The drive for greatly increased power densities in switch mode power supplies (SMPS) is dramatically pushing the switching frequency up as a method for increased power density. This increase in switching frequency now puts severe limitations on the output filter capacitor’s electrical parameters and how it is physically mounted in the circuit.

Historically, ESR (equivalent series resistance) has been the primary output filter capacitor parameter that dominated output ripple voltage. The amount of capacitance required to meet this ESR requirement has been 10 - 1000 times the minimum capacitance that is needed for the power supply and the large number of capacitors used to get low ESR significantly reduced the effective ESL (equivalent series inductance). This has changed with switching frequencies departing from the 200 KHz range and moving into the 1 MHz and beyond. ESR is no longer the single dominant factor in output filter ripple and noise voltage, now ESL also becomes a limiting parameter for output filter capacitors.

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