Kangping Wang

A Maximum Efficiency Point Tracking Control Scheme for Wireless Power Transfer Systems Using Magnetic Resonant Coupling

With a good balance between power transfer distance and efficiency, wireless power transfer (WPT) using magnetic resonant coupling is preferred in many applications. Generally, WPT systems are desired to provide constant output voltage with the highest possible efficiency as power supplies. However, the highest efficiency is not achieved by the reported closed-loop WPT systems that maintain constant output voltage against coupling and load variations. In this paper, an efficiency evaluation method is put forward to evaluate the closed-loop control schemes. Furthermore, a maximum efficiency point tracking control scheme is proposed to maximize the system efficiency while regulating the output voltage. This control scheme is unique and prominent in that it fixes the operating frequency at the receiving-side resonant frequency and converts both the input voltage and the load resistance at the same time. Thus, the maximum efficiency point on the constant output voltage trajectory can be tracked dynamically. Therefore the systems output voltage can be maintained constant and its efficiency is always the highest. The experimental results show that the maximum efficiency point is tracked and a very high overall efficiency is achieved over wide ranges of coupling coefficient and load resistance.

An Analytical Switching Process Model of Low-Voltage eGaN HEMTs for Loss Calculation

This paper proposes an improved analytical switching process model to calculate the switching loss of low-voltage enhancement-mode Gallium Nitride high-electron mobility transistors (eGaN HEMTs). The presented eGaN HEMTs models are more or less derived from silicon MOSFETs models, whereas eGaN HEMTs are different from three aspects: higher switching speed, much more reduced parasitic inductance in switching loop, and absence of reverse recovery. Applying the traditional model to eGaN HEMTs results in inaccurate prediction of switching waveforms and losses. The proposed model considers the effect of low-parasitic inductances, nonlinearity of junction capacitances, and nonlinearity of transconductance. The turn-on and turn-off switching processes are described in detail and the resulting equations can be easily solved. The accuracy of the proposed model is validated by comparing the predicted switching waveforms and converters efficiency with the experimental results, respectively. Based on the analytical model, the effects of gate resistance, gate supply voltage, and parasitic inductances on switching losses are investigated. Meanwhile, a novel current measuring method based on magnetic coupling is proposed to measure the switching current waveform with improved accuracy.

Dynamic Modeling Based on Coupled Modes for Wireless Power Transfer Systems

A novel dynamic modeling method based on the concept of coupled modes is proposed for wireless power transfer (WPT) systems which use magnetic resonant coupling. The proposed method aims on the dynamics of the overall WPT system, including the nonlinear inverter and rectifier. It uses the slowly varying amplitudes and phases of coupled modes rather than resonant currents and voltages to describe the coupled resonances. Three analytical models-averaged model, small signal model, and conductance network model are developed sequentially by using the proposed method. The orders of the developed models are equal to or lower than that of the discrete state space model. In contrast, the existing dynamic modeling methods for WPT systems and resonant converters have to transform the discrete state space model into a higher order model or use complex currents and voltages in order to adopt the averaging method and obtain an analytical model. Simulation and experimental results give a firm support to the proposed method and models. The concept employed in this paper provides a deeper insight into the dynamic behaviors of coupled resonances.

Research and realization of a novel active common-mode EMI filter

Voltages and currents of high power density power electronic circuits vary rapidly for its high speed switching, which results in serious electromagnetic interference (EMI) problems. The conventional used EMI filters are made of passive inductors and capacitors. Its size is usually comparable to that of the converter itself. As to this, active EMI filters (AEF) made of amplifiers and transistors are proposed. In this paper, a novel AEF topology used for DC-DC power converters is proposed. The most distinguished feature of this filter is that it shares the same power supply with the DC-DC converter. Thus, the size and weight of the whole filter is reduced to a great extent. The experimental results show that the proposed filter worked well and could attenuate the noise with high effectiveness.

A discrete time-domain model for fast simulation of MMC circuits

In this paper, a novel discrete time-domain MMC Sub Module (SM) model is proposed for fast simulation of MMC circuits. Firstly, by using Modified Nodal Approach (MNA), the switch in SM is replaced by a changeable voltage source in series with a resistor. Then the SM model is simplified in form and mathematics expression for fast simulation. Secondly, stability of these algorithms are discussed in detail. It is because applying various numerical integration algorithms to the SM model will get different difference expressions, which may result in stability problem. Finally, the efficiency and accuracy of the model are validated using a 20-level MMC circuit by comparison with commercial simulation software.

