/4 IN. HEAT SINK
NO HEAT SINK
(continued)
(400)
2.0
(continued)
(500)
2.5
8-1153 (C)
(600)
3.0
Then solve the following equation:
Use Figure 17 to determine air velocity for the 1/2 inch
heat sink.
The minimum airflow necessary for the JW100M
module is 1.0 m/s (200 ft./min.).
Custom Heat Sinks
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 18).
Figure 18. Resistance from Case-to-Sink and
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:
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.
θ
θ
θ
θ
sa
ca
ca
ca
P
=
D
=
=
=
∅
Sink-to-Ambient
3.1 °C/W
(
------------------------ -
T
(
----------------------- -
(
----------------------- -
T
85 40
C
T
P
14.3
C
–
P
C
D
–
–
T
D
A
T
)
A
)
)
–
θ
cs
θ
cs
T
S
θ
sa
T
A
8-1304 (C)
11