Design of wireless charger (schematic diagram + main material bom)

Introduction
Radio technology has been used for communication for nearly a hundred years. From the original radio broadcasting and wireless telegraph, it has developed into satellite and microwave communication, as well as mobile communication, wireless network, GPS, etc. that are popularized to almost every individual in the world. Wireless communication has greatly changed people's production and lifestyle. Without wireless communication, the goal of information society is unthinkable.
However, wireless communication transmits weak information, not high-power/energy. Therefore, 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 sealing process with waterproof performance, 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. It is also easy to achieve sealing and waterproofing. This goal must require that energy can be transmitted wirelessly like information.
The transmission of energy and the transmission of signals 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 transmission of energy has long been around, it has not been able to enter the practical field because it has not been able to break through the bottleneck of efficiency.
At present, there is still no substantial breakthrough in this bottleneck. However, if there is no strict requirement for the transmission distance (not compared with wireless communication), for example, within a few centimeters (referred to as macro distance in this article), its 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, mobile electronic devices that can be seen everywhere in today's society may face a new change.
Working principle
Convert direct current into high-frequency alternating current, and then realize wireless feeding of electric energy through mutual inductance coupling between the primary and secondary coils without any wired connection. The basic scheme is shown in Figure 1.

This wireless charger consists of two parts: power transmission circuit and power receiving and charging control circuit.
1 Power transmission part
As shown in Figure 2, there are two power supplies for the wireless power transmission unit: 220V AC and 24V DC (such as car power), which are selected by relay J. According to the principle of AC priority, the normally closed contact of relay J in the figure is connected to DC (battery BT1). Under normal circumstances, S3 is in the on state.
Wireless charging module
When there is AC power supply, the rectified and filtered 26V DC makes relay J close, and the transmission circuit unit works in AC power supply mode. At this time, the DC power supply BT1 is disconnected from the power transmission circuit, and LED1 (green) lights up to show this state.
The +24V DC selected by relay J 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 of J does not affect the stable operation of the transmission circuit, the capacity of capacitor C3 must not be less than 2200uF.

Figure 2 Circuit diagram of wireless power transmission unit

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 form 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 into a 1.6MHz sine.
2 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 close to coaxial, so a higher transmission efficiency can be obtained.
The principle of the power receiving and charging control circuit unit is shown in Figure 3.
The effective value of the 1.6MHz sine voltage induced by L2 is about 16V (no load). After bridge rectification (composed of 4 1N4148 high-frequency switching diodes) and C5 filtering, a DC of about 20V is obtained. As the only power supply for the charging control part.
The precise reference voltage 4.15V (charging termination voltage of lithium-ion batteries) composed of R4, RP2 and TL431 is connected to the in-phase input terminal 3 of the op amp IC through R12. When the inverting input terminal 2 of IC2 is lower than 4.15V (during the charging process), the high potential output by IC3 saturates Q4, thereby obtaining a stable voltage of about 2V across LED2 (the forward conduction of LED has a voltage-stabilizing characteristic), and Q5, R6, and R7 thus form a constant current circuit I0=2-0.7R6+R7. On the other hand, R5 turns off Q3, and LED3 does not light up.

Performance Testing

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 can still be performed at 6cm.

2 Charging Control

Keep L1 and L2 coaxial and fixed at a distance of 2cm, connect the battery to be charged, and connect the voltmeter.
Disconnect SW, the ammeter reading is 10mA, this is the slow charging working mode; connect SW, the ammeter reading is 30mA, this is the fast charging working 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 automatically stops charging, and this state is displayed.
During the test, the battery to be charged can be replaced by a 20000uF capacitor to shorten the charging time and facilitate testing.
3 Energy conversion efficiency
Still keep L1 and L2 coaxial and 2cm apart, and the charger works in fast charging, slow charging and stop charging respectively, and measures.

4 Power switching
Disconnect S1, the relay is reset, and the power is supplied by the DC power supply BT1; connect S1, the relay is energized, and the power is supplied by the AC power supply, and BT1 is disconnected at this time.
The two power supply methods have exactly the same test results for the above.
S3 is used for manual switching of the two power supply methods or forced use of DC, and is generally in the on state.
Conclusion
As a prototype for feasibility exploration experiments, 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 relatively long and difficult work, such as electromagnetic radiation leakage problems, cost control and product technology, as well as market entry and consumer startup.