Loosely Coupled Transformer Research Articles

Misalignment tolerance improvement of loosely coupled transformer with ferromagnetic materials via genetic algorithm

AbstractIn wireless power transfer (WPT) systems, misalignment between the primary and secondary coils can decrease the transmission efficiency of the loosely coupled transformer (LCT). Adding ferromagnetic materials or a relay coil is an effective solution for efficiency and misalignment tolerance improvement if extra eddy current loss and hysteresis loss on the ferromagnetic materials or the conduction loss caused by the relay coil is well controlled. The effect of ferrite cores on the transmission efficiency of two‐coil and three‐coil LCTs is analysed from the perspective of loss. By finite element analysis, the dimension of ferrite cores is optimised via the genetic algorithm to improve the efficiency and misalignment tolerance of the LCT. Optimal ferrite core dimension is also verified in a 1‐kW WPT prototype, and experimental results show that, after optimisation, transmission efficiency of the two‐coil LCT can be improved by 2.681% and 8.930% under alignment and the 200‐mm misalignment, and with the proposed control strategy, transmission efficiency of the LCT is always above 94.925% under various misalignment conditions.

Decoupled dual‐channel loosely coupled transformer with hybrid nanocrystalline and ferrite core for current sharing of modular wireless power transfer system

AbstractCross‐coupling and current sharing are the main problems faced by modular wireless power transfer (WPT) systems for electric vehicle (EV) applications. In this study, a decoupled dual‐channel wireless power transfer loosely coupled transformer (LCT) with a hybrid core composed of nanocrystalline and ferrite is proposed for modular WPT systems, as well as WPT systems with specific hybrid topology or detuned compensation. Firstly, the requirements of the LCT for modular parallel dual‐channel WPT system and detuned compensation are discussed. For these requirements, the structure and corresponding optimization design method of the decoupled dual‐channel LCT with a hybrid core is then proposed, and an LCT is designed with this method for the case study. Furthermore, the LCT is analysed by the finite element method (FEM). It is revealed that the mutual inductances of the proposed LCT between transmitting and receiving coils can be symmetrical under misalignment in multiple directions, while the cross‐coupling is eliminated. Finally, the designed LCT is manufactured and verified in a modular parallel dual‐channel WPT system. The experimental results show that the current and power sharing is realized under misalignment in multiple directions.

A CV‐type WPT system based on CLC‐N compensation with compact receiver

AbstractThe demand for wireless power transfer (WPT) systems that can maintain a constant voltage (CV) output characteristic is gradually increasing in some practical application areas of WPT, such as industrial power supplies. However, existing CV‐type systems are often limited by the parameter design of the loosely coupled transformer (LCT), and the configuration of the receiver side is not compact enough. Therefore, this paper proposes a strongly coupled system with a CV output function composed of capacitor‐inductor‐capacitor‐none (CLC‐N) topology. The system's transmitter uses a CLC compensation, while the receiver conforms to a minimalist design without the need for compensation components such as capacitors and inductors. The initial segment of the paper pertains to the discussion of the conditions required to satisfy both the zero‐phase‐angle (ZPA) and CV output functionality of the suggested system. Subsequently, an in‐depth approach to parameter design is outlined, accentuating that the output voltage remains unrestricted by the LCT's parameters. Additionally, this paper examines the realization of zero‐voltage switching (ZVS) within the suggested system as well as the influence that variations in compensation components have on the CV output function. To underscore the benefits of the suggested CV‐type WPT system that employs CLC‐N compensation, the paper presents a comparison with a range of other CV‐type WPT systems. Ultimately, an experimental prototype is fabricated to test the theoretical framework, and the results from these experiments confirm the feasibility of the suggested CV‐type WPT system based on CLC‐N compensation.

