RATING, SAFE OPERATION AREA AND POWER HANDLING CAPABILITY OF DEVICES

If the modern power control engineer is to make the correct choice of switching device for a given application it is necessary to be aware of the characteristics and limitations of the devices that are currently available. The obvious limitations to any device are its maximum voltage and current ratings, which must not be exceeded. It is hence essential to have some knowledge of device construction and fabrication.

Power handling capability (PH)

The maximum power handling capability PHmax of a semiconductor switch is related to the product of Vbus and Imax. For any semiconductor device the maximum voltage to which the device can be subjected is related to the avalanche breakdown value of the silicon p–n junction, while the maximum device current is limited to the chip current density.The maximum current density is affected by various factors, including temperature and mechanical stress.

Both the maximum voltage and maximum allowable current are affected by the impurity levels present in the chip. Moreover, they are affected inversely in such a way that any increase of one parameter will result in reduction of the other. When endeavours are made to increase a device power handling capability, limiting factors arise as with most engineering problems. A compromise has to be made as invariably the load current flows through the same junctions that have to withstand the supply voltages without unintentional avalanching.