Three-phase LLC resonant converters can handle much higher power compared to half-bridge and full-bridge LLC converters, which makes them suitable for level 2 and level 3 battery charger applications. In addition to the unique features of LLC resonant converters, the three-phase structure provides higher power capacity and higher power density at higher power levels in comparison with single-phase structures. Although single-phase LLC resonant converters were thoroughly investigated in the literature for many different applications, limited work has been done on three-phase LLC converters. Unexplored problems with three-phase LLC resonant converters include issues with their limited gain range and also their poor light-load efficiency. In this paper, a modified three-phase LLC resonant converter with a new phase-shedding strategy is proposed. With the proposed modified topology and phase-shedding strategy, the wide gain range needed for covering the recovery zone of charging Li-ion batteries is realized. Moreover, with the proposed phase-shedding strategy, a significant efficiency improvement with light-load absorption charging is achieved. A 3kW prototype was developed to validate the performance of the proposed converter.

A novel cell voltage equalizer using a series LC resonant converter is proposed for series connected energy storage devices, namely battery, or super (or ultra) capacitor cells. The proposed circuit is an active voltage equalization circuit for energy storage devices that is low cost, small in size and equalizes the voltages quickly. Compared to state of the art solutions, the proposed series LC resonant circuit eliminates the complexity of multi-winding transformers and it can balance series connected energy storage devices in a short time by transporting energy successively between the cells having highest voltage difference. The detailed circuit operation and theoretical analysis are provided. Simulation and experimental results are presented to demonstrate the voltage equalization process from an initial voltage difference of 527 mV to a final difference of 10 mV in 900 seconds for three seriesconnected super capacitor cells.

Three-phase LLC resonant converters can handle very high power levels beyond the capabilities of half-bridge and full-bridge LLC topologies. Among other characteristics, three-phase LLC structures reduce output current ripple (small output filter), enable parallel power processing (low peak current), and provide good thermal distribution. However, all these key advantages can be severely compromised due to passive components tolerances, leading to undesired current balance issues in three-phase LLC resonant converters. Tolerances in resonant tank passive components are inevitable and lead to unequal peak currents between phases, uneven temperature distribution, and large output current ripple. This paper investigates imbalances in three-phase LLC converters and proposes a novel Trigonometric Current Balancing (TCB) technique using phasor analysis. In this strategy the required input voltage phase-angles are calculated to achieve balanced phase currents, even under severe unbalanced conditions. In some cases, the output filter current ripple is reduced to less than half. The methodology is verified with a 3 kW experimental prototype, which validates the analytical framework and effectiveness of TCB.

Rectification (SR) for LLC resonant converters has been developed to enhance the power conversion efficiency. Conventional SR driving strategies can be categorized as current-driven methods, vDS_ON sensing methods, and alternative approaches. The current-driven methods widely adopt current transformers (CTs) to sense the rectifier current and generate the SR driving signals. The CTs are lossy and bulky, which is unfavorable to high power density design. The vDS_ON sensing methods remove the current sensors by sensing the voltage across the on-state resistor of the SR MOSFET, yet the sensed signal is small and prone to be offset by the inductive voltage induced from parasitic components. This paper proposes a SR driving strategy based on the resonant capacitor voltage (RCV) to address those issues. The sensed resonant capacitor voltage is insensitive to the parasitic effects. In addition, the RCV strategy controls the SR on-time effectively over a wide range of operating frequency and loading conditions. Simulation and experimental results of a 650 W/24 V LLC converter are presented to validate the effectiveness of the RCV strategy. Compared with the conventional vDS_ON sensing method, the on-time error is greatly reduced, which improves the power conversion efficiency and reduces the SR MOSFET temperature.

This paper presents novel single-phase single-stage buck-boost inverters. The proposed inverters provide buck-boost operation for a wide variation of the input dc voltage. In addition, the proposed inverters are bidirectional and therefore can provide reactive power. Further, they require only one inductor. The proposed inverters also eliminate the common-mode leakage current problem by connecting output neutral to the midpoint of capacitors or directly to input voltage sources. Therefore, they are well suitable for photovoltaic applications. Although six switches are required, two switches are working at line frequency resulting in negligible switching loss. Of the four remaining switches, only two are switching under high frequency at a time depending on the half cycle of the output voltage. The circuit operations are demonstrated through the analysis of the proposed inverters. A 120 Vrms/ 60 Hz/ 400 W hardware prototype was constructed and tested. The experimental results verify the theoretical analysis.

In this paper, two single-phase single-stage split-inductor buck-boost inverters are proposed: type-I and II. The proposed type-I is unity power factor inverter, while the proposed type-II inverter is suitable for any power factor. Both topologies remove the shoot-through problem. Furthermore, the reverse recovery issues for all switches are eliminated; therefore, MOSFETs are used to obtain higher efficiencies. The proposed inverters operated at higher switching frequencies confer the additional benefit of smaller passive components. The current stresses of the two switches in the proposed type-I and II inverters are lower of the conventional boost inverter. The dead-time can be eliminated at both the high and line switching frequencies. In addition, the inductor conduction loss is minimized to improve efficiency. Moreover, the voltage stresses of the two switches in the type-I inverter can be much lower of its counterpart boost inverter. Most importantly, unlike the conventional high reliability inverters, the magnetic volume of the proposed high reliability type-I and II inverters is same of its counterpart boost inverter. The experimental results obtained for 500 W, 110 Vrms, 60 Hz hardware prototypes verify the analysis.