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.

Extensive solutions have been proposed to damp LCL filter resonance. However, the time delay caused by computation and modulation process aggravates the complexity of current loop analysis. In this paper, a generalized virtual impedance model for six kinds of commonly used active damping methods is proposed, and the physical meaning is established by introducing the metric of “damping factor”. To ensure system robustness in a wide range of grid conditions, a hybrid active damping strategy that combines inverter current feedback and capacitor voltage feedforward is proposed. Importantly, this method only requires inverter current and capacitor voltage sensors, which are the basic variables for over-current protection, power control and synchronization. The superposition theorem is also utilized to analyze the damping capability provided by parasitic resistances, current feedback loop and voltage feedforward loop. Finally, experimental results verify the feasibility and robustness of the proposed damping method.

In the past decade, extensive active damping solutions based on state variable feedback have been proposed to damp LCL resonance. In this letter, an innovative control strategy, which can achieve overcurrent protection, injected power control, and active damping at the same time, is proposed with the inverter-side current feedback only. Moreover, to avoid the mathematical limitations when solving characteristic roots of a third-order system, the damping-factor-based design method is proposed for the first time, by analyzing its equivalent circuit. The optimal damping factor, which is recommended to be positive and as large as possible, is searched to achieve superior resonance suppression effect. Finally, the proposed control and design method is verified by experimental results and shows good robustness both in stiff and ultraweak grid conditions.

Axes cross-coupling in dq-frame will deteriorate the current control performance, especially for applications with low switching/sampling frequency. Decoupling methods based on inductor current state feedback and complex vector proportional-integrator controllers are usually performed to solve this problem. In this paper, an alternative decoupling method based on preprocessed reference current feed-forward is proposed, which can provide excellent decoupling performance even with considerable control delay. The current control of grid-connected voltage source converters is analyzed with complex-coefficient transfer functions. It is revealed that the decoupling performance of current control schemes is closely related to the symmetry of current closed-loop transfer function bode diagrams. The current closed-loop transfer function should show a standard symmetrical bode diagram with respect to 0 Hz, if axes cross-coupling is well decoupled. A quantitative index named cross-coupling function is then proposed to analyze and comprehensively compare the decoupling performance of the proposed method and existing ones in terms of the influence of the control delay, inductance estimation error, grid frequency deviation, and voltage disturbance. Experimental tests are implemented to validate the performance of the proposed method and comparison results.

Dual active bridge (DAB) converters either adopt single-degree-of-freedom (SDOF) modulation or multiple-degree-of-freedom modulation. The SDOF single phase shift (SPS) modulation has low power transmission efficiency under light load, and the multiple-degree-of-freedom modulation has high requirements for model accuracy and controller design. Focusing on these limitations, this paper proposes an SDOF hybrid modulation method to optimize transmission efficiency, switching device current stress, and zero-voltage-switching (ZVS) implementation under the light-load condition. To this end, first, the SDOF SPS, primary-side internal phase shift (PS-IPS), and secondary-side internal phase shift (SS-IPS) modulation methods are comprehensively analyzed in terms of current stress, ZVS feature, and transmission power. Second, by comparing the current stress, ZVS, and loss features of SDOF modulation methods under the light-load condition, the case-dependent optimal modulation method is determined. On this basis, an SDOF hybrid modulation strategy within the light-load range is developed by resorting to a proper switching rule, which judges the voltage ratio between the two ends of the inductor and the size of the transmission power. Finally, a 600 W experimental prototype is built, and the results show that the proposed strategy effectively improves the transmission efficiency of the converter under the light-load condition.

Diode-capacitor-based dc-dc converters provide a simple and low cost solution for high step-up voltage regulation in solar and fuel cell generation. Transient modeling analysis reveals their worse influence of nonlinear and nonminimum-phase system characteristic due to right-half-plane (RHP) zero, especially in high voltage gain application. However, the process of energy transfer for diode-capacitor-based dc-dc converter is different from basic dc-dc converter. Based on the unique feature, this paper proposes an improved main circuit structure with parallel connection of resistive-capacitive damping network across the intermediate capacitor to achieve good dynamic performance. By optimal parameter design according to Routh-Hurtwitz criterion, all the RHP zeros in the transfer function of control-to-output voltage are eliminated completely. Then, by the case of diode-capacitor-based boost converter, the adaptive PI controller is designed to deal with nonlinear characteristic of voltage gain. It gets good dynamic performance under wide range output voltage. All the theoretical findings and design approaches are verified by simulation and experiment results. The existing diode-capacitor-based high step-up dc-dc converters with slight main circuit modification are more promising in renewable energy application.