An algorithm to analyze circulating current for multi-phase resonant converter

The inductor inductor capacitor (LLC) resonant topology has become more popular for deployment in high power density and high-efficiency power converter applications due to its ability to maintain zero voltage switching (ZVS) over a wider input voltage range. Due to their ease of operation and acceptable accuracy, frequency domain-related analytical methods using fundamental harmonic approximation (FHA) have been frequently utilized for resonant converters. However, when the switching frequency is far from the resonant frequency, the circuit currents contain a large number of harmonics, which cannot be ignored. Therefore, the FHA is incapable of guiding the design when the LLC converter is used to operate in a wide input voltage range applications due to its inaccuracy. As a result, a precise LLC converter model is needed. Time domain analysis is a precise analytical approach for obtaining converter attributes, which supports in the optimal sizing of LLC converters. This work strives to give a precise and an approximation-free time domain analysis for the exact modeling of high-frequency resonant converters. A complete mathematical analysis for an LLC resonant converter operating in discontinuous conduction mode (DCM)—i.e., the boost mode of operation below resonance—is presented in this paper. The proposed technique can confirm that the converter operates in PO mode throughout its working range; in addition, for primary MOSFET switches, it guarantees the ZVS and zero current switching (ZCS) for the secondary rectifier. As a function of frequency, load, and other circuit parameters, closed-form solutions are developed for the converter’s tank root mean square (RMS) current, peak stress, tank capacitor voltage, voltage gain, and zero voltage switching angle. Finally, an 8 KW LLC resonant converter is built in the hardware-in-loop (HIL) testing method on RT-LAB OP-5700 to endorse the theoretical study.

The effects of the transformer leakage inductance in a LLC converter that exploits the integrated magnetic concept are investigated. The vast literature on this subject always models the transformer considering its magnetizing inductance, together with a lumped leakage inductance located at the primary side. However, when the transformer windings are arranged so as to increase the leakage magnetic flux with the aim of achieving the desired value, two leakage inductances are indeed created, one associated to each winding. Using the Fundamental Harmonic Approximation (FHA), the converter behavior at steady-state is revised so as to highlight the effects of a split leakage inductance, while the dynamic behavior is investigated using the Generalized Averaging Method. A suitable design procedure is proposed that accounts for the more complex transformer model. Measurements taken on a 100 W rated LLC converter prototype confirm the theoretical forecasts.

In high-power applications, parallel operation is required to improve both efficiency and reliability. In an LLC converter, it is difficult to achieve parallel operation due to the gain sensitivity to component tolerance. This paper proposes a new structure of a common resonant capacitor for a multiphase LLC converter, which can achieve very good current-sharing performance with no additional components or control. To evaluate the current-sharing performance, a new first harmonic approximation (FHA) model is built in this paper to analyze the LLC converter with coupled resonant tanks. Both FHA analysis and powersim (PSIM) simulation results demonstrate the load-sharing improvement of the proposed common capacitor LLC resonant converter over the conventional two-phase LLC converter. The current-sharing performance from the FHA analysis and the PSIM simulation are compared, and the error of the FHA analysis is evaluated and explained. A 600 W two-phase LLC converter prototype based on the proposed method is also built to verify the feasibility. Excellent current-sharing performance (6% resonant current sharing error at a wide load range) has been achieved.

In this paper, a new common inductor current sharing method is proposed for a multiphase LLC resonant converter for high-power applications. Automatic current sharing is achieved by using a common resonant inductor connecting the resonant inductors in each LLC phase in parallel. The current sharing performance of the proposed method is evaluated under first harmonic approximation (FHA) assumption. The proposed method can automatically share the primary resonant current and the load current for all phases without any additional circuit and control strategy. A 600-W two-phase LLC converter prototype based on the proposed method is built to verify the feasibility. Excellent current sharing performance (less than 0.5% current sharing error at full load) has been achieved.

With development in the field of research, there is a need of improvement in present equipments. This paper is about improving the efficiency of DC-DC converter that will be operated with renewable sources like PV panel, fuel cell, etc. In the proposed model, a full bridge LLC converter has been developed for bidirectional operation. A PV array is connected to LLC converter which may be inverted using an inverter in Simulink. The low efficiency of PV array, Fuel Cell due to their non-linear characteristics have been improved by using MPPT controller which feeds the reference voltage in the converter's voltage regulator. The regulated DC voltage from LLC converter can be used with an inverter to provide a portable AC voltage source for electrical equipment.

