Stability Analysis of Constant Current Controlled Primary-Side Regulation Flyback Converter

In low power applications, primary-side regulation (PSR) flyback converter with discontinuous conduction modulation (DCM) mode is widely used to realize constant output current for its simple structure and low cost. In peak current control method, the output current can be predicted from the primary peak current, the demagnetization time and the switching period. Based on the current prediction, a pulse width modulation (PWM) mode and a pulse frequency modulation (PFM) mode are used. However, as the output current cannot be sampled directly in PSR flyback converter, traditional small-signal modelling method based on the control and output variables cannot be used. And there is no stability analysis for these constant current (CC) controlled PSR flyback converters. In this paper, a discrete matrix analysis method is proposed to analyze the stability of CC controlled PSR flyback converter. An iterative matrix of the control and output variable perturbations is obtained and the stability can be acquired from the eigenvalues. Oscillation occurs in above PWM mode and stable control is realized in PFM mode. An improved PWM mode is thus proposed and analyzed for stable control. The analysis is verified in a PSR flyback converter with 1.92A.