Resonant DC Transformer Research Articles

Soft-Switching Resonant DC Transformer for Class-D Switching Amplifier With Symmetric Bipolar Outputs

In this letter, soft-switching resonant DC transformer (DCX) for the class-D switching amplifier with the symmetric bipolar outputs is proposed. The proposed DCX consists of a single transformer primary inverter circuit and two transformer secondary dual series resonant half-bridge rectifier circuits, which reduces the voltage stress of the rectifier diodes. In the proposed DCX, the duty cycle of the primary inverter circuit is 0.5, and the symmetric bipolar output voltage gains only depend on the transformer. By dual series resonance between the leakage inductance and the output resonant capacitors, ZVS turn-on of switches and ZCS turn-off of the diodes can be achieved, which improves the overall efficiency. The characteristic of the DCX is elaborated and analyzed in this letter. Finally, a 60 W prototype is established and the experimental verified the theoretical analysis of the proposed DCX.

Modular Multilevel Resonant DC Transformer With Inherent Balancing Capability

As the demand for dc/dc converters in medium voltage applications is increasing in recent years, more and more attention has been paid to modular multilevel technology to reduce device stress. Derived from modular multilevel converter and LLC converter, this paper proposes a modular multilevel resonant dc transformer. Compared with the multi-stack structures, the proposed dc transformer has a simpler single-stack structure, which reduces the number and cost of sub-modules (SMs). Moreover, through the co-design of the boost inductor and the transformer, the proposed scheme can realize zero voltage switching of the primary switches and zero current switching of the secondary rectifier. With circulant modulation, not only a flexible step-down ratio is obtained, but also the SM capacitor voltages can be self-balanced without adding the control complexity. In addition, a high operating frequency is synthesized through a low switching frequency of the SM, thereby reducing the size of passive components and switching losses. The operating principle and theoretical analysis of the dc transformer are presented. A downscaled hardware prototype is built, and experimental results validate the correctness of the theoretical analysis and the feasibility of the converter.

Partial-Power Post-Regulated LLC Resonant DC Transformer

This article presents a partial-power postregulated LLC resonant converter. In the proposed topology, the input power is first transferred to the secondary side of the transformer through the LLC-DC transformer converter to achieve galvanic isolation and voltage conversion functions. Then, most of the power is directly transmitted to the load side, and only a small portion of the power is converted to the output side by the partial-power processing converter. Therefore, unlike the two-stage solution, the converter proposed in this article achieves output voltage regulation while minimizing the efficiency loss caused by the two-stage conversion. The operating principle of the proposed converter is analyzed, and the system prototype is constructed. Experimental result validates the correctness of the analysis and proves the feasibility of the proposed converter.

Switching Transition Analysis and Optimization for Bidirectional CLLC Resonant DC Transformer

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.

Power Reversal Algorithm for Resonant Direct Current Transformers for DC Networks

The bidirectional resonant DC transformer has all the desired features to drive the development of the DC power distribution networks of the future. This power-electronics converter provides galvanic isolation between DC buses, has desired open loop properties, and follows the system’s power flow naturally, requiring only a power reversal algorithm to allow a bidirectional operation and smooth transitions. This paper proposes a solution to tackle the power reversal of the DCT with a dynamic algorithm, considering the system behaviour and the power converter characteristics. The proposed algorithm executes the power reversal in few switching periods and removes the DCT losses during no-load condition. Then, using low voltage prototype of DC transformer, experimental results along the paper demonstrate how this power reversal algorithm is applied under various operating conditions.

