Wireless power transmission (WPT) systems are becoming increasingly popular, with significant applications in the consumer electronics, medical devices and electric vehicle charging sectors. Among the various aspects involved in the design of WPT systems, one of the most important relates to the configuration or architecture of the coil connection. For high-frequency resonant WPT systems, two- and four-winding (without ferrite) configurations are commonly used. However, in the case of midrange WPT systems, ferrites are used to improve wireless power efficiency in near-field wireless charging applications, with a consequent increase in the weight and cost of the solution. Finally, closely related WPT systems usually do not contain ferrites, thus achieving reduced cost and weight at the expense of reasonably lower efficiencies. Coil design plays a key role in WPT systems as it addresses several aspects, including system behavior, efficiency, mismatch tolerance, and operating frequency bandwidth.

This paper analyzes and evaluates different configurations of WPT windings for short and closely connected WPT systems, focusing on the efficiency of wireless power transmission and the lightweight design to reduce the total mass of the WPT system. Read the original article here.

Coil configurations

A 3D view of two different configurations with two windings is shown in Figure 1. The conventional configuration of a WPT winding consisting of two identical flat helical windings, well aligned and separated at approximately zero distance, is shown in Figure 1a. The spiral WPT (IH-WPT) configuration shown in Figure 1b consists of a pair of coaxial helical coils, one coil being inserted inside the other having a slightly larger diameter. The two windings can be easily aligned and maintain alignment even in vibrations and shocks due to the small air gap between them. Finally, the configuration obtained by inserting a flat helical coil inside another helical coil is shown in Figure 1c.

Figure 1: Different types of WPT two-winding configurations

To provide a better comparison and evaluation, all of these configurations are rated at frequencies below 1 MHz (30 kHz to 1 MHz) and for short-distance values ​​using coils made with almost the same self-inductance and diameter values. An experimental setup based on a vector network analyzer (VNA) was used to obtain the S-parameters (Figure 2).

VNA-based experimental setup.
Figure 2: Experimental setup based on VNA

Modeling of WPS system with two windings

Because two-winding WPT configurations are typically designed and analyzed as a single-input and single-output system, their model can be reduced to a two-port network model. The model of the assembled circuit is shown in Figure 3a, where VS and HS are the voltage source and impedance, respectively, while C1 and C2 are optional tuning capacitors placed in series with wireless windings, L1 and L2 are the self-inductances of the two windings, and M is the mutual inductance between the two windings. ESR is the equivalent series resistance given by the sum of the ESR of the coil and the resistance of the tuning capacitor. The two-port network model is shown in Figure 3b, where the WPT system is represented by the S-parameter matrix.

Model of the inductive WPT system.
Figure 3: Model of the inductive WPT system

Through proper calibration and system settings, the relationship between the source impedance, the load impedance and the nominal impedance of the characteristics, respectively, is as follows:

Зs = Зl = З0 = 50 Ω

The resulting waves of ports 1 and 2 are given by:

Similarly, the reflecting wave of port 2 is given by:

While the parameter S21 is expressed by:

Now, replacing equations 1 and 3 in equation 4, S21 the parameter can be obtained as given by:

Since the impedance of the source (Zs) and the nominal characteristic impedance (Z0) are the same, the maximum available power from the source (Pava) can be obtained using the concept of maximum power given by the following equation:

While the load power is given by:

Finally, the effectiveness of the WPT system can be obtained as follows:

Experimental results and comparison

To evaluate the efficiency of the WPT system, the scattering parameter S21 can be used and measured by VNA. Its value can be measured with the network analyzer equipment in all three corresponding WPT winding configurations shown in Figure 1. The graph of power transmission efficiency as a function of frequency, for different coil configurations, is shown in Figure 4. Most The best results are obtained with the IH-WPT configuration, which achieves peak efficiency values ​​above 70%, followed by flat to flat and flat in spiral configurations.

Efficiency as a function of frequency for different coil configurations.
Figure 4: Efficiency as a function of frequency for different coil configurations

Figure 5 shows how the power transfer efficiency of the conventional flat pair configuration is affected by the mismatch conditions, limited to a range between 0 and 3 cm. As expected, the peak value of efficiency is obtained when the discrepancy is zero, while the efficiency decreases with increasing size of the discrepancy. The highest efficiency values ​​for these three configurations are measured in the frequency range from 300 kHz to 600 kHz.

Effect of non - compliance on power transmission efficiency.
Figure 5: Effect of mismatch on power transmission efficiency

As shown in Figures 4 and 5, the conventional flat helix-coil-pair configuration achieves a peak efficiency of about 68% (without ferrite and when the coil pairs are well aligned) at a frequency of 490 kHz. Nevertheless, the efficiency value decreases rapidly as the mismatch between the transmitting winding and the receiving winding increases. The IH-WPT configuration archives a peak efficiency value of about 72% at a frequency of 530 kHz.

Conventional silicon-based switching power supplies (such as MOSFET, IGBT, and power diodes) as well as common inverter and rectifier circuit configurations (such as half-bridge and H-bridge inverters and rectifiers) can operate efficiently in this frequency range, providing high values ​​of power transmission efficiency.

Due to its specific and unique geometry, the IH-WPT configuration offers the advantage of easily achieving and maintaining alignment between the transistor coil and the receiver coil. The configuration shown in Figure 1c, the flat helical coil placed inside the helical coil, offers much lower efficiency than the other two configurations.

In conclusion, we can say that the proposed IH-WPT configuration, consisting of one helical coil inside another larger helical coil, offers two main advantages over the conventional flat coil pair configuration. The first advantage is that it provides higher values ​​of wireless power transmission efficiency, while the second advantage is that it can easily achieve and maintain alignment between the transistor coil and the receiver coil.

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