Based on the fact that inductance and capacitance are of fractional-order, the nonlinear dynamic characteristics of a fractional-order Boost converter are studied. The predictor-corrector model of the Boost converter is established using the predictor-corrector algorithm of fractional-order calculus. On this basis, the bifurcation diagrams with the reference current, input voltage and orders of capacitance and inductance as bifurcation parameters are obtained. The period doubling bifurcation and chaotic behaviors of the fractional-order Boost converter are studied, and its nonlinear dynamic behavior is compared with that of an integer-order Boost converter at the same time. Results show that under certain operating conditions, some nonlinear phenomena such as bifurcation and chaos will appear in the fractional-order Boost converter with changes in some circuit parameters. Under the condition of the same circuit parameters, the parameter stability domains of integer-and fractional-order converters are different. Compared with that of the integer-order converter, the parameter stability region of the fractional-order converter is smaller, which more truly reflects the nonlinear dynamic characteristics of the Boost converter.

Aimed at the poor voltage regulation performance and large circulating current of the traditional frequency-controlled LLC resonant converter in wide voltage applications, a six-switch dual-resonant LLC converter is designed, which is a hybrid combination of two full-bridge LLC resonant converters sharing one bridge arm. Compared with the traditional frequency control, fixed-frequency phase-shift control is employed in this converter to adjust the output voltage and reduce the switching frequency range. According to different connection modes of two transformers, the converter has two topological forms. When the two transformers are connected in series in the forward polarity, the converter's gain range is 0-1, which can achieve an ultra-wide output voltage range. When the two transformers are connected in series in the reverse polarity, the gain range is 1-2, and the circulating current loss during the working process is small. Under the two topological forms, the ZVS turn-ON of the primary switch tube and ZCS turn-OFF of the secondary diodes can be achieved, respectively. Finally, the validity of the research in this paper was verified by Simulink simulations and experimental results.

For a traditional Boost power factor correction (PFC) converter, its output voltage must be greater than its input voltage, which limits its applications in light emitting diode (LED) drivers to a certain degree. Meanwhile, due to the existence of a rectifier bridge at the input end of the traditional LED driver, the improvement of its efficiency is also confined. In this paper, a bridgeless Boost LED driver based on passive current balancing is proposed, which is based on the topology of a resonant Boost PFC converter. With the introduction of a resonant capacitive current-balancing network, the output current of each LED string can be balanced. In addition, the elimination of the rectifier bridge further improves the system efficiency. Finally, a 140 W prototype with a peak efficiency of 93.64% was built, and experimental results verified the correctness and feasibility of theoretical analysis.

The large grid inductance in weak grid may cause a grid-connected converter to be unstable. Therefore, an impedance model of grid-connected converter is built at first, and the influence of grid inductance on the stability of grid-connected converter is analyzed according to the impedance ratio criterion. Then, aimed at the problem of low adaptability of the grid-connected converter to inductive grid impedance, a virtual impedance control strategy based on band-pass filter is proposed, and the influence of virtual resistance value on the adaptability of grid-connected converter to weak grid is studied. Furthermore, a selection principle for the virtual resistance value is also given. Finally, a system simulation model is built, and simulation results verify the correctness of theoretical analysis and the effectiveness of the proposed control strategy.

As a commonly used islanding detection method, the active detection method can reduce the non-detection zone by increasing the disturbance intensity, but it will also increase the degree of harmonic pollution to the grid at the same time. Aimed at this problem, a hybrid fuzzy low-harmonic islanding detection method without non-detection zone is proposed in this paper. First, the non-detection zone is judged, and the passive islanding detection method which has no harmonic pollution to the grid is used preferentially. If this zone is within the non-detection zone of the passive detection method, then a fuzzy low-harmonic islanding detection method with fuzzy disturbance control is used to disturb the output current in its second and fourth quarter periods. Finally, the corresponding simulation model is built to verify the effectiveness of the proposed method.