The LLC converter is considered the most efficient topology in server and telecom applications. And, it has been demonstrated that three interleaved LLC converters can achieve further efficiency improvement at the 3KW power level. However, the magnetic components for multiphase LLC are complex, bulky, and difficult to manufacture in a cost-effective manner. In this paper, a high-frequency GaN based three-phase LLC converter is utilized to attempt to address these challenges. With GaN operating at 1 megahertz, all magnetic components, namely three inductors and three transformers, can be integrated in one common structure while all magnetic windings are contained in a compact 4-layer PCB with 3 oz. copper. The proposed structure can be easily and cost effectively manufactured in high quantities. Furthermore, up to a 20db reduction of common-mode noise, from 150KHz up to 30MHz, can be realized if 2 additional PCB layers are employed for proposed CM noise shielding. A 3kW 400V/48V 3-phase prototype is demonstrated, with an expected peak efficiency of 97.4% and a power density of 450W/in3 (28 kW/L).

In low-voltage, high-current, and high-distance-diameter (d-D)-ratio applications, the magnetically coupled resonant wireless power transfer (MCR WPT) has been confronted with some new challenges, including low coupling coefficient between transmission coils, high-voltage/current stresses on resonant components, large conduction loss of the coil branch, and power switches, which further deteriorate the system efficiency and reliability. In view of above issues, a novel three-stage scalable multimodule parallel MCR WPT architecture is proposed in this article, which includes the input stage, the coil stage, and the output stage, and the specific circuit topology of each power stage is determined subsequently. The transfer characteristics are analyzed by the model of a multimodule parallel system with two sub-modules using the fundamental harmonic approximation (FHA) method. Meanwhile, three different output-stage circuits and the corresponding control schemes are compared to optimize the system performance. Furthermore, a system parameter design guideline is presented from the aspects of high coupling coefficient, low-voltage/current stress on resonant components, and wide-range soft-switching realization. Finally, a 1 kW prototype with the d-D ratio of 1 (3030 cm) is built and the experiments on three different output-stage topologies are carried out and compared, and the optimal system efficiency is 85.2%.

This paper investigates a wireless power transfer (WPT) system based on the LCC-S compensation topology and proposes a natural implementation method for achieving constant-current (CC) and constant-voltage (CV) outputs without requiring additional control mechanisms. A system mathematical model is established through fundamental harmonic approximation, and analytical expressions for the output current and voltage are derived. The conditions for realizing constant-current operation (B = 0) and constant-voltage operation (satisfying specific resonance constraints) are clarified. Furthermore, the zero-phase-angle (ZPA) characteristics are analyzed to ensure unity power factor operation in both modes.Based on the theoretical analysis, key system parameters are designed and validated through simulation. At 100 kHz, stable constant-voltage output is achieved (output voltage of 15.95 V with a voltage gain of 1.0638). At 118 kHz, constant-current operation is obtained, and the output characteristics remain unaffected by load variations while still satisfying the ZPA condition. An experimental prototype is constructed for testing, yielding a measured receiver-side voltage of 15.16 V, which aligns closely with the simulation results. The observed deviation primarily arises from component tolerances, parasitic parameters, and measurement accuracy.The results demonstrate that the LCC-S topology offers excellent output stability and high efficiency potential in lithium-battery wireless charging applications, providing both theoretical guidance and practical reference for achieving efficient and reliable CC/CV charging.

Fringing losses play a detrimental role in high-frequency transformers. The tendency toward higher power density and miniaturization of power converters leads to higher switching frequency and enforces the use of low-profile components such as gapped planar transformers. Due to the air gap, lower magnetizing inductance and better regulation of LLC (L $_m$ , L $_s$ , C $_s$ ) can be achieved, but fringing fluxes can also be induced, resulting in extra magnetic losses and more hotspots. These losses are highly dependent on the frequency and the core material, so it is critical to model them for gapped planar transformers in LLC resonant converters, which operate at high frequencies and use ferrite material. In this article, fringing losses for gapped ferrite transformers in LLC converters are thoroughly modeled in order to provide a precise prediction regardless of the materials and core geometries. The proposed method provides an accurate and compact formula for predicting the fringing losses of planar transformers. This formula is obtained based on the finite-element method so as to consider and evaluate different design parameters. An LLC resonant converter with different planar transformers is implemented to show the compatibility of the proposed model. Experimental results show the higher accuracy of the proposed model compared to traditional ones and confirm that the proposed loss model can be applied to diverse core shapes. In addition, the gradient descent method is used to calibrate the theoretical and experimental results. Moreover, temperature deviations of the transformer due to the fringing losses are measured and evaluated both experimentally and theoretically to show the accuracy of the proposed loss formula. Due to the proposed model's higher accuracy, an improved design procedure for planar transformers is obtained, adding substantial value for design engineers.

