How to choose a good MOS DIODE

How to choose a good MOS tube

The first step 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 branch line voltage, the MOS tube forms a high-voltage side switch. In the high-voltage side switch, N-channel MOS transistors should be used, which is based on the consideration of the voltage required to open or conduct the device. When the MOS tube is connected to the bus and the load is grounded, a low-voltage side switch is used. Generally, P-channel MOS transistors are used in a certain topology, which is also due to the consideration of voltage drive.

Determine the additional voltage required, or perhaps the maximum voltage the device can accept. The greater the extra voltage, the higher the profit of the machine. According to theoretical experience, the rated voltage should be greater than the branch voltage or bus voltage. That ability requires the rest of the cover, so that the MOS tube will not work. As far as the selection of MOS tubes is concerned, it is necessary to determine the maximum acceptable voltage between the drain and the source, that is, the maximum VDS. It is very important to know that the maximum voltage that the MOS tube can accept will vary with the measurement. We have to test the variation range of the voltage in the whole working measurement range. The additional voltage must have a margin to cover a certain range of transitions to ensure that the path does not fail. Other insurance factors to consider include voltage transients induced by switching electronics such as generators or transformers. The additional voltage used by different devices is also different; generally, 20V for portable devices, 20-30V for FPGA power supplies, and 450-600V for 85-220VAC applications. The MOS tube designed by KIA semi-superconductor has strong pressure resistance and wide application range, and is favored by customers.

Two: Determine the extra DC current of the mosfet 

This rated DC should be the maximum DC that the load can accept under all conditions. Similar to the voltage situation, ensure that the selected MOS tube can withstand a certain rated DC, even when the system generates a peak DC. Two DC conditions to consider are continuous mode and pulsed spikes. In the continuous conduction mode, the MOS tube is in a steady state, and at this time, the direct current passes through the device continuously. Pulse spike refers to a small amount of surge (or peak current) flowing through the device. Once the maximum direct current in the environment is determined, it is only necessary to directly select the parts that can withstand a certain maximum direct current.

After selecting the rated DC power, it is also necessary to calculate the conduction consumption. In practice, the MOS tube is not an actual component, because there will be kinetic energy loss during the heat conduction process, which is called conduction loss. When the MOS tube is "on", it is like a variable resistor, which is determined by the RDS (ON) of the device and changes significantly with the measurement. The power loss of the device can be calculated by Iload2×RDS(ON), because the lead-back resistance changes with the measurement, so the power loss will also change accordingly. The higher the voltage VGS applied to the MOS tube, the smaller the RDS(ON); otherwise, the higher the RDS(ON). Note that the RDS(ON) resistance will drop slightly with DC.

Three: The next step in choosing a MOS tube is the systematic heat dissipation requirements

Two different scenarios must be considered, the worst scenario and the actual scenario. It is recommended to adopt a design effect aimed at the worst case, because a certain effect requires a larger insurance margin to ensure that the system does not fail. The junction temperature of the device is equal to the maximum condition measurement plus the product of thermal resistance and power dissipation (junction temperature = maximum condition measurement + [thermal resistance × power dissipation]). According to a certain formula, the maximum power dissipation of the system can be solved, which is the same as I2×RDS(ON) by definition. We are going to pass the maximum direct current of the parts, and we can calculate RD(ON) under different measurements. In addition, the heat dissipation of the access board and its MOS tube must be done well.

Landslide breakdown means that the reverse voltage on the semi-superconductor device exceeds the maximum value and forms a strong magnetic field to increase the DC current in the device. The increase in the size of the chip will improve the ability to prevent wind and collapse, and ultimately improve the stability of the machine parts. Therefore, choosing a larger package can effectively prevent landslides.

Four: The last step in choosing a MOS tube is to determine the switch function of the MOS tube

There are many parameters for the response switch function, but the most important ones are electrode/drain, electrode/source and drain/source storage capacity. This storage capacity creates switch losses in the mechanism, as they are inflated each time they are switched. The switching speed of the MOS tube is thus increased, and the frequency of the machine is also reduced. In order to calculate the total consumption of the parts during the switching process, it is necessary to calculate the consumption during the opening process (Eon) and the consumption during the opening process (Eoff). The total power of the mosfet  switch can be expressed by the following equation: Psw = (Eon+Eoff) × frequency of the switch. The electrode point charge (Qgd) has the greatest response to the switch function.