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How to choose MOSFET

MOSFET is the abbreviation of Metal-Oxide-Semiconductor Field-Effect Transistor, and it is a field-effect transistor that can be widely used in analog and digital circuits. With the development and progress of manufacturing technology, system designers must keep up with the pace of technological development in order to select the most suitable electronic devices for their designs. MOSFET is the basic component in the electrical system, engineers need to understand its key characteristics and indicators in order to make the right choice.

This article Shenzhen PCBA manufacturer-SysPCB will discuss how to select the correct MOSFET based on RDS(ON), thermal performance, avalanche breakdown voltage and switching performance indicators.

There are two types of MOSFETs: N-channel and P-channel. In power systems, MOSFETs can be regarded as electrical switches. When a positive voltage is applied between the gate and source of the N-channel MOSFET, 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 MOSFET is a high-impedance terminal, so a voltage must always be applied to the gate. If the gate is left floating, the device will not work as designed, and may be turned on or off at an inappropriate moment, causing potential power loss in the system. When the voltage between the source and gate is zero, the switch is turned off and 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 first step: choose N channel or P channel

The first step in choosing the right device for the design is to decide whether to use an N-channel or a P-channel MOSFET. In a typical power application, when a MOSFET is grounded and the load is connected to the mains voltage, the MOSFET forms a low-side switch. In the low-side switch, N-channel MOSFET should be used, which is based on the consideration of 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, usually a P-channel MOSFET, which is also based on consideration for voltage driving.

To select a suitable device for the application, you must determine the voltage required to drive the device and the easiest method to implement in the design. Then determine the required rated voltage, or the maximum voltage the device can withstand. 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. In this way, sufficient protection can be provided so that the MOSFET will not fail. As far as selecting MOSFET is concerned, the maximum voltage that can be withstood from the drain to the source must be determined, that is, the maximum VDS. It is important to know that the maximum voltage that a MOSFET can withstand changes with temperature. The designer must test the voltage variation range over the entire operating temperature range. The rated voltage must have enough margin to cover this variation range to ensure that the circuit will not fail. Other safety factors that design engineers need to consider include voltage transients induced by switching electronics such as motors or transformers. The rated voltages of different applications are also different; generally, portable devices are 20V, FPGA power supplies are 20-30V, and 85-220VAC applications are 450-600V.

The second step is to select the rated current of the MOSFET.

Depending on the circuit structure, the rated current should be the maximum current that the load can withstand under all conditions. Similar to the voltage situation, the designer must ensure that the selected MOSFET can withstand this rated current, even when the system generates peak currents. The two current conditions considered are continuous mode and pulse spikes. In continuous conduction mode, the MOSFET is in a steady state, and current flows continuously through the device at this time. 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, simply select the device that can withstand this maximum current. 

After selecting the rated current, the conduction loss must be calculated. In reality, MOSFET is not an ideal device, because there will be power loss in the conduction process, which is called conduction loss. The MOSFET is like a variable resistor when it is "on", determined by the RDS(ON) of the device, and changes significantly with temperature. The power loss of the device can be calculated by Iload2×RDS(ON). Since the on-resistance changes with temperature, the power loss will also change proportionally. The higher the voltage VGS applied to the MOSFET, the smaller RDS(ON) will be; on the contrary, the higher RDS(ON) will be. For system designers, this is where the trade-off depends on the system voltage. For portable designs, it is easier (more common) to use lower voltages, while for industrial designs, higher voltages can be used. Note that RDS(ON) resistance will rise slightly with current. Various electrical parameter changes of RDS(ON) resistance can be found in the technical data sheet provided by the manufacturer.

Technology has a significant impact on the characteristics of the device, because some technologies tend to increase RDS(ON) when increasing the maximum VDS. For such a technology, if you plan to reduce VDS and RDS(ON), you have to increase the chip size, thereby increasing the package size and related development costs. There are several technologies in the industry that try to control the increase in wafer size, the most important of which are channel and charge balancing technologies.

