Junction to Case Temperature ΘJc
The junction to case thermal resistance
parameter, qJC, is defined as
the ratio of the difference in temperature from the die junction to
the case and the power dissipated by the die. qJC
is expressed in units of °C/W.
qJC is not dependant on the case to the ambient thermal resistance as is the qJA parameter. qJC is a better indicator of thermal resistance when the package is to be thermally sinked to other components in the assembly.
qJC is listed in the data sheet for different packages. It is useful for evaluating which package is least likely to overheat and to determine what the die temperature is when the case temperature and power dissipation are known.
The following equation more fully defines the math of heat dissipation.
TA(C°)=Temperature of Ambient Air
TJ(C°)=Temperature of Semiconductor Junction
PD(W)=Power Disipated in Semiconductor
qJC(°C/W)=Thermal Resistance (Junction to Case)
qCH(°C/W)=Thermal Resistance (Case to Heat Sink)
qHA(°C/W)=Thermal Resistance (Seat Sink to Ambient Air)
qJA(°C/W)=Thermal Resistance (Junction to Ambient Air)
The design of a heat sink is based on empirical measurements made
by the heat sink manufacturer, and empirical measurements made
within the enclosure of the final system. The actual math is
straightforward. It looks like a simple electrical circuit. Take
temperature to be the equivalent of voltage, thermal resistance is
now just a simple resistor and the power is considered the same as
current. Now this looks like a linear circuit, and in fact the
resistances add linearly.
The figure and chart above compares the performance of two different heat sinks, each at two different power levels. A place to start is at ground or at the ambient temperature. Since the temperature within an enclosure or in some environmental areas can range widely, the expected maximum temperature should be used for TA.
The power is multiplied by the heat sink thermal resistance and added to the ambient temperature to find the temperature at the interface between the heat sink and the case. Each step is the same as this one, finding the temperature at the next point.
Now, the power has to pass through the mounting resistance of either 0.1 or 0.5°C/W. The 10W dissipation can either result in 37°C junction temperature or 158°C junction temperature. This points out the advantage of using a highly-efficient heat sink to get the heat out of the device and into the air.
Installing a fan on the heat sink will significantly increase its effectiveness. For example, nearly all PCs use a fan attached to the processor's heat sink for this reason.