An Adaptive Variable-Frequency Control With Constant Crossover Frequency Achieving Fast Transient Response for Wide-Operation-Range Flyback Converter

Flyback converters with universal serial bus power delivery (USB-PD) specification are widely used in low-power adapter application. The wide output voltage range operation of USB-PD makes the loop gain vary under different working conditions. To ensure stability in all working conditions, the loop gain has to be compensated based on the worst case of control-to-output transfer function. However, in other conditions, the crossover frequencies of loop gain are sacrificed, resulting in poor transient responses. This paper proposes an adaptive variable-frequency peak current mode control with constant crossover frequency to mitigate the tradeoff between stability and transient response while possessing the variable frequency characteristic to improve light load efficiency. The adaptive control law is derived from the proposed small-signal model. Experimental and simulation results verify the analysis. The proposed control reduces the output voltage variation at load transient by 43% and 34% at 5 and 20 V output voltage, respectively.