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How To Choose A Power Mosfet?

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By Author: UTMEL
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MOSFETs are divided into two categories: N-channel and P-channel. MOSFETs can be thought of as electrical switches in power systems. The N-channel MOSFET's switch is activated when a positive voltage is placed between the gate and the source. When the switch is turned on, the current can flow from the drain to the source. Between the drain and the source, there is an internal resistance known as the on-resistance RDS (ON). Because the gate of the MOSFET has a high-impedance terminal, a voltage must be provided to it at all times. If the gate is left floating, the device will not function as intended, and it may turn on or off at an inopportune time, potentially causing system power loss. The switch is turned off and the current stops flowing through the device when the voltage between the source and gate is zero. 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 constitutes a low-side switch. In the low-side switch, N-channel MOSFET should be used, which is out of consideration for 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, which is also out of consideration for voltage drive.
Step 2: Determine the rated current
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. A 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.
Step 3: Determine thermal requirements
The next step in selecting a MOSFET 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. Some measurement data 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.
Step 4: Determine the switch performance
The final step in choosing a MOSFET is to determine the switching performance of the MOSFET. Many parameters affect switching performance, but the most important ones are the 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. 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 (Eff). The total power of the MOSFET switch can be expressed by the following equation: Psw = (Eon + E0ff) x switching frequency.
The gate charge (QGD) has the greatest impact on switching performance.
In 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 electrical systems, choosing the right MOSFET plays a key role in the success of the entire design.
This is the specify MOSFET which help you to learn more about MOSFET,

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