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Wireless charging technology has grown in importance over the past few years. Wireless charging technology does not require cables, bringing great convenience to users when charging their devices. Wireless charging technology has another advantage. Thanks to regular wireless charging, the battery module can be designed smaller [1], thus making the device smaller. Wireless charging technology covers different power levels, from electric vehicles with megawatt-level charging power, to watt-level consumer products, to milliwatt-level biomedical implant devices. Wireless charging applications are very wide.

There are many ways to implement wireless charging technology, such as transmitting electrical energy through magnetic fields, capacitance, radio frequency ( RF ) , ultrasonic waves or lasers [1]. However, electromagnetic induction or electromagnetic resonance is the most widely used wireless charging solution. Electromagnetic charging schemes rely on magnetic field coupling between two coils to transfer electrical energy. The antenna module assembly includes a compensation circuit composed of these two coils and capacitors. The difference between inductive charging and electromagnetic resonance systems lies in the magnetic field coupling coefficient and operating frequency. Wireless charging standards Qi and AirFuel are representative inductive and electromagnetic resonance charging technologies respectively. The Qi standard is characterized by strong coupling of the coils (coupling coefficient k is usually around 0.7 ), an operating frequency in the range of 100kHz-300kHz , and a generally low resonant frequency of the power receiving end ( RX ) antenna, while the opposite is true for the AirFuel standard, where the antenna coil There is loose coupling, the resonant frequency is 6.78MHz or an integral multiple thereof, and the resonant frequency of the transmitting end antenna and the receiving end ( RX ) antenna is the same.

Given that these two standards have been accepted by the market, wireless charging antennas and power receiving modules that support Qi and AirFuel dual standards, as well as their related technical specifications, have attracted the attention of the industry. In the literature, some research works can be found [3]-[6] that propose solutions for dual-standard power reception. However, the main research direction of these works is the design of power receiving end circuits, especially active rectifiers, without detailed introduction to antenna configuration and coil parameter settings. Although there is a discussion of supporting dual-standard antenna configurations in [2], the final coupling coefficient is not introduced in detail. This article proposes an innovative antenna configuration scheme and active rectification circuit. The rectifier circuit is designed using a 90nm BCD process and supports two wireless charging standards: Qi and AirFuel. The output power covers the output power commonly used in consumer devices such as laptops and smartphones.

In order to evaluate the performance of the design proposed in this article, we used the Cadence Virtuoso simulation tool to evaluate the complete wireless charging system shown in Figure 1, which includes the input battery, power transmitter (TX) module, antenna coil, active rectifier, filter output capacitor and load. In order to simulate the effect of the output voltage stabilizing module (not included in the solution proposed in this article), we consider connecting a battery to the load and setting the output voltage V _OUT to the target voltage value, that is, Qi charging is 12V and AirFuel charging is 20V.

The antenna parameters used in the antenna configuration simulation are actual measured values ​​of coils currently on the market. Align the two coils while continuously changing the coil spacing, measure the coupling coefficients of the coils at different spacings, and then simulate these coupling coefficients in a simulation tool. In this way, the performance of the antenna configurations and active rectifier circuits proposed in this article can be comprehensively evaluated, resulting in detailed efficiency information for each module of the wireless charging system. Taking everything into account, the efficiency of the active rectifier is over 93% and the efficiency of the antenna module is between 67.4% and 95.6%.

The structure of this paper is as follows: the second part describes the antenna configuration proposed in this paper, the third part introduces the active rectifier circuit proposed in this paper, the fourth part reports the simulation verification results, and the fifth part is the conclusion.

In order to develop a dual-standard wireless charging reception system compatible with AirFuel and Qi, two independent antennas need to be used. By individually selecting the inductance value and Q factor, efficiency can be maximized in two different operating frequency ranges. According to the structure proposed in literature [2], the two antennas can be integrated into a dual-antenna structure, see Figure 2 (a). Capacitor C ₂ can be approximately an open circuit at low frequencies; large capacitor C ₁ acts as a short circuit at high frequencies, and the impedance of L ₁ is the largest in the circuit.

