MOSFET Selection Basics

MOSFET is widely used in analog circuits and digital circuits, and is inseparable from our lives. The advantages of MOSFET are: first, the driving circuit is relatively simple. The driving current required by MOSFET is much smaller than that of BJT, and it can usually be directly driven by CMOS or open collector TTL driving circuit; second, the switching speed of MOSFET is relatively fast, and it can work at a higher speed because there is no charge storage effect; in addition, MOSFET has no secondary breakdown failure mechanism. The higher the temperature, the stronger its endurance is, and the lower the possibility of thermal breakdown is. It can also provide better performance in a wider temperature range. MOSFET has been widely used and can be seen everywhere in consumer electronics, industrial products, electromechanical equipment, smart phones and other portable digital electronic products.

In recent years, with the rapid development of industries that use MOSFET products in large quantities, such as automobiles, communications, energy, consumption, and green industries, power MOSFET has attracted much attention. It is predicted that the overall compound annual growth rate of China's power MOSFET market will reach 13.7% from 2010 to 2015. Although market research company iSuppli said that the power MOSFET market will slow down this year due to macro investment and economic policies and wafer and raw material supply problems caused by the Japanese earthquake, the demand for consumer electronics and data processing is still strong. Therefore, in the long run, the growth of power MOSFET will continue for a considerable period of time.

Technology has been advancing, and the power MOSFET 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 performance of silicon power MOSFETs can be improved. However, GaN's challenge to the power MOSFET market is still in a very early stage. MOSFET still has obvious advantages in terms of technology maturity and supply. After more than 30 years of development, the MOSFET market will not be easily replaced by new technologies.

For five years or even longer, MOSFET will still occupy a dominant position. MOSFET will still be a device that many engineers who have just entered the industry will come into contact with. This issue of the semi-monthly talk will start from the basics and explore some basic knowledge of MOSFET, including selection, introduction of key parameters, system and heat dissipation considerations, etc.; Finally, we will introduce some of the most common popular applications.
MOSFET selection basis

There are two major types of MOSFETs: N-channel and P-channel. In power systems, MOSFETs can be viewed as electrical switches. When a positive voltage is applied between the gate and source of an N-channel MOSFET, the 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 the source, which is called the on-resistance RDS(ON). It must be understood that the gate of the MOSFET is a high impedance terminal, so a voltage must always be applied to the gate. If the gate is floating, the device will not work as designed and may turn on or off at inappropriate times, resulting in potential power loss in the system. When the voltage between the source and the gate is zero, the switch is turned off and the current stops passing through the device. Although the device is turned off at this time, there is still a small current, which is called leakage current, or IDSS.

As a basic component in electrical systems, how can engineers make the right choice based on parameters? This article will discuss how to choose the right MOSFET in four steps.

1) Channel selection. The first step in choosing the right device for your design is to decide whether to use an N-channel or P-channel MOSFET. In a typical power application, when a MOSFET is grounded and the load is connected to the mains voltage, the MOSFET constitutes a low-side switch. In the low-side switch, an N-channel MOSFET should be used, which is due to the voltage required to turn off or turn on the device. When the MOSFET is connected to the bus and the load is grounded, a high-side switch is used. P-channel MOSFETs are usually used in this topology, also due to voltage drive considerations.

2) Voltage and current selection. The higher the rated voltage, the higher the cost of the device. As a rule of thumb, the rated voltage should be greater than the mains voltage or bus voltage. This provides adequate protection so that the MOSFET does not fail. When selecting a MOSFET, the maximum voltage that can be sustained between the drain and source, i.e., the maximum VDS, must be determined. Other safety factors that designers need to consider include voltage transients induced by switching electronic devices (such as motors or transformers). The rated voltage varies for different applications; typically, it is 20V for portable devices, 20-30V for FPGA power supplies, and 450-600V for 85-220VAC applications.

In continuous conduction mode, the MOSFET is in steady state, and current flows continuously through the device. A pulse spike is a large surge (or spike current) flowing through the device. Once the maximum current under these conditions is determined, it is straightforward to select a device that can withstand this maximum current.

3) Calculate conduction losses. The power loss of a MOSFET device can be calculated by Iload2×RDS(ON). Since the on-resistance changes 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 increase slightly with current. The various electrical parameter changes of the RDS(ON) resistance can be found in the technical data sheet provided by the manufacturer.

4) Calculate the heat dissipation requirements of the system. Designers must consider two different situations, namely the worst case and the real situation. It is recommended to use the calculation results for the worst case because this result provides a larger safety margin to ensure that the system will not fail. There are also some measurement data that need to be paid attention to in the MOSFET data sheet; such as the thermal resistance between the semiconductor junction of the packaged device and the environment, and the maximum junction temperature.
Switching loss is actually a very important indicator. As can be seen from the figure below, the voltage-current product at the moment of conduction is quite large. To a certain extent, it determines the switching performance of the device. However, if the system has high switching performance requirements, a power MOSFET with a relatively small gate charge QG can be selected.