Instruction Manual

Design Guide & Applications Manual
F
or Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
Maxi, Mini, Micro Design Guide Rev 4.9 vicorpower.com
Page 29 of 88
Apps. Eng. 800 927.9474 800 735.6200
6. Thermal Performance Information
EXAMPLE 1
D
etermine the maximum output power for a Maxi module without a heat sink delivering 5 V in 400 LFM
airflow at a maximum ambient temperature of 40ºC.
Maximum output power = (T
b
m
ax
– Ta
m
ax
)/[θba • (1/η – 1)]
Tb
m
ax
= 100ºC
Ta
max
= 40ºC
For Maxi module without a heat sink @ 400 LFM, θba = 2.17ºC/W
For the 5 V Maxi module the typical value for η = 0.83
Maximum output power = (100 – 40) / [2.17 (1/0.83 – 1)] ~135 W
Or, the same answer could be obtained by using the output power versus ambient temperature derating
curves for the Maxi modules.
For the case with no heat sink the baseplate chart for the 5 V module would be used. At a 40ºC ambient
and 400 LFM airflow this chart indicates a maximum output power of approximately 135 W.
For full output power of 400 W the required thermal resistance is;
θba = (100 – 40) / [400 (1/0.83 – 1)] = 0.73ºC/W
What size heat sink would be necessary to operate at full output power (400 W) under the same conditions?
From the θba versus airflow charts for the Maxi, the thermal resistance at 400 LFM airflow requires the use of
a 0.9"(22,8 mm) transverse fin heat sink.
EXAMPLE 2
Determine the maximum ambient for a Mini module with a 0.9” (22,8 mm) heat sink in 400 LFM of airflow
delivering 200 W at 5 V.
From the output power versus ambient temperature chart for the 5 Vout Mini with a 0.9’’ (22,8 mm) heat
sink, the 200 W at 400 LFM data point results in a Ta
max
of approximately 48ºC.
Typical Examples — Thermal Equations (Maxi, Mini, Micro)
5V Mini Thermal Performance Curves - .9" Heat Sink
0
20
40
60
80
100
120
140
160
180
200
0 5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
100
Ambient Temperature (deg C)
Output Power (Watts)
5 V Mini with
0.9” (22,8 mm)
heat sink