Inductive Wireless Power Transfer Link Research Articles

Extending the design space of minimized VA rating inductive wireless power transfer links operating in restricted sub-resonant frequency region with constant current output

Wireless power transfer systems are typically required to operate under varying geometrical positioning (vertical distance, misalignment) between the transmitter and receiver coils while supporting a load with non-constant characteristics. This creates the need for robust systems that can operate over a range of coupling coefficients under time-varying load. Widespread control methods utilized to support such an operation typically suffer from volt-ampere (VA) over-rating of the inductive wireless power transfer link (IWPTL) transmitter. Sub-resonant frequency control has proven to be an effective solution to this problem, attaining robust operation and minimized transmitter VA rating by utilizing operational frequency as the control variable for system output regulation under above-mentioned constraints. However, allowed operating frequency range is usually restricted in practical applications (e.g. to 79 kHz–90 kHz by SAE J2954 EV charging standard), limiting the misalignment tolerance of sub-resonant control method. Recently, extension of the classical sub-resonant control methodology was proposed in order to overcome this limitation. However, no clear guidelines were given for designing practical IWPTLs operating under extended sub-resonant control. To this end, this work bridges the gap between academical findings and practical applications by establishing generalized guidelines for designing sub-resonant controlled series-series (SS) compensated IWPTL with minimized transmitter VA rating, allowing to maximize misalignment tolerance under given operating frequency and output voltage ranges. The proposed methodology is accurately validated by simulations and experiments, demonstrate close correlation with analytical predictions. Experimental framework reveals that IWPTL operating under extended sub-resonant control methodology attains 50 % increase of tolerable coupling coefficients range compared to a classical system designed to operate under sub-resonant control under identical operating conditions while achieving nearly similar DC-to-DC efficiency.

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.

Simple Enhancement of Series–Series-Compensated Inductive Wireless Power Transfer Links Operating With Load-Independent Voltage Output at Fixed Frequency to Attain Zero Inverter Phase Angle

It is well-known that series-series compensated resonant inductive wireless power transfer links (SS-IWPTL) may be designed to attain load independent voltage output (LIVO) for given operating frequency and coupling coefficient. Unfortunately, corresponding inverter output impedance is either highly-inductive or highly-capacitive (inductive region is typically preferred due to natural zero-voltage switching (ZVS) of inverter transistors), significantly increasing inverter volt-ampere (VA) rating. The letter demonstrates that designing the SS-IWPTL for operation in the capacitive region and then adding a shunt inductor across AC-side terminals of either inverter or rectifier while allows attaining inverter zero phase angle (ZPA) thus minimizing its VA rating.

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.

Output characteristics of none-series compensated inductive wireless power transfer link operating at load-independent-voltage-output frequency

The most important performance merits of inductive wireless power transfer links (IWPTL) operating at load-independent-voltage-output (LIVO) frequency are efficiency and output characteristics. It was recently demonstrated that none-series (NS) compensated IWPTL operating at LIVO frequency outperforms its series-series (SS) and series-none (SN) compensated counterparts in terms of efficiency. In order to complete the comparison, this paper presents output characteristics modeling and analysis (namely, relation between output voltage and load power) of a practical NS-compensated IWPTL operated at LIVO frequency. It is demonstrated that even though the system is operated at LIVO regime, its output voltage remains load-dependent due to practical issues. The lower bound of the output voltage range corresponds to operation under rated loading and is derived using phasor domain equivalent circuit, assuming continuous conduction mode (CCM) of the receiving-side diode rectifier without neglecting equivalent series resistances. On the other hand, upper bound of the output voltage range corresponds to operation under zero loading (inevitably imposing discontinuous conduction mode (DCM) of the receiving-side diode rectifier) and is obtained using time-domain solution of corresponding differential equations system due to the fact that classical phasor analysis fails to accurately describe DCM operating NS-compensated IWPTL. The proposed method is applied to a 1 kW, 400 V NS-compensated IWPTL operated at LIVO frequency and is well-verified by simulations and experiments. Comparison with output voltage characteristics of SS and SN-compensated IWPTLs assert the advantage of NS-compensated topology.

