21
EN/LZT 146 033 R1D Ericsson Power Modules, June 2004
PKB 4000 Datasheet
Thermal Consideration
General
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
operation. Heat is removed by conduction, convection and
radiation to the surrounding environment. Increased airflow
enhances the heat transfer via convection. The available
load current vs. ambient air temperature and airflow at Vin
=53 V for each model is according to the information given
under the output section. 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 6 layer PCB with a size of 254 x 254
mm, each layer with 105 m (3 oz) copper. Proper cooling
can be verified by measuring the temperature of selected
devices. Peak temperature can occur at position P1 and
P2. The temperature at this position should not exceed the
recommended max value.
Calculation of ambient temperature
By using the thermal resistance the maximum allowed
ambient temperature can be calculated.
1. The powerloss is calculated by using the formula
((1/
η) - 1) × output power = power losses.
η = efciency of converter. E.g 88% = 0.88
2. Find the value of the thermal resistance for each product in
the diagram by using the airow speed at the output section
of the converter. Take the thermal resistance x powerloss to
get the temperature increase.
3. Max allowed calculated ambient temperature is: Max
TPCB of DC/DC converter – temperature increase.
B. 8.52 W × 5.11 °C/W = 43.54 °C
C. 110 °C - 43.5 °C = max ambient temperature is 66.5 °C
The real temperature will be dependent on several factors,
like PCB size and type, direction of airow, air turbulence
etc. It is recommended to verify the temperature by testing.
A. ((
) - 1) × 62.5W = 8.52 W
1
0.88
E.g PKB 4619 PI at 1m/s:
Position
Device
TC
Recommended
max value
P1
mosfet
Tsurface
120
°C
P2
mosfet
Tsurface
110
°C