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.>