JW050F, JW075F, JW100F, JW150F Power Modules:
dc-dc Converters; 36 to 75 Vdc Input, 3.3 Vdc Output; 33 W to 99 W
Data Sheet
April 2008
Example
Thermal Considerations (continued)
Heat Transfer with Heat Sinks (continued)
If an 85 °C case temperature is desired, what is the
minimum airflow necessary? Assume the JW100F
module is operating at VI = 54 V and an output current
of 20 A, maximum ambient air temperature of 40 °C,
and the heat sink is 1/2 inch.
Thermal derating with heat sinks is expressed by using
the overall thermal resistance of the module. Total
module thermal resistance (θca) is defined as the max-
imum case temperature rise (ΔTC, max) divided by the
module power dissipation (PD):
Solution
Given: VI = 54 V
IO = 20 A
(TC – TA)
ΔTC, max
θca = --------------------
=
------------------------
PD
PD
TA = 40 °C
The location to measure case temperature (TC) is
shown in Figure 26. Case-to-ambient thermal resis-
tance vs. airflow is shown, for various heat sink config-
urations and heights, in Figure 32. These curves were
obtained by experimental testing of heat sinks, which
are offered in the product catalog.
TC = 85 °C
Heat sink = 1/2 in.
Determine PD by using Figure 29:
PD = 15.8 W
Then solve the following equation:
8
(TC – TA)
θca = ------------------------
1 1/2 IN. HEAT SINK
7
PD
1 IN. HEAT SINK
1/2 IN. HEAT SINK
1/4 IN. HEAT SINK
6
5
4
3
2
1
0
(85 – 40)
-----------------------
θca =
15.8
NO HEAT SINK
θca = 2.8 °C/W
Use Figure 32 to determine air velocity for the 1/2 inch
heat sink.
The minimum airflow necessary for the JW100F
module is 1.1 m/s (220 ft./min.).
0
0.5
1.0
1.5
2.0
2.5
3.0
(100)
(200)
(300)
(400)
(500)
(600)
AIR VELOCITY, m/s (ft./min.)
Custom Heat Sinks
8-1153 (C)
Figure 32. Case-to-Ambient Thermal Resistance
Curves; Either Orientation
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 33).
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 32 had a
thermal-conductive dry pad between the case and the
heat sink to minimize contact resistance. The use of
Figure 32 is shown in the following example.
T
C
T
S
TA
∅
D
P
θ
cs
θsa
8-1304 (C)
Figure 33. Resistance from Case-to-Sink and
Sink-to-Ambient
Lineage Power
15
LINEAGEPOWER [ LINEAGE POWER CORPORATION ]
