The secrets of close-range wireless charging design: detailed explanation + schematic diagram

　　Want to get rid of the sockets and connecting cables? Here is a detailed design process of a close-range wireless charger! Within a few centimeters, the energy transmission efficiency can be easily improved to a satisfactory level. This design is for small-capacity lithium-ion batteries and lithium-polymer batteries of about 100mAh, suitable for pocket digital products such as MP3, MP4 and Bluetooth headsets. Of course, there is no principled obstacle to extending this to large-capacity batteries.

　　Radio technology has been used for communication for nearly a hundred years. However, wireless communication transmits weak information rather than high-power energy, so many portable mobile products that are extremely convenient to use have to be connected to the power grid for charging from time to time, and therefore have to leave various sockets and connecting cables. This makes it difficult to achieve a waterproof sealing process, and this personalized cable makes it difficult for chargers of different products to be universal. If these tails are completely removed, mobile terminal devices can gain real freedom and are easy to achieve sealing and waterproofing. This goal must require that energy be transmitted wirelessly like information.

　　The requirements for energy transmission and signal transmission are obviously different. The latter requires the integrity and authenticity of its content, and does not require efficiency, while the former requires power and efficiency. Although the idea of ​​wireless energy transmission has been around for a long time, it has not been able to enter the practical field because it has not been able to break through the bottleneck of efficiency.

　　If there is no strict requirement for the transmission distance (not compared with wireless communication), for example, within the range of several centimeters (referred to as macro distance in this article), the transmission efficiency can be easily improved to a satisfactory level. If relatively simple equipment can be used to achieve wireless energy transmission under macro distance conditions and form commercial promotion and application, the mobile electronic devices that can be seen everywhere in today's society may face a new transformation.

　　As a prototype, this design is only for small-capacity lithium-ion batteries and lithium-polymer batteries of about 100mAh, suitable for pocket digital products such as MP3, MP4 and Bluetooth headsets. There is no principled obstacle to extending it to large-capacity batteries. Of course, from laboratory prototypes to products on the market, there may be a relatively long and difficult task, such as electromagnetic radiation leakage, cost control and product technology, as well as market entry and consumer launch.

　　How close-range wireless charging works

　　The DC power is converted into high-frequency AC power, and then the wireless power feeding is realized through the mutual inductance coupling between the primary and secondary coils without any wired connection. The basic scheme is shown in Figure 1.

Figure 1 Wireless power transmission scheme

　　The wireless charger consists of two parts: a power transmission circuit and a power receiving and charging control circuit.

　　1 Wireless charger power transmission part

Figure 2 The circuit diagram of the wireless power transmission unit is shown in Figure 2. There are two types of power supply for the wireless power transmission unit:

　　220V AC and 24V DC (such as car power supply) are selected by the relay. According to the principle of AC priority, the normally closed contact of the relay in the figure is connected to the DC (battery BT1). Under normal circumstances, S3 is in the on state.

　　When there is AC power supply, the 26V DC after rectification and filtering causes the relay to energize, and the sending circuit unit works in AC power supply mode. At this time, the DC power supply BT1 is disconnected from the power sending circuit, and LED1 (green) lights up to display this state.

　　The +24V DC power selected by the relay is mainly used to power the transmitting coil L1. In addition, it is used to power the integrated circuit IC2 after being stepped down by IC1 (78L12). To ensure that the action does not affect the stable operation of the transmitting circuit, the capacity of capacitor C3 must not be less than 2200uF.

　　The wireless transmission of electric energy is actually achieved through the mutual inductance of the transmitting coil L1 and the receiving coil L2. Here, L1 and L2 constitute the primary and secondary coils of a coreless transformer. In order to ensure sufficient power and the highest possible efficiency, a higher modulation frequency should be selected. At the same time, the high-frequency characteristics of the device should be taken into consideration. After experiments, 1.6MHz is more suitable.

　　IC1 is a CMOS six-inverter CD4069. Only three inverters are used here. F1 and F2 form a square wave oscillator to generate a square wave of about 1.6MHz. After buffering and shaping by F3, a square wave with an amplitude of about 11V is obtained to excite the VMOS power amplifier tube IRF640. It is enough to make it work in the switching state (class D) to ensure the highest possible conversion efficiency. To ensure that it is consistent with the resonant frequency of the L1C8 loop. C4 can be set to 100pF and R1 is to be adjusted. For this reason, R1 is temporarily set to 3K and connected in series with the adjustable resistor RP1. In the resonant state, although the excitation is a square wave, the voltage in L1 is a sine wave of the same frequency.

　　It can be seen that this part is actually a frequency converter, which converts the 50Hz sine wave into a 1.6MHz sine wave.

　　2 Wireless charger power receiving and charging control part

　　Under normal circumstances, the receiving coil L2 and the transmitting coil L1 are only a few centimeters apart and are nearly coaxial, so a higher transmission efficiency can be obtained. The principle of the power receiving and charging control circuit unit is shown in FIG3 .

Figure 3 Wireless power receiver circuit diagram

　　The effective value of the 1.6MHz sinusoidal voltage sensed by L2 is about 16V (no-load). After bridge rectification (composed of 4 1N4148 high-frequency switching diodes) and filtering by C5, a DC of about 20V is obtained, which serves as the only power supply for the charging control part.

　　The precise reference voltage of 4.15V (charging termination voltage of lithium-ion battery) formed by R4, RP2 and TL431 is connected to the non-inverting input terminal 3 of op amp IC through R12; when the inverting input terminal 2 of IC2 is lower than 4.15V (during charging), the high potential output by IC3 saturates Q4 and obtains a stable voltage of about 2V across LED2 (the forward conduction of LED has a voltage-stabilizing characteristic), Q5, R6 and R7 thus form a constant current circuit I0=2-0.7R6+R7, and on the other hand, R5 cuts off Q3 and LED3 does not light up.

