
14
Lineage Power
18 Vdc to 36 Vdc Input, 3.3 Vdc Output; 165 W
FC250F1 Power Module: dc-dc Converter;
Data Sheet
April 2008
Thermal Considerations (continued)
8-1321 (C)
Figure 21. Case-to-Ambient Thermal Resistance
Curves; Transverse Orientation
8-1320 (C)
Figure 22. Case-to-Ambient Thermal Resistance
Curves; Longitudinal Orientation
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.
22 had a thermal-conductive dry pad between the case
and the heat sink to minimize contact resistance.
To choose a heat sink, determine the power dissipated
as heat by the unit for the particular application.
Figure 23 shows typical heat dissipation for a range of
output currents and three voltages for the FC250F1.
8-2885 (F)
Figure 23. FC250F1 Power Dissipation vs. Output
Current at 25 °C
Example
If an 85 °C case temperature is desired, what is the
minimum airow necessary? Assume the FC250F1
module is operating at VI = 28 V and an output current
of 40 A, maximum ambient air temperature of 40 °C,
and the heat sink is 1.0 inch.
Solution
Given: VI = 28 V
IO = 40 A
TA = 40 °C
TC = 85 °C
Heat sink = 1.0 inch
PD = 35 W
Then solve the following equation:
0.0
0.5
3.0
3.5
4.0
4.5
2.5
2.0
1.0
1 1/2 IN. HEAT SINK
1 IN. HEAT SINK
1/2 IN. HEAT SINK
1/4 IN. HEAT SINK
NO HEAT SINK
1.5
AIR VELOCITY, m/s (ft./min.)
0
0.5
(100)
1.0
(200)
1.5
(300)
2.0
(400)
2.5
(500)
3.0
(600)
CASE-TO-AMBIENT
THERMAL
RESISTANCE,
θCA
(
°C/W)
0
0.5
(100)
1.0
(200)
1.5
(300)
2.0
(400)
2.5
(500)
3.0
(600)
0.0
0.5
3.0
3.5
4.0
4.5
2.5
2.0
1.0
1 1/2 IN. HEAT SINK
1 IN. HEAT SINK
1/2 IN. HEAT SINK
1/4 IN. HEAT SINK
NO HEAT SINK
1.5
AIR VELOCITY, m/s (ft./min.)
CASE-TO-AMBIENT
THERMAL
RESISTANCE,
θCA
(
°C/W)
50
45
0
OUTPUT CURRENT, IO (A)
POWER
DISSIPATION,
P
D
(W)
40
35
30
25
0
5
101520253035
50
20
15
10
5
40
45
VI = 18 V
VI = 28 V
VI = 36 V
θca
TC
TA
–
()
PD
------------------------
=
θca
85
40
–
()
35
------------------------
=
θca 1.3 °C/W
=