Output Voltage and Resistance Assessment of Load-Independent-Voltage-Output Frequency Operating Inductive Wireless Power Transfer Link Utilizing Input DC-Side Measurements Only

The paper puts forward a method for predicting output voltage and resistance of a series-series (SS) compensated inductive wireless power transfer (IWPT) link operating at load-independent-voltage-output (LIVO) frequency. The link is a part of the static system (reported by the authors in earlier works), wirelessly delivering power into an enclosed compartment without any secondary-to-primary feedback. The proposed algorithm employs input DC-side quantities (which are slow-varying and nearly noise-free, thus measured utilizing low-cost, low-bandwidth sensors) only to monitor output DC-side quantities, required for protection and/or control. It is shown that high estimation accuracy is retained as long as system parameter values are known and the phasor-domain equivalent circuit is valid (i.e., upon continuous-conduction mode (CCM) of the diode rectifier, where the proposed methodology utilizes the recently revealed modified diode rectifier equivalent model for enhanced accuracy). Under light loading (i.e., in discontinuous conduction mode (DCM)), a nonlinear correction is combined with the proposed technique to retain accuracy. The proposed methodology is well-verified by application to a 400 V to 400 V, 1 kW static IWPT link by simulations and experiments.

Quadrature Demodulator-Assisted Estimation of Load Voltage and Resistance Based on Primary-Side Information of a Wireless Power Transfer Link

This paper proposes an algorithm for the extraction of primary-side first harmonic voltage and current components for inductive wireless power transfer (WPT) links by employing quadrature demodulation. Such information allows for the accurate estimation of corresponding receiver-side components and hence permits the monitoring of the output voltage and resistance necessary for protection and/or control without using either sensors or feedback communication. It is shown that precision estimation is held as long as the parameter values of the system are known and the phasor-domain equivalent circuit is valid (i.e., in continuous conduction mode). On the other hand, upon light load operation (i.e., in discontinuous conduction mode), the proposed technique may still be employed if suitable nonlinear correction is employed. The methodology is applied to a 400 V, 1 kW inductive WPT link operating at a load-independent-voltage-output frequency and is well-verified both by simulations and experiments.

A novel contactless, feedbackless and sensorless power delivery link to electromagnetic levitation melting system residing in sealed compartment

The paper presents practical considerations and guidelines for designing an improved-efficiency contactless inductive power transfer link, delivering DC power in step-down mode to an electromagnetic levitation melting system residing in a sealed compartment. The proposed arrangement employs none-series compensated inductive power transfer link operating at the so-called “load-independent-voltage-output” frequency, allowing feedback-less and sensor-less design. However, the output voltage of a system operating at such frequency remains influenced by the load to some extent, residing within a certain range with boundary values (minimum and maximum) corresponding to maximum (rated) and minimum load, respectively. Consequently, the proposed contactless power delivery system is first analyzed using both first harmonic approximation (frequency-domain) based approach and differential equations (time-domain) based method to obtain corresponding output voltage bounds. Then, coil-to-coil efficiency and optimal load matching factor are determined. Comparison with both typically utilized symmetrical series-series and recently proposed series-none compensation topologies reveals the superiority of none-series compensation topology in terms of coil-to-coil efficiency for the whole range of coupling coefficients. Simulations and experiments of 400 V, 1 kW none-to-series compensated inductive wireless power transfer link, delivering contactless energy to a 250 V electromagnetic levitation melting system, demonstrate excellent matching and accurately validate the presented analysis.

Multiple Input Multiple Output Resonant Inductive WPT Link: Optimal Terminations for Efficiency Maximization

In this paper a general-purpose procedure for optimizing a resonant inductive wireless power transfer link adopting a multiple-input-multiple-output (MIMO) configuration is presented. The wireless link is described in a general–purpose way as a multi-port electrical network that can be the result of either analytical calculations, full–wave simulations, or measurements. An eigenvalue problem is then derived to determine the link optimal impedance terminations for efficiency maximization. A step-by-step procedure is proposed to solve the eigenvalue problem using a computer algebra system, it provides the configuration of the link, optimal sources, and loads for maximizing the efficiency. The main advantage of the proposed approach is that it is general: it is valid for any strictly–passive multi–port network and is therefore applicable to any wireless power transfer (WPT) link. To validate the presented theory, an example of application is illustrated for a link using three transmitters and two receivers whose impedance matrix was derived from full-wave simulations.

Accurate first-harmonic-approximation-based model of the diode rectifier in series-series compensated inductive wireless power transfer link at load-independent-voltage-output frequency

The brief proposes a modification of classical pure resistive first harmonic approximation (FHA) based equivalent of diode rectifier in series-series compensated inductive wireless power transfer link (SS-IWPTL), operating at load-independent-voltage-output (LIVO) frequency. It is shown by means of time-domain analysis that a load-independent inductance should be added in parallel with the typically utilized load-dependent resistance to accurately represent input rectifier impedance at operating frequency. It is also shown that the worst case (minimal) inductance value is obtained when equivalent series resistances (ESRs) are ignored. In practical systems, non-zero ESRs are typically present and are typically desired to be as low as possible to increase the power transfer efficiency. It is shown that increasing the ESRs leads to escalation of equivalent resistance and inductance values (i.e. reducing the influence of the latter), without altering the load-independence characteristics of equivalent inductance. Simulations and experiments based on a 400 V, 1 kW strongly-coupled SS-IWPTL operating at LIVO frequency validate the proposed analysis.

