Three-level Research Articles

A Zero‐Voltage Switching Three‐Level Nonisolated Bidirectional DC/DC Converter With a Lossless Passive Component Auxiliary Circuit and Design‐Oriented Analysis

ABSTRACTA three‐level bidirectional buck/boost converter (TLBBBC) is suitable for power electronic systems with a high‐voltage DC link for its switches with half voltage. In order to achieve higher efficiency, a novel zero‐voltage switching (ZVS) TLBBBC is proposed in this paper to enable operation with higher switching frequencies. In the proposed ZVS TLBBBC, two identical ZVS cells, each composed of a resonant inductor, two snubber capacitors, and two resonant capacitors, are integrated with the conventional three‐level (TL) topology to enable soft switching in all four switches in both buck and boost modes. Here, one advantage is that soft‐switching conditions are ensured under conventional control methods without using the auxiliary switch or complex control methods. In addition, the soft‐switching region is derived, clearly illustrating the relationship between the duty cycle ratio and the power inductor current that ensures the soft‐switching condition. Hence, the soft‐switching region is beneficial for assisting in ZVS cell design and guiding the converter in running in ZVS. Then, the operation and design considerations of the proposed topology are analyzed in detail. Finally, the experimental results of an 800‐W prototype for both the boost and buck modes are presented to confirm the theoretical analysis.

Double-Loop Controller Design of a Single-Phase 3-Level Power Factor Correction Converter

This paper presents a study on the double-loop controller design technique for a single-phase power factor correction (PFC) three-level (TL) boost converter. Designing a double-loop controller using conventional methods is challenging due to the 120 Hz voltage ripple in the output voltage. This study proposes new double-loop control design methods using a band-stop filter and MATLAB SISOTOOL, detailed in a step-by-step sequence. A band-stop filter with a 120 Hz stop band is applied to the double-loop controller design. Modeling based on the state-space equation, applicable to the full duty range, is constructed to obtain the transfer function. The double-loop controller structure is then designed, and optimal gains that satisfy the design requirements are obtained through automatic tuning using the MATLAB SISOTOOL library. The simulation results demonstrate the performance of the proposed method, which is further verified by experimental results.

Compensation of Second Harmonic Current Based on Bus Voltage Ripple Limitation in Single-Phase Photovoltaic Grid-Connected Inverter

In the two-stage single-phase photovoltaic (PV) grid-connected inverter, the flying-capacitor-clamped (FCC) boost three-level (TL) converter is adopted to achieve maximum power point tracking (MPPT) of the PV panel. To restrain the second harmonic current (SHC) in the PV panel and remove the undesired electrolytic capacitors, the flying capacitor is employed to compensate the SHC. To achieve this goal, the flying capacitor average voltage control incorporated bus voltage ripple limitation scheme (AVC-IBVRLS) is proposed in this letter. The closed-loop parameters design for the bus voltage ripple limitation loop is given. Finally, a prototype of 3-kVA PV grid-connected inverter is fabricated and tested in the lab, and the experimental results verify the validity of the proposed scheme.

Hybrid Control for Three-Level LLC Resonant Converter of Dual-Bridge for Wide Output Range

A three-level (TL) LLC resonant converter of dual-bridge (DB) with hybrid control modulation strategy is proposed for the wide output-voltage range and high input-voltage applications. The topology integrates two neutral-point-clamped arms (NPCs) through an auxiliary transformer, which forms a new controllable double-voltage structure and makes all MOSFETs hold half of the input voltage. Based on the characteristics of the new topology, a hybrid modulation strategy is proposed corresponding to three gain modes, which include conventional pulse frequency modulation (PFM) and fixed frequency phase shift modulation (FF-PSM). Compared to conventional modulation strategies, the proposed hybrid modulation strategy, which improves the output voltage gain range, achieves zero-voltage switching (ZVS) of all the MOSFETs and zero-current switching (ZCS) of rectifier diodes under wider operating voltage and full load range. To minimize the frequency window and improve the efficiency of the converter, the parameter optimization method is designed, which based on the characteristics of the modulation strategy and the relationship among the maximum output voltage gain, resonant current and turned-off current. At last, the experimental results of the 600W prototype are presented to confirm the theoretical and characterization analysis.

