This paper proposes a new bidirectional buck-boost converter, which is a key component in the photovoltaic and energy storage system (PV-ESS) for smart grid. The conventional bidirectional buck-boost converters for ESS operate in discontinuous conduction mode (DCM) to achieve zero-voltage-switching turn-on for switches. However, the operation in DCM causes a high output voltage/current ripples and a low power-conversion efficiency. To improve the performance of the conventional converter, the proposed converter has the new combined structure of a cascaded buck-boost converter and an auxiliary capacitor. The combined structure of the proposed converter reduces the output current ripple by providing a current path. Therefore, the output voltage ripple is reduced and the efficiency is increased. To verify the effectiveness of the proposed converter, a prototype was built and tested. The proposed converter with a maximum efficiency of 98% had an output voltage ripple of less than 5.14 V $_{p.p}$ , and an output current ripple of less than 7.12 A $_{p.p}$ at input voltage of 160 V, switching frequency of 45 kHz, output voltage of 80 ~ 320 V, and output power of 16 ~ 160 W. These experimental results showed that the proposed converter improved the performance of the conventional converter.

The basic and adaptive maximum power point tracking algorithms have been studied for distributed photovoltaic systems to maximize the energy production of a photovoltaic (PV) module. However, the basic maximum power point tracking algorithms using a fixed step size, such as perturb and observe and incremental conductance, suffer from a trade-off between tracking accuracy and tracking speed. Although the adaptive maximum power point tracking algorithms using a variable step size improve the maximum power point tracking efficiency and dynamic response of the basic algorithms, these algorithms still have the oscillations at the maximum power point, because the variable step size is sensitive to external factors. Therefore, this paper proposes an enhanced maximum power point tracking algorithm that can have fast dynamic response, low oscillations, and high maximum power point tracking efficiency. To achieve these advantages, the proposed maximum power point tracking algorithm uses two methods that can apply the optimal step size to each operating range. In the operating range near the maximum power point, a small fixed step size is used to minimize the oscillations at the maximum power point. In contrast, in the operating range far from the maximum power point, a variable step size proportional to the slope of the power-voltage curve of PV module is used to achieve fast tracking speed under dynamic weather conditions. As a result, the proposed algorithm can achieve higher maximum power point tracking efficiency, faster dynamic response, and lower oscillations than the basic and adaptive algorithms. The theoretical analysis and performance of the proposed algorithm were verified by experimental results. In addition, the comparative experimental results of the proposed algorithm with the other maximum power point tracking algorithms show the superiority of the proposed algorithm.

This paper proposes a quasi-resonant boost half-bridge DC-DC converter with high power conversion efficiency and a wide input voltage range for photovoltaic micro-inverter. The proposed converter has a combined structure of a boost integrated half-bridge converter on the primary side of the transformer and a voltage doubler with a snubber capacitor on the secondary side. By using the quasi-resonance techniques, it achieves zero-voltage switching (ZVS) turn-on and low turn-off losses for the switches, as well as ZVS turn-on for the diodes. In addition, the proposed converter has no DC-magnetizing current for the transformer due to the DC blocking capacitor, and it doubles the voltage gain with half the voltage stress of diodes through the voltage doubler. The effectiveness of the converter was verified with a 300-W prototype with input voltage range of 13 ~ 50 V, output voltage of 400 V, and switching frequency of 50 kHz. The experimental results show that the converter had more than 90.0% power conversion efficiency for the output power range of ~ 300 W, and its maximum power conversion efficiency was 97.5% for an output power of 160 W.

This paper proposes an asymmetric forward-flyback dc-dc converter that has high power-conversion efficiency η _e over a wide output power range. To solve the problem of ringing in the voltage of the rectifier diodes and the problem of duty loss in the conventional asymmetric half-bridge (AHB) converter, the proposed converter uses a voltage doubler structure with a forward inductor L _f in the second stage, instead of using the transformer leakage inductance, to control output current. L _f resonates with the capacitors in the voltage doubler to achieve a zero-voltage turnon of switches and a zero-current turn-off of diodes for a wide output power range. The proposed converter could operate at a wider input voltage range than the other AHB converters. η _e was measured as 95.9% at output power P _O = 100 W and as 90% at P _O = 10 W, when the converter was operated at input voltage 390 V, output voltage 142 V, and switching frequency 100 kHz.

This paper presents a control method that can prevent audible noise generation in an LLC resonant half-bridge dc-dc converter at a light load condition, while achieving the same input voltage range and power conversion efficiency $\eta_e$ as the burst-control method that generates an audible noise. The proposed method reduces switching and conduction losses at light load by skipping several pairs of switch-control pulses, while varying the switching frequency less than the normal-control method does. This skip-control method enables the LLC resonant converter to have a large magnetizing inductance, and to have high $\eta_e$ over a wide range of load variation. At an input voltage of 385 V, an output voltage of 24 V, and a resonant frequency of 180 kHz, the proposed method achieved $\eta_e\geq 85.46\%$ for an output power range of $7.2 \sim 360 W$; the highest $\eta_e$ was 96.08% at $P_o= 336$ W.