Perspectives on control strategies for CC-CV charging in Resonant Inductive Power Transfer systems: a review of SS-RIPT configuration

Wireless power transfer, especially via resonant inductive power transfer (RIPT), is becoming increasingly popular for charging various devices such as Electric Vehicles (EVs), Unmanned Aerial Vehicles (UAVs), and consumer electronics such as cellphones^[1,2]. This technology offers users a safe and efficient charging experience. With the charging process automated and free from constant user intervention, wireless charging addresses the range anxiety problem in electric vehicles and extends the flight time of UAVs^[3,4]. For consumers, eliminating charging cables means no more hassle and clutter, keeping their charging areas clean and organized. Figure 1 shows the block diagram of a typical RIPT-based wireless charger.

Figure 1. 

Block diagram of RIPT based wireless charger.

In Fig. 1, power is transferred from the transmitter to the receiver through resonant inductive coupling. It consists of two inductors that are magnetically coupled through the principle of electromagnetic induction. The air gap between the transmitter and receiver inductors is significantly larger than a conventional transformer. To improve the system's efficiency, both inductors are made to resonate with a bank of capacitors in the transmitter and receiver circuits. These capacitors can be connected to the inductors in a series, parallel, or a combination of series-parallel configuration, resulting in multiple resonant topologies.

For charging applications, load on the RIPT system is some kind of energy storage device. Due to their high energy density and power density, Li-ion batteries are a popular choice for energy storage devices in electric vehicles, UAVs, and consumer electronics^[5−8]. Li-Ion batteries are charged using a Constant Current Constant Voltage (CC-CV) method which is shown in Fig. 2.

Figure 2. 

CC-CV charging operation.

In Fig. 2, V represents the battery's rated voltage, I denotes the battery current and R indicates the equivalent resistance variation during the CC-CV charging operation of a lithium-ion (Li-Ion) battery. The battery is initially charged with a constant current until its terminal voltage reaches the rated voltage. Subsequently, the charger transitions to a constant voltage output, maintaining the terminal voltage until the battery achieves full charge. This charging technique, known as CC-CV (constant current-constant voltage), leads to an exponential rise in the terminal resistance observed by the charger, depicted by the red curve in Fig. 2. Consequently, the RIPT (Resonant Inductive Power Transfer) system must be controlled to operate in CC-CV mode.

This paper presents a review of control methods applicable to RIPT systems for charging batteries using the CC-CV mode. Referring to Fig. 1, RIPT systems can be implemented in various configurations depending on the choice of the power converter and compensation topology, it is impractical to address control strategies for every circuit configuration. Therefore, this paper focuses solely on the control method for the Series-Series RIPT (SS-RIPT) system. For the DC-AC converter, H-Bridge topology and for AC-DC converter a Full-Bridge rectifier will be considered. The main contribution is identifying the control variables and utilizing them to achieve the CC-CV mode of operation.

The control strategies for Resonant Inductive Power Transfer (RIPT) systems present a significant concern, and existing literature has yet to address all control strategies comprehensively in a single paper. There is a need for a thorough discussion of these strategies, including their advantages, limitations, and implementation examples, within a single paper. Addressing this gap is essential for advancing future research in this area, which underscores the importance of this review.

Figure 3 shows the SS-RIPT-based wireless charger^[9−11]. In Fig. 3, only the DC to DC stage has been considered, and the front-end converter (EMI + PFC) stage has been neglected. In Fig. 3, the symbols V_P and I_P denote the primary side voltage and current, while V_S and I_S represent the secondary side voltage and current, respectively. L_P and L_S refer to the primary and secondary side inductances, and C_P and C_S indicate the primary and secondary side capacitances.

Figure 3. 

SS-RIPT based charger.

Since the transmitter and receiver sides are mutually coupled through electromagnetic field, therefore battery voltage (V_Batt) and battery current (I_Batt) can be controlled directly by sensing the V_Batt and I_Batt or it can be controlled indirectly by sensing the current and voltage of the resonant tank such as I_P, V_P, I_S, and Vs. Figure 4 shows the electrical equivalent circuit of the SS-RIPT link.

Figure 4. 

Simplified equivalent circuit of the SS-RIPT system.

In Fig. 4, R_L is the equivalent resistance of the battery seen by the secondary side, given by Eqn (1).