Table 1: Actual measured parameters of the coil

In this way, two series resonant circuits can be formed. One is a Qi standard low-frequency resonant circuit with an operating frequency range of 100kHz-300kHz. The resonant frequency range is determined by L _S1 + L _S2 and C ₁ ; the other is an operating frequency of 6.78MHz or other An integer multiple of the Airfuel standard high-frequency resonant circuit, the operating frequency is determined by L _S2 and C ₂ . In order to satisfy the two series resonant frequencies, this paper proposes the antenna configuration shown in Figure 2 (b). In the case of Qi, the series resonant frequency is determined by L _S1 and C ₁ ; in the case of AirFuel, the series resonant frequency is determined by L _S2 and C ₂ decided. The value of L ₁ is mainly optimized to work in the Qi frequency range; the value of L _{2 is mainly optimized to work in the Airfuel frequency range; then, the capacitors C 1} and C ₂ corresponding to the two inductors are selected to obtain the required two Resonant frequency.

Figure 1 shows the complete antenna configuration proposed in this article. The coils use standard coils sold on the market: the coils L _S1 and L _S2 selected at the receiving end are 760308101150 inductor coils from Wurth Elektronik Company. The inductances are 6.3µH and 1.2 respectively. µH [8]; in the case of Qi, the transmitter coil LP _uses a 760308101141 10µH inductor coil [9]. In the case of AirFuel, consider using a 760308101150 1.2µH inductor coil or a 3.55µH induction plate charger.

_{In order to estimate the coupling coefficients k 1} and k ₂ between different coil pairs , align the two selected coils according to the configuration shown in Figure 2 ( c ) , and use an LCR meter to measure the coupling degree between the two coils. The coupling conditions at different spacings are taken into account to derive the mutual inductance M and the coupling coefficient k ₁ . When calculating the AirFuel system parameters, select the maximum frequency of 1MHz on the LCR measuring instrument, because when the resonant frequency is set at 6.78MHz, the frequency will not change significantly under normal circumstances. The coil measurement parameters are shown in Table 1 .

The rectifier circuit design adopts 90nm BCD process and consists of four reasonably controlled power switch tubes. These four NMOS transistors function as equivalent diodes. When the transistors are turned on, positive current flows from source to drain, realizing a so-called active rectification circuit, as shown in Figure 1.

The reason for using power switching tubes instead of ordinary diodes is that the voltage drop of the power switching tubes is lower and the efficiency is higher. In particular, the quality factor of NMOS devices is higher than that of PMOS devices. Figure 3 (a) and Figure 3 (b) are the internal circuit diagrams of the active rectification scheme of the high-side power switch and the low-side power switch respectively. In the example we discussed the operation of power transistors M ₁ and M ₃ and switching node S _{1 , the same applies to M 2} , M ₄ and S ₂ . The comparator is used to detect the voltage drop and current direction on the switch M _{1. The comparator output also needs to be processed by a filter circuit similar to the one proposed in literature [7], and finally the control signal CTRL i} of the power switch is obtained . The purpose of the filter circuit is to remove glitches and spurious commutation signals from the comparator output. The comparator can limit the driving voltage of the high-side switch tube and the low-side switch tube. Because node S ₁ switches between the power receiving end ground GND and the designed output voltage node OUT, it may exceed the safe operating area of ​​the power MOS, S ₂ The functions are similar.

In particular, in the high-side switching circuit topology, since the operating voltage range of the comparator is between the output voltage node OUT and the node gndHV, gndHV is the design output voltage minus the voltage drop of a Zener diode. Therefore, it is necessary to compare The positive input of the converter is limited to ensure that the operating voltage is not lower than gndHV. The limiting function is realized through the transistor _MP1 and the resistor R1 _{: when S1} switches to OUT, _MP1 works in the transistor area, which is equivalent to closing the switch; when S1 _switches to GND, MP1 _conducts in the saturation area, and Ensure that the comparator positive input node never falls below the sum of gndHV and the source-gate voltage of _{MP1 ; R1} must be of the correct value to limit the current flowing through _MP1 .

In the case of low-side switches, the negative input of the comparator needs to be limited so as not to exceed the local supply voltage vddLV (assumed to be 5V): this function is implemented by _MP3 and R3 _. In fact, similar to the high-side MP1 _, when S ₁ switches to GND, MP3 _works in the transistor safe area and functions as a closed switch; when S ₁ switches to OUT, the transistor works in the saturation area, Limit the comparator negative input voltage to the gate-source voltage of _MP3 . The high-side switching circuit requires a level shifter because the supply voltage range of the comparator is between OUT and gndHV, while the supply voltage of the filter and driver circuit is between the bootstrap supply voltage vddHV _i and _Si .

