March 27, 2008
QBK025A0B DC-DC Power Modules:
36-55Vdc Input; 12Vdc Output Voltage; 25A Output Current
The power modules operate in a variety of thermal
environments and sufficient cooling should be
provided to help ensure reliable operation.
Thermal considerations include ambient temperature,
airflow, module power dissipation, and the need for
increased reliability. A reduction in the operating
temperature of the module will result in an increase in
reliability. The thermal data presented here is based
on physical measurements taken in a wind tunnel.
Heat-dissipating components are mounted on the top
side of the module. Heat is removed by conduction,
convection and radiation to the surrounding
environment. Proper cooling can be verified by
measuring the thermal reference temperature (TH).
Peak temperature (TH) occurs at the position indicated
in Figure 13. For reliable operation this temperature
should not exceed the listed temperature threshold.
Figure 13. Location of the thermal reference
The output power of the module should not exceed
the rated power for the module as listed in the
Ordering Information table.
Although the maximum TH temperature of the power
modules is 110 °C - 115 °C, you can limit this
temperature to a lower value for extremely high
Please refer to the Application Note “Thermal
Characterization Process For Open-Frame Board-
Mounted Power Modules” for a detailed discussion of
thermal aspects including maximum device
Heat Transfer via Convection
Increased airflow over the module enhances the heat
transfer via convection. The thermal derating figures
(14-16) show the maximum output current that can be
delivered by each module in the respective orientation
without exceeding the maximum TH temperature
versus local ambient temperature (TA) for air flows of 1
m/s (200 ft./min) and 2m/s (400 ft./min).
Note that the natural convection condition was
measured at 0.05 m/s to 0.1 m/s (10ft./min. to 20
ft./min.); however, systems in which these power
modules may be used typically generate natural
convection airflow rates of 0.3 m/s (60 ft./min.) due to
other heat dissipating components in the system. The
use of Figures 14 - 15 are shown in the following
What is the minimum airflow necessary for a
QBK025A0B operating at VI = 48 V, an output current
of 12A, and a maximum ambient temperature of 70 °C
in transverse orientation.
Given: VI = 48V, Io = 12A, TA = 70 °C
Determine required airflow (V) (Use Figure 14):
V = T1 m/sec. ( 200 ft./min.) or greater.
LOCAL AMBIENT TEMPERATURE, TA (°C)
Figure 14. Output Current Derating for the
QBK025A0B in the Transverse Orientation with no
baseplate; Airflow Direction from Vin(+) to Vin(-); Vin