Extending the lower bound of attainable load-independent voltage gain values range in contactless, feedbackless and sensorless power delivery links

It is well-known that series(capacitive)-series(capacitive) compensated inductive wireless power transfer links (SS(CC)-IWPTL) operating at fixed frequency with constant coupling coefficient may be designed to attain arbitrary load independent voltage gain (LIVG) value, residing within a region defined only by loosely coupled transformer (LCT) parameters. Such a characteristic allows designing the system to be sensorless and operate without feedback, which is an extremely useful feature in hostile environment applications. Unfortunately, LCT parameters are not selected arbitrarily in practice; hence, the desired LIVG value may reside outside the attainable region, calling for an additional power conversion stage. In particular, region of attainable LIVG values is especially narrow when LCT inductances ratio is imposed by the application and thus cannot be freely selected. One of existing challenges is regulating the system output to low DC voltage while it is being fed from a much higher valued voltage source, i.e. calling for an extremely low LIVG value. The paper demonstrates that adopting an inductor rather than a capacitor as primary series compensation element allows to extend the lower bound of attainable LIVG values region for given LCT and operating frequency, potentially eliminating the need for an additional step-down power converter. Resulting coil-to-coil efficiency is quantified and guidelines for sizing corresponding compensation elements pair are established as well. Simulations and experiments accurately verify the proposed methodology.

Wireless Power Transfer with Magnetic Flux Concentrator and Its Equivalent Circuit

This paper proposes a novel loosely coupled transformer (LCT) with a planar magnetic flux concentrator (MFC) for the wireless power transfer (WPT) in smart grid, which can improve the power of the receiving (Rx) coil. The MFC is a planar copper plate with a radial slit and a center hole, which is made of a printed circuit board (PCB). A simulation model and an experimental platform are built to investigate the performance of the LCT with and without an MFC. The results show that in the case of the transmitting (Tx) side with an MFC, the power of the receiving (Rx) coil increase by 33.82%. Besides, we propose the equivalent T-circuit for the LCT with an MFC.

An improved auxiliary circuit for IPT systems to achieve inherent CC‐to‐CV transition and load fault‐tolerant operation

AbstractThree‐coil inductive power transfer (IPT) charging system can not only achieve inherent constant current (CC) to constant voltage (CV) transition but also tolerate load open‐circuit. However, due to the cross‐coupling between the auxiliary coil and the receiver coil, the output voltage in CV mode may deviate from the CV voltage significantly, which makes the design freedom of the three‐coil coupler low. By replacing the auxiliary coil with a transformer, an improved auxiliary circuit is proposed in this paper to address the cross‐coupling issue and to increase the design freedom of the loosely coupled transformer (LCT). The improved auxiliary circuit can achieve load short‐circuit and open‐circuit protection. Besides, the optimization design of LCT is easy to achieve by load impedance matching. The operating principle and parameters design of the improved auxiliary circuit are discussed. A 1 kW experimental prototype is built to verify the feasibility of the proposed method.

Analysis and Design of a CLC/N Compensated CC-Type WPT System with Compact and Low-Cost Receiver

Wireless power transfer (WPT) has been extensively studied by technicians for its advantages of safety, convenience and aesthetics. The load-independent constant current (CC) output is the focus of WPT research and has been initially applied in various fields, such as light-emitting diodes (LEDs) driving, CC charging of electric vehicles (EVs), etc. However, the existing CC-type WPT system has problems in that the output current is constrained by the loosely coupled transformer (LCT) parameters, the receiver is bulky, and the development cost is high. Therefore, this manuscript proposes a new CLC/None (CLC/N) compensated WPT system with a CC output function that eliminates the receiver-side compensation components, ensures the compactness of the receiver, and saves on production costs. The conditions for satisfying the CC output and zero-phase-angle (ZPA) operation of the proposed system are first discussed. Then, the detailed parameter design method is provided, and the characteristic that the output current is unconstrained by the LCT parameters is illustrated. In addition, the implementation of zero-voltage switching (ZVS) operation of the proposed system and the sensitivity of the changes of compensation components to the output current are analyzed in detail. Furthermore, to demonstrate the superiority of the proposed system, several other typical CC-type WPT systems are introduced for comparison. Finally, a confirmatory experimental prototype with an output current of 2 A is fabricated, and the experimental results are consistent with the theoretical analysis.

