Say goodbye to the selection problem: the 4-step golden rule of MOSFET selection

    The next step in choosing a MOSFET is to calculate the heat dissipation requirements of the system. Designers have to consider two different scenarios, the worst case and the real case. A worst-case calculation is recommended because it provides a greater margin of safety to ensure that the system does not fail. There are also some measurement data that need to be noted on the MOSFET data sheet; For example, the thermal resistance between the semiconductor junction of the packaged device and the environment, and the maximum junction temperature.

    The junction temperature of the device is equal to the product of the maximum ambient temperature plus thermal resistance and power dissipation (junction temperature = maximum ambient temperature +[thermal resistance x power dissipation]). According to this equation, the maximum power dissipation of the system can be solved, which is equal to I2×RDS(ON) by definition. Since the designer has determined the maximum current that will pass through the device, RDS(ON) at different temperatures can be calculated. It is worth noting that when dealing with simple thermal models, the designer must also consider the heat capacity of the semiconductor junction/device housing and the housing/environment; That is, the printed circuit board and the package will not heat up immediately.

    The final step in selecting a MOSFET is to determine the switching performance of the MOSFET. There are many parameters that affect the performance of a switch, but the most important are the gate/drain, gate/source, and drain/source capacitance. These capacitors create switching losses in the device because they are charged each time they are 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 switching, the designer must calculate the loss during switching on (Eon) and the loss during switching off (Eoff). The total power of the MOSFET switch can be expressed by the following equation: Psw=(Eon+Eoff) x switching frequency. The grid charge (Qgd) has the greatest influence on the performance of the switch.