How to choose MOS tube? Four rules for selecting MOS tube

How to choose MOS tube is a problem often encountered by novice engineers. Understand the selection rule of MOS tube, then engineers can use these rules to select the desired MOS tube, so that the whole circuit can work smoothly. . It will not affect the subsequent work and matters because of the inappropriateness of the MOS tube. The following summarizes the four rules of how to correctly select the MOS tube.

One of the rules: using N-channel orP channel

The first step in choosing a MOS device is to decide whether to use an N-channel or a P-channel MOS transistor. In a typical power application, when a MOS transistor is grounded and the load is connected to the mains voltage, the MOS transistor forms a low side switch. In the low side switch, an N-channel MOS transistor should be used for the consideration of the voltage required to turn the device off or on. When the MOS tube is connected to the bus and the load is grounded, the high side switch is used. P-channel MOSFETs are often used in this topology, also for voltage drive considerations.

Rule 2: Determine the rated current of the MOS tube

This rated current should be the maximum current the load can withstand under all conditions. Similar to the voltage case, ensure that the selected MOS transistor can withstand this rated current even when the system generates a spike current. The two considered current conditions are continuous mode and pulse spikes. In continuous conduction mode, the MOS transistor 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.

Rule 3: The next step in choosing a MOS tube is the system's thermal requirements.

There are two different situations to consider, 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.

Rule 4: The final step in choosing a MOS tube is to determine the switching performance of the MOS tube.

There are many parameters that affect the performance of the switch, but the most important are the gate/drain, gate/source and drain/source capacitances. These capacitors create switching losses in the device because they are charged every time they are switched. The switching speed of the MOS transistor is thus reduced, and the device efficiency is also lowered. To calculate the total loss of the device during the switching process, the loss (Eon) during turn-on and the loss (Eoff) during turn-off are calculated.

The above are the four rules when choosing a MOS tube. I hope that we can help everyone and improve the efficiency of design.