Derating

Derating is the technique employed in power electrical and electronic devices wherein the devices are operated at less than their rated maximum power dissipation taking into consideration the case/body temperature, ambient temperature and the type of cooling mechanism used.

In electronics

Derating curve of a hypothetical power device.

Power electronic devices have a maximum power dissipation rating usually quoted at a case temperature of 25°C. The datasheet for the device also includes a derating curve which indicates how much a device will dissipate without getting damaged at any given case temperature and this must be taken into account while designing a system. As can be seen from the derating curve image for a hypothetical BJT, the device (rated for 100 W at 25°C) cannot be expected to dissipate anything more than about 40 W if the ambient temperature is such that the temperature at which the device's case will stabilise (after heat-sinking) is 65°C. This final case temperature is a function of the thermal resistance between the device's case and the heat-sink; and the heat-sink and the ambient (this includes the heat-sink's temp/watt rating - with lower values implying better cooling characteristics).

In electrical installations

All dimmers rely on heat conduction and convection to keep the electronic components cool. Derating is the reduction of the maximum capacity (load) a unit can reliably handle when fins/side sections are removed.

References

Datasheet of power transistor MJL0281A on OnSemi (PDF)

See also

• Safe operating area