Soft-switching Operation Research Articles

A High Step-Up Interleaved Current-Fed Resonant Converter for High-Voltage Applications

A current-fed LLC resonant converter is proposed for high-voltage and high-power applications. Here, a two-phase interleaved structure is used in the input stage under the continuous conduction mode (CCM) to effectively reduce the input current and output voltage ripple values and the input filter and the output capacitors volumes. Due to the expandable structure and high voltage gain, the proposed configuration is suitable for high voltage applications since low voltage stresses are applied across its components. In fact, voltage stresses across the power semiconductors, i.e., MOSFETs and output diodes, along with the output capacitors, are almost one-third of the output voltage in the implemented three-stage configuration of the proposed converter. Here, the switching frequency is chosen close to, but less than, the converter series resonant frequency to reduce its different components’ current stresses and perform soft-switching operation for all power devices under wide input voltage and output power variations. Therefore, conduction and switching losses, and EMI noises are effectively reduced. Consequently, efficiency is improved and high-frequency operation is possible, which reduces the volume of passive components to achieve high-power density. The given topology is thoroughly analyzed mathematically. Also, a 1 kW prototype converter has been implemented to validate the given simulations and analyses. Here, wide input voltage (100 V-200 V) and output power (100 W-1000 W) variations are applied, and an asymmetric pulse width modulation (APWM) technique is used at 143 kHz switching frequency to regulate the output voltage at 1 kV. The obtained maximum efficiency value is 95.3%.

A Multichannel High-Frequency Current Link Based Isolated Auxiliary Power Supply for Medium-Voltage Applications

The auxiliary power supply (APS) is one of the critical components inside medium-voltage (MV) power converters. Besides high insulation capability and small footprint, low common-mode (CM) coupling capacitance and multichannel output are the desired features of APS in the emerging silicon-carbide-based MV converters due to their fast switching speed. This article presents the design and optimization procedure of a high-density isolated APS using an LCCL-LC resonant topology with an operating frequency of 1 MHz. The proposed design procedure attains consistent soft-switching operation under a random number of output channels. The galvanic isolation is realized by a current-fed single-turn 1 MHz transformer that can achieve a breakdown voltage of over 20 kV while maintaining a small size. Design optimization on the insulation system of the current transformer is proposed to obtain both high partial-discharge inception voltage (PDIV) and low coupling capacitance. Finally, two versions of APSs are developed, using air and silicone as dielectric materials, which can reach PDIV of over 5 and 16 kV, respectively. The corresponding coupling capacitances are 1.86 pF and 3.6 pF. Both designs can provide a maximum power of 20 W on the receiving side, and 120 W on the sending side.

Constant-Current, Constant-Voltage Operation of a Dual-Bridge Resonant Converter: Modulation, Design and Experimental Results

To meet the requirements of charging the mainstream rechargeable batteries, in this work, a dual-bridge resonant converter (DBRC) is operated as a battery charger. Thanks to the features of this topology, the required high efficiency can be achieved with a wide range of battery voltage and current by using different modulation variables. Firstly, a typical charging process including constant-voltage stage and constant-current stage is indicated. Then, two different modulation methods of the DBRC are proposed, both of which can realize constant-voltage charging and constant-current charging. Method I adopts phase-shift modulation with constant switching frequency while Method II adopts varying frequency modulation. Furthermore, as guidance for practical application, the design principles and detailed design procedures of the DBRC are customized for the two modulation methods respectively in order to reduce the switching loss and conduction loss. Consequently, the full soft-switching operation with low rms tank current is achieved under the two modulation methods, which contributes to the high efficiency of the whole charging process. At last extensive simulation and experimental tests on a lab prototype converter are performed, which prove the feasibility and effectiveness of the proposed modulation strategies.

