Active damping of a 6th order 3LC output filter with state-feedback

Standard solution to reduce switching ripple in power converters is to employ LC(L) output filters, with higher attenuation requiring higher switching frequency and/or larger filter size. High switching frequency can increase the converter losses, whereas large filter reduces the bandwidth and increases cost of the power converter. The alternative path of increasing the filter order is taken in this paper. To be more exact, a 6th order filter consisting of 3 series connected LC blocks is employed to achieve very high ripple attenuation and increased converter bandwidth without increasing the switching frequency. In contrast to previously proposed high-order filter solutions active damping is employed yielding no penalty to the converter efficiency. The active damping is achieved via state-feedback control, i.e., Linear Quadratic Regulator, with the state information reconstructed only from capacitor voltage measurements with a Luenberger observer. The complete design is evaluated in simulations and validated on the experimental synchronous buck converter achieving in excess of −80 db ripple attenuation in one decade.