芯联微电子

There are two types of FETs: n-channel and P-channel. In the power system, the FET can be regarded as an electrical switch. When a positive voltage is added between the gate and the source of the n-channel FET, the switch is turned on. When on, the current can flow from the drain to the source through the switch. There is an internal resistance between the drain and the source, called the on-resistance RDS (ON). It must be made clear that the gate of the FET is a high impedance end, so a voltage must always be added to the gate. If the gate is extremely suspended, the device will not work as intended and may be turned on or off at an inappropriate time, resulting in potential power loss in the system. When the voltage between the source and gate is 00:00, the switch is turned off and the current stops passing 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.

The choice of channels. The first step in choosing the right device for design is to decide whether to use N-channel or P-channel FET. In a typical power application, when a FET is grounded and the load is connected to the trunk voltage, the FET forms a low-voltage side switch. In the low-voltage side switch, the N-channel FET should be used, which is due to the consideration 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, the high voltage side switch should be used. The P-channel FET is usually used in this topology, which is also due to the consideration of voltage drive.

The choice 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 trunk voltage or bus voltage. Only in this way can adequate protection be provided so that the FET will not fail. As for the selection of field effect transistors, it is necessary to determine the maximum voltage that may be borne between the drain and the source, that is, the maximum VDS. Other safety factors that design engineers need to consider include voltage transients caused by switching electronic devices such as motors or transformers. The rated voltages of different applications are also different. In the continuous conduction mode, the FET is in a steady state, and the current continues to pass through the device. A pulse spike refers to a large amount of surge (or spike current) flowing through the device. Once the maximum current under these conditions is determined, you only need to directly select the device that can withstand the maximum current.

Calculate the conduction loss. The power loss of FET devices can be calculated by Iload2 × RDS (ON). Because the on-resistance varies with temperature, the power loss will 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 increases slightly with the current. Various changes in electrical parameters of RDS (ON) resistors can be found in the technical data sheet provided by the manufacturer.

Calculate the heat dissipation requirements of the system. The designer must consider two different scenarios, the worst-case scenario and the real-world situation. The worst-case calculation is recommended because it provides a greater margin of safety and ensures that the system does not fail.

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