Multitime Scale Frequency Regulation of a General Resonant DC Transformer in a Hybrid AC/DC Microgrid

The high-frequency resonant dc transformer (DCT) becomes popular in hybrid ac/dc microgrids, thanks to its good bidirectional power transmission (PT) ability and high power density. However, its application is limited by either the complex systematic control or the complicated parameter design. To solve this problem, in this article, a multitime scale frequency regulation approach is put forward for a general CLLC-DCT (GCLLC-DCT). By establishing the general model of the resonant DCT, the relationship between the transmitted power and the resonant frequency is derived at first. Afterward, the power trigger mechanism is introduced to assist the implementation of the proposed approach. If the long-time scale control is triggered, only once calculation based on the PT requirement will be introduced to avoid redundant computations. If the short-time scale control is triggered, the frequency regulation based on the voltage conversion gain feature of GCLLC-DCT will be executed till it reaches a new steady state, which is beneficial to ensure the real-time control precision. The cooperation of the short-time and long-time scale control is also presented to make the proposed approach easily implement at hybrid ac/dc microgrids. The lab-scale prototype of GCLLC-DCT has been set up based on a hybrid ac/dc microgrid to verify the superiority of the proposed approach.

Design Methodology for Symmetric CLLC Resonant DC Transformer Considering Voltage Conversion Ratio, System Stability, and Efficiency

With the popularity of dc microgrid, the symmetric CLLC resonant dual active bridge converter usually works as a dc transformer (DCT) when it is applied in dc microgrid. It has received increasing attention as it provides galvanic isolation and also enjoys high power density. In such application, the open-loop control with 50% duty ratio is commonly adopted for the CLLC resonant DCT (CLLC-DCT) for simple control scheme and high efficiency. However, this will bring about difficulties for the design of CLLC-DCT. First, the fluctuating values of the practical inductors or capacitors because of temperature or power variation may lead to deviated voltage conversion ratio (VCR) of CLLC-DCT. Second, chances are that system instability will happen when CLLC-DCT and a constant power load connect in a cascaded structure because of impedance interactions, which will also be affected when parameters fluctuate. Third, high efficiency of the DCT is also expected even when the inductance or capacitance fluctuate. With all the above concerns, the five-stage design methodology for the CLLC-DCT converter is put forward to meet design objectives regarding VCR, system stability, and power efficiency simultaneously. And the effects of fluctuating inductance and capacitance have been fully taken into account by the assistance of particle swarm optimization. Besides, a 1-kW hardware prototype has experimentally validated that the proposed design methodology is feasible.

Optimal Design of LLC Resonant DC Transformer under Adaptive Frequency Tracking Strategy

In recent years, the LLC (inductor–inductor–capacitor) DC transformer has been widely used in communication and computer power supply because of its advantages of zero voltage conduction of primary switch and zero current turn off concerning the output rectifier diode. To obtain higher transmission efficiency and make the LLC DC transformer always run at the optimal operating point, the switching frequency of the LLC DC transformer should work at the resonance frequency of the circuit. In actual conditions, the optimal operating frequency of the LLC DC transformer will be changed due to the influences of the working condition on the circuit parameters and the load change. Therefore, the LLC DC transformer controlled by the fixed frequency mode will not be in the best working condition. In this paper, an adaptive frequency tracking method is used to control the circuit; when the circuit parameters change, the LLC DC transformer can always be in the best working state. Then, the influence of circuit parameters such as output power and excitation inductor on the optimal working point of the LLC DC transformer is analyzed in detail. Finally, a 1 kW LLC resonant converter prototype is designed under laboratory conditions to verify the feasibility of the control strategy.

Dual‐output AC–DC converter with three‐port bridgeless PFC and non‐isolated DCX

This study proposed a dual-output AC–DC converter based on three-port bridgeless power factor corrector (TPB-PFC) and a non-isolated resonant DC-transformer (Ni-DCX) for dual-output DC voltage applications. A constant DC output voltage and a variable DC output voltage are provided simultaneously by the proposed AC–DC converter. Based on the TPB-PFC converter, a stacked power conversion structure with reduced power conversion stage is presented. Therefore, lower power losses and higher efficiencies are achieved. Meanwhile, soft switching of all the power devices in the Ni-DCX is achieved with open-loop controlled and optimised operation of the Ni-DCX, resulting in easy control and higher efficiency. In addition, switching losses of the TPB-PFC are reduced thanks to the three-level characteristic. Operation principles and characteristics of both TPB-PFC and Ni-DCX converter, and modulation and control strategies of the proposed AC–DC converter are analysed in detail. Feasibility and effectiveness of the proposed solutions are verified experimentally with a 2 kW dual-output AC–DC prototype.

