Baifeng Chen

High Reliability and Efficiency Single-Phase Transformerless Inverter for Grid-Connected Photovoltaic Systems

This paper presents a high-reliability single-phase transformerless grid-connected inverter that utilizes superjunction MOSFETs to achieve high efficiency for photovoltaic applications. The proposed converter utilizes two split ac-coupled inductors that operate separately for positive and negative half grid cycles. This eliminates the shoot-through issue that is encountered by traditional voltage source inverters, leading to enhanced system reliability. Dead time is not required at both the high-frequency pulsewidth modulation switching commutation and the grid zero-crossing instants, improving the quality of the output ac-current and increasing the converter efficiency. The split structure of the proposed inverter does not lead itself to the reverse-recovery issues for the main power switches and as such superjunction MOSFETs can be utilized without any reliability or efficiency penalties. Since MOSFETs are utilized in the proposed converter high efficiency can be achieved even at light load operations achieving a high California energy commission (CEC) or European union efficiency of the converter system. It also has the ability to operate at higher switching frequencies while maintaining high efficiency. The higher operating frequencies with high efficiency enables reduced cooling requirements and results in system cost savings by shrinking passive components. With two additional ac-side switches conducting the currents during the freewheeling phases, the photovoltaic array is decoupled from the grid. This reduces the high-frequency common-mode voltage leading to minimized ground loop leakage current. The operation principle, common-mode characteristic and design considerations of the proposed transformerless inverter are illustrated. The total losses of the power semiconductor devices of several existing transformerless inverters which utilize MOSFETs as main switches are evaluated and compared. The experimental results with a 5 kW prototype circuit show 99.0% CEC efficiency and 99.3% peak efficiency with a 20 kHz switching frequency. The high reliability and efficiency of the proposed converter makes it very attractive for single-phase transformerless photovoltaic inverter applications.

Zero-Voltage-Switching PWM Resonant Full-Bridge Converter With Minimized Circulating Losses and Minimal Voltage Stresses of Bridge Rectifiers for Electric Vehicle Battery Chargers

This paper presents a zero-voltage-switching (ZVS) full-bridge dc-dc converter combing resonant and pulse-width-modulation (PWM) power conversions for electric vehicle battery chargers. In the proposed converter, a half-bridge LLC resonant circuit shares the lagging leg with a phase-shift full-bridge (PSFB) dc-dc circuit to guarantee ZVS of the lagging-leg switches from zero to full load. A secondary-side hybrid-switching circuit, which is formed by the leakage inductance, output inductor of the PSFB dc-dc circuit, a small additional resonant capacitor, and two additional diodes, is integrated at the secondary side of the PSFB dc-dc circuit. With the clamp path of a hybrid-switching circuit, the voltage overshoots that arise during the turn off of the rectifier diodes are eliminated and the voltage of bridge rectifier is clamped to the minimal achievable value, which is equal to secondary-reflected input voltage of the transformer. The sum of the output voltage of LLC resonant circuit and the resonant capacitor voltage of the hybrid-switching circuit is applied between the bridge rectifier and the output inductor of the PSFB dc-dc circuit during the freewheeling phases. As a result, the primary-side circulating current of the PSFB dc-dc circuit is instantly reset to zero, achieving minimized circulating losses. The effectiveness of the proposed converter was experimentally verified using a 4-kW prototype circuit. The experimental results show 98.6% peak efficiency and high efficiency over wide load and output voltage ranges.

A High-Efficiency MOSFET Transformerless Inverter for Nonisolated Microinverter Applications

State-of-the-art low-power-level metal-oxide-semiconductor field-effect transistor (MOSFET)-based transformerless photovoltaic (PV) inverters can achieve high efficiency by using latest super junction MOSFETs. However, these MOSFET-based inverter topologies suffer from one or more of these drawbacks: MOSFET failure risk from body diode reverse recovery, increased conduction losses due to more devices, or low magnetics utilization. By splitting the conventional MOSFET-based phase leg with an optimized inductor, this paper proposes a novel MOSFET-based phase leg configuration to minimize these drawbacks. Based on the proposed phase leg configuration, a high efficiency single-phase MOSFET transformerless inverter is presented for the PV microinverter applications. The pulsewidth modulation (PWM) modulation and circuit operation principle are then described. The common-mode and differential-mode voltage model is then presented and analyzed for circuit design. Experimental results of a 250 W hardware prototype are shown to demonstrate the merits of the proposed transformerless inverter on nonisolated two-stage PV microinverter application.

