Teach you to choose the right MOS diode, you can start from six aspects

Teach you to choose the right MOS diodee, you can start from six aspects.
In the design of some circuits, not only MOS transistors are often used in switching power supply circuits, correct selection of MOS transistors is a problem often encountered by hardware engineers, but also a very important link. Poor selection of MOS transistors may affect the efficiency and cost of the whole circuit. Here is a summary of how to select MOS transistors correctly, which can be considered from six aspects.
1. Channel type.
The first step in selecting MOS devices is to decide whether to use N-channel or P-channel MOS transistors. In a typical power application, when a MOS transistor is grounded and the load is connected to the trunk voltage, the MOS transistor forms a low-voltage side switch. The N-channel MOS transistor should be used in the low-voltage side switch, which is due to the consideration of the voltage required to turn off or turn on the device. When the MOS diodee is connected to the bus and the load is grounded, the high voltage side switch should be used. P-channel MOS transistors are usually used in this topology, which is also for the consideration of voltage drive.
2. Rated voltage.
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 trunk voltage or bus voltage. Only in this way can sufficient protection be provided so that the MOS diodee will not fail.
For the selection of MOS transistors, it is necessary to determine the maximum possible voltage between the drain and the source, that is, the maximum VDS. It is important to know that the maximum voltage that a MOS diodee can withstand varies with temperature. We have to test the range of voltage variation over the entire operating temperature range. The rated voltage must have sufficient margin to cover this range of variation to ensure that the circuit does not fail. Other safety factors to consider include voltage transients caused by switching electronic devices such as motors or transformers. The rated voltage varies from application to application; typically, the portable device is 20V, the FPGA power supply is 2030V, and the 85~220VAC application is 45000V.
3. Rated current.
The rated current should be the maximum current that the load can withstand in all cases. Similar to the voltage situation, ensure that the selected MOS diodee can withstand this rated current, even when the system produces a peak current. The two current cases considered are continuous mode and pulse spikes. In the continuous conduction mode, the MOS transistor is in a steady state, and the current continues to pass through the device. 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, you only need to directly select the device that can withstand the maximum current.
4. Conduction loss.
In practice, MOS transistors are not ideal devices, because there will be power loss in the conductive process, which is called conduction loss. The MOS transistor is like a variable resistor when it is turned on, which is determined by the RDS (ON) of the device and varies significantly with temperature. The power loss of the device can be calculated by Iload2 × RDS (ON). Because the on-resistance varies with temperature, the power loss varies proportionally. The higher the voltage VGS applied to the MOS diodee, the smaller the RDS (ON) will be; conversely, the higher the RDS (ON) will be. Note that the RDS (ON) resistance increases slightly with the current. Various changes in electrical parameters of RDS (ON) resistors can be found in the technical data sheet provided by the manufacturer.
5. Heat dissipation of the system.
Two different situations have to be considered, namely, the worst-case scenario and the real situation. The worst-case calculation is recommended because it provides a greater margin of safety and ensures that the system does not fail. There are also some measurement data that need to be paid attention to in the data sheet of MOS diodee; 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 formula, the maximum power dissipation of the system can be solved, that is, by definition, it is equal to I2 × RDS (ON). We are about to pass through the maximum current of the device, and we can calculate the RDS (ON) at different temperatures. In addition, it is necessary to do a good job in the heat dissipation of the circuit board and its MOS diodee.
Avalanche breakdown means that the reverse voltage on a semiconductor device exceeds the maximum and a strong electric field is formed to increase the current in the device. The increase of wafer size will improve the avalanche resistance and ultimately improve the robustness of the device. Therefore, choosing a larger package can effectively prevent avalanches.
6. Switch performance.
There are many parameters that affect the performance of the switch, but the most important ones are gate / drain, gate / source and drain / source capacitance. These capacitors cause switching loss in the device because they are recharged each time they are switched. As a result, the switching speed of the MOS transistor is reduced, and the device efficiency is also reduced. In order to calculate the total loss of the device in the switching process, it is necessary to calculate the loss in the turn-on process (Eon) and the loss in the closing process (Eoff). The total power of MOSFET switch can be expressed by the following equation: Psw= (Eon+Eoff) × switching frequency. The gate charge (Qgd) has the greatest influence on the performance of the switch.