Llcc secondary overtone resonant power converter

Abstract

An LLCC Secondary Overtone Resonant (LLCC-SOR) power converter obtains dramatically higher efficiency with light loads by providing a resonance in the transformer secondary that is approximately tuned to an odd order overtone of the upper primary switching frequency, an upper frequency limit of the primary switching frequency, and a secondary duty cycle control that engages once the upper primary switching frequency limit is reached. The transformer circuit resonates in an LLCC-SOR mode that regulates the output voltage when the maximum frequency limit is reached. In operation, the gain of the resonant circuit is raised above its regulation point under light loads, forcing the controller into duty cycle mode. The secondary current completes an odd number of oscillations per single oscillation of the primary current, and the primary current returns to near zero after each switching transition. Also, a zero-voltage switching condition is maintained on the primary switch.