In trench technology, a deep trench is embedded in the wafer, usually reserved for low voltage, to reduce the on-resistance RDS(ON). In order to reduce the impact of the maximum VDS on RDS(ON), an epitaxial growth column/etching column process is used in the development process. For example, Fairchild Semiconductor has developed a technology called SuperFET, which adds additional manufacturing steps to reduce RDS(ON).

This attention to RDS(ON) is very important, because when the breakdown voltage of a standard MOSFET increases, RDS(ON) will increase exponentially and cause the chip size to increase. The SuperFET process turns the exponential relationship between RDS(ON) and wafer size into a linear relationship. In this way, SuperFET devices can achieve ideal low RDS(ON) in a small wafer size, even when the breakdown voltage reaches 600V. The result is that the wafer size can be reduced by up to 35%. For end users, this means a significant reduction in package size.

Step 3: Determine the thermal requirements

The next step in selecting MOSFETs is to calculate the heat dissipation requirements of the system. The designer must consider two different situations, the worst case and the real case. It is recommended to use the calculation result for the worst case, because this result provides a greater safety margin and can ensure that the system will not fail. There are also some measurement data that need attention on 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.

The junction temperature of the device is equal to the maximum ambient temperature plus the product of thermal resistance and power dissipation (junction temperature = maximum ambient temperature + [thermal resistance × power dissipation]). According to this equation, the maximum power dissipation of the system can be solved, which is equivalent to I2×RDS(ON) by definition. Since the designer has determined the maximum current that will pass through the device, the RDS(ON) at different temperatures can be calculated. It is worth noting that when dealing with simple thermal models, designers must also consider the heat capacity of the semiconductor junction/device housing and housing/environment; that is, the printed circuit board and package are required not to heat up immediately.   Avalanche breakdown means that the reverse voltage on the semiconductor device exceeds the maximum value and a strong electric field is formed to increase the current in the device. This current will dissipate power, increase the temperature of the device, and may damage the device. Semiconductor companies will conduct avalanche tests on devices, calculate their avalanche voltage, or test the robustness of devices. There are two methods for calculating the rated avalanche voltage; one is statistical method, and the other is thermal calculation. The thermal calculation is widely used because of its practicality. Many companies provide details of their device testing. For example, Fairchild Semiconductor provides "Power MOSFET Avalanche Guidelines" ( Power MOSFET Avalanche Guidelines--you can download it from Fairchild's website). In addition to calculations, technology also has a great influence on the avalanche effect. For example, an increase in wafer size will improve avalanche resistance and ultimately improve device robustness. For end users, this means using larger packages in the system.

Step 4: Determine the switch performance

The final step in choosing a MOSFET is to determine the switching performance of the MOSFET. There are many parameters that affect switching performance, but the most important ones are gate/drain, gate/ source and drain/source capacitance. These capacitors will cause switching losses in the device because they must be charged each time it is switched. The switching speed of the MOSFET is therefore reduced, and the device efficiency is also reduced. In order to calculate the total loss of the device during the switching process, the designer must calculate the loss during the turn-on process (Eon) and the loss during the turn-off process (Eoff). The total power of the MOSFET switch can be expressed by the following equation: Psw=(Eon+Eoff)×switching frequency. The gate charge (Qgd) has the greatest impact on switching performance.

Based on the importance of switch performance, new technologies are constantly being developed to solve this switch problem. The increase in chip size will increase the gate charge; and this will increase the device size. In order to reduce switching losses, new technologies such as channel thick bottom oxidation have emerged to reduce gate charge. For example, the new technology of SuperFET can minimize conduction loss and improve switching performance by reducing RDS (ON) and gate charge (Qg). In this way, the MOSFET can cope with high-speed voltage transients (dv/dt) and current transients (di/dt) during switching, and can even work reliably at higher switching frequencies.

Conclusion 

By understanding the types of MOSFETs and understanding and determining their important performance characteristics, designers can select the correct MOSFET for a specific design. Since MOSFET is one of the most basic components in an electrical system, choosing the correct MOSFET plays a key role in the success of the entire design.