Analysis of a Time-Length Compensation Algorithm for Elimination of Subharmonic Oscillation and Application in a Digitally Controlled Primary-Side Regulation Flyback Converter

Subharmonic oscillation mostly happens in the continuous conduction modulation mode. Digital slope compensation and digital reposition compensation are two digital realized compensation methods to eliminate subharmonic oscillation. In the above two methods, a fast digital to analog converter (DAC) for slope voltage shaping or an analog to digital converter (ADC) for current sampling is required. In order to reduce the cost and simplify the compensation, a matrix analysis method is first put forward to analyze the generation mechanism of subharmonic oscillation in the primary-side regulation (PSR) flyback converter with peak current control. Based on the matrix analysis method, a simple time-length compensation algorithm is presented to eliminate subharmonic oscillation. Only the switch-on time is applied to compensate the primary peak current or the switching period of the next switching cycle. No subharmonic oscillation occurs and only a low-speed DAC is required. The cost is low as the fast DAC or ADC is removed. A 48-W PSR flyback converter is implemented to verify the above analysis. Experiments prove that the matrix analysis method is feasible, and the time-length compensation algorithm is effective to eliminate subharmonic oscillation. It can be promoted to eliminate subharmonic oscillation in different control methods and other power topologies.