There are two types of FETs: N-channel and P-channel. In power systems, FETs can be seen as electrical switches. When a positive voltage is applied between the gate and source of the n-channel FET, its switch is turned on. When turned on, current can flow from the drain to the source through the switch. There is an internal resistance between the drain and source, called the on-resistance RDS(ON). It must be clear that the gate of the FET is a high impedance terminal, therefore, a voltage must always be applied to the gate. If the gate is left floating, the device will not work as designed and may turn on or off at inappropriate times, resulting in potential power losses in the system. When the voltage between the source and gate is zero, the switch turns off and current stops flowing through the device. Although the device has been turned off at this time, there is still a small current, which is called leakage current, or IDSS.

Channel selection. The first step in choosing the right device for a design is deciding whether to use an N-channel or P-channel FET. In a typical power application, when a FET is grounded and the load is connected to the mains voltage, the FET constitutes a low-side switch. In low-side switches, N-channel FETs should be used because of the voltage required to turn off or turn on the device. When the FET is connected to the bus and the load is grounded, a high-side switch is used. Usually a P-channel FET is used in this topology, which is also due to the consideration of voltage drive.

Selection of voltage and current. The higher the voltage rating, the higher the cost of the device. According to practical experience, the rated voltage should be greater than the mains voltage or bus voltage. This will provide enough protection so that the FET will not fail. As far as the selection of a FET is concerned, it is necessary to determine the maximum voltage that can be tolerated between the drain and the source, that is, the maximum VDS. Other safety factors that design engineers need to consider include voltage transients induced by switching electronics such as motors or transformers. Voltage ratings vary from application to application. In continuous conduction mode, the FET is in steady state, where current flows continuously through the device. A pulse spike is when there is a large surge (or peak current) flowing through the device. Once the maximum current under these conditions is determined, simply select the device that can handle this maximum current.

Calculate the conduction loss. The power dissipation of the FET device can be calculated by Iload2×RDS(ON). Since the on-resistance varies with temperature, the power dissipation also changes proportionally. For portable designs, it is easier (and more common) to use lower voltages, while for industrial designs, higher voltages can be used. Note that the RDS(ON) resistance will rise slightly with current. Various electrical parameter variations for RDS(ON) resistors can be found in the technical data sheet provided by the manufacturer.

Calculate the cooling requirements of the system. Designers have to consider two different scenarios, worst-case and real-world. It is recommended to use the calculation result for the worst case, because this result provides a larger safety margin to ensure that the system does not fail.

switching losses. The voltage-current product at the turn-on instant is quite large. To a certain extent, it determines the switching performance of the device. However, if the system has higher switching performance requirements, a power MOSFET with a smaller gate charge QG can be selected.