Optimal Terminations for a Single-Input Multiple-Output Resonant Inductive WPT Link

This paper analyzes a resonant inductive wireless power transfer link using a single transmitter and multiple receivers. The link is described as an (N+1)–port network and the problem of efficiency maximization is formulated as a generalized eigenvalue problem. It is shown that the desired solution can be derived through simple algebraic operations on the impedance matrix of the link. The analytical expressions of the loads and the generator impedances that maximize the efficiency are derived and discussed. It is demonstrated that the maximum realizable efficiency of the link does not depend on the coupling among the receivers that can be always compensated. Circuital simulation results validating the presented theory are reported and discussed.

Additional Two-Capacitor Basic Compensation Topologies for Resonant Inductive WPT Links

The letter reveals two additional basic compensation topologies for resonant inductive wireless power transfer links (supplementary to the existing four) created by placing both compensation capacitors either on primary or secondary side, leaving the opposite coil uncompensated. This produces parallel-series/none and none/series-parallel topologies, with the latter being more practical due to its suitability to operation with typically utilized voltage-source inverters.

Analysis and design of inductive wireless power transfer link for feedback-less power delivery to enclosed compartment

The paper presents analytical investigation and hands-on design of inductive wireless power transfer link for a through-glass AC power delivery system into enclosed compartment such as industrial glove box or vehicle cabin. The system is fed from AC grid and creates a standard AC outlet within the enclosed compartment, capable of supplying loads of up to 1 kW. The system consists of three (namely, AC/DC, DC/DC and DC/AC) conversion stages, with the former and the latter realized by standard off-the-shelf units. The intermediate DC-DC power conversion stage, which is the paper focus, is realized by a from-scratch-built inductive wireless power transfer link, operating in load-independent voltage output regime without any feedback. It was recently shown that the output voltage of such a system, even when operating at load-independent frequency, remains influenced by the load due to practical issues. As a result, the output voltage resides within a certain range rather than remains constant, obtaining minimum/maximum values under rated load/no-load conditions, respectively. Since coils separating glass width typically possesses some finite uncertainty, values of both coupling coefficient and coils self-inductances may be different than nominal (design) ones and hence the output voltage would drift from its nominal range. Hence, due to the fact that upper and lower bounds of an inductive wireless power transfer link output voltage range are limited by DC link capacitor and DC/AC power stage constraints, tolerated glass width uncertainty is obtained in the paper. Moreover, compensating capacitors pair symmetrizing uncertainty tolerance of the inductive wireless power transfer link is also revealed. Simulations and experiments based on 400 V, 1 kW-rated inductive wireless power transfer link are presented to validate the analysis.

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.

Compensation Capacitors Sizing for Achieving Arbitrary Load-Independent Voltage Gain in Series-Series Inductive WPT Link Operating at Fixed Frequency

The letter reveals that for a given operating frequency, infinite amount of compensation capacitor pairs exists, yielding load independent voltage gain of a typical series-series compensated resonant inductive wireless power transfer link (WPTL). Closed-form analytical expression is derived, linking the values of compensating capacitors with the desired load independent voltage gain based on given coil inductances, coupling coefficient and operating frequency. Feasible range of achievable voltage gains is shown to be bounded by the value of coupling coefficient based on a novel 4-parameter generalized loosely coupled transformer model. Experimental results are given to validate the presented findings.

On the Minimal Loading of Sensorless Series-Series Compensated Inductive WPT Link Operating at Load Independent Voltage Output Frequency Without Feedback

It is well-known that continuous conduction mode (CCM) operation at certain frequency yields nearly load-independent voltage gain in practical series-series compensated inductive wireless power transfer links (SS-IWPTL). Such an operation is usually referred to as “load independent voltage output” (LIVO). Residual load dependence is due to the presence of non-zero equivalent series resistances in both transmitter and receiver. On the other hand, it has been shown that voltage gain remains load-dependent at the same operating frequency in discontinuous conduction mode (DCM) irrespectively of equivalent series resistances values. Consequently, the output voltage of SS-IWPTL operated at LIVO frequency resides within a range with lower bound corresponding to rated load and upper bound corresponding to zero loading for a given input voltage. The upper bound expression (typically attained in CCM) is derived using the first harmonic approximation (FHA) based equivalent circuit and is well-established by previous works. On the contrary, derivation of the lower bound (achieved in DCM) requires differential equation (DE) based analysis and was barely treated in the literature. The paper demonstrates that lumped capacitance of receiving side coil and input capacitance of receiving side rectifier must be taken into account during DE-based analysis since they impose significant receiving side voltage oscillations, which may give rise to safety issues. In order to damp these oscillations and keep the output DC voltage below a predetermined limit, minimal loading should be applied at the SS-IWPTL output, creating an RCD-snubber equivalent. Guidelines for the minimal load sizing are derived in the paper analytically. Simulations and experiments based on a 400V, 1kW SS-IWPTL operating at LIVO frequency demonstrate excellent matching, validating the proposed analysis.

