A detection method for use in a primary unit of an inductive power transfer system, the primary unit being operable to transmit power wirelessly by electromagnetic induction to at least one secondary unit of the system located in proximity to the primary unit and/or to a foreign object located in said proximity, the method comprising: driving the primary unit so that in a driven state the magnitude of an electrical drive signal supplied to one or more primary coils of the primary unit changes from a first value to a second value; assessing the effect of such driving on an electrical characteristic of the primary unit; and detecting in dependence upon the assessed effect the presence of a said secondary unit and/or a foreign object located in proximity to said primary unit.

Inductive power transfer

Inductive power transfer (IPT) was an engineering curiosity less than 30 years ago, but, at that time, it has grown to be an important technology in a variety of applications. The paper looks at the background to IPT and how its development was based on sound engineering principles leading on to factory automation and growing to a $1 billion industry in the process. Since then applications for the technology have diversified and at the same time become more technically challenging, especially for the static and dynamic charging of electric vehicles (EVs), where IPT offers possibilities that no other technology can match. Here, systems that are ten times more powerful, more tolerant of misalignment, safer, and more efficient may be achievable, and if they are, IPT can transform our society. The challenges are significant but the technology is promising.

Inductive Power Transfer

In this paper, resonant tank design procedure and practical design considerations are presented for a high performance LLC multiresonant dc-dc converter in a two-stage smart battery charger for neighborhood electric vehicle applications. The multiresonant converter has been analyzed and its performance characteristics are presented. It eliminates both low- and high-frequency current ripple on the battery, thus maximizing battery life without penalizing the volume of the charger. Simulation and experimental results are presented for a prototype unit converting 390 V from the input dc link to an output voltage range of 48-72 V dc at 650 W. The prototype achieves a peak efficiency of 96%.

An LLC Resonant DC–DC Converter for Wide Output Voltage Range Battery Charging Applications

High-power inductive power transfer (IPT) systems operate at power levels of 100 kW or more. However, existing high-power IPT power supplies are typically designed for one power level and are expensive to make, due to the use of high-power electronic components. This paper presents a parallel IPT power supply topology that can achieve high output power levels in a cost effective manner. The parallel topology can minimize uneven power sharing due to component tolerance, and does not require any additional reactive components for parallelization. In addition, it can continue to operate when a faulty parallel unit is electronically shut down, dramatically improving the availability and reliability of the systems. Furthermore, flexible output power levels may be achieved by connecting identical modules in parallel. A 6 kW parallel power supply has been constructed by connecting three 2 kW power supplies in parallel. The maximum efficiency of the power supply and track is measured to be 94%.

A Parallel Topology for Inductive Power Transfer Power Supplies

Renewable energy sources, such as offshore wind farms, require high voltage gains in order to interface with power transmission networks. These conversions are normally made using bulky, complex, and costly transformers and high-voltage ac-dc converters with unnecessary bidirectional power flow capability. Multiple modules of single-switch single-inductor dc-dc converters can reach high gains without transformers in these applications due to low semiconductor conduction loss in high-power devices. This paper describes a new approach for high-gain high-voltage dc-dc converters using multiple modules of single-switch single-inductor transformerless converters. Results for low-voltage experimental prototypes show gains of up to 29 p.u. and demonstrate the potential of the approach as high-gain dc-dc converters for offshore wind farms. This paper then demonstrates the viability of multiple-module converters compared to a conventional high-voltage dc converter and a theoretical full-bridge converter due to fewer devices and valves, comparable isolation levels, and ease of interleaving for increased reliability.

Multiple-Module High-Gain High-Voltage DC–DC Transformers for Offshore Wind Energy Systems

An immittance converter (IC), in general, is a two-port network, in which input impedance is proportional to the load admittance connected across the output terminals and is useful in transforming a voltage source into a current source and vice versa. In this paper, a family of lumped-element resonant IC (RIC) topologies is identified by investigating the transmission parameters of various topological structures of electrical networks. In all, 24 RIC topologies have been identified with three and four reactive elements. The operating point and the design condition, under which these topologies exhibit immittance conversion characteristics, are derived. The suitability of these topologies in terms of absorbing parasitic components and providing inherent dc blocking to the transformer is examined. The analysis and design is illustrated with a newly identified four-element RIC topology as a constant-current power supply and validated with experimental results on a 250-W 105-kHz prototype converter.

