Micro-distance wireless charger solution circuit analysis

At the Nokia New York conference a few years ago, Nokia showed us how to put your phone on the charging pad after returning home and listen to music while charging. Lumia920 has a built-in wireless charging receiver, and soon wireless chargers became available in Coffee Bean stores in the United States and London Heathrow Airport.

Wireless charging has moved from a dream to a reality, from a concept to a commodity. In recent years, it has led a new trend in mobile phones, electric vehicles and other fields. All prove that wireless charging technology has a very broad market prospect. The circuit scheme of a micro-distance wireless charger is introduced below. This scheme is a feasibility exploration experiment.

Circuit scheme and principle

The direct current is converted into high-frequency alternating current, and then 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 the figure.

Schematic diagram of wireless power transmission scheme

This wireless charger consists of a power sending circuit and a power receiving and charging control circuit.

Power transmission part

As shown in the figure, there are two power supplies for the wireless power sending unit: 220V AC and 24V DC (such as car power supply), 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 on.

Wireless power sending unit circuit diagram

When there is AC power supply, the rectified and filtered DC of about 26V causes the relay J to close, and the sending circuit unit works in the AC power supply mode. At this time, the DC power supply BT1 is disconnected from the power sending circuit, and at the same time, LED1 (green) lights up to display this state.

The +24V DC selected by relay J mainly supplies power to the transmitting coil L1. In addition, it supplies power to 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 transmitting circuit, the capacity of capacitor C3 must not be less than 2200uF.

The wireless transmission of electric energy is actually realized 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 core-less 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 appropriate.

IC1 is a CMOS six-NOT gate CD4069. Only three NOT gates are used here. F1 and F2 form a square wave oscillator, which generates a square wave of about 1.6MHz. After being buffered and shaped by F3, a square wave with an amplitude of about 11V is obtained for excitation. The VMOS power amplifier tube IRF640 is enough to make it work in the switching state (Class D) to ensure the highest possible conversion efficiency. In order to ensure that it is consistent with the resonant frequency of the L1C8 loop. C4 can be set to 100pF, and R1 needs to be adjusted. For this reason, R1 is tentatively set to 3K, and the adjustable resistor RP1 is connected in series. 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.

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 close to coaxiality. In this case, higher transmission efficiency can be obtained. The principle of the power reception and charging control circuit unit is shown in Figure 2-3.

The effective value of the 1.6MHz sinusoidal voltage induced by L2 is about 16V (no load). After bridge rectification (composed of four 1N4148 high-frequency switching diodes) and C5 filtering, a DC of about 20V is obtained. As the only power source for the charging control section.

The precision reference voltage 4.15V (charge termination voltage of lithium-ion battery) composed of R4, RP2 and TL431 is connected to the non-inverting input terminal 3 of the 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 at both ends of LED2 (the forward conduction of LED has voltage stabilizing characteristics ), Q5, R6, and R7 form a constant current circuit I0=2-0.7R6+R7 accordingly. On the other hand, R5 cuts off Q3 and LED3 does not light up.

Wireless power receiver circuit diagram

When the battery is fully charged (slightly greater than 4.15V), the inverting input terminal 2 of IC3 is slightly above 4.15V. 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 does not receive any bias current, thus stopping charging. At the same time, the low potential output of the op amp turns on Q3 through R8, lighting up LED3 as a full status indicator.

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

As a prototype for feasibility exploration experiments, this design is only for small-capacity lithium-ion batteries and lithium-polymer batteries of about 100mAh, and is suitable for pocket-sized 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 in the market, there may still be relatively long and difficult tasks, such as electromagnetic radiation leakage issues, cost control and product technology, as well as market entry and consumption startup, etc.