Wide bandgap (WBG) devices offer the advantages of high frequency, high efficiency and high power density to power converters due to their excellent performance. However, their low parasitic capacitance and fast switching speed also make them more susceptible to switching oscillations. The switching oscillations can cause voltage and current overshoots, shootthrough, electromagnetic interference (EMI), additional power loss, and even device damage, which can seriously affect the performance of power converters and systems. However, a comprehensive and in-depth overview is lacking on this topic. This paper reviews the types, the causes and negative effects, the effects of parasitic parameters and suppression methods of these switching oscillations, which is helpful for practical engineering. Firstly, the switching oscillations are divided into different types, and their causes and negative effects are reviewed. Then, the effects of different parasitic parameters on the switching oscillations are overviewed. It is found that due to the different physical structures of silicon carbide metal-oxide-semiconductor field-effect transistors (SiC MOSFETs), enhancement-mode gallium nitride high-electron mobility transistors (eGaN HEMTs) and cascode GaN HEMTs, the effects are also different. Finally, the main methods of suppressing the switching oscillations are summarized, and the advantages and disadvantages of these methods are presented.

In this paper, a dual half-bridge LLC resonant converter with magnetic control is proposed for the battery charger application. The primary switches are shared by two LLC resonant networks, and their outputs are connected in series. One of the LLC resonant converter is designed to operate at the series resonant frequency, which is also the highest efficiency operating point, and the constant output voltage characteristic is achieved at this operating point. The second LLC resonant converter adopts magnetic control to regulate the total output current and voltage during both constant current (CC) charge mode and constant voltage (CV) charge mode. Meanwhile, the function decoupling idea is adopted to further improve the system efficiency. The significant amount of the power is handled by the LLC resonant converter operating at the series resonant frequency, while the second LLC resonant converter fulfills the responsibility to achieve closed-loop control. By carefully designing the resonant networks, the zero-voltage switching (ZVS) for primary switches and zero-current switching (ZCS) for secondary diodes can be achieved for whole operation range. A 320W experimental prototype is built to verify the theoretical analysis, and the maximum efficiency is measured about 95.5%.

The LLC resonant converters commonly adopt Frequency Modulation (FM) or combination of FM with Phase-shift Modulation (PSM) to regulate its output voltage. However, in these control schemes, a variable switching frequency range is required, which makes the magnetic components design complicated. Therefore, in this paper, magnetic control (or variable inductor control) is adopted to make the converter operating at constant switching frequency and constant duty cycle. The Fundamental Harmonic Analysis (FHA) is commonly used because of its characteristic of simplicity. However, the accuracy of this method is reduced and considerable errors occur when the switching frequency or output power changes. Therefore, an optimal design methodology based on time domain analysis of the LLC resonant converter with magnetic control is proposed in this paper. The proposed methodology can assure that the converter will operate in PO or OPO modes within the whole operating range, and Zero Voltage Switching (ZVS) operation for primary switches and Zero Current Switching (ZCS) operation for secondary rectifier will be guaranteed. In addition, by limiting the resonant tank Root-mean-square (RMS) current, the system efficiency is improved. A 200W experimental prototype is built and the effectiveness of the proposed optimal design methodology is verified.

The DC-Link capacitors are important components that tend to be one of the weakest part of power converters. Thus it is necessary to monitor the aging parameters of DC-Link capacitors in valuable power electronic systems. This paper proposes a simple yet effective variable electrical network (VEN) condition monitoring method for DC-Link capacitors in three-phase PWM AC/DC/AC power converters. The capacitance(C) and equivalent series resistance (ESR) of DC-Link capacitors are estimated through the designed VEN unit during the discharging process. This VEN method does not need the current sensor or injecting signals into the control loop. The above property is favorable for the hardware and controller design. Besides, the designed monitoring circuit can be an external unit or a built-in unit which offers a flexible solution for electrical devices. When the designed VEN is used as an external unit, neither the hardware design nor the programs in the controller needs to change, which is practical and economic for electrical devices in use. Experiments are carried out in a 55kW AC/DC/AC inverter with different capacitors. The experiment results prove the method to be effective and accurate.

This paper presents a theory for the creation of harmonics by resonance in converter-grid systems. A harmonic of this type is usually not present or observable under normal operation, but may suddenly appear, with a growing magnitude and at a frequency that coincides with a system resonance frequency. The growth may be unbounded, leading eventually to shutdown of the converter or other components in the system due to protection or physical damage, but often it stops after certain time and the harmonics sustain themselves at a constant magnitude. The latter is the focus of this work and is identified as a result of system operation changing from an initially unstable mode to a critically stable mode, achieved via a self-adjusting process. Small-signal sequence impedance models are used to explain this self-adjusting process and to determine the resulting harmonic responses. Experimental measurements are presented to validate the theory and to demonstrate the characteristics of this type of harmonics.

Asymmetric duty modulation (ADM), as a recently proposed modulation scheme, provides new chance to improve the performance of dual active bridge (DAB) converter. On this basis, an optimal asymmetric duty modulation (OADM) scheme is proposed in this paper to minimize the inductor peak-to-peak current over the whole voltage and power range. Firstly, the DAB converter adopting ADM scheme is analyzed in frequency-domain and time-domain, and four operating modes are established. Then, the minimum peak-to-peak inductor current for each operating mode is obtained under Karush—Kuhn—Tucker (KKT) condition, and the corresponding optimal closed-loop control with low calculation complexity is derived. Then, the key values and features of the OADM scheme are compared with recent state-of-the-art, mainly including the inductor root mean square (RMS) current and soft-switching performance. Finally, an experimental prototype was built to validate the efficiency enhancement of the converter by adopting the proposed OADM scheme.