Design Space of Sub-Resonant Frequency- Controlled Series–Series-Compensated Inductive Wireless Power Transfer Links Operating With Constant Output Current Under Frequency Constraints

Output current of inductive wireless power transfer links (IWPTLs) with battery loads is influenced by coupling coefficient and input–output voltage. To regulate the output current while keeping the operational frequency near resonance, dc–dc converters are often added at IWPTL input and/or output, increasing system complexity and dimensions while reducing lifetime and reliability. On the other hand, a sub-resonant frequency control method was shown to eliminate the need for dc–dc converters addition as well as preventing IWPTL overrating. The technique is based on imposing bifurcation (or frequency splitting) and varying the operational frequency within certain region below resonance to regulate IWPTL output current. Due to the fact that some applications restrict the operational frequency band, methodology for deriving the design space of sub-resonant frequency-controlled series–series (SS)-compensated IWPTL (SS-IWPTL) operating under frequency constraints with constant current (CC) output is proposed in this work. The algorithm identifies all feasible combinations of loosely coupled transformer (LCT) coils self-inductances and coupling coefficients for expected ranges of input–output voltages and prescribed operational frequency band, allowing to maintain CC output. Simulations and experiments are carried out to verify the proposed methodology.

Structural Optimization Design of Rotary Loose Coupler

The loose coupling transformer, the core component of the rotary wireless excitation system, has a large leakage inductance due to the existence of a large gap, and the coupling coefficient cannot be effectively improved, thus limiting the improvement of the system efficiency. In order to solve this problem, firstly, the equivalent modeling and leakage inductance analysis of the rotary loosely coupled transformer are carried out, and the improved design is carried out according to the defects of common windings, and an improved shaft rotary coupler is proposed. The feasibility of this optimized structure was determined using Ansys/Maxwell simulation analysis software. The S-S resonance compensation scheme is designed, and the co-simulation with Simplorer and Maxwell is used to compare the coupling coefficient and transmission efficiency of the loosely coupled transformer before and after the improvement under the condition of large air gap, and the rationality of the proposed scheme is verified.

Voltage-Gain Design and Efficiency Optimization of Series/Series-Parallel Inductive Power Transfer System Considering Misalignment Issue

Compensation is key to an inductive power transfer (IPT) system in terms of voltage transfer function and efficiency optimization. Basic compensation is simple, but not suitable, for the achievement of variable load-independent voltage-gains without changing the design of the loosely-coupled transformer (LCT). On the other hand, higher-order compensation circuits enable greater design freedom to achieve variable load-independent voltage-gains while keeping the LCT unchanged, but it requires a variety of compensation components, especially the inductive components, which incur significant copper and core losses. This paper proposes a comprehensive design of the series/series-parallel (S/SP) IPT system. The design methodology for variable load-independent voltage-gains is studied to keep the LCT unchanged and achieve zero phase angle input over the whole load range. Design consideration includes the effect of misalignment issue on the voltage-gain and, thus, a design criteria can be derived to ensure an acceptable sensitivity to the misalignment when taking efficiency optimization. The experimental results are presented for verification.

High-Misalignment-Tolerant IPT Systems With Solenoid and Double D Pads

The capability of misalignment tolerance is vital for an inductive power transfer (IPT) system. In this article, a new loosely coupled transformer (LCT) structure with series solenoid and DD pads (SDDP) is proposed to improve the misalignment tolerance of the IPT system. The designed LCT not only has strong misalignment-tolerant capability in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> -direction but also has strong misalignment-tolerant capability in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> -direction. The fluctuation of mutual inductance is 3.15% within 67.0% misalignment distance of the largest size of the SDDP in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> -direction and 44% misalignment distance of the largest size of the SDDP in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> -direction. Finally, an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> /S compensated IPT system with 300 W power is built as an example to demonstrate the performance of the proposed LCT. The output voltage fluctuates with the rated load, which is less than 3.2%, and the efficiency is 90.2–91.3%.

