Owing to the advantages of high efficiency, high energy density, electrical isolation, low electromagnetic interference (EMI) and harmonic pollution, magnetic integration, wide output ranges, low voltage stress, and high operation frequency, the LLC resonant converters are widely used in various sectors of the electronics-based industries. The history and development of the LLC resonant converters are presented, their advantages are analyzed, three of the most popular LLC resonant converter topologies with detailed assessments of their strengths and drawbacks are elaborated. Furthermore, an important piece of research on the industrial applications of the LLC resonant converters is conducted, mainly including electric vehicle (EV) charging, photovoltaic systems, and light emitting diode (LED) lighting drivers and liquid crystal display (LCD) TV power supplies. Finally, the future evolution of the LLC resonant converter technology is discussed.

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LLC Resonant Converter Topologies and Industrial Applications - A Review*

The demand for isolated bidirectional dc transformer (DCX) is driven by the fast development of energy storage system, data center power supply and transportation electrification. Due to zero voltage switching (ZVS) and small rms current, the open-loop CLLC resonant converter operating at the resonant frequency is considered a promising candidate for DCX with a constant voltage transfer ratio. To solve unsmooth bidirectional power flow and current distortion in traditional CLLC-DCX with synchronization rectification (SR) modulation, a dual-active-synchronization (DAS) modulation is adopted with identical driving signals on both sides. Firstly, the switching transition of this modulation is fully analyzed with the consideration of large device output capacitances. After comparison of different transitions, a so-called Sync-ZVS transition is found more desirable with ZVS, no deadtime conduction loss, and load-independent voltage gain. In order to achieve this switching transition, an Axis and Center Symmetric (ACS) method is proposed. Based on this method, an overall design procedure of CLLC-DCX with DAS modulation is also proposed. Finally, Sync-ZVS transition and the proposed ACS method are both verified by three 18 kW 500 kHz prototypes with a 98.8% peak efficiency. This article is accompanied by one video demonstrating the load changing test. 

Switching Transition Analysis and Optimization for Bidirectional CLLC Resonant DC Transformer

Energy management schemes, challenges and impacts of emerging inverter technology for renewable energy integration towards grid decarbonization

LLC resonant converter can achieve zero voltage switching (ZVS) for primary-side devices and zero current switching (ZCS) for secondary-side rectifiers. However, the reverse recovery and junction capacitance ( Cj ) of secondary-side diode critically affect the ZVS condition of primary-side switches. The effect of Cj has been discussed in literature, but not the reverse recovery. In this paper, the reverse recovery charge ( Q rr) is converted to an equivalent capacitance $(C_{{\rm rr}\_{\rm eq}})$ for the study of primary-side ZVS performance. An accurate model during deadtime is derived and further applied to characterize ZVS performance with different reverse recovery charges in different regions. The concept of establishing parameter C total to consider both Cj and $C_{{\rm rr}\_{\rm eq}}$ is proposed to evaluate the effect of the secondary-side rectifiers. This concept provides the guideline for diode and synchronous rectification mosfet selection to ensure ZVS condition for LLC converters. To verify the concept and the derived model, a 200/400 V 400 W LLC resonant converter prototype operating from 200 to 700 kHz is built and its ZVS performances with different diodes are compared. Two issues caused by Q rr effect, including $V_{\rm ds}$ reverse charging and asymmetrical waveform during deadtime, are explained thoroughly as well.

Analysis of Diode Reverse Recovery Effect on ZVS Condition for GaN-Based LLC Resonant Converter

Extensive research in wide-bandgap material technology such as silicon carbide (SiC) has led to the development of medium-voltage (MV) power semiconductor devices with blocking voltages of 3.3 to 15 kV. When these devices are used in various applications, they are exposed to a high peak voltage stress and a very high $dv/dt$ (50–100 V/ns). These impose stringent requirements on the gate driving stage for these devices in terms of featuring a high isolation voltage capability along with a high $dv/dt$ ruggedness, which makes it necessary to have an ultralow coupling capacitance between primary and secondary sides of the gate drivers. One of the key issues in achieving this MV insulation pertains to the necessary clearance and creepage requirements, as defined in IEC 61800-5-1 standards. While the successful operation of these gate drivers is demonstrated in MV converter applications such as solid-state transformers, and MV grid-connected inverters, substantial research needs to be carried out to improve the gate drivers’ performance and provide a plug-and-play solution. This article aims to comprehensively review these gate drivers and consolidate various required design features concerning their galvanic isolation stage, based on normal and short-circuit operation of MV high-power converter systems. Different device short-circuit protection schemes for these gate drivers are explored in detail. Additional applications and functionalities of the gate drivers, including gate drivers used in the series-connection of MV devices and intelligent gate drivers, are also provided in brief. Based on prior research, this review aims to provide design choices and guidelines for the gate drivers, accelerating the growth and deployment of MV SiC devices for field applications.

