A novel single-switch high-gain converter with no transformers and no coupled inductors is studied in this paper. Since the voltage lifting unit is added to the Boost converter, the voltage gain of the converter is improved, the voltage stresses of the switch and diodes are reduced, and the conduction loss of the switch is reduced under the condition of a small duty cycle. As a result, the efficiency of the converter is improved. To further improve the dynamic performance and anti-disturbance capability of the converter, the immune feedback mechanism is introduced based on the analysis of a single neuron controller. A fuzzy immune-single neuron PID control strategy is studied in this paper, in which the fuzzy immune control is combined with the single neuron smart controller to realize self-tuning of the single neuron proportional coefficient. Finally, a simulation study of the proposed converter and control strategy was carried out, and an prototype with an output of 200 V/0.5 A was designed for experimental verification. Both the simulation and experimental results show that the proposed converter can obtain a higher voltage gain under a smaller duty cycle. Compared with the traditional PID control strategy, the proposed fuzzy immune-single neuron PID control strategy can more effectively suppress system disturbances and improve the dynamic performance of the converter, indicating a stronger adaptive capability and a stronger robustness.

In order to improve the DC voltage gain and reduce the electrical stress, a novel high voltage gain DC-DC converter based on Z-source is proposed. Theoretically, the ratio of output voltage to input voltage can reach (2-D)/(1-2D). Compared with the traditional diode capacitor filter Z-source DC-DC converter, the proposed topology can provide a higher DC voltage gain at the same duty cycle. It has lower voltage stress and inductance current stress when the DC voltage gain is the same. In addition, the input port and output port of the proposed DC-DC converter share the common ground, which helps to reduce the electromagnetic interference of the system. On this basis, the steady-state principle and characteristics of the proposed DC-DC converter are introduced, and the parameter design and theoretical efficiency calculation are also carried out. Finally, an experimental prototype with a power level of 200 W was fabricated, and experimental results proved the feasibility and superiority of the proposed circuit topology.

Owing to their merits including continuous input and output current, high efficiency and high power density, non-isolated Superboost converters are widely applied in spacecraft power systems. However, the switching loss of the device will increase in a scenario with a high step-up ratio, resulting in a decrease in the converter efficiency. To solve this problem, a zero-voltage switching pulse-width modulation (ZVS-PWM) Superboost converter with low voltage stress is proposed. By introducing a resonant tank, the main switch can be turned on or off under ZVS, and the auxiliary switch can be turned on under zero current switching and turned off under ZVS. Besides, all the diodes are operating under soft-switching. As a result, the switching loss is reduced effectively, and the converter efficiency is improved without increasing the voltage and current stress of the main power device. The operation principle, soft-switching conditions and device stress are analyzed in detail, and the state-space averaging approach is used to estimate the steady-state and dynamic characteristics of the proposed converter. In addition, its feasibility was verified by a prototype with 100 kHz and 400 W.

The active-clamped soft-switching inverter can realize the soft-switching of power devices, which is conducive to improving the power density and dynamic performance of the inverter. However, when overcurrent occurs, if the conventional cycle-by-cycle(CBC) current limit strategy( i.e., a strategy under which power devices will be blocked once overcurrent occurs) is adopted, the DC bus current will change its direction from flowing to the inverter bridge to flowing to the DC side. Due to the existence of a resonant inductor, both the DC bus current and the current flowing through the resonant inductor flow through the auxiliary switch, so there is high current stress on the auxiliary switch. In this paper, an improved CBC current limit strategy is proposed. By changing the switching state of the inverter bridge after the CBC current limit strategy is triggered, the DC bus current flowing to the DC side is reduced, thus significantly suppressing the current stress. In addition, the protection strategy was verified by an experiment of 3 kW active-clamped soft-switching inverter.

Aimed at the problem that the particle swarm optimization (PSO) algorithm has a poor global search capability and is easy to fall into local optimum when it is applied to the selective harmonic elimination pulse width modulation (SHEPWM) of an inverter, an improved PSO algorithm is proposed. The vertical crossover operation of genetic algorithm is introduced to the search process, and the elite retention strategy is used to improve the global and local search capabilities of the algorithm and retain excellent individuals, thereby increasing the accuracy of switching angle and improving the performance of SHEPWM. This algorithm is used to solve the SHEPWM nonlinear transcendental equation of a novel asymmetric cascaded switched capacitor multi-level inverter, which overcomes problems such as the high dependence of traditional numerical methods on initial value and the low accuracy of the traditional PSO algorithm in solving the switching angle. Finally, the feasibility of the proposed topology and the effectiveness of the proposed algorithm applied to SHEPWM were verified by simulation and experimental results.

