The inductive power transfer (IPT) technique in battery charging applications has many advantages compared to conventional plug-in systems. Due to the dependencies on transformer characteristics, loading profile, and operating frequency of an IPT system, it is not a trivial design task to provide the battery the required constant charging current (CC) or constant battery charging voltage (CV) efficiently under the condition of a wide load range possibly defined by the charging profile. This paper analyzes four basic IPT circuits with series–series (SS), series–parallel (SP), parallel–series (PS), and parallel–parallel (PP) compensations systematically to identify conditions for realizing load-independent output current or voltage, as well as resistive input impedance. Specifically, one load-independent current output circuit and one load-independent voltage output circuit having the same transformer, compensating capacitors, and operating frequency can be readily combined into a hybrid topology with fewest additional switches to facilitate the transition from CC to CV. Finally, hybrid topologies using either SS and PS compensation or SP and PP compensation are proposed for battery charging. Fixed-frequency duty cycle control can be easily implemented for the converters.

Hybrid IPT Topologies With Constant Current or Constant Voltage Output for Battery Charging Applications

Inductive power transfer is a highly attractive option for powering unmanned aerial or underwater vehicles, in harsh environments and while in continuous motion. This study presents a variable frequency controller for series–series compensated contactless chargers operating in dynamic conditions. The controller tracks, in real time, the frequency for which the output voltage is load independent. The criterion for that is the zeroing of the phase difference between the secondary current and the primary-side inverter output voltage. Control is performed by a phase-locked loop with optical communication between the two sides. Experimental results on a 1-kW prototype, for power transfer while in motion, show fast frequency response, along with steady output voltage, despite load variations. Comparison is performed with two other fixed frequencies of operation; the natural frequency of the primary resonant circuit and the maximum output power frequency at nominal gap. The proposed control is proven superior in terms of output power level and stability, as well as safety to highly misaligned conditions.

Variable Frequency Controller for Inductive Power Transfer in Dynamic Conditions

Series/series (S/S) and series/parallel (S/P) compensations are widely used in contactless power transfer systems. However, the inductive input impedance of the S/S compensated converter operating at an output voltage gain intersection point impairs the system’s efficiency, and the output voltage of the S/P compensated converter operating at resonant frequency is quite sensitive to variation of transformer’s parameters caused by air gap change or misalignment. This paper proposes a novel series/series–parallel (S/SP) compensation topology to achieve both high efficiency and good output controllability. The frequency-domain and time-domain analyses of the proposed resonant converter are performed in this paper. Similar to the conventional resonant converters, both phase-locked-loop (PLL) control and fixed-frequency control can be employed. A 60-W prototype with PLL control and a 1.5-kW prototype with fixed-frequency control are built to verify the theoretical analysis. To perform the input-to-output voltage gain comparison, an S/P compensated converter is also fabricated. Experimental results verify the effectiveness of the proposed compensation method. The efficiency of the fabricated S/SP prototype reaches 95.2% at an output power of 1.5 kW with a 10-cm air gap between windings.

Analysis and Control of Series/Series-Parallel Compensated Resonant Converter for Contactless Power Transfer

A major challenge for practical wireless power transfer (WPT) applications is to attain stable power transfer with high and constant transfer efficiency under a dynamic change of coupling condition. In order to address the issue, this paper proposes a novel nonlinear parity-time (PT) symmetric model, wherein the nonlinear saturable gain is provided by a self-oscillating controlled inverter. In this paper, the transfer performance and stability criterion of the nonlinear PT-based WPT system are analyzed based on the coupled-mode theory. The theoretical analysis shows that the proposed system automatically achieves constant output power with constant transfer efficiency against the variation of coupling coefficient. Moreover, based on the gain saturation mechanism, the control strategy for the inverter needs to detect only the current in the transmitter, which eliminates auxiliary circuits of wireless communication for feedback control from the receiver. As a case study of dynamic charging, a drone-in-flight wireless charging platform is improved by applying the nonlinear PT-symmetric model. Experimental results show that when the flying drone hovers in a confined three-dimensional volume of space above the WPT platform, a stable output power is maintained with approximately constant transfer efficiency of 93.6%.