A hybrid full-bridge three-level DC-DC converter with crossing auxiliary capacitors

This paper presents a high-power medium-frequency hybrid full-bridge (FB) three-level (TL) DC-DC converter with crossing auxiliary capacitors. In this converter, two auxiliary capacitors are deployed between phases a and b, replacing the flying capacitor. The converter’s three-level leg switches are with low voltage stress, while the two-level leg switches are with high voltage stress. The topology and modulation method, as well as the switching characteristics of the hybrid FBTL DC-DC converter with crossing auxiliary capacitors are described. The feasibility and performance of the above-mentioned converter is validated by a laboratory prototype. Based on the mechanism analysis and experiments, the zero-voltage switching (ZVS) and zero-switching loss (ZSL) of power switches, self-balance of the clamped capacitors voltages, and the voltage self-balance of the switches in the three-level leg can be achieved.

Design of hybrid SiC/Si based T‐type three‐level LLC$ LLC$ resonant converter with wide‐input range and low conduction loss for automotive auxiliary power module

The auxiliary power module (APM) in the electrical vehicle converts the high voltage of battery pack to the low voltage to supply the electric power for the internal loads, including electric power steering, electric turbo, headed windshield and cooling pumps. The increasing demands on charging time and mileage have made the battery packs with even higher voltage (e.g. 800 V) a promising option. However, it brings a new challenge for the APM, and either the 1.2-kV silicon carbide (SiC) device or the three-level (TL) topology are used conventionally. This work proposes a cost-effective hybrid SiC/Si-based T-type TL L L C $ LLC$ resonant converter for the vehicular APM application with a high step-down ratio. The 1.2-kV SiC and 650-V Si devices are used as the main and auxiliary switches, respectively. It fully utilizes the voltage rating of device, and also improves the efficiency compared with conventional TL topology. To improve the gain range and electromagnetic interference performance, a variable frequency and adjustable phase-shift modulation scheme is adopted. At the same time, the steady-state time-domain model is established to elaborate the operation principles, topology features, and boundary conditions for soft-switching. Furthermore, the precise solutions for switching frequency and phase-shift angle are figured out. Finally, the effectiveness of model is verified by the simulation as well as the experiment. A 1-kW prototype for the application in the APM is demonstrated with the input voltage of 640–840 V and a constant output of 48 V/ 21 A.

Design and Control of a Three-Level Rectifier in LCC/S-Compensated IPT for Wide Output Voltage Regulation over Various Magnetic Couplings

Conventional inductive power transfer (IPT) employs primary control via phase shift, frequency tuning, or voltage tuning, whereas closed-loop control requires real-time wireless feedback communication. However, the long propagation delay results in small bandwidth. In this paper, a three-level (TL) rectifier is studied to implement secondary control and wide output voltage regulation in an inductor–capacitor–capacitor/series (LCC/S)-compensated IPT system over various magnetic couplings. The periodical operation behavior is analyzed, and a generic analytical expression of the system voltage gain including the TL rectifier is derived based on the Fourier series. A control strategy of an optimal control trajectory is proposed to maximize the power factor in the TL rectifier. The control variables are the duty cycles of the zero-level and one-level voltage in the TL rectifier. Either one remains at zero, while another one is utilized to modulate the output voltage in the proposed control strategy. A 2 kW prototype is designed and built to validate the theoretic analysis. The wide output voltage range between 100 V and 200 V under different magnetic coupling coefficients (0.16 and 0.23), a peak efficiency of 95.8% at 100 V and misaligned position, as well as a faster response of 1.3 ms are experimentally validated.

Passive Capacitor Voltage Balancing of SiC-Based Three-Level Dual-Active-Bridge Converter Using Hybrid NPC-Flying Capacitor Structure

Three-level (TL) dual-active-bridge (DAB) converter serves a critical role in the medium-voltage (MV) solid-state-transformers in which high voltage rating and bidirectional power flow are required. The regular neutral-point-clamping (NPC) topology is easily subjected to capacitor voltage unbalance due to nonideal operating conditions. In this article, a hybrid structure incorporating NPC and flying capacitor (FC) is presented to resolve the voltage unbalance issue. The key advantages include minimal additional hardware efforts and no need to resort to active control. The FC behaves as a buffer to leverage the upper and lower capacitor so that passive voltage balance between the two dc-link capacitors can be achieved on a switching cycle basis. Closed-form analysis further reveals the impact of FC value on voltage unbalance. Moreover, the appropriate modulation scheme, switching condition, and commutation loop are evaluated to provide detailed rule of thumb to the implementation of FC circuit. Analysis shows the FC also brings favorable switching loss performance and is friendly to employ upon fast switching of wide bandgap devices such as SiC. Finally, a 1.6 kV input, 400 V output, 8 kW scaled-down hybrid NPC-FC-based DAB converter is built to validate the above analysis.

