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Introduction

Wireless power transfer technology holds significant importance in various aspects of our modern world due to its convenience and high flexibility for wireless charging experience. It enhances safety by reducing the risk of electrical accidents and damage to devices as it offers several advantages over traditional wired charging methods. However, it also poses several design challenges that need to be addressed for its effective implementation. Some of the key design challenges of wireless power transfer technology for charging are studied in this analysis over two WPT applications.
 

Fig. 1. General Block Diagram of WPT System for EV Charging [1]
 

Design Challenges of Wireless Power Transfer Technology

WPT technology poses several significant design challenges related to the charging process, primarily encompassing the following aspects:

• The effect of air gap distance on the WPT efficiency
• The effect of misalignment on the WPT efficiency
• Electromagnetic Interference and Compatibility
• The size and weight of the charger
• The generated losses and Heat
• The effect of the shielding material on the field distribution
• Design adaptability and integration to power applications

Wireless charger for Implantable Pacemaker Application

The studied model [2] uses an inductive coupling to recharge a pacemaker battery. The simulated model, depicted in Figure 2, consists of transmitter and receiver coils, two aluminum plates, and two ferrite cores. To comply with electromagnetic field (EMF) standards, the studied wireless power transfer (WPT) system operates at a low frequency of 20kHz. However, operating at such a low frequency reduces the efficiency of WPT. To address this issue and improve system efficiency, the aluminum plates and ferrite cores are incorporated into the design.

Fig. 2. 3D Model of WPT Design used in Pacemaker Application
 

Air Gap and Misalignment Effect on Field Distribution

To evaluate the effect of air gap distance and the misalignment on the magnetic field distribution, a parametric AC Magnetic analysis is used.
The following animation plots show the variation of the magnetic flux density for the different scenarios.
 

Fig. 3. Animation of the Magnetic Flux vs Airgap




Fig. 4. Animation of the Magnetic Flux vs Lateral Misalignment




Fig. 5. Animation of the Magnetic Flux vs Angular Misalignment
 

Air Gap Effect on Coil Parameters (R, L and K)

One of the primary challenges is improving the efficiency of the inductive wireless power transfer system which increases with the coupling coefficient of the transmitter-receiver coils. This coefficient depends in turn on the resistance and the inductance of the used coils and it is defined by the following relation:

The following figures show the variation of the coil and coupling parameters versus the air gap distance between the transmitter and receiver sides.
 

Fig. 6. Self and Mutual Inductances Results versus Air Gap




Fig. 7. AC Resistance Results versus Air Gap




Fig. 8. Coupling Coefficient versus Air Gap
 

Shielding Effect on Field Distribution

Shielding plays a crucial role in wireless power transfer systems by containing and controlling the distribution of electromagnetic fields. It helps confine the fields within a desired region, preventing their spread to surrounding areas.
Shielding materials can concentrate the fields in specific directions or areas which allows optimizing power transfer efficiency. They can also reflect or deflect the fields, redirecting them toward the receiver and enhancing the coupling between the transmitter and receiver coils.
The following simulated scenarios confirm the importance of the shielding role in improving the effectiveness and efficiency of wireless power transfer by controlling the field distribution and minimizing energy loss.

As noticed from the field distribution in the WPT design, using metallic shielding allowed field confinement and concentration within the region between the transmitter and receiver coils.
 

 
 

Fig. 9. Magnetic Flux Density Distributions with and without Shielding
 

A second visualization of the field distribution within the human body with and without the shielding is shown in the following plots:
 

Fig. 10. Magnetic Flux Density Distributions with and without Shielding across Human Body
 

Wireless Power Charger for Electric Vehicle Application

The second WPT design is a wireless charging system for an electric vehicle made of a double-sided bipolar pad which is presented by two overlapped coils for transmitter and receiver sides made of 5 turns each and operates at the frequency of 85 kHz. The coils are carried by Ferrite support bars and shielded with Aluminum plates.
 

Fig. 11. 3D Design of Bipolar Pad for WPT EV Charger
 

The following figure is showing the EMS Equivalent circuit used for the LCC Compensation Network presented in [3].
 

Fig. 12. Equivalent EMS Circuit Schematic for LCC Network
 

Shielding Effect on Field Distribution

The frequency of a wireless power transfer (WPT) system is a crucial factor for its efficiency, range, size, and compatibility. Higher frequencies can lead to increased energy losses and shorter effective ranges but enable smaller and more portable designs. Lower frequencies may provide longer-range power transfer but require larger components.
Selecting the right frequency is important to avoid interference with other devices and comply with regulations. Balancing these factors is essential to optimize the performance and practicality of the WPT system.
An analysis of the effect of the operating frequency on the WPT system efficiency using parametric analysis of AC Magnetic study with circuit coupling allowed obtaining the following results:
 

Fig. 13: Input and Output Power versus Frequency




Fig. 14. Power Efficiency versus Frequency
 

The results obtained enabled the determination of the resonant frequency of the investigated WPT design, which operates at 85kHz. This frequency corresponds to a maximum power efficiency of 96%, calculated based on the maximum input and output power values.
The following animation plot shows the vector distribution of magnetic flux lines between the transmitter and receiver WPT sides.
 

Fig. 15. Vector Plot Animation of the Magnetic Flux Density
 

Heat and Loss Analysis

An investigation of the loss quantities generated within the studied WPT device allowed us to compute and visualize the different loss distributions across the Aluminum shielding, Ferrite bars, and copper coils, as shown below.
 

Fig. 16. Solid Loss Distribution in the Al Shield




Fig. 17. Core Loss Distribution in the Ferrite Support




Fig. 18: Winding Loss Distribution in the Coils
 

Lastly, the heat generated is determined by the temperature distribution throughout the entire WPT design, reaching a maximum steady-state temperature of 67°C. 
 

Fig. 19. Temperature Distribution across the WPT System





Fig. 20. Temperature Variation versus Time 
 

Conclusion

Wireless power transfer technology is continuously advancing, with ongoing research and development endeavors dedicated to enhancing efficiency, range, and standardization. As this technology progresses, we anticipate witnessing its increased adoption and integration across diverse industries and everyday applications. Through this application note, an investigation of the main design challenges of WPT system for two different applications allowed us to analyze the performance of this technology over various design scenarios.

References