Steps for correct selection of MOS tube

The correct selection of MOS transistors is an important part. Poor selection of MOS transistors may affect the efficiency and cost of the entire circuit. Understanding the subtle differences between different MOS transistor components and the stress in different switching circuits can help engineers avoid many problems. Let's learn the correct selection method of MOS tube below.

　　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 N-channel or P-channel MOS transistors. In a typical power application, when a MOS tube is grounded and the load is connected to the mains voltage, the MOS tube forms a low-side switch. In the low-voltage side switch, N-channel MOS transistors should be used. This is due to the consideration of the voltage required to turn off or turn on the device. When the MOS tube is connected to the bus and the load is grounded, a high-voltage side switch is used. P-channel MOS transistors are usually used in this topology, which is also due to the consideration of voltage drive.

　　To choose a suitable device for the application, it is necessary to determine the voltage required to drive the device and the easiest method to implement in the design. The next step is to 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 the bus voltage. In this way, sufficient protection can be provided so that the MOS tube will not fail. In terms of choosing a MOS tube, it is necessary to determine the maximum voltage that can withstand from the drain to the source, that is, the maximum VDS. It is very important to know that the maximum voltage that the MOS tube can withstand will change 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 range of variation 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; usually, portable devices are 20V, FPGA power supplies are 20-30V, and 85-220VAC applications are 450-600V.

　　 Step 2: Determine the rated current

　　 The second step is to select the rated current of the MOS tube. 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 MOS tube can withstand this rated current, even when the system generates peak currents. The two current conditions considered are continuous mode and pulse spikes. In the continuous conduction mode, the MOS tube is in a steady state, and current flows through the device continuously 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 the actual situation, MOS tube is not an ideal device, because there will be power loss during the conduction process, which is called conduction loss. The MOS tube is like a variable resistor when it is "on", which is determined by the RDS (ON) of the device and changes significantly with temperature. The power loss of the device can be calculated by Iload2&TImes; RDS (ON). Since the on-resistance changes with temperature, the power loss will also change proportionally. The higher the voltage VGS applied to the MOS tube, the smaller the RDS (ON) will be; on the contrary, the higher the 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 the RDS (ON) when increasing the maximum VDS. For such a technology, if you plan to reduce VDS and RDS (ON), then 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 chip, 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 chip size, even when the breakdown voltage reaches 600V. The result is that the wafer size can be reduced by up to 35%. For the end user, this means a significant reduction in package size.

　　The third step: Determine the thermal requirements

　　The next step in choosing a MOS tube 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 data sheet of the MOS tube; 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 & TImes; power dissipation]). According to this equation, the maximum power dissipation of the system can be solved, which is equivalent to I2&TImes;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 to not heat up immediately.

"Avalanche breakdown" refers to the fact 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 voltages, or test the robustness of the devices. There are two methods for calculating the rated avalanche voltage; one is the statistical method, and the other is the thermal calculation. The thermal calculation is widely used because of its practicality. 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 the end user, this means using a larger package in the system.

　　 Step 4: Determine the switch performance

　　The final step in choosing a MOS tube is to determine the switching performance of the MOS tube. 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 MOS tube 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)&TImes; switching frequency. The gate charge (Qgd) has the greatest impact on switching performance.