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Say goodbye to the selection problem: the 4-step golden rule of MOSFET selection

Time:2024-06-13 Views:54
Today you will learn 4 steps to choose a suitable MOSFET.。

Step 1: Choose N channel or P channel

    The first step in choosing the right device for your design is deciding whether to use an n-channel or P-channel MOSFET. In typical power applications, when a MOSFET is grounded and the load is connected to the main line voltage, the MOSFET constitutes a low-voltage side switch. In low-voltage side switches, n-channel MOSFETs should be used because of the voltage required to turn off or on the device. When the MOSFET is connected to the bus and the load is grounded, the high-voltage side switch is used. P-channel MOSFETs are often used in this topology, again for voltage drive considerations.

    To select the right device for the application, it is necessary to determine the voltage required to drive the device and the easiest way to perform it in the design. The next step is to determine the desired voltage rating, or the maximum voltage that 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 main line voltage or bus voltage. This provides sufficient protection so that the MOSFET does not fail. For the selection of MOSFETs, it is necessary to determine the maximum voltage that can be experienced between the drain and the source electrode, that is, the maximum VDS. It is important to know that the maximum voltage a MOSFET can withstand will vary with temperature. The designer must test the range of voltage variation over the entire operating temperature range. The rated voltage must have enough margin to cover this range of variation to ensure that the circuit does not fail. Other safety factors that design engineers need to consider include voltage transients induced by switching electronics, such as motors or transformers.

Step 2: Determine the rated current

    The second step is to select the rated current of the MOSFET. Depending on the circuit structure, this current rating should be the maximum current that the load can withstand under all circumstances. Similar to the case of voltage, designers must ensure that the selected MOSFETs can withstand this current rating, even when the system produces spikes. The two current cases considered are continuous mode and pulse spikes. In continuous on-mode, the MOSFET is in steady state, where current flows continuously through the device. A pulse spike is when there is a large surge (or spike current) flowing through the device. Once the maximum current under these conditions is determined, it is simply necessary to directly select a device that can withstand this maximum current.

    After selecting the rated current, the on-off loss must also be calculated. In practice, MOSFETs are not ideal devices, because there is a loss of power in the process of conducting electricity, which is called the on-loss. The MOSFET acts like a variable resistor when it is "ON", determined by the RDS(ON) of the device, and varies significantly with temperature. The power loss of the device can be calculated from the Iload2×RDS(ON), and since the on-resistance varies with temperature, the power loss also varies proportionally. The higher the voltage VGS applied to the MOSFET, the smaller the RDS(ON) will be; ON the contrary, the RDS(ON) will be higher. For the system designer, this is where the tradeoff depends on the system voltage. 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) resistance rises slightly with the current.

Step 3: Determine thermal requirements

    The next step in choosing a MOSFET is to calculate the heat dissipation requirements of the system. Designers have to consider two different scenarios, the worst case and the real case. A worst-case calculation is recommended because it provides a greater margin of safety to ensure that the system does not fail. There are also some measurement data that need to be noted on the MOSFET data sheet; For example, 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 product of the maximum ambient temperature plus thermal resistance and power dissipation (junction temperature = maximum ambient temperature +[thermal resistance x power dissipation]). According to this equation, the maximum power dissipation of the system can be solved, which is equal to I2×RDS(ON) by definition. Since the designer has determined the maximum current that will pass through the device, RDS(ON) at different temperatures can be calculated. It is worth noting that when dealing with simple thermal models, the designer must also consider the heat capacity of the semiconductor junction/device housing and the housing/environment; That is, the printed circuit board and the package will not heat up immediately.

Step 4: Determine the switch performance

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








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