Design and optimization of 3‐kW inductive power transfer charging system with compact asymmetric loosely coupled transformer for special applications

SummaryInductive power transfer (IPT) technology has become popular in wireless charging applications. Compared with the traditional 3‐kW wireless charging application, the diameter for the primary coil and the secondary coil in this special design are required to be smaller than 260 and 110 mm, respectively. In order to achieve this target, an approach to design and optimize the loosely coupled transformer (LCT) is proposed to match the strict geometry limitation, while achieving the high conversion efficiency. Different from the traditional design method, the low resonant current and voltage stress are considered as the input of the design flowchart, especially the LCT. Thus, the overall conversion efficiency can be improved. Based on series–series (S–S) compensation topology, the parameter range of the LCT is determined by analyzing transformer‐based equivalent model. To achieve the desired coil size, an asymmetric LCT with single‐layer primary coil and three‐layer secondary coil is designed and optimized by the proposed approach. Furthermore, the closed‐loop controller with narrow frequency variation range is design to realize constant current (CC) charge and constant voltage (CV) charge. A 3‐kW IPT charging prototype is built to demonstrate the validity of the proposed method. Experimental results show that the peak efficiency from input to battery load is 95.69% at 3‐kW output power in the transition point from CC mode to CV mode.

Analysis and design of a S/PS compensated IPT system with constant current output

Load-independent constant current (CC) output with high-system efficiency is required in many cases of Wireless power transfer (WPT) applications. However, the existing topologies with CC output are hard to achieve both simple structure and enough design freedom. In this study, the series/parallel–series (S/PS) topology, which has three compensation capacitors, is presented to achieve CC output. The constant output current of the proposed S/PS topology is free from the constraint of the coils' self-inductance of the loosely coupled transformer (LCT). A thorough analysis of the proposed S/PS topology that implements CC and ZPA is provided. Besides, ZVS can also be obtained by reasonably setting the value of compensation capacitors. An experimental prototype with 50 V input voltage and 3 A output current is fabricated to validate the practicability and rationality of the proposed topology, and 92.18% power transfer efficiency is achieved. Furthermore, a design example with 2 A output current and the corresponding experimental results are provided to verify that the CC output of the proposed S/PS topology is free from the constraint of the LCT parameters. Finally, a comprehensive comparison with several existing topologies for CC output is conducted to reveal the advantages of the proposed S/PS topology.

Multi-regional modeling and operational analysis of LC/S-compensated inductive wireless power transfer link with load-independent current output

When feeding a constant-resistance or constant-voltage type load, inductive wireless power transfer link (IWPTL) operating as current source is typically desired. Moreover, in case coupling coefficient between transmitter and receiver is constant, load-independent current output (LICO) characteristics are beneficial, allowing the system to operate without feedback. Moreover, near zero phase angle (ZPA) operation is also valuable, allowing to minimize volt-ampere rating of the transmitting-side inverter. A new LC/S compensated IWPTL topology, capable of simultaneously achieving LICO and ZPA, was recently revealed in the literature. The proposed compensation network consists of one inductor and two capacitors. This paper generalizes the LC/S compensated IWPTL topology by first deriving the compensating elements set based on a novel loosely-coupled transformer (LCT) equivalent circuit and then enlightening that for certain values of primary compensation inductor, secondary compensation capacitor should either be omitted or replaced by an inductor in order to preserve LICO and ZPA. Thus, an LC/S compensated IWPTL may actually operate in three different regions, each calling for specific sub-type of LC/S compensation. Design guidelines for compensation parameters selection are provided in this paper, allowing to achieve virtually any value of load-independent current output for a given LCT coils pair. Moreover, expressions for calculating normalized component stresses are obtained for all three regions of operation, taking into account nonlinear nature of transmitting-side inverter and receiving-side rectifier, as opposed to typical phasor-domain based approach, which ignores higher-order harmonics. Analytical results based on obtained expressions are well-supported by their real-time-domain counterparts.