The two standard operating frequencies and target output power values ​​of AirFuel and Qi are different (Qi is 40W and Airfuel is 10W). In order to be compatible with these two wireless charging standards, the power MOS transistor and its driving circuit must be able to be reconfigured. On the one hand, in the case of Qi, the operating frequency is low and switching losses are negligible. Because the design goal is to achieve higher output power, large-size MOS devices are required to minimize conduction losses; on the other hand, in the case of AirFuel, the operating frequency is higher and the switching loss is large, so it is preferred to use Smaller transistor size to minimize parasitic capacitance.

The reconfigurable power switch and driver circuit proposed in this article is designed using 90nm BCD process, as shown in Figure 4 (a). The circuit consists of four drivers and four power MOS modules. The driver and power module can be selected according to the digital signal fse through the AND gate. There are three parallel sub-modules in the power MOS module. Each sub-module contains a gate width of 6.72-mm, a gate length of 250 nm, and 56 fingers (nf) as shown in Figure 4 (b). The driver module consists of a 4-stage inverter chain, the number of fingers (nf) is 1-3-8-16, the NMOS gate width is 24μm, the PMOS gate width is 41.3μm, and the transistor lengths of NMOS and PMOS are both 1 μm, as shown in Figure 4 (c) is shown.

In the case of Qi, fsel is high level, all four modules work, and the control signal CTRL drives the module to work, forming an equivalent power switch composed of 12 parallel sub-modules; in the case of AirFuel, fsel is low level, Only the first module is activated, and the control signal CTRL drives the module to work, while the remaining three modules are closed. Therefore, these three power switches are in an off state. This approach enables the active rectifier to adapt to the power requirements of both charging standards.

We used the Cadence Virtuoso simulation tool to evaluate the antenna configuration scheme proposed in this article according to the test bench schematic shown in Figure 2 (d), using the actual measurement data of the coils sold on the market, and adjusted the resonant capacitance according to the standard specifications. capacity value. The input voltage V _IN is 12V and the load resistance R _LOAD is variable. We also tested the antenna configuration proposed in [2] using the same method.

Table 2: Simulation test results for different antenna configurations

Figure 5 and Figure 6 respectively describe the output power and efficiency curves of the two antenna configuration simulation tests in different load ranges. It is not difficult to find that the performance of the antenna configuration proposed in this article in the Qi case is comparable to the antenna configuration proposed in the literature [2]; in the AirFuel case, the efficiency performance is higher, showing higher efficiency in the entire load resistance range, And when the load resistance is high and the load current is low, the output power is significantly higher, which is very consistent with the wireless charging design goals of consumer applications. Put the antenna configuration and active rectifier circuit proposed in this article into a complete wireless charging system, as shown in Figure 1, and then use the Cadence Virtuoso simulation tool to test the wireless transmission system.

Considering the difference in power transmission structure, the dual-standard transmitter used in the simulation test uses the same reconfigurable driver and power MOS switch tube as the receiver proposed in this article. Simulation testing also used a single-standard transmitter whose driver and power MOS dimensions were specifically customized to Qi or AirFuel technology requirements. In addition, we also simulated and tested two complete single-standard dedicated wireless charging systems, in which the transmitter and receiver were specially designed based on Qi or AirFuel. This single-standard simulation provides a reference benchmark for dual-standard performance evaluation. The input voltage of the simulation test is 12V, and the load is the battery voltage. In the case of AirFuel, it is 20V, and in the case of Qi, it is 12V.

The antenna parameters used for system simulation are actual measurements of the coil. Table 2 summarizes the simulation results for each case, providing detailed efficiency data for the different modules. It is evident that the dual-standard active rectification circuit maintains excellent efficiency in all cases, being 1.5% lower than the standard-specific solution. In addition, the antenna configuration proposed in this article performs well in most cases (above 82%), and is only less efficient when the magnetic field coupling is very low.

This article proposes an innovative antenna configuration and active rectification circuit that supports Qi and AirFuel wireless charging standards. When realizing the antenna configuration, we consider using standard coils sold on the market, characterizing the coil characteristics by measuring the coils, and obtaining the parameters of the simulated antenna. The antenna configuration and reconfigurable active rectifier proposed in this article were comprehensively tested using the Cadence Virtuoso simulation tool and compared with the corresponding single-standard system, proving that the scheme design proposed in this article retains good efficiency and output power while providing different The detailed efficiency data of the module comprehensively analyzes the performance of the antenna and active rectifier circuit of the dual-standard wireless charging receiving system, thus filling the gaps and deficiencies in the literature in this regard.

END

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