Development and design of wireless charger control board
Smart phones are popular nowadays, which consume a lot of power and charge frequently. Traditional mobile phone chargers can easily damage the USB interface of the mobile phone and are very troublesome. The wireless mobile phone charger avoids repeated plugging in the USB interface, which is convenient and high-end.
1. Introduction to the principle of wireless charger
Using the principle of electromagnetic induction, when the transmitting coil passes alternating current, an alternating magnetic field will be generated around the coil. At this time, the induced electromotive force generated in the receiving coil can be rectified, filtered, and stabilized to charge the mobile phone. According to this principle, the mobile phone wireless charger is mainly composed of a transmitting part and a receiving part. The transmitting part includes an oscillation circuit, a power amplifier circuit and a transmitting coil, and the receiving part includes a receiving coil and a rectifier filter circuit. The oscillating circuit generates electromagnetic oscillation, which is amplified by the power amplifier circuit. The oscillating signal (alternating current) passes through the transmitting coil, and an alternating magnetic field is generated around the transmitting coil; the induced electromotive force generated by the receiving coil is rectified, filtered, and stabilized. Then you can charge the phone.
Second, the wireless charger design process
2.1 Block diagram design
The mobile phone wireless charger is composed of 5 parts: power supply circuit, oscillating circuit, power amplifier circuit, transmitting coil, receiving coil and rectifier filter circuit. The system framework is shown in Figure 1, and finally the mobile phone battery is charged. From the perspective of the principle of wireless circuit transmission, electric energy and magnetic energy propagate to space in the form of electromagnetic waves with the periodic changes of electric and magnetic fields. To generate electromagnetic waves, electromagnetic oscillation is first necessary. The higher the frequency of electromagnetic waves, the strength of the ability to radiate into space. The larger, the frequency of electromagnetic oscillation must be at least higher than 100KHZ in order to have enough electromagnetic radiation.
2.2 Oscillation circuit design
At first, an LC oscillation circuit was used, but the oscillation frequency was not enough. Later, the crystal oscillator circuit composed of the crystal oscillator circuit CD4069 of the CMOS circuit six inverter CD4069 is shown in Figure 2. The effect of using CD4069 to generate high frequency oscillation is better than the effect of the LC oscillator circuit.
2.3 Design of power amplifier
At first, the triode structure was used to successfully discharge the circuit, but the temperature performance was not stable enough, and then the field effect tube was used. The field effect transistor is a voltage control element, which is a kind of transistor similar to the electronic tube. Compared with the bipolar transistor, the field effect transistor has high input impedance, low input power consumption, good temperature stability, good signal amplification stability, and signal The characteristics of low distortion and low noise, and its amplification characteristics are better than electronic transistors. In Figure 3, the three resistors R1, R2, and R3 in the power field effect tube circuit are connected in parallel to the grid G of the field effect tube, and the high frequency oscillation of the previous stage The circuit is also connected to G; the primary S is directly grounded; the drain D is connected to the LC oscillation circuit, and its resonance frequency is the same as the high-frequency oscillation frequency of the previous stage. The circuit is shown in Figure 3.
2.4 Design of transmitting and receiving coil circuit
Transmitting and receiving coils are made of enameled wire with a diameter of about 0.5ram around 12 turns, and the coil diameter is about 80r. The function of the transmitting module is to efficiently convert the DC energy into a radio frequency power signal so that the receiving circuit can make full use of the energy. The receiving module is a module that processes the energy of the previous stage after receiving it. In order to meet the needs of practical applications, the received radio frequency signal needs to be rectified, filtered, stepped down and stabilized, and the processed DC voltage can be used by other loads. The module mainly includes a rectifier circuit and a step-down circuit.
2.5 Overall schematic diagram of mobile phone wireless charger
A wireless charging circuit composed of CD4069 crystal oscillator circuit and power FET. The limit voltage of the CMOS circuit of the CD400 series of the charging circuit is 18v. The no-load voltage after rectification and filtering of the unstable AC 12v voltage may exceed 18v, so the power supply voltage of CD4069 is provided by a three-terminal voltage regulator integrated circuit 7805. All unused input terminals of the CMOS circuit are connected to appropriate logic levels, the crystal oscillator is connected to a single-gate oscillator, and the oscillation output is sent to the gate G of the power FET after the secondary buffer, and it starts to protect the FET. The bias voltage and leakage circuit are set on the pole circuit to ensure the stable operation of the circuit.
Summary and outlook
The product has been completed and can wirelessly charge the mobile phone. The wireless distance is 3cm. If you need a larger distance, you need to increase the number of coil turns and increase the number of power amplifier circuits to generate a stronger alternating magnetic field around the transmitter coil for the receiving coil Receive at greater distances. The receiving coil can be embedded in the phone, and the coil can receive signals through the back cover (tested successfully). If this product can be produced on the market, it is believed that various mobile phones will have embedded receiving coils during production to cater for mobile phone wireless chargers, which will be more convenient to use, and even mobile phone manufacturers may buy out the property rights of wireless chargers and use wireless charging as mobile phones. Unique selling point.