Switching MOSFET Selection

The buck converter is a hard-switched topology. The switching MOSFET has to resemble an ideal switch, i.e. being low ohmic and fast switching. As with the synchronous rectifier MOSFET, the FOM is setting limits to as far as one can come to an ideal switch. For a buck converter switch, the following are major MOSFET selection criteria:

 Low FOMs for - RDSON·QG and RDSON·Qoss 

 Ratio QGD/QGS<1 for immunity against dv/dt induced turn-on 

 Fast turn-on/off switching, gate plateau near middle of gate drive range 

 Switching and conduction losses must be balanced for minimum total loss at desired point of peak efficiency 

 VDS rating to handle spikes (voltage overshoot)

 Low thermal resistance RthJC 

 Package selection must consider the importance of RthJA 

 Body diode speed and reverse recovery charge are not important, since body diode rarely conducts in a buck converter (only during forced output voltage down transitions)

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