A Dead-Time Compensation Method for Parabolic Current Control With Improved Current Tracking and Enhanced Stability Range

Hysteresis current control is an attractive nonlinear current-control method for voltage source inverters when a fast system response is required. A well-known disadvantage of hysteresis current control is that the system has to operate over a wide switching frequency range. This causes an increase in the switching losses of the system and increases the difficulty in designing the output filter. The recently proposed parabolic current control solves this problem by employing a pair of parabolic carriers as the control band. Through the use of parabolic current control, constant switching frequency can be achieved. In the implementation of parabolic current control, dead time is employed to prevent shoot through of the inverter leg. The employment of dead time impacts the current-tracking precision and the stability range of the parabolic current-control method. Another side effect of using dead time is that the switching frequency deviates from the desired value. In this paper, the effects of dead time on parabolic current control are analyzed, and a compensation method is proposed for voltage source inverters that use parabolic current control. Using the output current direction of the voltage source inverter, a new pair of improved parabolic carriers is derived. As a result, the current error can be well controlled and the effects of dead time can be eliminated. The improvement in the current tracking of the system comes with an added benefit where the duty cycle range is extended. The effectiveness of the proposed dead-time compensation method is experimentally verified by the use of a full-bridge voltage source inverter.

Control of electrolyte-free microinverter with improved MPPT performance and grid current quality

This paper presents a control technique to improve the MPPT performance and grid current quality for two-stage electrolyte-free microinverter. The proposed method rejects the PV-side double-line frequency oscillation and reduces the grid current distortion with the PV dc-dc converter controlled with a high loop gain while intermediate dc bus voltage loop of grid-tie dc-ac inverter controlled with a low loop gain both at the double-line frequency. The dc-bus film capacitors with the small capacitance allow to have high ripple voltage to buffer the double-line energy while the PV-side could not see this doubleline frequency oscillation due to the control of dc-dc converter with high double-line frequency ripple rejection capability. As a result of high reduction of double-line frequency of dc bus voltage loop, the distortion of grid current reference is reduced and the total harmonic distortion (THD) of the grid current is improved. The PV-side capacitance can also be greatly reduced because it is only required to filter the high-frequency transistor switching ripple. Experimental results justify the effectiveness of the proposed control method.

Hybrid Transformer ZVS/ZCS DC–DC Converter With Optimized Magnetics and Improved Power Devices Utilization for Photovoltaic Module Applications

This paper presents a nonisolated, high boost ratio dc-dc converter with the application for photovoltaic (PV) modules. The proposed converter utilizes a hybrid transformer to incorporate the resonant operation mode into a traditional high boost ratio active-clamp coupled-inductor pulse-width-modulation dc-dc converter, achieving zero-voltage-switching (ZVS) turn-on of active switches and zero-current-switching turn-off of diodes. As a result of the inductive and capacitive energy being transferred simultaneously within the whole switching period, power device utilization (PDU) is improved and magnetic utilization (MU) is optimized. The improved PDU allows reduction of the silicon area required to realize the power devices of the converter. The optimized MU reduces the dc-bias of magnetizing current in the magnetic core, leading to smaller sized magnetics. Since the magnetizing current has low dc-bias, the ripple magnetizing current can be utilized to assist ZVS of main switch, while maintaining low root-mean-square (RMS) conduction loss. The voltage stresses on the active switches and diodes are maintained at a low level and are independent of the wide changing PV voltages as a result of the resonant capacitor in series in the energy transfer loop. The experimental results based on 250 W prototype circuit show 97.7% peak efficiency and system CEC efficiencies greater than 96.7% over 20 to 45 V input voltages. Due to the high efficiency over wide power range, the ability to operate with a wide variable input voltage and compact size, the proposed converter is an attractive design for PV module applications.

