Thermal Calculation and Measurement
Junction Temperature Rise Above
Ambient Divided by Package Power
Board Temperature Rise Above Ambient Divided by Package Power
Figure 2. Effect of Board Temperature Rise on Thermal Behavior
If the board temperature is known, an estimate of the junction temperature in the environment can be made
using the following equation:
T
J
= T
B
+ (R
θJB
×
P
D
)
where:
R
θJB
= junction-to-board thermal resistance (ºC/W)
T
B
= board temperature (ºC)
P
D
= power dissipation in package
If the board temperature is known and the heat loss from the package case to the air can be ignored,
acceptable predictions of junction temperature can be made. For this method to work, the board and board
mounting must be similar to the test board used to determine the junction-to-board thermal resistance,
namely a 2s2p (board with a power and a ground plane) and by attaching the thermal balls to the ground
plane.
7.4
Estimation Using Simulation
When the board temperature is not known, a thermal simulation of the application is needed. The simple
two-resistor model can be used with the thermal simulation of the application [2], or a more accurate and
complex model of the package can be used in the thermal simulation.
7.5
Experimental Determination
To determine the junction temperature of the device in the application after prototypes are available, the
thermal characterization parameter (Ψ
JT
) 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:
T
J
= T
T
+ (Ψ
JT
×
P
D
)
MPC860 PowerQUICC™ Family Hardware Specifications, Rev. 8
Freescale Semiconductor
13
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