A Hybrid-Bridge-Based Dual Active Bridge Converter With Reduced Device Count

In this paper, a hybrid-bridge-based dual-active-bridge (DAB) converter with a three-level (TL) half-bridge and a two-level full-bridge is proposed, which can be utilized to interface the medium voltage and low voltage DC. In the proposed converter, there are only eight power switches in total. Compared with the traditional hybrid-bridge-based DAB converter with a neutral-point-clamped (NPC) type TL half-bridge, a fewer power device count and higher efficiency can be realized in the proposed converter due to removing clamping diodes. Thus, a more compact circuit structure and a lower cost can be obtained. Moreover, zero-voltage-switching (ZVS) of all switches in primary-side and secondary-side can be achieved in the proposed converter. The operating principle, performances, and characteristics of the proposed converter are analyzed in detail. Also, a specification design case with a detailed key parameter design is given. Finally, a 1-kW experimental prototype is established to validate the proposed converter, and obtained experimental results are highly consistent with the theoretical analysis.

Euler–Lagrange passivity-based controller for the three-level Ćuk PFC converter for electric vehicle battery charging application

This paper intends a Euler-Lagrange passivity-based controller (EL-PBC) for the three-level (TL) Ćuk PFC converter for EVs battery charging application. The TL Ćuk converter has features of reduced filter size and voltage stress of switches. First, the Euler-Lagrange formulation is used to develop the mathematical modelling. Then, the EL-PBC methodology is implemented by injecting a virtual resistor in series with the input inductor. Further, the stability analysis is also carried out. To improve the performances against input perturbations and reduce the steady state error, a PI controller is integrated with the aforesaid controller and its performances are investigated. Furthermore, the power quality of the charger is assessed and it satisfies IEC 61000-3-2 Class C standard. This controller is also compared with the benchmark PI controller. A prototype model of same specifications is tested in hardware in loop. Computer simulations and test results are presented to validate the theoretical aspects.

Interleaved hybrid control with an upper optimizer of parallel three‐level LLC converters for battery charging applications

This letter proposes an interleaved hybrid control for parallel three-level (TL) LLC converters in battery charging applications. The duty cycle and the switching frequency are used to adjust the voltage gain; the phase shift control is adopted to realize voltage sharing of the divided capacitors. A virtual equivalent frequency-gain control is adopted to coordinate multiple control variables. And the current ripples are reduced for the interleaved structure. Based on the performance analysis, an upper optimizer is proposed to update the upper limit frequency and interleaving arrangement in real time. Accordingly, it achieves a wide range of output voltage, and it further improves the system efficiency under light load and low output voltage conditions. A prototype is presented to verify the effectiveness of the proposed method.

Port-Controlled Hamiltonian Based Controller for Three-Level Ćuk PFC Converter for Battery Charging Application

This paper reveals energy shaping passivity-based control methodology for a three-level (TL) Ćuk PFC converter. The proposed control methodology is based on the dynamic model of the system along with the idea of energy shaping and damping injection. First, a dynamic model of the TL Ćuk PFC converter is developed using the port-controlled Hamiltonian formulation and the state-space averaging technique. Then, the PCH control technique is implemented in the system and the stability analysis is carried out. In order to eliminate the steady state error, a PI controller is integrated with the PCH-PBC control scheme. Further, the performances of aforesaid system are investigated for battery charging application with the help of MATLAB/Simulink. To validate the simulation study, a prototype model of TL Ćuk PFC converter with proposed controller is built using OP-5142 real-time simulator and test results are recorded. Furthermore, the power quality features of TL Ćuk PFC converter are assessed through monitoring of input current THD under different operating conditions. To assess the system performance in terms of efficiency, input p.f., input current THD, and controller parameters, the proposed controller is compared with benchmark PI controller under dynamic variations at the input voltage and the load.

