The drive for greatly increased power densities in switch mode power supplies (SMPS) has pushed switching frequencies to one megahertz and beyond. This switching increase now puts severe limitations on output filter capacitor electrical performance, capacitor technology, and mounting techniques. Traditional capacitor solutions no longer work at these higher frequencies because ESR (equivalent series resistance) is not the limiting electrical parameter; performance or ripple voltage is now limited by ESL (equivalent series inductance).

The amount of capacitance required for older low frequency switchers has been 10-1000 times the minimum capacitance required to achieve desired ESR values. Switching noise due to inductance in output networks is now limiting output ripple and not resistance. Output filter inductance values are now limited to only a few nanohenries or less forcing designers to consider surface mount technology (SMT) to eliminate leads and its associated inductance. Shifting to SMT will reduce output filter capacitance values to actual minimums for physical size and inductance reasons. Also, traditional output filter technologies must be abandoned in favor of new capacitors with configurations and electrical characteristics that meet those future needs.

Read more: http://www.avx.com/docs/techinfo/pgsmps.pdf