The fundamental components of V_S and I_S are related to V_Batt and I_batt respectively using Eqns (2) and (3).

Referring to Fig. 4, at resonance,

By utilizing Eqns (1)−(5), the relationship between battery voltage and current can be established with primary side voltage and current, as demonstrated in Eqns (6) and (7).

Relationships in Eqn (2), (3), (6) and (7) can be utilized to control the battery voltage and current from the resonant tank variables.

It should be noted that battery current/voltage can also be controlled indirectly by sensing DC link voltage V_DC and DC link current I_DC. However, for this article, the sole focus was on direct control by sensing V_Batt and I_Batt and indirect control by sensing resonant tank variables. Figure 5 shows the overall block diagram of the CC-CV controller.

Figure 5. 

CC-CV controller.

In Fig. 5, a voltage/current controller enables the comparison between the measured output voltage/current and the desired values. The outputs of the controller determine the duty cycle for the PWM (Pulse Width Modulation) modulator. This duty cycle is then compared with a ramp signal to generate PWM pulses for the inverter or rectifier, which charges the battery. The PWM block generates a quasi-square wave at the output of the converter as shown in Fig. 6.

Figure 6. 

Quasisquare output voltage waveform of a resonant inverter.

As shown in Fig. 6, output voltage and thus current can be regulated by four control variables α+, α-, and β, and the switching period (T_s). Three types of PWM are possible namely: Symmetric Voltage Cancellation (SVC) also known as Phase shift (PWM), Asymmetrical Voltage Cancellation (AVC), and Asymmetrical Duty Cycle (ADC). Detailed descriptions of these PWM techniques have been previously reported^[12,13]. In Fig. 5, a selection logic is incorporated to switch between Constant Current (CC) and Constant Voltage (CV) modes based on the battery's state of charge.

Control strategies in resonant inductive power transfer (RIPT) systems are primarily classified into three types, depending on which side's converter is being controlled: primary side control, secondary side control, and dual side control. Primary side control involves regulating the converter of the transmitter (i.e., the DC-AC converter), while secondary side control focuses on managing the converter of the receiver side (i.e., the AC-DC converter). Dual side control, on the other hand, entails controlling both the transmitter and receiver converters simultaneously.

Based on the choice of control signal (reference signals), Table 1 categorizes the control method for SS-RIPT chargers. In Table 1, direct refers to sensing the battery voltage and current directly for feedback and indirect control refers to sensing signal other than load (battery) voltage and current.

Existing primary side direct control strategies typically rely on wireless communication modules, such as Wi-Fi and ZigBee. In these strategies, charging information is wirelessly transmitted from the secondary side to the primary side and is then used as reference signals for the controller. Although this is a well-established technical solution, many studies have thoroughly analyzed the primary side direct control methods^[14−17], and certain challenges persist. These challenges include the additional cost of hardware and software for wireless communication modules, potential communication instability due to strong magnetic field interference, and the unsuitability of wireless communication for specific applications such as underwater or aerospace environments.

To address these issues, some studies have introduced primary side indirect control methods. In Madawala et al.^[18], the output power is regulated through the power-frequency droop characteristic of the wireless charging system, eliminating the need for a dedicated communication link. In Chow et al.^[19], transmitter-side electrical information is used to regulate the power consumption on the receiver side. However, these methods focus solely on adjusting the output power without addressing the control of charging current and voltage. In Madawala & Thrimawithana^[20], a phase-shifted full-bridge inverter is used to achieve constant voltage (CV) charging for the RIPT system, based on the estimated charging voltage from the primary side. However, no further analysis of constant current (CC) charging is provided. In Song et al.^[21], while the primary side controller is capable of CC/CV charging for the SS/SP compensated RIPT system, it requires the estimation of load resistance and mutual inductance to control the charging current and voltage. This, in turn, necessitates additional hardware for load resistance estimation and mutual inductance identification, increasing system complexity and overall cost. In Li et al.^[22], a primary side indirect control method is proposed to achieve CC/CV charging for the SS compensated RIPT system without the need for load identification, mutual inductance estimation, or dual-side communication.

This review discusses both direct and indirect control strategies. For the indirect control strategy, rather than employing the complex approaches mentioned in the references, a more simplified method is adopted. This method involves deriving equations that establish the relationship between battery current and voltage with the primary and secondary tank currents and voltages, enabling effective control of the battery voltage and current.