Hybrid Inductive-Power-Transfer Battery Chargers for Electric Vehicle Onboard Charging With Configurable Charging Profile

Inductive power transfer (IPT) technologies have gained a wide acceptance in onboard battery charging applications due to some significant advantages over traditional plug-in systems. An IPT battery charger is expected to provide a configurable charging profile consisting of an initial constant current (CC) and a subsequent constant voltage (CV) efficiently. With a wide load range during the charging process, two sets of IPT topologies with the inherent load-independent CC and CV at the same zero-phase angle (ZPA) frequency are commonly combined into a hybrid topology to avoid sophisticated control schemes, while maintaining nearly unity power factor and soft switching of power switches simultaneously. However, the load-independent CC and CV are usually constrained by parameters of a loosely coupled transformer (LCT), making the LCT hard to design. To solve it, this paper systematically presents a method to derive such effective hybrid IPT converters, which starts from some existing topologies having the configurable CC or CV output and cascades a general T network for mode transition. Design principles with fewer mode switches and compensation components are proposed and some available hybrid topologies regardless of the constraint of LCT parameters are given in this paper. Control logic and sensitivities of compensation parameters to the input impedance and the load-independent output are also discussed. Finally, a 1 kW hybrid IPT battery charger prototype based on LCC-LCC and LCC-S topologies is built to verify the theoretical analysis.

Comparison of Basic Inductive Power Transfer Systems With Linear Control Achieving Optimized Efficiency

Inductive power transfer (IPT) systems with front-side and load-side converters are generally developed for better overall system efficiency than a single IPT converter by providing maximum efficiency tracking and output regulation against variations of coupling coefficient ( $k$ ) and load. The optimum efficiency point of an IPT converter is at a particular loading resistance, which varies with $k$ and is hard to measure directly for the purpose of control. Perturb and observe control is normally implemented to iteratively track the optimum efficiency point. However, this heuristic algorithm results in slow response to the variations of $k$ and load. Recently, a much faster linear control for optimum efficiency tracking has been developed for the series-series (SS) IPT system only. This paper proposes a general linear control scheme for all four basic IPT systems to track the optimum efficiency point. Unlike the SS IPT converter, it is found that additional control of either frequency or adaptive compensation is needed for the other three basic IPT converters to operate against the variation of $k$ . Different input to output transfer functions and their $k$ - and load-independent properties at optimum efficiency are identified for all four basic IPT systems. Thus, a general fast linear control can be applied in the front-side converter to achieve optimum efficiency, while the load-side converter regulates the output power independently. Furthermore, the maximum efficiencies of the IPT converters with these four basic compensations for an identical loosely-coupled transformer are compared theoretically and verified experimentally. The parallel–parallel IPT converter gives the best efficiency. Its additional frequency control in conjunction with the optimum efficiency tracking and the output power regulation is also experimentally validated.

Electro-mechanical modeling of electromagnetic levitation melting system driven by a series resonant inverter with experimental validation

In this paper, decoupled electro-mechanical modeling of a resonant-inverter-fed electromagnetic levitation melting system (in which a conductive body is floated by Lorentz force and heated-up by high frequency magnetic field-created eddy currents) is carried out and validated experimentally. The work piece considered is formed by a metallic sphere placed in a conical-shaped coil. The proposed model consists of an electrical side subsystem formed by series resonant circuit with time-varying resistance and inductance, a typical second-order mechanical subsystem and a highly nonlinear link between the two. It is also shown that the power conversion process is similar to that of a loosely-coupled transformer, on which any wireless power transfer system is based. Therefore, the system should be powered by a resonant high-frequency AC source to both stabilize vertical axis dynamics and maximize the transferred power. Moreover, operation under resonant conditions yields an explicit relation between operation frequency and vertical position of levitating sphere, leading to possibility of position-sensorless implementation. 1 kW-rated experimental prototype, based on 210Vdc-fed inverter, operating at switching frequency of around 25 kHz is built and tested to validate the derived model. The inverter forces 300A current flowing through 8-turns conical coil in which a 2.27 g sphere is levitated. Simulation and experimental results are shown to accurately resemble each other, verifying well the proposed modeling approach.