Experimental investigation on the design of a three-phase power factor corrector utilizing a SEPIC converter

LLC resonant DC-DC converters have recently attracted attention for use in distributed renewable energy generation systems, considering the effects of isolation and high efficiency. However, it faces the problem of high voltage gain beyond regulation at light load condition and the challenge of DC bus voltage control during disturbance. In this study, a full-bridge LLC converter is transformed into a half-bridge LLC converter at light load conditions, by utilizing a simple transform method of drive signals. In this way, DC voltage is regulated while ripple is eliminated and working frequency is reduced. According to the proposed method, a new control strategy is proposed, which is used for voltage regulation and current sharing. Active Disturbances Rejection Controller (ADRC) is used to get good dynamic performance. This strategy is confirmed with experiments in MATLAB and a 7-kW full-bridge LLC prototype based residential photovoltaic power system.

This paper presents a reverse-feeding hold-up time strategy to improve the performance of two-stage grid-interface power factor correction (PFC) circuit with a rectifier-coupled boost inductor. During the hold-up time, instead of delivering the energy only through the dc-dc converter (e.g., a LLC converter, or a phase-shift full-bridge (PSFB) converter), the energy stored in the energy-buffer capacitor is also transferred through an extra-winding on the boost inductor to the secondary side. This additional energy transfer path regulates the output voltage during the hold-up time and offers extra flexibility to optimize the system performance during normal operation and hold-up time. With this configuration, the dc-dc converter can be designed as a almost fixed ratio dc transformer. The reverse-feeding strategy can extend the HUT by 25%, reduce the energy-buffer capacitor size by 25%, improve the system peak efficiency (96.5%) and enhance the power density. The effectiveness of the proposed method is verified by a PFC prototype with 100V-240V rms ac input and 800W, 12V/66.7A dc output.

LLC resonant converters are used in renewable energy applications to achieve high power efficiency conversions between different energy sources, buses, and energy storage elements. The traditional design methods for LLC resonant converters are based on the simple frequency domain analysis (FDA). However, the accuracy of FDA is not satisfactory, especially at wide voltage range applications. Considering the flexible power control requirements for photovoltaic systems, a wide voltage range operation is desired for the DC/DC converter. To overcome the drawbacks of traditional FDA method in wide range applications, the time domain analysis is adopted to achieve accurate analysis of LLC converters in this paper. Efficiency is normally selected as the optimization objective, where the circuit components and an accurate power loss model are required beforehand. To achieve a more general optimal design method for LLC converters, a simple root-mean-square (RMS) current and time domain analysis based optimal design method is proposed. The optimal design is achieved by minimizing the converter RMS current and ensuring some key design considerations. Moreover, an automated design tool for the proposed method is developed. Finally, a 2.5 kW experimental prototype is built with the estimated optimal circuit parameters. Experimental results under different operating conditions are demonstrated and the results are consistent with the theoretical analysis.

Two-phase Three-dimension (3D) common inductor LLC resonant converter is proposed to achieve automatic current sharing. The series inductor of each phase are connected by a Coupled impedance which is inductor, capacitor or short-circuit. A Coupled index is indicated to analyze the current sharing performance under three types converter based on the Fundamental Harmonic Analysis (FHA). The previous method only work at Coupled index equals to 1 and 0, However, the Coupled index can be design any value in (−∞, +∞) for two-phase 3D common inductor LLC converter, and the current sharing performance is better if the Coupled index is designed in negative value. Four steady-state operations under Coupled index is 1, 0.5, 0, −1 are discussed to verify the current sharing improving. A 600W, 12V two-phase LLC converter with short-circuit Coupled impedance prototype is built. The prototype verified the feasibility and demonstrated advantages of the proposed converter.

We propose frequency and voltage control method for a hybrid high-voltage direct current (HVDC) system for integrating an offshore wind farm into transmission networks. Conventionally, DC voltage level is maintained as constant regardless of the wind velocity and the grid frequency, so AC voltage fluctuates with active power variation in a hybrid HVDC system due to the innate reactive power absorption of inverter side line-commutated converter (LCC). However, in the proposed method, the DC voltage changes according to the wind velocity. Furthermore, DC voltage is simultaneously regulated to participate in frequency control in a communication-free manner. These two characteristics can be achieved by using five droop control methods proposed here. Additionally, by determining droop constants using the proposed determination methods, AC voltage can be maintained as constant during active power fluctuations. Therefore, using the proposed method, an offshore wind farm can be stably connected to mainland transmission networks and successfully used as frequency supporting resources, because voltage characteristics of the AC network are not destabilized. A small-signal state-space (SS) model of the target system was also developed to investigate stability of the proposed control. The utility of the proposed method is demonstrated by case studies using PSCAD and SS models.