The authors acknowledge that dual-side control is also widely used, especially in bidirectional power transfer applications^[23−25]. However bidirectional control is not the focus of the current paper and will be covered separately in a different study.

In direct control, the battery is charged by directly sensing the battery voltage and current, utilizing them as feedback signals for the controller. Depending on whether PWM is controlling the primary-side converter or the secondary-side converter, direct control can be classified into two categories: Primary-side direct control and secondary-side direct control. Both types are elaborated on in the subsequent subsection.

Primary side direct control

This is a very straightforward and easy-to-implement control technique. Figure 7 shows the block diagram of the primary side direct control of the SS-RIPT charger. V_Batt and I_Batt are used as the feedback signal for the CC-CV controller shown in Fig. 4. PWM signals generated from the CC-CV controller are used to adjust the primary side inverter output to control the voltage and current applied to the battery^{[15,26,27−29]}. The secondary side converter could be a simple uncontrolled rectifier or should be switched to 50% duty cycle in the case of an active rectifier.

Figure 7. 

Primary side direct contro.

By utilizing Eqns (6) and (7), V_Batt and I_Batt can be manipulated by controlling I_P and V_P, respectively. To achieve this, a voltage control loop will generate PWM to regulate the output voltage of the inverter, ensuring that I_P remains constant, thus operating the charger in CV mode. Similarly, for CC mode, a current control loop will generate PWM to maintain a constant V_P, thereby supplying the battery with a constant I_Batt.

Secondary side direct control

Figure 8 shows the block diagram of the secondary side direct control scheme of the SS-RIPT system.

Figure 8. 

Secondary side direct control.

In Fig. 8, the primary side converter operates at a fixed 50% duty cycle to achieve a square waveform. The secondary side converter functions as an active rectifier^[30−34]. For this control method to operate effectively, the secondary resonant tank must be slightly inductive^[12]. PWM regulates the I_Batt and V_Batt through Eqns (2) and (3).

In indirect control, battery voltage and current are not used as feedback signals. Instead, primary, or secondary side resonant network variables are used as feedback signals of the CC-CV controller. Depending on whether PWM is controlling the primary-side converter or the secondary-side converter, direct control can be classified into two categories: Primary-side indirect control and secondary-side indirect control. Both types are elaborated on in the subsequent subsection.

Primary side indirect control

Figure 9 shows the block diagram of the primary side indirect control scheme of the SS-RIPT system.

Figure 9. 

Primary side indirect control.

In Fig. 9, the average, RMS, or peak value of V_P and I_P are detected and used as feedback signals for the CC-CV controller. Figure 10 shows the block diagram representation of the average voltage control of the CC-CV controller^[35,36]. Similarly, a current controller can be implemented. Equations (6) and (7) are applicable in this control.

Figure 10. 

Inverter output voltage regulation based on duty cycle control.

Secondary side indirect control

Figure 11 shows the block diagram of the secondary side indirect control scheme of the SS-RIPT system.

Figure 11. 

Secondary side indirect control.

In Fig. 11, the primary side converter is operated at 50% duty cycle to obtain square wave output. The secondary side converter is operated as an active rectifier. Here the average, RMS, or peak values of V_S and I_S are detected and used as feedback signals for the CC-CV controller.

Existing literature indicates that both direct and indirect control strategies have distinct advantages and limitations. This section provides a comparison of these methods, focusing on their respective advantages and limitations.

Implementing all four control strategies is technically possible; however, thoroughly addressing them in a single paper is not feasible. Therefore, this paper will concentrate on showcasing the simulation and Hardware in Loop Validation of two specific strategies: primary side indirect control for CC-CV charging of Li-Ion batteries and primary side direct control for CC-CV charging of Li-Ion batteries. The simulation involves an SS-RIPT based charger simulated in PLECS, with parameters outlined in Tables 3 & 4. A comprehensive simulation circuit for the primary side indirect control is illustrated in Fig. 12.

Figure 12. 

PLECS simulation circuit of primary side indirect control of an Li-Ion charger using CC-CV charging.

In Table 2, the CC-CV charging profile has been decided based on 5,200 mAh 5S 25C/50C (18.5 V) Lithium Polymer battery pack^[37,38].

In Fig. 12, the RMS value of I_P is sensed and used as feedback in the CV controller. For the CC controller, the RMS value of V_P is sensed and used as feedback. As stated earlier, the average, RMS, or peak value could be used.

