Thermal Consideration
General
Calculation of ambient temperature
The PKB 4000 series DC/DC converters are designed to operate
in a variety of thermal environments, however sufficient cooling
should be provided to help ensure reliable operationꢀ Heat
is removed by conduction, convection and radiation to the
surrounding environmentꢀ Increased airflow enhances the heat
transfer via convectionꢀ The available load current vsꢀ ambient air
temperature and airflow at Vin =53 V for each model is according
to the information given under the output sectionꢀ The test is
done in a wind tunnel with a cross section of 305 x 305 mm, the
DC/DC converter vertically mounted on a 8 layer PCB with a size
of 254 x 254 mm, each layer with 35 µm (1 oz) copperꢀ Proper
cooling can be verified by measuring the temperature of selected
devicesꢀ Peak temperature can occur at positions P1, P2 and
P3ꢀ The temperature at these positions should not exceed the
recommended max valuesꢀ
By using the thermal resistance the maximum allowed
ambient temperature can be calculatedꢀ
1ꢀ The powerloss is calculated by using the formula
((1/η) - 1) × output power = power lossesꢀ
η = efficiency of converterꢀ Eꢀg 89ꢀ5% = 0ꢀ895
2ꢀ Find the value of the thermal resistance for each product in the
diagram by using the airflow speed at the output section of the con-
verterꢀ Take the thermal resistance x powerloss to get the temperature
increaseꢀ
3ꢀ Max allowed calculated ambient temperature is: Max
TPCB of DC/DC converter – temperature increaseꢀ
Eꢀg PKB 4610 PINB at 1m/s:
1
Note that the recommended max value is the absolute maximum
rating (non destruction) and that the electrical output data is
guaranteed up to TPcb +90 °Cꢀ
Aꢀ ((
) - 1) × 66 W = 7ꢀ74 W
0ꢀ895
Bꢀ 7ꢀ74 W × 5ꢀ0 °C/W = 38ꢀ7 °C
Cꢀ 110 °C - 38ꢀ7 °C = max ambient temperature is 71ꢀ3 °C
The real temperature will be dependent on several factors, like PCB
size and type, direction of airflow, air turbulence etcꢀ It is recommen-
ded to verify the temperature by testingꢀ
Position
Device
Tcritical
Recommended
max value
P1
P2
P3
Pcb
110 °C
120 °C
120 °C
mosfet
mosfet
Tsurface
Tsurface
Output side
P1
P2
AIRFLOW
P3
Input side
EN/LZT 146 033 R9A ©Ericsson Power Modules, March 2007
40
PKB 4000 Datasheet
ERICSSON [ ERICSSON ]
