A Digitally Current Mode Controlled Non-Inverting Buck-Boost Converter for Fast Voltage Transitions

Mode configurations in a non-inverting buck-boost (NIBB) converter offer wide voltage variations and make it useful for reference tracking applications. However, the reference transient performance is limited by the presence of right-half-plane (RHP) zero in the boost-mode. Further, the controller design based on linear small signal models often ignores switching dynamics and fails to capture the performance under large signal transients. This paper proposes a digital current mode control (CMC) tuning method in a NIBB converter for fast voltage transitions from buck-mode to the boost-mode and back in dynamic voltage scaling (DVS), LED driving, and envelope tracking. This considers a mixed-signal current mode control architecture with a simple PI voltage controller in the digital domain along with a reference feed-forward. Using phase-plane geometry, the objective is to tune the proportional gain, which drives the large signal transient recovery from buck-mode to the boost-mode and vice-versa. The proposed tuning method achieves near time optimal transient recovery using a fixed-frequency modulator throughout. Moreover, a reference feed-forward also ensures smooth mode transitions by adjusting the average inductor current in each mode of operation. A NIBB prototype is made and the proposed tuning method is experimentally verified using a Xilinx Virtex-5 FPGA device.