　　When the battery is fully charged (slightly greater than 4.15V), the inverting input terminal 2 of IC3 is slightly higher than 4.15V, and the op amp outputs a low potential. At this time, Q4 is cut off, and the constant current tube Q5 is cut off because it cannot get any bias current, so charging stops. At the same time, the low potential output by the op amp turns on Q3 through R8, lighting up LED3 as a full status indication.

　　The two charging modes are determined by R6 and R7. This non-sequential value can be found in the E24 series resistor with a nominal value of 918, so just use 918.

　　If it is designed as a product, this part of the circuit should be miniaturized as much as possible (the ammeter and voltmeter are only used for debugging in the experiment, not in the product), and it is best to make it an auxiliary circuit of the battery 　　.

　　●Power transformer T1: 5VA18V, here we use the existing dual 18V, after rectification and filtering, we get about 24V DC

　　●Relay J: DC24V, its reliable pull-in current is measured to be 13mA

　　●FUSE: Quick response 1A

　　●Adjustable resistors RP1 and RP2: Use precision adjustable

　　●Resonant capacitor C8: Ceramic dielectric capacitor with a withstand voltage of no less than 63V

　　●Rectifier bridge D5-D8: use high frequency switch tube 1N4148

　　●Precision voltage source: TL431

　　●Op amp IC3: OPA335, TI's rail-to-rail precision single op amp

　　● Transistors Q3, Q4 and Q5: leakage current is required to be less than 0.1uA, and the amplification factor is greater than 200. The model numbers are marked in the figure

　　●LED 2: Normally bright (red), forward VA characteristic is as steep as possible (small dynamic resistance, good voltage stabilization characteristics)

　　● Transmitting coil L1: Use U1mm enameled wire to wind 20 turns on a U66mm cylinder (just the size of a can), and use 502 glue to bond it properly to form a barrel-shaped coil.

　　● Receiving coil L2: Use U0.4mm enameled wire to wind 20 turns on the same cylinder, and then trim it into a dense ring shape and glue it in place. This is to make the receiving unit as thin as possible.

　　Key points for debugging short-distance wireless chargers

　　Connect an ammeter in series to the FUSE1 circuit of the sending unit to maintain monitoring. Debug in the following order.

　　1 Adjust the operating frequency

　　Adjust PR1 to make the square wave frequency generated by F1-F2 consistent with the resonant frequency of C8L1. At this time, the ammeter reading is the smallest, and the induced voltage obtained by the receiving coil L2 is the largest. Do not connect the charged battery BT2 for the time being.

　　2 Adjust the reference voltage

　　Keep L1 and L2 2cm apart and coaxial, the DC voltage across C5 should be 18-20V. Adjust RP2 to make the voltage across it 4.15V, which is the charging termination voltage of the lithium-ion battery. Change the distance between L1 and L2, the reference voltage should be constant at 4.15V between 0-6cm. Any debugging must be done while keeping other conditions unchanged.

　　3. Adjust charging control

　　Increase the distance between L1 and L2 (about 55mm) to reduce the DC voltage across C5 to 8V. Or turn off the sending unit and connect an 8V experimental power supply to both ends of C5.

　　When the op amp outputs a high potential, replace R10 with a 5M potentiometer and adjust it from large to small. While ensuring that Q4 is fully saturated, take 3/4 of its maximum resistance to become the adjusted R10. This is to ensure reliable control and save power as much as possible.

　　4-tone full display

　　●When the op amp outputs a high potential, R5 takes the maximum value while ensuring that Q3 is cut off (LED3 is not lit).

　　●When the op amp outputs a low potential, connect an ammeter in series to LED3 and adjust R8 so that the ammeter reading is 0.5mA. At this time, LED3 has sufficient brightness (the method is the same as 4-3, and the purpose is the same as 4-3).

　　●In this way, the total power consumption of the charging control circuit of the receiving unit is less than 2mA. The R4 branch consumes about 1mA, Q3 and Q4 consume 0.5mA (Q3 and Q4 will not be turned on at the same time), and IC2 consumes even less power (less than 0.01mA). 　　Short-distance wireless charger performance test

　　Make sure there is no other metal or magnetic medium near L1 and L2.

　　1 Coupling performance

　　When the receiving unit is unloaded (not connected to the charged battery), keep L1 and L2 coaxial, change the distance between L1 and L2, and measure the voltage DCV across C5 of the receiving unit; within 5cm, the charging control circuit can ensure accurate and reliable operation, and charging is still possible at 6cm.

　　2 Charging Control

　　●Keep L1 and L2 coaxial and fixed 2cm apart, connect the battery to be charged, and connect the voltmeter.

　　●When SW is disconnected, the ammeter reading is 10mA, which is the slow charging mode; when SW is connected, the ammeter reading is 30mA, which is the fast charging mode.

　　●When charging makes the voltmeter reading reach 4.15V, LED3 goes out and LED2 lights up, and the ammeter reading is zero, indicating that the battery BT2 has been fully charged and charging stops automatically, and this status is displayed.

　　●During the test, the battery being charged can be replaced by a 20000uF capacitor to shorten the charging time and facilitate the test.

　　3 Energy conversion efficiency

　　L1 and L2 are still kept coaxial and 2 cm apart, and the charger is operated in fast charging, slow charging and no charging respectively, and measured.

　　4 Power Switching

　　●Disconnect S1, the relay is reset and powered by DC power supply BT1; connect S1, the relay is energized and powered by AC power supply, and BT1 is disconnected at this time.

　　●The two power supply modes have exactly the same test results. S3 is used for manual switching of the two power supply modes or forced use of DC, and is generally in the on state.