Thermal
20.2.3 Experimental Determination of Junction Temperature
To determine the junction temperature of the device in the application after prototypes are available, use
the thermal characterization parameter (Ψ ) to determine the junction temperature and a measure of the
JT
temperature at the top center of the package case using the following equation:
T = T + (Ψ × P )
J
T
JT
D
where:
T = junction temperature (°C)
J
T = thermocouple temperature on top of package (°C)
T
Ψ
= junction-to-ambient thermal resistance (°C/W)
JT
P = power dissipation in the package (W)
D
The thermal characterization parameter is measured per the 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.
20.2.4 Heat Sinks and Junction-to-Case Thermal Resistance
Some application environments require a heat sink to provide the necessary thermal management of the
device. When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-case
thermal resistance and a case-to-ambient thermal resistance:
R
= R
+ R
θJA
θJC θCA
where:
R
R
R
= junction-to-ambient thermal resistance (°C/W)
= junction-to-case thermal resistance (°C/W)
= case-to-ambient thermal resistance (°C/W)
θJA
θJC
θCA
RθJC is device-related and cannot be influenced by the user. The user controls the thermal environment to
change the case-to-ambient thermal resistance, RθCA. For instance, the user 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.
The thermal performance of devices with heat sinks has been simulated with a few commercially available
heat sinks. The heat sink choice is determined by the application environment (temperature, air flow,
adjacent component power dissipation) and the physical space available. Because there is not a standard
application environment, a standard heat sink is not required.
MPC8347EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 12
Freescale Semiconductor
87
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