Design Considerations
RΘJC = Package junction-to-case thermal resistance (°C/W)
RΘCA = Package case-to-ambient thermal resistance (°C/W)
RΘJC is device related and cannot be adjusted. You control the thermal environment to
change the case to ambient thermal resistance, RΘCA. For instance, you can change the
size of the heat sink, the air flow around the device, the interface material, the mounting
arrangement on printed circuit board, or change the thermal dissipation on the printed
circuit board surrounding the device.
To determine the junction temperature of the device in the application when heat sinks
are not used, the thermal characterization parameter (YJT) can be used to determine the
junction temperature with a measurement of the temperature at the top center of the
package case using the following equation:
TJ = TT + (ΨJT x PD)
Where,
TT = Thermocouple temperature on top of package (°C/W)
ΨJT = hermal characterization parameter (°C/W)
PD = Power dissipation in package (W)
The thermal characterization parameter is measured per JESD51–2 specification using a
40-gauge type T thermocouple epoxied to the top center of the package case. The
thermocouple should be positioned so that the thermocouple junction rests on the
package. A small amount of epoxy is placed over the thermocouple junction and over
about 1 mm of wire extending from the junction. The thermocouple wire is placed flat
against the package case to avoid measurement errors caused by cooling effects of the
thermocouple wire.
When heat sink is used, the junction temperature is determined from a thermocouple
inserted at the interface between the case of the package and the interface material. A
clearance slot or hole is normally required in the heat sink. Minimizing the size of the
clearance is important to minimize the change in thermal performance caused by
removing part of the thermal interface to the heat sink. Because of the experimental
difficulties with this technique, many engineers measure the heat sink temperature and
then back-calculate the case temperature using a separate measurement of the thermal
resistance of the interface. From this case temperature, the junction temperature is
determined from the junction-to-case thermal resistance.
MC56F844xx Advance Information Data Sheet, Rev. 2, 06/2012.
Freescale Semiconductor, Inc.
59
Preliminary
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