The effects of MOSFET output capacitance in high frequency applications

In high-frequency power converters (those with switching frequencies >200 kHz) losses associated with charging and discharging the MOSFET´s output capacitance begin to affect system efficiency. It is noted that knowing these losses improves predictions of the frequency at which zero-voltage-switched resonant topologies become more efficient than zero-current-switched converters or related square-wave devices. Ways to estimate these losses are shown. Emphasis is placed on determining the losses due to C/sub dg/, since they are especially troublesome to calculate. It is observed that the concept of gate charge greatly improved the understanding of the MOSFET´s input characteristics, and there is some benefit to applying similar charge concepts to the output capacitances. A quick summary of gate charge concepts lays the foundation for an in-depth look at how gate charge data and losses due to C/sub dg/ are related.<>