Limit-Cycle Free Digitally Controlled Power Converters.

Control of switch-mode power converters, which has been traditionally carried out using analog circuits, is nowadays preferably performed digitally due to greater flexibility, higher integrability, less sensitivity to thermal changes, lower costs and possibility to implement advanced control techniques and, for these reasons, it has become a major research focus in power electronics field. Despite the mentioned advantages digitally controlled power converters could cause the onset of non-linear issues due to inherent quantization effect of analog to digital converter (ADC) and digital pulse width modulator (DPWM) which converts the controller output from digital into analog signals. Such quantization effects cause steady-state limit cycle oscillations (LCOs) that are a major concern. The mitigation or the reduction of these problems have been a topic of extensive research in the field of power electronics and design guidelines for LCO-free operation have been formulated. In particular, it has been shown that the resolution of DPWM should be higher than that of ADC for LCO-free operation. Meeting the above guideline, unfortunately, results either in a limited DC accuracy and/or in increased cost and complexity especially for converters operating at high switching frequency (MHz range) taking advantage of emerging semiconductor device technology (GaN, SiC power transistors). In this thesis, an innovative technique intended to increase the resolution of the DPWM for LCO-free operation of switch-mode power converters is analyzed and experimentally evaluated. More precisely, the novel Dyadic Digital PWM (DDPWM) is adopted as a systematic approach that generalizes and extends the standard PWM dithering techniques to achieve accurate, LCO-free operation in a digital converter at negligible cost and design effort and without any detrimental effect on the output ripple voltage. A digitally controlled DC-DC Boost converter is considered to validate the approach proposed in the thesis. This choice is justified by the growing importance of the Boost converter in a variety of practical applications concerning renewable energy technologies, power efficiency and power quality. Among the various applications, the boost is employed as one of the main building blocks in photovoltaic power systems. In this thesis, the converter is designed to operate efficiently over a range of the input voltages and to supply a constant current to the external load, and the effectiveness of the DDPWM in mitigating the onset of the LCOs is verified versus different operating conditions and digital control parameters. More specifically, the converter is designed to be operated in continuous-conduction-mode (CCM) with a switching frequency in the MHz range while voltage-mode digital control algorithms are considered by referring to the implementation form of the PID compensator. Simulation tools, in these phases, are employed to optimize the design of the power stage and the controller with the aim to analyze both the static and the dynamic performance of the converters. A printed circuit board (PCB) is designed to accommodate the power stage employing the last generation of Enhancement Mode Gallium Nitride (eGaN) power transistors while the digital control algorithm has been prototyped and implemented on a field programmable gate array (FPGA) interfaced to the power stage by adopting state-of-the art analog to digital converter (ADC).