A Sub-harmonic Oscillation Elimination Method with PI Modulation in PSR Flyback Converter

To increase the power density and satisfy the higher power requirements, primary-side regulation (PSR) flyback converter working in continuous conduction modulation (CCM) mode is often used. However, with peak current control, the magnetizing inductor current will diverge from the stable value when the duty ratio is larger than 0.5, resulting in sub-harmonic oscillation and unstable system. In this article, a sub-harmonic oscillation elimination method with PI modulation is proposed. The stable control parameters of PI modulation are specially designed to eliminate sub-harmonic oscillation. And the stability of PSR flyback converter in CCM mode is analyzed from the eigenvalues and eigenvectors of the discrete iterative matrix, which describes the iterative relationship of the control and output variable perturbations. Slope compensation or current sensing with ADC device is not required, which greatly simplifies the control. The stability analysis and designed control parameters are experimentally verified on an field programmable gate array (FPGA) controlled 20-V/3-A PSR flyback prototype with CCM mode. Experiments prove that sub-harmonic oscillation is eliminated with the parameter-designed PI control method.