In a weak grid, due to the existence of grid impedance, the natural resonant frequency of a new energy grid-connected LCL filter will shift, and the traditional active damping control strategy cannot guarantee the system stability. Moreover, as the proportion of new energy power generation in the power system continues to grow, how to reduce the operating costs is a hot topic for research. Therefore, a novel control strategy based on grid-connected current and common coupling voltage feedback is proposed in this paper, which not only provides active damping to suppress LCL resonance, but also reduces the use of sensors. In addition, it has a strong adaptability under wide-ranging changes in grid impedance. Simulation and experimental results show that, compared with that under the traditional control strategy, the practical range of weak grid under the improved strategy increases, the system stability is enhanced, and the capability to suppress harmonics is raised, indicating that the quality of grid-connected current is well improved.

Aimed at the problem that DC-DC converters with coreless transformers must operate at high frequencies, a DC-DC converter with a coreless transformer based on free decay oscillation is proposed. First, a circuit model in series-series topology with no excitation sources is established, the evolution of the eigenvalue is discussed, and the free decay oscillation behavior of the system is analyzed. Second, the DC-DC converter with a coreless transformer based on free decay oscillation is designed. The switching frequency of this converter can be significantly lower than the operating frequency of the coreless transformer, and the output power from the converter and the operating frequency of the coreless transformer can be described by the eigenvalue of the free decay oscillation system. Finally, an experimental prototype was constructed, which was under constant duty control. The switching frequency of the converter was reduced to one half(103 kHz) and one third (69 kHz) of the operating frequency of the coreless transformer (206 kHz), respectively. The converter efficiency achieved 91.2%, and the transformer efficiency was always higher than 96%.

In this paper, a Boost converter model in the pseudo-continuous conduction mode(PCCM) of inductive current based on the definition of R-L fractional-order is studied. On this basis, the state-space averaging model of the converter is derived. Then, the expressions for the step-up ratio, DC static operating point, inductive current ripple, and output voltage ripple are derived. Results show that compared with the corresponding expressions derived on the basis of the definition of Caputo fractional-order, the derived expressions for DC static operating point and step-up ratio under the definition of R-L fractional-order are related to both the duty cycle and the orders of the capacitor and inductor. The expression for output voltage ripple is not only related to the order a of fractional-order inductor, but also related to the order ? of fractional-order capacitor. Therefore, the orders of inductor and capacitor obviously affect the DC component of state variables and the stable-state characteristics of factional-order PCCM Boost converter. Finally, a mathematical model and a circuit model of the R-L fractional-order PCCM Boost converter are built in MATLAB/SIMULINK, and simulation results show that the analysis result of the R-L fractional-order PCCM Boost converter model is more stable than that of the other PCCM Boost model, and the corresponding error is smaller.

Aimed at the low conversion efficiency and all the load power that needs to be processed by converters at different stages in the two-stage architecture of a DC transformer cascaded PWM converter, a quasi single-stage DC-DC conversion method with partial power active regulation based on a split-sigma structure is proposed. By splitting the output port of the DC transformer into two ports, the power of one port is directly transmitted to load without being processed by the back-stage PWM converter, thus realizing the quasi single-stage power conversion equivalently. In addition, the power capacity and loss of the PWM converter are effectively reduced, and the system efficiency is improved. The principle and circuit implementation method for the split-sigma structure are analyzed in detail, and the principle and characteristics of system output voltage regulation are also studied. One of the circuits is taken as an example, and its working principle, voltage regulation characteristics and design method for key parameters are analyzed. Finally, experimental results verified the effectiveness and correctness of the proposed scheme.

Aimed at the problems of complex topologies, high total standing voltage of switches, and unbalanced series capacitor voltages in the existing multilevel inverters, a novel seven-level inverter based on switched-capacitor is proposed. By adopting the phase disposition pulse width modulation strategy, three capacitors are reasonably controlled in series/parallel with the DC source, so that the seven-level output with a triple boost gain is realized. The proposed inverter uses only one single DC source, so it has advantages such as a simple structure, fewer components, high boost gain and self-balanced capacitor voltage. In addition, it does not use the H-bridge to change the polarities of output levels, thus reducing the total standing voltage of switching devices. The topology, working principle, capacitor voltage self-balance, modulation strategy, capacitor parameter and current stress are analyzed, and five aspects including the numbers of switches, diodes and capacitors, the total standing voltage and the boost gain are compared with those of the existing topologies, which fully proves the practicality of the proposed topology. Finally, the feasibility of this topology was verified by experimental results.