Maximum Power Dissipation (Power Transistors)

Maximum Power Dissipation

High currents and voltages in power devices produce very high internal power loss. This loss occurs in the form of heat that must be dissipated; otherwise, the device can be destroyed as a result of overheating.

 

The maximum power dissipation P_max indicates a device's maximum capability to transfer and conduct this power loss without overheating.
The above figure compares the power dissipation of different devices. As a device type is selected, the circular area changes to show the device's maximum power dissipation. As illustrated in the figure, larger metal cases are more efficient in removing heat from the transistor.

Thermal Resistance

Thermal resistance R_thj-a indicates the ability of the device to conduct and transfer heat from the junction (with temperature T_j) to ambient (with temperature T_a). More resistance means that less heat will dissipate.
The junction-to-ambient thermal resistance R_thj-a consists of two resistances: R_thj-c and R_thc-a in series. The R_thj-c is the junction-to-case thermal resistance. The R_thc-a is the case-to-ambient thermal resistance. The R_thc-a is many times higher than R_thj-c.
The above figure shows the values of R_thj-a for different packages. The particular value of each resistance depends on the type of materials and on the case size. Smaller values are typical for large metal cases.

Maximum Power Dissipation vs. Temperature

 

The maximum power dissipation P_max of the transistor depends on the highest junction temperature that will not destroy the device T_jmax, the ambient temperature T_a, and the thermal resistance R_thj-a according to the equation shown in the above figure.
If the ambient temperature is less than or equal to 25°C the device reaches its maximum specified power rating. When the ambient temperature increases, the power rating decreases. If the ambient temperature T_a reaches the maximum junction temperature T_jmax, maximum power P_max becomes zero.

Heat Sink

One way to increase the power rating of the device is to diminish the thermal resistance R_thj-a. A heat sink, which is usually a metal construction with a large surface area, is used to allow heat to dissipate to ambient more easily.
When a heat sink is present, the global thermal resistance R_thj-a decreases because there are more paths available for heat dissipation. The case-to-heat-sink thermal resistance R_thc-s and the heat-sink-to-ambient thermal resistance R_ths-a both facilitate heat dissipation. As a result, the power rating increases as illustrated in the above figure.