How to select Field-effect transistor

How to select Field-effect transistor

There are two types of Field-effect transistor: N channel and P channel. In power system, Field-effect transistor can be regarded as electrical switch. When a positive voltage is applied between the gate and the source of the n-channel field-effect transistor, the switch is turned on. When conducting, the current can flow from the drain to the source through the switch. There is an internal resistance between the drain and source, called the conduction resistance RDS (ON). It must be clear that the grid of Field-effect transistor is a high impedance terminal, so a voltage is always applied to the grid. If the gate is suspended, the device will not operate as designed and may turn on or off at inappropriate times, leading to 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 already turned off at this time, there is still a small current present, which is called leakage current, or IDSS.

The selection of channels. The first step in choosing the right device for 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 main line voltage, the Field-effect transistor constitutes a low-voltage side switch. In the switching of low-voltage side, N-channel field-effect transistor shall be used, which is based on the consideration of the voltage required to turn off or turn on the device. When the field-effect transistor is connected to the bus and the load is grounded, the high-voltage side switch should be used. P channel FETs are usually used in this topology, which is also due to the consideration of voltage driving.

Selection of voltage and current. The higher the rated voltage, the higher the cost of the device. Based on practical experience, the rated voltage should be greater than the main line voltage or bus voltage. In this way, sufficient protection can be provided so that the Field-effect transistor will not fail. For the selection of Field-effect transistor, it is necessary to determine the maximum voltage that may be borne between the drain electrode and the source electrode, that is, the maximum VDS. Other safety factors that design engineers need to consider include voltage transients induced by switching electronic devices such as motors or transformers. The rated voltage of different applications is also different. In the continuous conduction mode, the Field-effect transistor is in steady state, and the current continuously flows through the device. Pulse spike refers to a large amount of surge (or peak current) flowing through the device. Once the maximum current under these conditions is determined, simply select the device that can withstand this maximum current directly.

Calculate the conduction loss. The power loss of Field-effect transistor devices can be determined by Iload2 × RDS (ON) calculation shows that due to the variation of conduction resistance with temperature, the power loss 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. The various electrical parameter changes regarding RDS (ON) resistance 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, namely 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.

Switch loss. The product of voltage and current at the moment of conduction is quite large. To some extent, it determines the switching performance of the device. However, if the system requires high switching performance, Power MOSFET with low gate charge QG can be selected.