Novel Output Regulation Method for Three-Phase Three-Level Wireless EV Charging System

Three-phase wireless power transfer (WPT) systems have compact coil structures, canceled triplen harmonics, and less dc -link current ripple, which are suitable for wireless electric vehicle (EV) charging systems. Most three-phase WPT systems are based on two-level inverters. Alternately, three-level (TL) inverters have the potential to further promote the performance of EV chargers by reducing the electromagnetic interference (EMI) and the voltage stress of power switches. However, the unbalance of the neutral point (NP) voltage is a problem for traditional TL inverters. This article proposes a TL three-phase WPT system for EV wireless charging, which is based on a T-type neutral point clamp (TNPC) inverter. The modeling, coil structure, and switching pattern of the proposed system are analyzed in detail. Furthermore, a novel output regulation method based on vector control is proposed for the WPT system. Also, a NP voltage balance strategy is proposed to enhance the reliability of the TL inverter. Both simulations and an experimental prototype based on SIC- MOSFET are given to verify the effectiveness of the proposed three-phase WPT system and control method.

Brayton-Moser Passivity Based Controller for Electric Vehicle Battery Charger

In this paper Brayton-Moser passivity-based control (BM-PBC) methodology is developed for an on-board battery charger for plug-in electric vehicles(PHEVs). The main features of this electric vehicle (EV) charger include improved power quality, reduced filter size and voltage stress across the switches and fast dynamic response. In this paper, a dynamic model of the three-level (TL) boost power factor correction (PFC) converter is developed using the Brayton-Moser formulation. Then, the Brayton-Moser based control technique is designed by injecting a virtual resistor in series with the input inductor. Further, the stability analysis of the proposed controller is also carried out using energy balance approach. To improve the dynamic performance and reduce the steady state error, a PI controller is integrated with the aforesaid controller. Therefore, the controller comprises of BM-PBC and the PI controller is implemented for the TL boost PFC converter as a battery charger and its performances are investigated under various operating modes with the help of MATLAB/Simulink. Furthermore, power quality of charger is assessed by monitoring source current total harmonic distortion (THD) under different operating conditions. It is also observed that the proposed system provides THD less than 5% in source current which satisfies IEC 61000-3-2 Class C standard. The performance of the aforesaid controller is also compared with the conventional PI controller. In order to validate the proposed controller, a prototype model of same specifications is tested in hardware in loop and obtained test results are also presented.

Dual-transformer-based hybrid resonant three-level ZCS converter

A hybrid resonant three-level converter comprised of dual transformers is proposed in this paper, which is suitable for the application of distributed photovoltaic power accessing the medium voltage dc distribution network. The proposed converter can be obtained by adding a control circuit into the traditional neutral point clamped (NPC) three-level (TL) circuit, achieving the basic TL circuit operating with a fixed duty cycle. Pulse width modulation (PWM) is adopted for the control circuit to realize zero current switchings for the basic TL circuit, which delivers most of the power, under full load range. As a result, the switching loss of the converter can be significantly reduced. The influences of the turn ratio of the second transformer and resonant capacitance on the switch current, the peak value of resonant voltage, and the value of the resonant inductance value are discussed in detail, and the parameters design principles are put forward. Finally, a prototype is built to verify the performance of the proposed converter.

High Performance Flexible Structure Three-Level DC-DC Converter: A Candidate DC Interface for Microgrids With Distributed Energy Resources

High-performance DC interfaces play a pivotal role in microgrids with distributed energy resources (DERs), which would significantly improve the utilization of DERs and the flexibility of the prosumers. For a candidate DC interface converter, how to maintain the performance over a wide operation range becomes the biggest challenge. This paper proposes a flexible structure soft-switching three-level (TL) dc-dc converter, which adds four high-speed MOSFETs and one low-speed electronic relay to achieve overall optimum performance over a wide range. Under high output or low input voltage conditions, the relay and the extra MOSFETs on the primary side are ON permanently. The proposed converter is a zero-voltage switching (ZVS) converter, which guarantees ZVS for all primary switches even under 0 load currents. Under low output or high input voltage conditions, the relay and the extra MOSFETs in the secondary side are OFF permanently. The proposed converter is a zero-voltage and zero-current switching (ZVZCS) converter, and two added MOSFETs provide ZVZCS operation not only for the main primary switches but also themselves over a wide range. Other advantages of the presented converter include reduced filter size, no primary side circulating currents, and reduced current stress of the clamping capacitor. Besides, the switching loss caused by the added power devices is low because of the full range of soft-switching operation and low on-state resistance of these devices. This paper discusses the circuit configuration, operation principle, soft switching characteristics, technical comparison, and experimental results from a 1-kW prototype prove the rightness of the presented converter.

