Lanhua Zhang

High-Efficiency Contactless Power Transfer System for Electric Vehicle Battery Charging Application

In this paper, a contactless charging system for an electric vehicle (EV) battery is proposed. The system consists of three parts: 1) a high-frequency power supply from a full-bridge inverter with frequency modulation; 2) a loosely coupled transformer that utilizes series resonant capacitors for both the primary and secondary windings; and 3) a rectification output circuit that uses a full-bridge diode rectifier. With carefully selected compensation network parameters, zero-voltage switching can be ensured for all the primary switches within the full range of an EV battery charging procedure, which allows the use of low ON-state resistance power MOSFETs to achieve high-frequency operation and system efficiency. The design of loosely coupled transformer is simulated and verified by finite element analysis software. For a 4-kW hardware prototype, the peak dc-dc efficiency reaches 98% and 96.6% under 4- and 8-cm air gap conditions, respectively. The prototype was tested with an electronic load and a home-modified EV to verify the performance of constant current and constant voltage control and their transitions.

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.

Design Considerations to Reduce Gap Variation and Misalignment Effects for the Inductive Power Transfer System

An inductive power transfer (IPT) system usually consists of four parts: an AC-DC power factor correction (PFC) converter, a high frequency DC-AC inverter, a compensation network comprising a loosely coupled transformer (LCT) and the resonant capacitors, and a rectification output circuit. Due to the relative large air gap, the magnetic coupling coefficient of the IPT system is poor, different from the closely-coupled IPT systems. As a result, the efficiency of the IPT system is always a main concern for different applications. To ensure high power transfer efficiency, these IPT systems should have high tolerance for different gap variation and horizontal misalignment conditions. In this paper, some design considerations to reduce gap and misalignment effects for the IPT system is proposed. By using finite element analysis (FEA) simulation method, the performance of different transmitter and receiver coil dimensions are compared. In order to validate the performance of the proposed design considerations, a hardware prototype is built and the corresponding experiments are carried out. The experimental results shows that the LCT prototype could maintain coupling coefficient between 0.237~0.212 within 40 mm horizontal misalignment.

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.

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.

A dual-buck based equalizer operating in burst-mode for split phase inverter

This paper presents a new method of current control that utilizes burst mode in order to balance the dc side capacitor voltages of split phase inverters. The outer voltage control loop employs hysteresis control, which provides fast transient response. When this voltage control loop is used in conjunction with the burst mode current loop, the efficiency at light load is improved and the capacitor voltages are more precisely balanced. A dual-buck converter is employed as the balancing auxiliary circuit, which has higher reliability due to the avoidance of shoot-through as well as avoiding the reverse recovery issue of the power MOSFET. No reverse recovery allows the use of super-junction MOSFETs instead of IGBTs in order to achieve higher efficiency. A 4 kW prototype with 400 V dc bus voltage is built to verify all the performance and a peak efficiency of 98.3% is achieved.

A capacitor voltage balancing method with zero-voltage switching for split phase inverter

This paper presents a new method of current control that employs burst mode in order to balance the dc-side capacitor voltages of split phase inverters. By alternating the inductor current in every switching period, zero-voltage switching (ZVS) can be achieved, which avoids the reverse recovery issue of power MOSFETs. As a result, super-junction MOSFETs instead of IGBTs can be employed to achieve higher efficiency. Combining the hysteresis current control with traditional pulse-width modulation (PWM), a new burst mode control method is proposed in this paper. The minimum operation time during a set modulation period is limited then the efficiency at the light load can be increased. A buck-and-boost converter is employed as the auxiliary circuit for the capacitor voltage balancing circuit. Due to the proposed ZVS method, inductor volume can be reduced. A prototype with 25 A current capacity is built to verify all the performance of the presented control method. The efficiency over the whole load range is over 98% with a peak efficiency of 98.4%.

A Novel Pulse-Width Modulation Method for Reactive Power Generation on a CoolMOS- and SiC-Diode-Based Transformerless Inverter

For efficiency considerations in the photovoltaic (PV) power generation, high-efficiency CoolMOS- and SiC-diode-based transformerless inverters have been proposed and studied in the previous literatures, but the reactive power generation capability to meet the upcoming standards has never been discussed. By reviewing the high efficiency converters with CoolMOS and SiC-diodes, this paper improves a previous transformerless inverter circuit and presents related operating modes for reactive power generation. A novel pulse-width modulation (PWM) method for this improved inverter topology is then proposed for reactive power generation. The ground-loop voltage of this inverter under the proposed PWM method is also derived with common mode (CM) and differential mode (DM) circuit analyses, which indicate that high-frequency voltage components can be minimized with symmetrical design of inductors. A 250-W inverter hardware prototype has been designed and fabricated. Steady-state and transient operating conditions are tested to demonstrate the validity of the improved inverter and proposed PWM method for reactive power generation, and the high-frequency-free ground loop voltage.