Research on Current Limiting Method Used for Short Circuit Fault Current of Resonant DC Transformer Based on Inverted Displacement Phase Control

Due to the characteristics of high efficiency and flexible transformation ratio, full-bridge CLLC resonant DC transformers have been increasingly used for DC grid in recent years. Based on the analysis of the working principle of CLLC resonant DC transformer circuit, this article analysis the current gain characteristics by using fundamental wave analysis method, the parameter influences of resonance network frequency and the transformer gain is also being concerned. On the basis of using soft switching method, this article will introduce a method of inverted displacement phase control. The working range will be further refined in the original circuit, and the linear control region will be expanded as well. In this way, the output current will have a precise control under the condition of short circuit, which will satisfy the low voltage ride-through requirements of DC transformers applied to the grid connection of renewable energy. Finally, the method proposed in this article is verified by simulation and hardware-in-the-loop experiment.

Two-Stage Parameter Design Methodology of a Generalized Resonant DC Transformer in Hybrid AC/DC Microgrid With Optimum Active Power Transmission

A semiregulated high-frequency resonant dc transformer (DCT) is usually recommended to work at its resonant frequency to ensure the desired active power transmission ratio (APTR) with stable voltage conversion gain (VCG) for hybrid ac/dc microgrid. Therefore, appropriately designed resonant parameters are vital in the DCT to acquire the required APTR and VCG under the changing power, temperature, and so on. However, the existing parameter design approach is not general enough to cover different resonant DCT topologies and it becomes rather complex with the increasing number of unknown parameters. To address this concern, a simplified two-stage parameter design methodology is proposed in this article by considering the parameter variation to optimize the APTR on the premise of satisfied VCG. First, a generalized resonant CLLC-type DCT (GCLLC-DCT) is established to cover CLL, LLC, symmetrical CLLC, as well as asymmetrical CLLC topologies. Afterward, based on the design requirements to maximize APTR under stable VCG, the design criteria are presented by analyzing the variation range of the designed intermediated parameters. Subsequently, a two-stage parameter design method is illustrated and facilitated with an example of a DCT prototype based on a simple MATLAB-assisted procedure. Finally, the proposed method is validated by experimental tests.

Implementation of Bidirectional Resonant DC Transformer in Hybrid AC/DC Micro-Grid

As a reliable device for voltage regulation and isolation, a line frequency transformer is commonly connected in series with bidirectional converter (BIC) to interlink the ac and dc networks of hybrid ac/dc microgrid. However, it may not be suitable for the applications where weight and space occupation are of important concerns such as hybrid ac/dc microgrid. In this paper, bidirectional resonant dc transformer (BRDT) is proposed to replace the conventional bulky transformer for bus voltage matching and galvanic isolation. In order to simplify systematic coordination with BIC and central controller, BRDT is designed as an ideal transformer with simple open-loop scheme. 50% duty ratio scheme is implemented as it has been proven to be better than the phase-shift scheme in order to ensure BRDT conversion efficiency. A generalized model for various BRDT topologies, including CLLC, CLL, and LLC high frequency transformer (HFT), is established for analysis. The impacts induced by the transformer leakage, magnetizing inductances, and extra resonant capacitors are considered when designing the HFT. The conversion gain of the designed BRDT has also been analyzed in the full power range. Lab-scale prototypes for BRDT and hybrid ac/dc microgrid have been developed for experimental verifications. The performances of BRDT and system in both steady and transient states have been confirmed.

1-MHz LLC Resonant DC Transformer (DCX) With Regulating Capability

This paper presents an MHz LLC resonant converter with load-regulating capability. Unlike the two-stage solution, most of input power is delivered to the load directly by the main transformer in the proposed topology. Moreover, only small portion of input power is converted by two stages: 1) auxiliary transformer and 2) a cascaded PWM dc–dc converter. Therefore, the sacrifice of the efficiency caused by the two-stage conversion is minimized. The operating principle of the proposed converter is analyzed, and a 1-MHz prototype based on the proposed converter is built-up to verify the high-frequency, high-efficiency, and output-regulating capability.