Combining peak current mode control with average current mode control using digitally assisted analog

Abstract

“Peak Current Mode Control” (CPM) is usually implemented in analogue and brings the advantage of cycle by cycle protection, higher bandwidth rejection of input voltage disturbance or equivalently increases a converter's input impedance. “Average Current Mode Control” (ACM) allows inductor current control to a “true average” and will facilitate an accurate current limit loop or Power Factor Correction with low Total Harmonic Distortion. ACM is however slower to react to line disturbance or over current (OC) faults and presents reduced converter input impedance burden on an input filter stage. In this work, analogue CPM is combined with a new digital average current mode control to provide the benefits of both control modes in a “Hybrid Current Controller” (HCM). The averaging and compensation functions traditionally implemented within an ACM compensated amplifier are separated out and implemented digitally. The digital control is designed to operate on the basis of one analog to digital conversion per PWM switching cycle and this combined with the low bias power required for a CPM comparator allows high performance PFC to be implemented in cost effective, low power and low pin count controller IC. Results in a 250W Boost Power Factor Correction Stage, capable of 250achieving less than 10mW in no load, employing this new HCM are presented.