The CLLC bidirectional resonant converter has significant potential in battery chargers and DC microgrids, due to its bidirectional power transfer capability. To ensure uniform characteristics for bidirectional operation, secondary LC resonant tank components are usually designed to equal the primary LC components after reflection. This conventional design method is regarded as the symmetric design. It is used in applications that do not require wide voltage regulation, such as CLLC DC transformers. However, for bidirectional battery charger applications, the battery has a wide voltage range variation, and gain requirements during charging and discharging are different. These asymmetric characteristics could lead to undesirable large switching frequency range if a conventional symmetric CLLC design is employed. To address this issue, a detailed Asymmetric Parameters Methodology (APM) is proposed in this paper. It can design gain curves for charging and discharging modes separately. This enables overlapping of the switching frequency range for both modes, thereby reducing the bidirectional frequency range variation. It brings lower switching loss caused by excessive high-frequency, and relieve the extra conduction loss and current stress of power switches as well. Finally, a detailed analysis of the proposed APM is provided and validated with simulations and experiments.

In recent years, railway power conditioner (RPC) has been used to improve the power quality of a traction power system. In engineering application of RPC, mismatches between parameters used in controller and actual values are inevitable, which will increase the difficulty of current controller design and deteriorate compensation performance. In this paper, a passivity-based control (PBC) system is studied for multilevel RPC to enhance its tolerance for mismatches. According to the topology of multilevel RPC, equivalent electrical and mathematical models are developed. To employ PBC, Euler-Lagrange system model of RPC is established by Park (αβ/dq) transformation, and the passivity of RPC and stability of PBC are proved. On this basis, the robustness of PBC is analyzed and criteria for damping selection are derived. Finally, simulation and experiments have been carried out to verify the structure and control method in the paper.

CLLC resonant converters are an attractive option for battery charging applications due to their bidirectional power flow and high efficiency. Pulse frequency modulation (PFM) is the simple method to drive CLLC converters. However, PFM cannot regulate the output voltage under the light-load condition properly, which leads to high switching and core losses. Phase-shift modulation techniques, such as dual-phase-shift (DPS) modulation, can overcome this challenge. However, more improvements are demanded due to high circulating currents in phase-shift modulations. In this article, an improved modulation technique based on triple phase shift (TPS) has been applied in CLLC converters to improve the light-load efficiency. TPS modulation like any other phase-shift modulation keeps the switching frequency of CLLC converters fixed at the resonant frequency under the light-load condition and regulates the output voltage by its three phase-shift parameters. The improved TPS modulation has lower circulating current at the same average output current compared to DPS modulation. Therefore, higher efficiency, smaller peak, and RMS current values can be achieved under the light-load condition. In addition, the proposed TPS modulation has a better flexibility in regulating the output voltage under the light-load condition due to an extra phase-shift parameter compared to DPS modulation. The proposed TPS modulation has been analyzed thoroughly in this article, and a proper selection of phase-shift values has been explained to improve the efficiency of the converter. Finally, advantages of the improved TPS modulation have been validated by simulation and a 1-kW experimental setup.

Unregulated CLLC converters have significant potential for dc transformers and two-stage battery chargers due to their high efficiency and bidirectional capability. In these applications, unregulated CLLC converters are usually designed to have their switching frequency fixed at resonance for maximum efficiency. However, the resonant frequency drifts from the designed value due to resonant tank parameter deviations, which leads to a decrease in efficiency for unregulated converters. To address this issue, this article proposes the bidirectional resonant frequency and a unified tracking algorithm. By analyzing the generalized resonant frequency equations in depth, an interesting feature is discovered for CLLC converters: forward mode and backward mode achieve maximum efficiency at an identical (but not constant) frequency. Thus, this unique frequency is defined as bidirectional resonant frequency (BRF). As the BRF varies with parameter deviations, a unified bidirectional tracking algorithm is proposed, which unifies the tracking of bidirectional maximum efficiency points into the tracking of a BRF. The introduction of the BRF combined with the unified tracking algorithm provides a simple, accurate, and low-cost solution for bidirectional resonance tracking of unregulated CLLC converters. Finally, experimental results show that unified bidirectional tracking and efficiency improvement are achieved under parameter deviations.

To manage the power quality of high-speed V/v traction system, a multilevel railway power conditioner (MRPC) is researched. According to the structure and equivalent models of MRPC, basically the MRPC is a symmetrical system, and its four clusters can be divided into two cluster-groups. According to the ac equivalent mathematical models of cluster-groups, it can be considered as the relationships in the αβ frame, but not completely symmetrical, so the positive- and negative-sequence dq models for cluster-groups can be derived. As the fluctuation and randomness of the locomotive loads, a sliding mode control system based on the exponential reaching law in the dq frame is explored to control the cluster ac currents, which can improve the dynamic performance and keep a good tolerance against parameter mismatches. Finally, an experimental prototype is developed, and the results show the feasibility and the robustness of the system.

Cascaded multilevel power factor correction (PFC) converters show great potential for applications that require high ac voltage input but low dc-bus voltage output, such as telecom power supply, battery formation, and Internet data centers. They are compact and put less voltage stress on power switches. To provide multidc outports with fewer power switches, a cascaded half-bridge-based multilevel multiport bridgeless PFC is proposed in this article. Compared with cascaded full-bridge multilevel PFC, the number of switches per power cell for the proposed PFC is reduced by half while maintaining the same dc outports. Due to the multilevel voltage, the volt–second on the boost inductor decreases, reducing the current ripple of the proposed PFC by $2n$ times with the same boost inductance as the conventional totem-pole PFC. By splitting the power into $2n$ cells with low voltage, the total switching losses are also reduced by $2n$ times with the same equivalent switching frequency. In addition, the proposed rectifier also reduces conduction losses as lower voltage switches with smaller conduction loss are adopted in half-bridge cells compared with conventional totem-pole PFC rectifiers. Finally, these benefits are analyzed and validated with experiments.