Research on efficiency optimisation of DAB converters considering current stress and soft‐switching

To solve the problem of low efficiency of the dual active bridge (DAB) converter under the traditional single-phase-shift control, a natural soft-switching current stress optimisation (NSS-CSO) method is proposed by considering the current stress optimisation and soft-switching operation under dual-phase-shift (DPS) control. This study establishes the current stress and the soft-switching analytical models of the DPS control global mode. Based on these models, the Karush–Kuhn–Tucker (KKT) condition is used to analyse the influence of the current stress optimisation methods on the efficiency when all switches achieve controlled soft-switching or some switches achieve natural soft-switching. It is found that the NSS-CSO method is more efficient. Furthermore, the optimal phase-shift ratio combinations and the corresponding current stress extremums of the NSS-CSO method under various ranges of transmission power and voltage conversion ratio are deduced completely. The results of this study show that the NSS-CSO method can effectively improve the operation efficiency of the DAB converter over the entire power range when the voltage conversion ratio does not match. Finally, experimental results confirm the accuracy of the theoretical analysis and the effectiveness of the proposed optimisation method.

A Voltage-Fed Soft-Switched Push–Pull Topology With Phase-Shifted Power Transfer Using Coupled LC Snubber

This article proposes a voltage-fed zero-voltage-switched topology based on the push-pull converter. The proposed converter uses the leakage inductance of the transformer to transfer power from input to output. Phase-shift modulation strategy is used to control the flow of this power. A coupled LC regenerative snubber-based energy recovery circuit reclaims the trapped energy in the leakage inductance. The snubber circuit also ensures soft-switched operation and reduces the voltage stress across the semiconductor switches. The power-flow equation is characterized and experimentally verified for the proposed converter. The gate control signals are implemented using an field-programmable gate array (FPGA). A 240-W prototype validates the steady-state behavior and snubber design equations.

An LLC Converter With Multiple Operation Modes for Wide Voltage Gain Range Application

Although LLC converters are attractive in industrial applications due to its advantages of soft switching, high efficiency, and simple control, in wide voltage gain range applications, large switching frequency operating range is required, which may cause the loss of soft switching operation and existence of control instability issue. In this article, a novel LLC converter with two LLC resonant tanks is proposed. The proposed converter has three operation modes, namely, dual full-bridge operation mode, full-bridge operation mode, and half-bridge operation mode, which can cover the high voltage gain range, medium voltage gain range, and low voltage gain range operations, respectively. The operational principles of the proposed converter are presented, and brief design considerations are discussed. A 250 W experimental prototype with six times input voltage range (50-300 V) was built to validate the theoretical analysis of the proposed converter. Comparisons among different LLC topologies for wide voltage gain range applications are made.

A New High-Gain, High-Efficiency SEPIC-Based DC–DC Converter for Renewable Energy Applications

This article proposes a new configuration of quasi-resonant high-gain high-efficiency single-ended primary inductor converter (QRHGHE-SEPIC)-based dc–dc converter with continuous input current. The presented single-switch topology uses a coupled-inductor (CI), a voltage multiplier integrated with a regenerative passive lossless clamp circuit to enhance the voltage conversion ratio. In the proposed converter, the main power switch turns <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> at zero current switching. Moreover, by adopting a quasi-resonance (QR) operation between the leakage inductor of the CI and the middle capacitors, the current value of the main switch at turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> moment is alleviated. In addition, the leakage inductance slows down the turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> slope of all diodes and hence there is no reverse recovery problem in the proposed converter. Due to soft-switching operation in all switching components, the power dissipations in the converter are significantly alleviated. Thus, the proposed QRHGHE-SEPIC can provide high voltage gain while achieving a high efficiency. Steady-state analysis, comprehensive comparisons with other related converters, and design considerations are discussed in detail. Finally, to verify the validity of the theoretical analysis, a 160 W/200 V sample prototype is demonstrated at the switching frequency of 60 kHz and with voltage gain of 10.

A Self-Compensated Planar Coil With Integrated Single-Switch Regulator for Wireless Power Transfer (WPT) Systems

In the receiver-side (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</sub> ) of the wireless power transfer (WPT) system, the R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</sub> is composed of compensation capacitors, a rectifier stage, and a dc/dc stage. Among them, the compensation capacitors are an essential component to minimize reactive power. Also, the dc/dc stage is an inevitable component to supply a stable output to the battery. However, to meet these aforementioned unavoidable requirements, a large number of components are required in R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</sub> of the WPT system, which increases the overall system size and cost. To relieve these drawbacks, a self-compensated planar coil with integrated single-switch regulator for the WPT system is proposed in this letter. The proposed converter can remove the many number of compensation capacitors by utilizing the intrinsic capacitance between adjacent layers of the planar coil as a self-compensated factor. Moreover, by integrating a single-switch into the rectifier stage, it is possible to supply a stable output to the battery with only a single power stage and low cost. Besides, the proposed structure can obtain reduced switching loss by securing the soft-switching operation under the rated load condition. The effectiveness of the proposed structure is verified experimentally by a prototype under 150 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_DC</i> input and 60 W/33 V output.