Output Voltage Range of a Power-Loaded Series–Series Compensated Inductive Wireless Power Transfer Link Operating in Load-Independent Regime

The article proposes an in-depth analysis of a series-series compensated resonant inductive wireless power transfer (WPT) link, operating at load-independent frequency while feeding a power load. It is shown that due to the presence of equivalent series resistances in practical systems, a load-independent operation point is only an approximation. The output voltage is still affected by the load, obtaining minimum and maximum values under rated load and no-load operation, respectively. While the output voltage value under rated load is typically well predicted by the commonly used first-harmonic-approximation-based equivalent circuit, the latter is unsuitable for obtaining the output voltage value under light loading due to nonlinearity caused by the discontinuous conduction mode of the secondary. Time-domain analysis is, therefore, used to accurately predict the WPT output voltage value under zero-load operation and accurately derive the expected WPT output voltage range in the load-independent regime. Simulations and experiments based on a 400-V 1-kW WPT link demonstrate excellent matching, validating the proposed analysis.

Efficiency optimization of a three-coil resonant energy link

AbstractThis paper presents an effective and time saving procedure for designing a three-coil resonant inductive wireless power transfer (WPT) link. The proposed approach aims at optimizing the power transfer efficiency of the link for given constraints imposed by the specific application of interest. The WPT link is described as a two-port network with equivalent lumped elements analytically expressed as function of the geometrical parameters. This allows obtaining a closed-form expression of the efficiency that can be maximized by acting on the geometrical parameters of the link by using a general purpose optimization algorithm. The proposed design procedure allows rapidly finding the desired optimal solution while minimizing the computational efforts. Referring to the case of an application constraining the dimensions of the receiver, analytical data are validated through full-wave simulations and measurements.

Gain expressions for resonant inductive wireless power transfer links with one relay element

In this paper, a resonant inductive wireless power transfer link using a relay element is analyzed. Different problems of practical interest are considered and solved by modeling the link as a lossy two-port network. According to the two-port network formalism, the standard gain definition (i.e. the power, the available, and the transducer gains) are used for describing the network behavior. Firstly, the case of a link with given parameters is considered and the analytical expressions of the optimal terminating impedances for maximizing the link gains are derived. Later on, the case of a link with given source and load is analyzed and the possibility of maximizing the performance by acting either on the transmitting or on the receiving side is investigated. It is shown that by using a single relay element, it is not always possible to maximize all the figures of merit that could be of interest in the WPT context. Theoretical data are validated by comparisons with circuital simulation results.

An NFC on Two-Coil WPT Link for Implantable Biomedical Sensors under Ultra-Weak Coupling.

The inductive link is widely used in implantable biomedical sensor systems to achieve near-field communication (NFC) and wireless power transfer (WPT). However, it is tough to achieve reliable NFC on an inductive WPT link when the coupling coefficient is ultra-low (0.01 typically), since the NFC signal (especially for the uplink from the in-body part to the out-body part) could be too weak to be detected. Traditional load shift keying (LSK) requires strong coupling to pass the load modulation information to the power source. Instead of using LSK, we propose a dual-carrier NFC scheme for the weak-coupled inductive link; using binary phase shift keying (BPSK) modulation, its downlink data are modulated on the power carrier (2 MHz), while its uplink data are modulated on another carrier (125 kHz). The two carriers are transferred through the same coil pair. To overcome the strong interference of the power carrier, dedicated circuits are introduced. In addition, to minimize the power transfer efficiency decrease caused by adding NFC, we optimize the inductive link circuit parameters and approach the receiver sensitivity limit. In the prototype experiments, even though the coupling coefficient is as low as 0.008, the in-body transmitter costs only 0.61 mW power carrying 10 kbps of data, and achieves a 1 × 10 bit error rate under the strong interference of WPT. This dual-carrier NFC scheme could be useful for small-sized implantable biomedical sensor applications.