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Overview of FET selection and application


Overview of FET selection and application

    FETs are widely used in analog and digital circuits and are inseparable from our lives. The advantage of the FET is that the driver circuit is relatively simple. The FET requires a much lower drive current than the BJT, and can usually be driven directly by a CMOS or open-collector TTL drive circuit. Secondly, the FET has a faster switching speed and can operate at higher speeds because there is no charge. Storage effect; in addition, the FET has no secondary breakdown failure mechanism. It tends to have higher endurance at higher temperatures, and the lower the possibility of thermal breakdown, it can also provide better temperature over a wider temperature range. performance. FETs have been used in a wide range of applications, in consumer electronics, industrial products, electromechanical devices, smartphones, and other portable digital electronics.

    In recent years, with the rapid development of the field of FET products in the automotive, communications, energy, consumer, and green industries, power FETs have received much attention. It is predicted that the overall compound annual growth rate of China's power MOSFET market will reach 13.7% in 2010-2015. Although market research firm iSuppli said that due to macro investment and economic policies and wafer and raw material supply problems caused by the earthquake in Japan, this year's power FET market will slow down, but consumer electronics and data processing demand remains strong, so long-term In view, the growth of power FETs will continue for a considerable period of time.

    Technology has been improving, and the power FET market has gradually been challenged by new technologies. For example, many companies in the industry have begun to develop GaN power devices, and assert that the space for improving the performance of silicon power FETs is very limited. However, GaN's challenge to the power FET market is still at a very early stage. FETs still have obvious advantages in terms of technology maturity and supply. After more than 30 years of development, the FET market is not. Will be quickly replaced by new technology.

    In five years or even longer, the FET will still dominate. The FET will still be the device that many newly-introduced engineers will come into contact with. The content of this issue will start from the basics and discuss some basic knowledge of FET, including selection, introduction of key parameters, system and heat dissipation considerations. Wait for everyone to do some introduction.

1. Basic selection of FET

    There are two types of FETs: N-channel and P-channel. In power systems, FETs can be viewed as electrical switches. When a positive voltage is applied between the gate and the source of the N-channel FET, its switch is turned on. When turned on, current can flow from the drain to the source via the switch. There is an internal resistance between the drain and the source, called the on-resistance RDS(ON). It must be clear that the gate of the FET is a high impedance end, so always apply a voltage to the gate. If the gate is left floating, the device will not operate as designed and may turn on or off at an inappropriate time, causing potential power loss to the system. When the voltage between the source and the gate is zero, the switch is turned off and current stops through the device. Although the device is turned off at this time, there is still a small current, which is called leakage current, ie IDSS.

    As a basic component in an electrical system, how do engineers make the right choices based on parameters? This article will discuss how to choose the right FET in four steps.

    1) Channel selection. The first step in choosing the right device for the design is to decide whether to use an N-channel or a P-channel FET. In a typical power application, when a FET is grounded and the load is connected to the mains voltage, the FET forms the low side switch. In low-side switches, N-channel FETs should be used for consideration of the voltage required to turn the device off or on. When the FET is connected to the bus and the load is grounded, the high side switch is used. P-channel FETs are often used in this topology, also for voltage-driven considerations.

    2) 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 will provide sufficient protection so that the FET will not fail. In choosing a FET, the maximum voltage that can be withstood between the drain and the source, the maximum VDS, must be determined. Other safety factors that design engineers need to consider include voltage transients induced by switching electronics such as motors or transformers. The rated voltages vary from application to application; typically, portable devices are 20V, FPGA power supplies are 20 to 30V, and 85 to 220VAC applications are 450 to 600V.
In continuous conduction mode, the FET is in a steady state, at which point current continues through the device. A pulse spike is one in which a large amount of surge (or spike current) flows through the device. Once the maximum current under these conditions is determined, simply select the device that can withstand this maximum current.

    3) Calculate the conduction loss. The power consumption of the FET device can be calculated by Iload2×RDS(ON). Since the on-resistance changes with temperature, the power consumption will also change 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) resistor will rise slightly with current. Various electrical parameter changes for RDS(ON) resistors can be found in the manufacturer's technical data sheet.

    Need to remind the designer, in general, the Id current marked in the MOS tube specification is the maximum normal current of the MOS tube chip, and the maximum normal current in actual use is also limited by the maximum current of the package. Therefore, the maximum current setting for the customer when designing the product should take into account the maximum current limit of the package. It is recommended that the maximum current setting of the customer when designing the product is more important to consider the internal resistance parameters of the MOS tube.

    4) Calculate the heat dissipation requirements of the system. Designers must consider two different situations, the worst case and the real situation. It is recommended to use the worst-case calculations because this result provides a greater margin of safety and ensures that the system does not fail. There are also some measurement data on the FET data sheet; such as the thermal resistance between the semiconductor junction of the packaged device and the environment, and the maximum junction temperature.

    Switching losses are also an important indicator. As can be seen from the figure below, the voltage-current product at the turn-on instant is quite large. To some extent, the switching performance of the device is determined. However, if the system requires a higher switching performance, a power MOSFET with a smaller gate charge QG can be selected.

Overview of FET selection and application


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