There are two main types of FETs: N-channel and P-channel. In power systems, field effect transistors can be considered as electrical switches. When a positive voltage is applied between the gate and source of an n-channel FET, its switch is turned on. During conduction, current can flow from the drain to the source via a 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 a field effect transistor is a high impedance terminal, so always apply a voltage to the gate. If the gate electrode is suspended, the device will not operate as designed, and may turn on or off at an inappropriate time, resulting in potential power loss in the system. When the voltage between the source and gate is zero, the switch closes and current stops flowing through the device. Although the device has been turned off at this time, there is still a small current present, which is called leakage current, or IDSS.

Selection of trenches. The first step in selecting the right device for your design is to decide whether to use N-channel or P-channel FETs. In typical power applications, when a field effect transistor is grounded and the load is connected to the mains voltage, the field effect transistor constitutes a low-voltage side switch. In low voltage side switching, N-channel FETs should be used because of the consideration of the voltage required to turn off or turn on the device. When field-effect transistors are connected to the bus and load ground, high-voltage side switches are used. Usually, P-channel FETs are used in this topology, which is also due to voltage driven considerations.

Selection of voltage and current. The higher the rated voltage, 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 can provide sufficient protection so that the FET does not fail. For selecting a field effect transistor, it is necessary to determine the maximum voltage that can be withstood between the drain and 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. The rated voltage varies for different applications. In the continuous conduction mode, the field effect transistor is in a steady state, where current continues to flow through the device. Pulse spikes refer to a large number of surges (or spikes in current) flowing through the device. Once the maximum current under these conditions is determined, it is only necessary to directly select the device that can withstand this maximum current.

Calculate the conduction loss. The power consumption of field effect transistor devices can be determined by Iload2 × RDS (ON) calculates that because the conduction resistance changes with temperature, the power consumption will also change proportionally. For portable designs, it is easier (more common) to use lower voltages, while for industrial designs, higher voltages can be used. Note that the RDS (ON) resistance will slightly increase with the current. Changes in various electrical parameters of the RDS (ON) resistor can be found in the technical data sheet provided by the manufacturer.

Calculate the heat dissipation requirements of the system. Designers must consider two different scenarios, the worst case scenario and the real situation. It is recommended to use a worst-case calculation result, as this result provides greater safety margin and ensures that the system will not fail.

Switching loss. The voltage current product at the moment of conduction is quite large. The switching performance of the device is determined to a certain extent. However, if the system requires high switching performance, a power MOSFET with a relatively small gate charge QG can be selected.