This section discusses the simulation of both direct and indirect primary side control methods. The results, exported from PLECS, are plotted in MATLAB for enhanced clarity. The findings are presented in two subsections: Primary side indirect control; and Primary side direct control.

Primary side indirect control

In primary side indirect control, the primary side RMS voltage and RMS current are used to control V_Batt and I_Batt. Adjusting IBatt_REF modifies VP_REF for the voltage controller while adjusting VBatt_REF modifies IP_REF for the current controller.

When IBatt_REF is changed from 5 to 3 A, the corresponding VP_REF is adjusted, and VP_RMS accurately tracks this change, as shown in Fig. 13a. Similarly, Fig. 14a demonstrates that VP_RMS follows VP_REF despite step changes in the load. Figures 13b & 14b illustrate the responses of V_P and I_P during these transitions.

Figure 13. 

Simulation result for voltage controller: (a) step change in IBatt_REF from 5 to 3 A to change the VP_REF, (b) primary side voltage and current waveform for the same.

Figure 14. 

Simulation result for voltage controller: (a) change in load from 4.2 to 2 to 5 Ω, (b) primary side voltage and current waveform for the same.

When VBatt_REF is changed from 21 to 12 V, the corresponding IP_REF is adjusted, and IP_RMS accurately tracks this change, as shown in Fig. 15a. Likewise, Fig. 16a demonstrates that IP_RMS continues to follow IP_REF despite step changes in the load. The responses of V_P and I_P throughout these transitions are shown in Figs 15b & 16b.

Figure 15. 

Simulation result for current controller: (a) step change in VBatt_REF from 21 to 12 V to change the IP_REF, (b) primary side voltage and current waveform for the same.

Figure 16. 

Simulation result for current controller: (a) change in load from 4.2 to 2 to 5 Ω, (b) Primary side voltage and current waveform for the same.

By replacing the load resistor with a Li-ion battery model designed in PLECS, as shown in Fig. 12, both controllers can operate together using logic to control the CC-CV mode of operation. The resulting performance is illustrated in Fig. 17.

Figure 17. 

CC-CV charging profile.

Primary side direct control

In the primary side direct control approach, the V_Batt and I_Batt serve as key feedback inputs for the CC-CV controller. This controller, in turn, orchestrates the generation of the appropriate PWM signals. These signals finely tune the output of the primary side inverter, thereby precisely regulating the voltage and current delivered to the battery, ensuring optimal charging performance.

When VBatt_REF is adjusted from 21 to 10 V and subsequently to 18 V, the corresponding V_Batt accurately tracks these changes, as illustrated in Fig. 18a. Similarly, Fig. 19a shows that I_Batt continues to follow IBatt_REF despite step changes in the load. The responses of V_P and I_P during these transitions are depicted in Figs 18b & 19b.

Figure 18. 

Simulation result for voltage controller: (a) step change in VBatt_REF from 21 to 10 to 18 V, (b) primary side voltage and current waveform for the same.

Figure 19. 

Simulation result for voltage controller: (a) change in load from 4.2 to 8 Ω, (b) primary side voltage and current waveform for the same.

Similarly, when IBatt_REF is adjusted from 5 to 2 A and subsequently to 4A, the corresponding I_Batt accurately tracks these changes, as illustrated in Fig. 20a. Likewise, Fig. 21a demonstrates that I_Batt continues to follow IBatt_REF despite step changes in the load. The responses of V_P and I_P during these transitions are shown in Figs 20b & 21b.

Figure 20. 

Simulation result for voltage controller: (a) step change in IBatt_REF from 5 to 2 to 4 A, (b) primary side voltage and current waveform for the same.

Figure 21. 

Simulation result for current controller: (a) change in load from 4.2 to 3 Ω, (b) primary side voltage and current waveform for the same.

By replacing the load resistor with a Li-ion battery model designed in PLECS, as shown in Fig. 12, both controllers can operate together using logic to control the CC-CV mode of operation. The resulting performance is illustrated in Fig. 22.

Figure 22. 

CC-CV charging profile.

The proposed control strategy is validated through Hardware-in-the-Loop (HIL) simulation using Typhoon HIL, as illustrated in Fig. 23. A digital twin of the RIPT system is developed in Typhoon HIL, with the controller implemented on a Texas Instruments TMS320F28335 DSP. The results obtained from the Typhoon HIL simulation are exported and plotted in MATLAB for enhanced clarity.

