GaN-based LLC Resonant Converter Research Articles

Smart Efficiency Tracking for Novel Switch—LLC Converter for Battery Charging Applications

In this study, a novel smart efficiency tracking (SET) algorithm for GaN-based LLC resonant converter for quick battery charging of light electric vehicles is proposed. Conventional frequency modulation (FM) method, single pulse width modulation method (S-PWM) and discontinuous operation mode method (DOM) are used to track the converter efficiency which varies for different loading conditions during the battery charging process. First, the charging modes are explained to define the efficiency variation clearly. Then, the theoretical background of GaN-based LLC resonant converter is presented and the details of design study are given. Third, the principles of three different switching control strategies, the modeling study and the simulation results are put forward. Finally, the details of the implementation of three different switching strategies are presented for the validation of the study. Furthermore, the implementation of the design and its impact on battery charging are discussed. The proposed efficiency tracking method is validated with a 20% efficiency increase for the low loading conditions of GaN-based LLC converter.

High Frequency, High Efficiency, and High Power Density GaN-Based LLC Resonant Converter: State-of-the-Art and Perspectives

Soft switching for both primary and secondary side devices is available by using LLC converters. This resonant converter is an ideal candidate for today’s high frequency, high efficiency, and high power density applications like adapters, Uninterrupted Power Supplies (UPS), Solid State Transformers (SST), electric vehicle battery chargers, renewable energy systems, servers, and telecom systems. Using Gallium-Nitride (GaN)-based power switches in this converter merits more and more switching frequency, power density, and efficiency. Therefore, the present paper focused on GaN-based LLC resonant converters. The converter structure, operation regions, design steps, and drive system are described precisely. Then its losses are discussed, and the magnets and inductance characteristics are investigated. After that, various interleaved topologies, as a solution to improve power density and decrease current ripples, have been discussed. Also, some challenges and concerns related to GaN-based LLC converters have been reviewed. Commercially available power transistors based on various technologies, i.e., GaN HEMT, Silicon (Si) MOSFET, and Silicon Carbide (SiC) have been compared. Finally, the LLC resonant converter has been simulated by taking advantage of LTspice and GaN HEMT merits, as compared with Si MOSFETs.

Analysis of ZVS Realization with Consideration of Secondary Parasitic Capacitance for High Frequency GaN-based LLC Resonant Converter under DCM Mode

LLC resonant converter can achieve zero voltage switching (ZVS) for primary side devices and zero current switching (ZCS) for secondary side rectifiers while operating in discontinuous conduction mode (DCM). However, the parasitic capacitance significantly affects the ZVS realization of primary side switches for high frequency GaN-based LLC converter. In order to reveal the influence, this paper analyses ZVS transition process with consideration of secondary parasitic capacitance in a high frequency GaN-based LLC resonant converter under DCM mode. Then, an accurate model during ZVS transition is derived. To verify the analysis and the derived model, a 250V rating input and 270V/6.6A rating output GaN-based LLC converter prototype operating at 500 kHz is built. The experimental results are consistent with the theoretical analysis.

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

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.