Two. Determine the rated current of MOS tube
The rated current shall be the maximum current the load can withstand under all circumstances. As in the case of voltage, ensure that the MOS tube selected can withstand this rated current, even when the system generates spikes in current. The two current cases considered are the continuous mode and the pulse spike. In the continuous conduction mode, the MOS tube is in a steady state, and the current continuously passes through the device. A pulse spike is when there is a large surge (or spike) flowing through the device. Once the maximum current under these conditions is determined, simply select the device that can withstand this maximum current.
After selecting the rated current, the conduction loss must also be calculated. In practice, MOS tubes are not ideal devices, because there will be electrical energy loss in the conduction process, which is called conduction loss. The MOS tube acts as a variable resistor when "ON", determined by the RDS (ON) of the device and varies significantly with temperature. The power loss of the device can be calculated from Iload2×RDS (ON), and since the on-resistance varies with temperature, the power loss also varies proportionally. The higher the voltage VGS applied to MOS tube, the smaller RDS (ON) will be. Otherwise, the RDS (ON) is going to be higher. Note that the RDS (ON) resistance rises slightly with the current. The various electrical parameters for RDS (ON) resistors can be found in the technical data sheet provided by the manufacturer.
Three. The next step in selecting MOS tubes is the heat dissipation requirements of the system
There are two different scenarios to consider, the worst-case scenario and the true scenario. Worst-case calculations are recommended because they provide a greater margin of safety and ensure that the system does not fail. There are also some measurements that need to be noted on the MOS tube data sheet; 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, which is equal to I2×RDS (ON) by definition. We have calculated RDS (ON) at different temperatures from the maximum current of the device. In addition, the circuit board and its MOS tube heat dissipation.
Avalanche breakdown refers to the reverse voltage on the semiconductor device exceeds the maximum value, and the formation of a strong electric field to increase the current in the device. Increasing chip size improves avalanche resistance and ultimately device robustness. Therefore, choosing a larger package can effectively prevent avalanches.
Four. The final step in selecting a MOS tube is to determine the switching performance of the MOS tube
There are many parameters that affect switch performance, but the most important are gate/drain, gate/source, and drain/source capacitance. These capacitors cause switching losses in the device because they are charged each time they are switched on or off. Therefore, the switching speed of MOS tube is reduced, and the device efficiency is also reduced. In order to calculate the total loss of devices in the switching process, the loss in the switching process (Eon) and the loss in the switching process (Eoff) should be calculated. The total power of the MOSFET switch can be expressed in the following equation: Psw= (Eon+Eoff) x switching frequency. Gate charge (Qgd) has the greatest influence on switch performance.
Follow WeChat public account