Stable DCM Design of a Novel Variable Frequency Peak-Current State-Machine Controller

Abstract

The peak-current control that is modeled in this paper is a novel variable-frequency control that maintains regulation in a flyback converter by enabling and disabling cycles instead of modulating the pulse-width, which allows for elimination of feedback compensation and can provide for peak power 3 times higher than the rated full load power without increasing the power supply component sizes. A state machine sets the peak current of enabled cycles, based on the load requirements. At light loads, the variable frequency nature of this control scheme reduces switching losses significantly, while the lowering of the peak current by the state machine reduces the output ripple and audible excitations of the transformer. The state machine control is completely modeled for the first time, including new modeling for the peak-current ratio between DCM states, the number of DCM states required for stability, the minimum number of counts for DCM state transitions required for stability, and DCM modeling for a new peak state with double the maximum switching frequency, which provides peak power during brief peak loads, without requiring an over-rated transformer [1-3]. Characterization of a supply that was designed around the monolithic IC that uses this control scheme is provided.