Field effect transistors (FETs) are found in both analogue and digital circuits. The most common type of FET is the metal-oxide semiconductor field effect transistor (MOSFET), which has an insulated gate electrode. This feature prevents static current from flowing through the device, which in turn minimises power consumption. In some applications, MOSFETs only dissipate significant power when they are "switching," or transitioning from the "on" state to the "off" state. In other applications, such as high-power circuits, the on-resistance of a MOSFET can dissipate large amounts of static power.
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Determine the current that is passing through the channel of the FET. This is usually referred to as the "drain current." If you have an accurate diagram for the circuit, you can use circuit analysis to find the drain current. Otherwise, measure the drain current by placing the leads of an ammeter in series with the drain.
Look in the FET's data sheet to find its on-resistance. In high-power applications, the gate of the FET is driven so the device is fully turned on. This minimises the resistance of the channel, making the FET act more like a mechanical switch. However, the on-resistance is not zero. The data sheet will provide values, and maybe even a graph, to indicate the on-resistance of the FET under your particular circuit conditions.
Square the drain current, and multiply this value by the on-resistance: Power = (drain current)^2 x (on-resistance).
Determine if your FET is primarily used in a CMOS arrangement. This is generally the case when the FET is part of a digital circuit.
Write down the supply voltage of your digital circuit and the frequency at which the circuit typically operates.
Refer to the data sheet for your FET and write down the gate capacitance. This capacitance is also the load capacitance, because the output of one FET usually drives the gate of another FET.
Calculate the dynamic power consumption of the FET using the following formula: Power = (load capacitance) x (supply voltage)^2 x (frequency). Note that this formula gives the power for an entire "inverter," which is a combination of two FETs, because almost every FET in a digital circuit is part of an inverter or of some similar combination of FETs.
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