Design of Bidirectional DC–DC Resonant Converter for Vehicle-to-Grid (V2G) Applications

In this paper, a detailed design procedure is presented for a bidirectional CLLLC-type resonant converter for a battery charging application. This converter is similar to an LLC-type resonant converter with an extra inductor and capacitor in the secondary side. Soft-switching can be ensured in all switches without additional snubber or clamp circuitry. Because of soft-switching in all switches, very high-frequency operation is possible; thus, the size of the magnetics and the filter capacitors can be made small. To reduce the size and cost of the converter, a CLLC-type resonant network is derived from the original CLLLC-type resonant network. First, in this paper, an equivalent model for the bidirectional converter is derived for the steady-state analysis. Then, the design methodology is presented for the CLLLC-type resonant converter. Design of this converter includes determining the transformer turns ratio, design of the magnetizing inductance based on zero-voltage switching condition, design of the resonant inductances and capacitances. Then, the CLLCtype resonant network is derived from the CLLLC-type resonant network. To validate the design procedure, a 3.5-kW converter was designed following the guidelines in the proposed methodology. A prototype was built and tested in the laboratory. Experimental results verified the design procedure presented.

High efficiency transformerless photovoltaic inverter with wide-range power factor capability

Photovoltaic (PV) inverter is the most important part for energy conversion, and the current research focus for PV inverter is high efficiency, high reliability, and low-output ac-current distortion. With considerable grid-connected PV installation in last few years, high penetration PV system is desired, which requires integrating PV inverter into grid regulation. New code in Germany already requires PV inverter providing system regulation and service for low voltage distribution system. One of obvious impact on PV inverter is reactive power generation requirement. Most state-of-the-art high-efficiency PV inverters that adopt single-stage topology only allow unity power generation but none of them are able to generate reactive power. This paper proposes a high-efficiency dual-buck full-bridge PV inverter for a wide range power factor operation. Additionally, a novel hybrid bipolar PWM method is proposed to achieve low ground leakage current and low output current distortion in the PV system. At last, simulation and experiment demonstrate that the proposed PV inverter system can provide a wide-range power factor operation with high efficiency, low ground leakage current, and low output current distortion.

A Sensorless Implementation of the Parabolic Current Control for Single-Phase Stand-Alone Inverters

Parabolic current control is an attractive current control method with fast transient response and constant switching frequency. Due to the good dynamics of the parabolic current control, it can be employed in voltage source inverters to improve the system performance such as minimizing the distortion of current waveforms or voltage waveforms. To implement the parabolic current control, a current sensor is required, associated with the current conditioning circuit and parabolic carrier generators. Since the parabolic current control is based on the real-time information of the inductor current, any phase delay or propagation delay of the sensor itself and the conditioning circuitry, or limited resolution of parabolic carrier generators, could impact the current control performance. Since the parabolic current control compares analog signals to generate the required control signals, noise from the control board impacts the control precision as well. This paper will explore solutions to these problems. First, the inductor current of the voltage source inverter is analyzed and the parabolic current control strategy is studied, then a sensorless parabolic current control method is proposed. The new sensorless parabolic control method utilizes a current emulator to rebuild the inductor current on a microcontroller. To avoid a dc offset on the ac-side output voltage caused by the current emulator, an additional control loop in the current emulator is added. The effectiveness of the proposed methods is experimentally verified by the use of an H-bridge voltage source inverter.

Design and optimization of 99% CEC efficiency soft-switching photovoltaic inverter

Driven by worldwide demands for renewable energy source, photovoltaic (PV) inverters, which are the most important part for energy conversion, have seen a considerable amount of innovations in recent years. The trends for PV inverters are toward high efficiency, high reliability, low ground leakage current, low-output ac-current distortion, and reactive power capability. This paper provides an efficiency optimized design of an Auxiliary Resonant Snubber with Coupled-Magnetic Reset Zero-voltage switching (ZVS) inverter for PV application. The main device is Power MOSFETs, which have low conduction loss and could achieve ZVS in all load condition. The auxiliary devices are low current, low cost IGBTs and Diodes, which could achieve zero-current switching (ZCS). To achieve high efficiency, resonant circuit is analyzed to optimally design the resonant components, which is based on the guarantee of full range ZVS, gating delay-time design, and lower power loss in auxiliary circuit. The power loss model is also analyzed to select suitable power device and further improve the efficiency. At last, experiment results of a 5kW single phase inverter in PV system was presented, which has more than 99% CEC efficiency and work in full load condition without cooling fan. Besides, reactive power capability is also demonstrated.