Nonlinear Parity-Time-Symmetric Model for Constant Efficiency Wireless Power Transfer: Application to a Drone-in-Flight Wireless Charging Platform

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.

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

In this paper, a self-oscillating control strategy is proposed for a series-series type contactless resonant converter (CRC). By detecting the secondary current phase of a CRC to control the inverter, better output controllability, dynamic response, and self-adaptability can be achieved. While the realization of self-oscillating control strategy depends on phase detection of secondary current. To provide a passive solution, a current transformer with shorted secondary, called phase detection contactless current transformer (PDCCT) is proposed. The proposed PDCCT can detect the current phase from the secondary side of a CRC and feedback to the primary side rapidly and accurately despite the changes in air gap. To guarantee the accuracy of self-oscillating control strategy, the time delay in control circuitry is studied and compensated. A 60-W self-oscillating CRC with PDCCT is then fabricated to demonstrate the validity of both PDCCT and self-oscillating control strategy. Besides, a testing circuit is designed to emulate the changes in converter parameters, and the dynamic performance shows that self-oscillating control strategy can respond to parameter changes in a switching period.

Self-Oscillating Contactless Resonant Converter With Phase Detection Contactless Current Transformer

A novel S/SP type compensation contactless resonant converter is proposed in this paper, which features the advantage of constant gain intersection point with zero phase angle of input impedance. Different from S/P compensation, the constant gain value is not only independent of load change but also independent of transformer's coupling coefficient change causing by the change of gap or misalignment. The characteristic analysis of the proposed resonant converter is performed in this paper. It is also found that with the proposed S/SP compensation the output voltage gain is not sensitive to the parameter change around the gain intersection point. Therefore, both phase locking control and fixed-frequency control can be employed for the proposed converter. A 60W prototype with PLL control and a 1.5kW prototype with constant frequency control are built to verify the analysis. Experimental results testify the theoretical analysis very well. The efficiency of the fabricated prototype reaches 95.2% with 1.5kW output at 10 cm gap.

Analysis and control of S/SP compensation contactless resonant converter with constant voltage gain

A secondary winding shorted current transformer (CT), called phase detection contactless current transformer (PDCCT) is proposed to provide passive current phase detection solution for self-oscillating contactless resonant converter (SOCRC). The proposed PDCCT can detect the current phase from the secondary side of a contactless resonant converter (CRC) and feedback to the primary side rapidly and accurately regardless of the variable air gap. To guarantee the operation of SOCRC, the time delay in control circuitry are studied and compensated. Steady state and dynamic state experimental results on a 60W SOCRC with PDCCT show the validity of both the accurate detection of PDCCT and the rapidly response of SOCRC.

Self-oscillating contactless resonant converter with phase detection contactless current transformer

The invention relates to a resonance circuit with a resonant inductor and a capacitor connected in segmented series and for a resonant converter, and belongs to the field of conversion of electrical energy. The resonance circuit with the resonant inductor and the capacitor connected in segmented series is characterized in that a resonant inductor winding to be connected in series and compensated by the capacitor is formed by connecting a plurality of winding sections in series, and a resonant capacitor is further connected between at least one pair of adjacent winding sections in series. All the series windings forming the resonant inductor can have a magnetic flux coupling relationship, and can also have no magnetic flux coupling relationship. The resonance circuit with the resonant inductor and the capacitor connected in segmented series can overcome the defect that the voltage stress of resonant inductor windings is too high and which cannot be overcome by a concentrated type compensation method adopted in an existing resonance circuit with an inductor and a capacitor connected in series. The resonance circuit with the resonant inductor and the capacitor connected in segmented series can be applied to all kinds of resonant converters for constructing resonance circuits with inductors and capacitors connected in series.

Kaiqin Yan

Papers

Self-Oscillating Contactless Resonant Converter With Phase Detection Contactless Current Transformer

Analysis and control of S/SP compensation contactless resonant converter with constant voltage gain

Self-oscillating contactless resonant converter with phase detection contactless current transformer

Resonance circuit with resonant inductor and capacitor connected in segmented series