Resonant Immittance Converter Topologies

The transformer winding capacitance, which is significant in high-voltage power supplies, is not gainfully utilized in an LCL-T resonant converter (RC). A simplified analysis presented in this paper predicts the severe degradation of output current regulation of an LCL-T RC due to transformer winding capacitance. The presence of winding capacitance, in fact, changes the third-order LCL-T resonant tank into fourth-order LC-LC topology. Using an AC analysis, it is shown that, under the derived design conditions, LC-LC RC also exhibits constant output current and in-phase source voltage and current, simultaneously at all loading conditions. Thus, the transformer leakage inductance and winding capacitance are gainfully utilized as a part of a resonant network, resulting in improved output characteristics. Closed-form expressions for the converter gain and component stresses are derived. The condition for converter design optimized for the minimum size of the resonant network is obtained. Experimental results on a prototype 100-mA 2-kV DC power supply confirm the observations of analysis.

Design of LCL-T Resonant Converter Including the Effect of Transformer Winding Capacitance

The characteristics of an asymmetrical duty cycle (ADC) controlled LCL-T resonant converter operating at the resonant frequency are studied by solving the state-space model of the converter. Four operating modes are identified having different circuit waveforms representing different device conduction sequences, thereby creating different conditions during the device switching. The mode boundaries are obtained and plotted on the D-Q plane. A region on the D-Q plane is identified for the converter design, where the switches operate under zero-voltage-switching condition. A prototype 500 W, 100 kHz converter is designed and built to experimentally demonstrate the operating modes, control characteristics, and performance of ADC-controlled LCL-T resonant converter.

Characteristics and Design of an Asymmetrical Duty-Cycle-Controlled LCL-T Resonant Converter

Resonant immittance converter (RIC) topologies can transform a current source into a voltage source (Type-I RICs) and vice versa (Type-II RICs), thereby making them suitable for many power electronics applications. RICs are operated at a fixed frequency where the resonant immittance network (RIN) exhibits immittance conversion characteristics. It is observed that the low-frequency response of Type-II RINs is relatively flat and that the state variables associated with Type-II RINs affect the response only at the high frequencies in the vicinity of the switching frequency. The overall response of a Type-II RIC is thus dominated by the filter response, which is particularly important for the controller design. Therefore, an approximate equivalent circuit model and a small-signal model of Type-II RICs are proposed in this paper, neglecting the high-frequency response of Type-II RINs. While the proposed models greatly simplify and speed-up the analysis, it adequately predicts the open-loop transient and small-signal ac behavior of Type-II RICs. The validity of the proposed models is confirmed by comparisons of their results with those obtained from a cycle-by-cycle simulation and with an experimental prototype.

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Approximate Equivalent-Circuit Modeling and Analysis of Type-II Resonant Immittance Converters

A higher-order resonant convertor (RC), named LCLCL-T RC, is proposed in this study. It is shown that the convertor behaves as a constant-current (CC) source under certain operating conditions. The proposed convertor additionally offers inherent dc blocking for the isolation transformer and integration of all magnetic components reducing the number of discrete passive components in the circuit. A detailed analysis of the proposed convertor is presented. Closed-form expressions for convertor gain, component stresses, condition for convertor operation as a CC power supply and component design values optimised for minimum size of resonant network are derived. An experimental 250 W prototype convertor was designed, built and tested to demonstrate its CC source behaviour, good conversion efficiency and feasibility of the use of integrated magnetic component.

M.S. Bhatia

Papers

Resonant Immittance Converter Topologies

Design of LCL-T Resonant Converter Including the Effect of Transformer Winding Capacitance

Characteristics and Design of an Asymmetrical Duty-Cycle-Controlled LCL-T Resonant Converter

Approximate Equivalent-Circuit Modeling and Analysis of Type-II Resonant Immittance Converters

Analysis and design of a higher-order T-type resonant convertor as a constant-current power supply