Gate Drivers for Medium-Voltage SiC Devices

LLC resonant converter can help achieve soft switching for both primary side and secondary side devices, which makes it a great candidate for today’s high-frequency, high-efficiency and high power density solid state transformer (SST) applications. For cases to achieve a constant DC output voltage, it is desired that the LLC converter works in the zero-voltage-switching (ZVS) region that the switching frequency (fs) is smaller than the resonant frequency (fr). However, when a much wider frequency modulation range with ZVS guaranteed is needed to achieve a wide output voltage range and a variable load, the region that fs is larger than fr is the better choice. This region, which is rarely analyzed in the literature, is investigated in this paper. Detailed analysis for charging/discharging process during deadtime is shown. A simple but accurate model to determine fully ZVS condition in this region is proposed, which can also be used for deadtime optimization. Moreover, a 1 MHz/2 kW GaN based LLC converter prototype is built and presented for verification.

An Investigation on Fully Zero-Voltage-Switching Condition for High-Frequency GaN Based LLC Converter in Solid-State-Transformer Application

This paper discusses the details of the control scheme and EMI filter design for a modular front-end solid-state transformer (SST). The multi-cell configuration is achieved by connecting the active-front-end (AFE) cells in series, with the voltage from the MV evenly distributed among the cells. The isolated outputs of the dc-dc converters are tied in parallel. The EMI filter is designed to reduce line impedance and switching noise. Simulations showed the EMI filter reduced noise below FCC Class A standards. The SST controller consists of a voltage controller, current controller, and a phase lock loop (PLL) with an ideal duty feedforward. The implementation of these subsystems is explained and its integration in the system is shown. The results show that at low voltage level, the input current waveform is nearly sinusoidal.

Controller and EMI filter design for modular front-end solid-state transformer

Accurate estimation of the switching loss for power MOSFETs is critical to predict the device junction temperature and the efficiency in power converters. In recent power electronics design, wide bandgap (WBG) devices are adopted with zero-voltage switching (ZVS) to achieve higher efficiency. For devices with ZVS, the only switching loss will be channel turn-off loss, and on the contrary to hard switching, the energy stored in $C_{\text {oss}}$ will be recycled. The models for the estimation of total turn-off energy have been discussed in the literature, whereas the model of channel turn-off energy is absent. In this article, a behavioral model of channel turn-off energy using a simple $\alpha $ – $\beta $ equation for curve fitting is proposed, which can be applied to an entire current and voltage range. The proposed model is verified by different GaN HEMTs and SiC MOSFETs with a double-pulse test circuit in simulation and experiment.

Channel Turn-Off Energy Model for Zero-Voltage-Switching Wide Bandgap Devices

The SiC MOSFETs are replacing Si IGBTs in power electronics applications due to its several intrinsic features, such as higher temperature operation availability, higher breakdown electric field and higher switching speed. However, for the medium voltage range above 1.7 kV, the SiC MOSFETs are still at their early development stage and are not ready for commercialization. This paper presents the characterization and evaluation of 3.3 kV, 5 A SiC MOSFETs. They are the samples from GeneSiC and are not commercialized yet. Static and dynamic tests are performed for the device at different working conditions. For static tests, I-V characteristics, Rds(on) and parasitic capacitors characteristics were recorded for different temperatures. The dynamic tests were done with a Double Pulse Tester (DPT) with an inductive load up to 1.8 kV. The switching speed and switching loss are evaluated and analyzed for different conditions. A simple energy effective and charge effective Coss calculation method through DPT test results is proposed. The test results show its potential in medium voltage applications, especially in multistage cascaded topology.

Characterization and Evaluation of 3.3 kV 5 A SiC MOSFET for Solid-State Transformer Applications

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