Aimed at the problem that the converter current ripple and electromagnetic interference (EMI) noise will increase due to the increasing switching frequency of a three-level neutral point clamped (NPC) converter, a variable switching frequency modulation strategy based on current ripple prediction is proposed to reduce the current ripple, harmonic noise and EMI noise of the three-level NPC converter. According to the requirements of current ripple, the switching cycle and sampling cycle are calculated to synthesize the latest switching cycle and form feedback, so as to realize variable frequency modulation. The random cycle is distributed around the expected cycle, so that the harmonic noise and electromagnetic noise of the converter are more evenly distributed in a wide frequency band. As a result, the electromagnetic noise of the converter is reduced, and the output inductance current ripple is improved. The relevant simulation and experimental results verified that compared with those under the traditional modulation strategy, the common mode noise was reduced by about 20 dB/µV under the proposed modulation strategy, the differential mode noise was reduced by about 10 dB/µV, and the amplitude of output inductance current ripple was also reduced accordingly.

A novel nine-level inverter topology with double boost and reduced components is proposed by combining the voltage boost capacity of switched capacitors with the voltage halving characteristics of a coupling inductor. This inverter consists of 12 switches, 2 switched capacitors and 1 reverse polarity coupling inductor. The charging and discharging of each capacitor in the inverter can maintain a self-balance, so the additional balance circuit is not needed. Compared with the topologies of most of the existing nine-level inverters that are based on switched capacitors, the proposed topology has a smaller capacity of switched capacitor. The output voltage levels are greatly increased by the adoption of the reverse polarity coupling inductor, and the current stresses on some switches are reduced by half, which can further reduce the switching loss. The working modes, modulation strategy, design of switched capacitors and loss analysis of the proposed topology are discussed in detail. In addition, the proposed topology is compared with those of other nine-level inverters, and its advantages are introduced. Finally, simulation and experimental results verified the effectiveness of the proposed topology.

For an LCL-type inverter connected to weak grid, the appearance of grid impedance often results in a decrease in the phase margin, serious distortion of grid-connected current and even system instability. To solve this problem, an improved grid-connected current control strategy is proposed, in which a multi-resonance controller is introduced in the voltage feedforward loop to suppress the voltage background harmonics and a phase compensator is added to the current feedforward loop to improve the system's phase margin, so as to avoid the risk that the resonance peak of the multi-resonance link intersects with the -180° line. Theoretical analysis and simulation results show that the proposed strategy can effectively suppress the harmonics of LCL-type grid-connected current, improve the current quality and enhance the stability of the grid-connected system.

The smooth switching between grid-connected and islanded microgrid operation modes and the stability of system frequency are important guarantees for the safe and stable operation of a master-slave microgrid system. Combined with the operating characteristics of the microgrid system, an off-grid switching method based on phase angle switching is proposed, and a pre-synchronization control module is added to make sure that the grid phase is quickly tracked and compensated when the islanded mode is switched to the grid-connected mode, thereby solving the problem of shocks in the output voltage and current from master and slave inverters in switching. To ensure the frequency stability of the microgrid system, a V-F frequency control strategy based on fuzzy droop control is proposed and further applied to the smooth switching based on phase angle switching, so that the system frequency is basically maintained at 50 Hz and the problems of large oscillations in voltage and current and frequency shocks in switching are avoided. Finally, an experimental platform was built to verify the effectiveness and stability of the proposed method.

The cascaded H-bridge is considered as one of the most suitable topologies for photovoltaic (PV) power generation. Aimed at the problems of the traditional three-phase cascaded H-bridge PV inverter such as a large capacitor volume, a short service life, inter-phase power mismatch and a complex control communication system, a novel modular three-phase PV inverter and its distributed control strategy are proposed based on the principle of magnetic flux cancellation. First, the basic structure of the proposed modular topology is introduced. Then, the basic principle of magnetic flux cancellation power decoupling and the influencing factors of double-line frequency voltage ripple are analyzed in detail, and a distributed control strategy is proposed to suppress the double-line frequency voltage ripple and ensure the balance of three-phase output power. Finally, the correctness of theoretical analysis and the feasibility of the proposed control strategy were verified by simulation and experimental results.