Improving Misalignment Tolerance for IPT System Using a Third-Coil

To improve misalignment tolerance for an inductive power transfer (IPT) system, a third-coil is employed to reversely connect with the primary coil of the loosely coupled transformer (LCT) in series. The LCT with a third-coil (LCT-TC) not only can improve the misalignment performance in x - and y -directions, but also does not affect the inherent output characteristics (such as constant current or voltage output) of original compensation topologies. In addition, the proposed approach can keep the IPT system operating with relatively high efficiency against misalignment. Then, a detailed design method of LCT-TC is presented. Finally, a double-side LCC-compensated IPT system is chosen as an example to verify the feasibility of proposed LCT-TC. A 3.4-kW prototype is constructed, and experimental results show that the proposed method has a good misalignment performance. The LCC-LCC IPT system with LCT-TC can retain 96% of the well-aligned power at 40% misalignment, and the end-to-end (dc–dc) overall system efficiency is at least 92%. Besides, the fluctuation of the output current is [–2%, 5%] while the load varies from 15 to 34 Ω within 40% misalignment.

Self-Compensation Theory and Design of Contactless Energy Transfer and Vibration System for Rotary Ultrasonic Machining

In contactless energy transfer and vibration system (CETVS) for rotary ultrasonic machining, the energy is delivered to an ultrasonic transducer equipped with a rotary spindle shank through a loosely coupled transformer (LCT). The purpose of this paper is to present a simplified construct, referred to as self-compensation system, to overcome the drawbacks of external compensation elements occupying the limited shank space during the LCT's leakage inductance compensating and the transducer's capacitance compensating. The advantage of this approach is that the secondary leakage inductive reactance of the LCT and the capacitive reactance of the transducer are mutually compensated without an intermediate element, which conserves the spindle shank space and reduces the operating load of an LCT. In this paper, we first derive the mutual compensation equation between a transducer and an LCT. Then, for a given transducer, the structural parameters of an LCT are designed with theoretical calculations and simulation analysis. Experiments are conducted with the self-compensation system excited by a 50 V ultrasonic signal. The results show that the energy transfer efficiency of LCT reached 92.0%, the amplitude of the transducer reached $10.7\,\mu {\rm{m}}$ , when the CETVS is self-compensated. These results indicate that this self-compensation theory and design are feasible.

Modified parameter tuning method for LCL/P compensation topology featured with load‐independent and LCT‐unconstrained output current

Inductive power transfer (IPT) has attracted a lot of attention in recent 30 years due to its advantages of convenience, safety, reliability and weather proof. This study proposes a modified parameter tuning method for primary inductor-capacitor-inductor, secondary parallel (LCL/P) compensation topology, which provides the characteristic of excellent constant current output (CCO). This characteristic is analysed on the basis of inductor-capacitor(LC) and capacitor-inductor (CL) resonant tanks. To lower the voltage stresses over capacitors and current stresses through inductors, the stress-based optimisation is conducted. The sensitivity of CCO characteristic with respect to the normalised deviation of compensation parameters is analysed; therefore, the performance of LCL/P compensated IPT system in practical scenarios can be fully understood. The loosely coupled transformer (LCT) is specially designed since it has a significant impact on system efficiency, component stress, power density etc. An exhaustive search algorithm is employed to find the optimal dimensions of the LCT. A 100 W LCL/P compensated IPT prototype is fabricated. The measured end-to-end efficiency, from DC input to DC output, is as high as 92.8%. The output current only increases by 2.02% when the load decreases by half. All experiment results agree well with the theoretical analysis.