Closed-Loop Black Start-Up of Dual-Active-Bridge Converter With Boosted Dynamics and Soft-Switching Operation

This letter proposes a closed-loop black start-up control for the dual-active-bridge (DAB) converter. With a simple combination of the extended phase-shift and triple phase-shift modulation schemes, the DAB converter can start up with the maximum allowable precharge current while realizing soft-switching operation in the trapezoidal current mode simultaneously. Compared to the conventional start-up procedure that usually requires multiple steps of operation and parameter fine-tuning, the proposed start-up method can be implemented simply into the existing closed-loop controller and can shorten the start-up transients of the DAB converter significantly. The presented methods are validated experimentally on a downscaled DAB converter prototype.

Full Soft-Switching Ultra-High Gain DC/DC Converter Using Three-Winding Coupled-Inductor with Modular Scalability for Renewable Energy Applications

This paper proposes a new non-isolated single-switch DC/DC converter with an ultra-high voltage gain, low input current ripple, low voltage stress, soft-switching operation, and modular scalability for renewable sources applications. Here, with the help of a Three-Winding Coupled-Inductor (TWCI) and Voltage Multipliers circuits, ultra-high voltage gains can be achieved without needing a large duty cycle. A regenerative clamp capacitor recycles the energy stored in the leakage inductor; thus, the maximum voltage across the single power switch is restricted. Moreover, at the turn-on instant of the power switch, a Zero Current Switching (ZCS) condition is achieved. By designing a resonant tank, the switched current value of the main switch at the turn-off instant is reduced significantly. Also, the leakage inductor of the TWCI helps all converter diodes to operate under the ZCS condition. Due to full soft-switching performance, the introduced topology can provide a wide output voltage range under a high conversion efficiency. The steady-state analysis and comprehensive comparisons are provided in this paper. A 160 W prototype with 24 V input and 250 V output voltage is developed to validate the theoretical analysis. Due to ZCS operation and low voltage stress (VDS ≈ 40 V), the power loss portion of the MOSFET is low. Moreover, the maximum voltage stresses of the diodes are measured 40 V, 60 V, 90 V, and 110 V that are well below the output voltage. Furthermore, at the full-load condition, the input current ripple is about 20 % and the measured efficiency is about 96.3%.

Soft-Switching Current-Source Rectifier Based Onboard Charging System for Electric Vehicles

In this article, a novel soft-switching current-source rectifier (CSR) based bidirectional on-board charger (OBC) system is proposed for electric vehicles (EVs) comprising multiple battery sets. Besides the CSR's inherent advantages of higher-quality input voltage waveforms, enhanced short-circuit-current tolerant capability and long lifetime of dc choke, the cascaded configuration of dc choppers can implement the higher dc-link voltage with multiple low-voltage battery sets, and the low-voltage devices can be used. With the auxiliary resonant circuit in dc link, the soft-switching operation can be achieved for all power switches in the proposed power conversion circuits. Moreover, the high dv/dt caused by high-speed switching silicon carbide devices can be reduced with the capacitor in the auxiliary circuit. The collaborative operation strategy is proposed for the CSR and the dc choppers, which enables reduction of switching frequency of dc choppers and current ripple in dc link simultaneously. The experimental results are presented to verify the performance of the proposed the OBC system.

A Novel Approach for the Modeling of the Dynamic ON-State Resistance of GaN-HEMTs

A compact model approach to enhance the accuracy and facilitate the parameter extraction of trapping models is presented. The proposed model replaces the conventional superposition of discrete trapping time constants by a Gaussian distribution of a particular range of time constants. This approach reflects the physically non-constant energy levels of the trap distribution of surface, interface, and bulk traps. Also, we propose a new equivalent circuit that takes the time-dependent charging and discharging of traps into account. We show that our model reduces the model complexity by 52%. The model is verified against dynamic ON-state resistance ( R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS,ON</sub> ) measurements of a commercial 100-V gallium-nitride (GaN) power transistor in soft-switching operation.

