1.8.6.2 Thermal Management Example
For preliminary heat sink sizing, the die-junction temperature can be expressed as follows:
T = T + T + (R +R + R ) * Q
j
a
r
jc
q
a
sa
Where:
T is the die-junction temperature
j
T is the inlet cabinet ambient temperature
a
T is the air temperature rise within the system cabinet
r
R
is the die-junction-to-top of die thermal resistance of the device
jc
q
R is the thermal resistance of the thermal interface material (thermal grease or thermal
a
compound)
R
is the heat sink-to-ambient thermal resistance
sa
Q is the power consumed by the device
Typical die-junction temperatures (T ) should be maintained less than 105 °C. The temperature of
j
the air cooling the component greatly depends upon the ambient inlet air temperature and the air
temperature rise within the computer cabinet. A computer cabinet inlet-air temperature (T ) may
a
range from 30 to 40 °C. The air temperature rise within a cabinet (T ) may be in the range of 5 to
r
10 °C. The thermal resistance of the interface material (R ) is typically about 1 °C/W. Assuming a
a
T of 30 °C, a T of 5 °C, and a power consumption (Q) of 7.5 watts, the following expression for
a
r
T is obtained:
j
Junction temperature: T = 30 °C + 5 °C + (0.03 °C/W + 1.0 °C/W + R ) * 7.5 W
j
sa
For a Thermalloy heat sink #2333B, the heat sink-to-ambient thermal resistance (R ) versus
sa
airflow velocity is shown in Figure 14.
8
Thermalloy #2328B Pin-fin Heat Sink
7
(25 x28 x 15 mm)
6
5
4
3
2
1
0
0.5
1
1.5
2
2.5
3
3.5
Approach Air Velocity (m/s)
26
PID9q-604e Hardware Datasheet
IBM
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
IBM [ IBM ]