Figure 23. 

Hardware in loop setup.

Primary side indirect control

Figures 24 & 25 display the HIL results for the voltage controller validation, demonstrating strong agreement with the simulation results depicted in Figs 13 & 14 from PLECS. Similarly, Figs 26 & 27 exhibit the HIL results for the current controller, which closely align with the PLECS simulation results shown in Figs 15 & 16.

Figure 24. 

HIL result for voltage controller: (a) step change in IBatt_REF from 5 to 3 A to change the VP_REF, (b) primary side voltage and current waveform for the same.

Figure 25. 

HIL result for voltage controller: (a) change in load from 4.2 to 2 to 5 Ω, (b) primary side voltage and current waveform for the same.

Figure 26. 

HIL result for current controller: (a) step change in VBatt_REF from 21 to 12 V to change the IP_REF, (b) primary side voltage and current waveform for the same.

Figure 27. 

HIL result for current controller: a) change in load from 4.2 to 2 to 5 Ω, (b) primary side voltage and current waveform for the same.

Primary side direct control

Figures 28 & 29 present the HIL results for the voltage controller validation, showing a strong correlation with the simulation results from PLECS depicted in Figs 18 & 19. Similarly, Figs 30 & 31 display the HIL results for the current controller, which closely align with the PLECS simulation results shown in Figs 20 & 21.

Figure 28. 

HIL result for voltage controller: (a) step change in VBatt_REF from 21 to 10 to 18 V, (b) primary side voltage and current waveform for the same.

Figure 29. 

HIL result for voltage controller: (a) change in load from 4.2 to 3 to 8 Ω, (b) primary side voltage and current waveform for the same.

Figure 30. 

HIL result for voltage controller: (a) step change in IBatt_REF from 5 to 2 to 4 A, (b) primary side voltage and current waveform for the same.

Figure 31. 

HIL result for current controller: (a) change in load from 4.2 to 2 to 3 Ω, (b) primary side voltage and current waveform for the same.

A model of a Li-ion battery was created in PLECS to validate the CC-CV charging technique. The emphasis during the HIL simulation was placed on showcasing how the system adapts to changing loads, thereby underlining the effectiveness and dependability of the implemented control approach.

In conclusion, wireless power transfer, particularly through resonant inductive power transfer (RIPT), is gaining momentum as a convenient and efficient method for charging a variety of devices including Electric Vehicles (EVs), Unmanned Aerial Vehicles (UAVs), and consumer electronics such as cell phones. This technology not only provides a safe and efficient charging experience but also addresses issues such as range anxiety in EVs and extending the flight time of UAVs by eliminating the need for constant user intervention during charging. Moreover, the absence of charging cables reduces clutter and keeps charging areas organized for consumers.

The paper reviewed control methods applicable to RIPT systems for charging batteries using the Constant Current Constant Voltage (CC-CV) mode, focusing primarily on the series-series RIPT (SS-RIPT) system. It presented various control strategies, including direct and indirect control methods, and discussed their implementation for both primary and secondary sides of the RIPT system. While acknowledging the widespread use of bidirectional power transfer, the paper concentrated solely on unidirectional power transfer and left bidirectional control for future research.

This paper primarily addresses the details of primary side indirect and primary side direct control for CC-CV charging of Li-ion batteries. Both control strategies were simulated using PLECS and verified through HIL results with the DSP controller TMS320F28335 and Typhoon HIL. The feasibility and effectiveness of both controls were demonstrated through simulations and analysis, providing valuable insights for practical implementation.

Overall, this study contributes to the understanding of control strategies for RIPT systems, particularly in the context of CC-CV charging, and provides a foundation for further research and development in this field. As wireless power transfer continues to evolve, such studies are crucial for optimizing charging efficiency, improving user experience, and advancing the adoption of wireless charging technology across various applications.

The authors confirm contribution to the paper as follows: study conception and design: Munsi A, Pradhan S; data collection: Munsi A, Pradhan S; analysis and interpretation of results: Munsi A, Pradhan S, Aditya K; draft manuscript preparation: Munsi A, Aditya K. All authors reviewed the results and approved the final version of the manuscript.

All data included in this study are available upon request from the corresponding author.