Hybrid resonant three‐level ZCS converter suitable for photovoltaic power MVDC distribution network

This study presents a hybrid resonant three-level (TL) zero-current-switching (ZCS) converter for distributed photovoltaic power applications accessing medium-voltage DC (MVDC) distribution network. The main constituents of the converter is a neutral point clamped TL circuit, an auxiliary circuit along with two transformers and six switches. The ZCS condition of the TL circuit is reached by operating the auxiliary circuit utilising pulse width modulation technique. It delivers the majority of the power and the switching loss is remarkably reduced. A section on the influence of the turn's ratio of the auxiliary transformer on the current is also presented. The piecewise linear electrical circuit simulation simulation results validate the designed parameter principles. For the sake of comparison, an experimental prototype is developed of 300–1500 V/1500 W to validate the performance of the proposed converter.

Second Harmonic Current Reduction for Flying Capacitor Clamped Boost Three-Level Converter in Photovoltaic Grid-Connected Inverter

In a two-stage single-phase photovoltaic (PV) grid-connected inverter, the second harmonic current (SHC) in the PV panel will affect the maximum power point tracking operation and degrade the energy harvesting efficiency. In order to solve this problem, a flying capacitor clamped (FCC) boost three-level (TL) converter is adopted in this article, and the flying capacitor is employed to compensate the SHC for removing a large electrolytic capacitor. The propagation mechanism of the SHC is analyzed from the perspective of impedance, and a virtual impedance in parallel with the intermediate dc bus is introduced, achieving a perfect SHC compensation performance. The implementation of the virtual parallel impedance is illustrated, and the key parameters design of the FCC boost TL converter is given. A 3-kW two-stage single-phase PV grid-connected inverter is fabricated and tested, and the experimental results verify the validity of the theoretical analysis and the proposed control scheme.

High-Efficiency Voltage Balancer Having DC–DC Converter Function for EV Charging Station

Due to the recent proliferation of electric vehicles, high-power charging stations are being actively studied. Among the main types of charging stations, bipolar dc-bus-fed ones outperform the ac-bus-fed ones because of easier interfacing with distributed generation and fewer conversion stages. Bipolar dc-bus-fed charging stations generally require voltage balancing control (VBC) between the dc-bus voltages; hence, they use a three-level (TL) dc–dc converter that serves as a fast charger while balancing the dc-bus voltages. However, the VBC range of the TL converter is limited by its output load power, which can cause voltage imbalance of the bipolar dc-bus. To address this problem, this paper proposes a high-efficiency voltage balancer (VB) having dc–dc converter function. The proposed VB has the ability of the dc–dc converter and can serve as a fast charger. Moreover, this VB has no limitations on the VBC range, unlike the TL converter, and features lower voltage and current stresses on the switches as well as higher conversion ratio and efficiency compared to the conventional VB with dc–dc converter function. The key features of the proposed and conventional schemes are compared, and simulated and experimental results are presented to demonstrate the effectiveness of the proposed VB.

A Hybrid Resonant ZVZCS Three-Level Converter Suitable for Photovoltaic Power DC Distribution System

A novel hybrid resonant zero-current- switching (ZCS) three-level DC-DC converter based on a dual transformer with two output filter capacitors is proposed in this paper, which is suitable for application in distributed photovoltaic power generation at medium voltage integrated with a DC distribution network. The proposed converter adds an auxiliary circuit to the traditional neutral point clamped (NPC) three-level (TL) circuit, which contains the basic NPC three-level circuit. Pulse width modulation (PWM) is adopted in the auxiliary circuit portion to realize the power and voltage regulation of the whole converter, and enable the main switches to operate with a fixed duty cycle. This has the advantages of simplified control. By reasonable design of the turn's ratio of the main transformer, the main three-level circuit can deliver most of the power and achieve ZCS within the under full load range that significantly reduced the switching losses. Therefore, the turns ratio of the auxiliary transformer can be optimized to reduce the loss of the converter further and improve the conversion efficiency as discussed in detail, while the parameters' design principles are put forward. Finally, a prototype of 300V-1.5kV/1.5 kW is built to verify the performance of the proposed converter.