An improved switching loss model for a 650V enhancement-mode GaN transistor

This paper proposes an improved switching loss model for a 650V enhancement-mode gallium nitride (GaN) transistor. The interpolation fitting method is used to fit the strong nonlinear capacitance and transconductance, and it shows a better accuracy than the given function or polynomial fitting method. Meanwhile, because the input capacitance has a strong nonlinear relationship with gate-source voltage and a weak nonlinear relationship with drain-source voltage, this paper uses Qc-Vgs curve instead of the Ciss-Vds curve in the proposed model to improve the accuracy. The parasitic inductance is also considered in the model. Then the switching processes are analyzed in detail, and they are described as a fifth-order nonlinear differential equation. Finally, the accuracy of the model is validated by experiment. The error of the predicted switching loss is within 20% when the load current changes from 3.5A to 20A.

Instability Analysis and Oscillation Suppression of Enhancement-Mode GaN Devices in Half-Bridge Circuits

This paper analyzes the problem of instability in enhancement-mode gallium nitride (GaN) transistors based half-bridge circuits. The instability may cause sustained oscillation, resulting in overvoltage, excessive electromagnetic interference (EMI), and even device breakdown. GaN devices operate in the saturation region when they conduct reversely during the dead time. Under the influence of parasitic parameters, the GaN-based half-bridge circuit exhibits positive feedback under certain conditions, thus, resulting in sustained oscillation. A small-signal model is proposed to study this positive feedback phenomenon. Like the second-order under-damped system, damping ratio is defined to determine the systems stability. Based on the model, the influence of circuit parameters on instability is investigated and guidelines to suppress the oscillation are given. Reducing the common-source inductance, increasing the gate resistance of the inactive switch or connecting a diode in parallel to the inactive switch are some effective ways to suppress the oscillation. Finally, the analyses are verified by both simulation and experiment.

Instability analysis of enhancement-mode GaN based half-bridge circuits

This paper analyzes the problem of instability in enhancement-mode gallium nitride (GaN) transistors based half-bridge circuits. The instability may cause sustained oscillation, resulting in overvoltage, excessive EMI, and even device breakdown. This problem does not occur in silicon or silicon carbide transistors based circuit because of their different reverse conduction characteristics from GaN devices. GaN devices operate in the saturation region when they conduct reversely during the dead time. Under the influence of parasitic parameters, the GaN-based half-bridge circuit exhibits positive feedback under certain conditions, thus resulting in sustained oscillation. A small-signal model is proposed to study this positive feedback phenomenon. Based on the model, the influence of circuit parameters on instability are investigated and guidelines to suppress the oscillation are given. Finally, the analyses are verified by experiment.

Pulse Density Modulation for Maximum Efficiency Point Tracking of Wireless Power Transfer Systems

Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on–off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils.

A High-Bandwidth Integrated Current Measurement for Detecting Switching Current of Fast GaN Devices

Gallium nitride (GaN) devices are suitable for high-frequency power converters due to their excellent switching performance. To maximize the performance of GaN devices, it is necessary to study the switching characteristics, which requires measuring the switching current. However, GaN devices have a fast switching speed and are sensitive to parasitic parameters, so the current measurement should have a high bandwidth and should not introduce excessive parasitic inductance into the power converters. Traditional current measurements are difficult to meet these requirements, especially for fast GaN devices. This paper presents a high-bandwidth integrated current measurement for detecting the switching current of fast GaN devices. By effectively utilizing the parasitic inductance in the circuit, a single-turn coil is embedded in the printed circuit board. This coil could pick up a sufficiently strong voltage signal, which is then processed to reconstruct the switching current. Moreover, corrections are carried out to further improve the accuracy. The current measurement has a small insertion impedance and a high bandwidth with a small influence on the parasitic inductance of the converter. The accuracy of the current measurement is experimentally verified by a 40 V GaN-based double pulse test circuit with a load current up to 25 A.