유한상태기계를 이용한 소프트스위칭 계통연계형 인버터 제어 알고리즘에 관한 연구

This paper present a finite state machine (FSM) model of a trajectory control algorithm for single-stage soft-switching grid-tied inverters with a fast dynamic response. A Grid-tied inverter for single photovoltaic (PV) panel that interfaces distributed energy sources and grid is targeted as an application in this paper. The single-stage soft-switching operation is implemented to reduce switching losses using a resonance filter between two switching bridges. The proposed FSM model effectively selects the switching state of a converter in complex DC / AC bidirectional operations with active/reactive power control. The control scheme directly limits the amount of energy in the resonant circuit and provides a fast transient response with minimal switching actions in all quadrants on v<SUB>ac</SUB> - i<SUB>ac</SUB> plane. The proposed model is digitally implemented on a TI digital signal processor and verified by Hardware-In-the-Loop (HIL) tests.

Analysis and Implementation of a Frequency Control DC–DC Converter for Light Electric Vehicle Applications

In order to realize emission-free solutions and clean transportation alternatives, this paper presents a new DC converter with pulse frequency control for a battery charger in electric vehicles (EVs) or light electric vehicles (LEVs). The circuit configuration includes a resonant tank on the high-voltage side and two variable winding sets on the output side to achieve wide output voltage operation for a universal LEV battery charger. The input terminal of the presented converter is a from DC microgrid with voltage levels of 380, 760, or 1500 V for house, industry plant, or DC transportation vehicle demands, respectively. To reduce voltage stresses on active devices, a cascade circuit structure with less voltage rating on power semiconductors is used on the primary side. Two resonant capacitors were selected on the resonant tank, not only to achieve the two input voltage balance problem but also to realize the resonant operation to control load voltage. By using the variable switching frequency approach to regulate load voltage, active switches are turned on with soft switching operation to improve converter efficiency. In order to achieve wide output voltage capability for universal battery charger demands such as scooters, electric motorbikes, Li-ion e-trikes, golf carts, luxury golf cars, and quad applications, two variable winding sets were selected to have a wide voltage output (50~160 V). Finally, experiments with a 1 kW rated prototype were demonstrated to validate the performance and benefits of presented converter.

Bidirectional Current-Fed CLLC Resonant Converter Employing Asymmetric PWM

A power systems of wearable application requires high power conversion efficiency and bidirectional power flows to obtain the compatibility with various power sources and loads. The conventional flyback converter has a simple structure and bidirectional power conversion capability. However, it shows poor power conversion efficiency caused by hard-switching operation, high DC offset current on the transformer, snubber losses, and large peak current. In this paper, a current-fed CLLC resonant converter is proposed to obtain the high power conversion efficiency and the bidirectional power conversion capability. The soft-switching operation, zero DC-offset current, and small peak current can induce high power conversion efficiency. The asymmetric pulse width modulation (APWM) regulates the bidirectional power flows and the output voltage. The design methodology of the proposed converter is analyzed by introducing its operational principles. The performance of the proposed converter is experimentally verified with a 20 W prototype.

Hybrid DC-DC Converter with Low Switching Loss, Low Primary Current and Wide Voltage Operation

A full-bridge converter with an additional resonant circuit and variable secondary turns is presented and achieved to have soft-switching operation on active devices, wide voltage input operation and low freewheeling current loss. The resonant tank is linked to the lagging-leg of the full bridge pulse-width modulation converter to realize zero-voltage switching (ZVS) characteristic on the power switches. Therefore, the wide ZVS operation can be accomplished in the presented circuit over the whole input voltage range and output load. To overcome the wide voltage variation on renewable energy applications such as DC wind power and solar power conversion, two winding sets are used on the output-side of the proposed converter to obtain the different voltage gains. Therefore, the wide voltage input from 90 to 450 V (Vin,max = 5Vin,min) is implemented in the presented circuit. To further improve the freewheeling current loss issue in the conventional phase-shift pulse-width modulation converter, an auxiliary DC voltage generated from the resonant circuit is adopted to reduce this freewheeling current loss. Compared to the multi-stage DC converters with wide input voltage range operation, the proposed circuit has a low freewheeling current loss, low switching loss and a simple control algorithm. The studied circuit is tested and the experimental results are demonstrated to testify the performance of the resented circuit.

Trapezoidal Current Modulation for a Compact DC Modular Multilevel Converter With ZVS of Submodules and ZCS of Voltage-Balancing Circuits

In this letter, a trapezoidal current modulation is proposed for a compact dc–dc modular multilevel converter with reduced submodule (SM) number and magnetic integration. The arm currents are shaped to fulfill the zero-voltage-switching conditions for all the SMs. Then, the diode–inductor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D–L</i> ) circuits between the adjacent SMs are further introduced to cooperate with the proposed modulation; hence, the SM capacitor voltages could be balanced without sensors and sorting algorithm. Besides, the diodes in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D–L</i> circuits could also achieve zero-current switching. Such a full soft-switching operation significantly contributes to the system volume reduction. The proposed method is verified by both simulation and experiment, and under steady-state, transient state, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D–L</i> circuit broken situations.

Single Input Single Output Two Level Isolated Dc-Dc Converter With Secondary Side Phase Shifting For Solar Applications

The isolated dc–dc converters with primary-side phase shifting (PPS) provides severely narrow soft-switching range for main devices in the primary side leg of full bridge converter. The leakage inductance of the high frequency transformer should be large enough for providing the energy needed for soft switching operations and also the idling power loss due to circulating current in the converter legs under large phase-shift angle, which makes reduction of conversion efficiency and complicated in designing the parameters of transformer. Furthermore, the turn-off diode commutations in the output-side rectifier are performed by hard-switching mode. To overcome all these drawbacks, a secondary side phase shifting (SPS) technique has been developed for two-level isolated DC-DC converter. This scheme provides wider soft switching range and reduced power loss due to elimination of circulating current in the primary side of high frequency transformer. In addition, SPS control also provide no reverse recovery current in diodes and hence no power losses in the secondary rectifier circuit. The control switches operate under soft switching even under rated load and short circuit conditions.&#x0D;

Bidirectional ZVS Buck–Boost Converter With Single Auxiliary Switch and Continuous Current at Low Voltage Source

In this article, a bidirectional zero voltage switching buck–boost dc–dc converter with a simple auxiliary circuit is presented. The auxiliary circuit employs only a single switch and a pair of coupled inductors to provide soft switching condition in both power flow directions. Moreover, the soft switching condition is independent of the load variation and duty cycle. Energy storage devices such as batteries and super capacitors are commonly utilized in systems using bidirectional converters as the low voltage source. Batteries require continuous current to prolong their life time and overall system efficiency. The proposed converter has continuous current at low voltage side in both operating modes, unlike the similar converters, which utilize coupled inductors in order to provide soft switching condition. Fully soft switching operation including the zero current switching condition of all diodes at turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> has significantly contributed to the converter overall efficiency. Operating principles, mathematical derivations and the soft switching conditions of the proposed converter are analyzed in both boost and buck operating modes. Furthermore, to confirm the main converter features and the theoretical analysis, a prototype of the proposed converter is implemented at 100 W–100 kHz and the experimental results are presented.

Analysis of a Series‑Parallel Resonant Converter for DC Microgrid Applications

An input-series output-parallel soft switching resonant circuit with balance input voltage and primary-side current is studied and implemented for direct current (DC) microgrid system applications. Two resonant circuits are connected with input-series and output-parallel structure to have the advantages of low voltage stresses on active devices and low current stresses on power diodes. A balance capacitor is adopted on high voltage side to balance two input capacitor voltages. The LLC (inductor–inductor–capacitor) resonant circuit cells are employed in the converter to have soft switching operation for power semiconductors. The magnetic coupling component is adopted on the primary-side to automatically realize current balance of the two resonant circuits. In the end, a laboratory hardware circuit is built and tested. Experiments demonstrate and prove the validity of the resonant converter.