Wireless power supply is an attractive production topic. Many electronic magazines and forums have introduced the introduction of wireless power supply circuits. Although these circuits have their own advantages, they all have a common shortcoming. First, the transmission efficiency is not ideal. Second, regardless of the presence or absence of the receiver, the transmitting part continues to emit energy continuously as it is, which is unsatisfactory.
The wireless power supply device designed by the author, in addition to the high transmission efficiency, has a remarkable feature: it can automatically detect the presence or absence of the receiving part of the work, and only when it detects that the receiver is working, it will continuously radiate the radio energy. Otherwise, it will always work only in the low-energy detection state. Its working block diagram is shown in Figure 1:
The combination of the CMOS circuit and the FET is used in the transmitting part. This combination is not only efficient but also easy to control. The transmitting coil adopts a Litz wire and a spider wire winding method to achieve higher conversion efficiency. The circuit is shown in Figure 2a:
Working principle :
Emission section
The oscillation source consists of a quarter crystal CD4011 and a crystal for the remote control. The measured oscillation frequency is 560 kHz. This frequency has two interferences in the middle band of the radio: 560 kHz and 1120 kHz. Because there is no more suitable crystal choice at hand, I have to do it.
4011 is a 2-input NAND gate, so whether the circuit works or not depends on the potential of the other input. The potential of this input is determined by the state of IC2 (555 circuit). The output duty of IC2 is approximately equal to 1/10. Wave, so the ratio of the operation of the high-frequency oscillating circuit to the intermittent time is also equal to 1/10.
The other three NAND gates of the 4011 are connected in parallel as a push stage to isolate the oscillation from the output stage. In order to output high enough power at low power, the output stage uses FET IRF634. The FET is a voltage control device. In principle, it does not consume excitation power, but its inter-electrode input, The output capacitance is very large, there are several hundred pF. If it is directly connected to the output of 4011, the rise time and fall time of the waveform will increase due to the small output current of the CMOS gate circuit, resulting in a decrease in efficiency. So I also added a pair of complementary triodes behind the CMOS gate. This complementary tube is connected to the emitter output and has a very small output resistance, which can greatly reduce the rise and fall times of the square wave. Practice has proved that the efficiency has been significantly improved after adding this level of circuit. Moreover, there is a significant difference in the current between no-load and load, which provides a simple and reliable detection basis for the intelligentization of wireless power supply.
When there is no load, that is, when the receiving part of the wireless power supply is not close to the transmitting coil, the source current of VT3 is small, and the voltage drop on R6 is not enough to make VT4 turn on, so there is no trigger on the second leg of IC3. Pulse, there is no high level output on the third pin; once the receiving part is close to the transmitting coil, enough energy is received from the transmitting stage, so that the source current of VT3 increases, and R6 also generates a sufficiently large voltage. Pushing VT4 on, a negative pulse of sufficient amplitude is generated at the collector of VT4, and IC3 is driven to output a high level. This high level is sent to the control terminal of the crystal through VD2, so that it operates in a continuous oscillation state, thus completing the task of load detection.
We say that this circuit is a smart wireless power supply circuit, because it can automatically detect the presence or absence of load. When there is no load, it works in an intermittent state to save energy, and once it detects the load, it works in a continuous state, making it work normally.
In Fig. 2, Rp is used as the detection sensitivity adjustment; LED is the working status indication (the red light intermittently flashes for the detection state, the green light is the continuous working state); SA is the maintenance switch, after the switch is closed, the red light is continuously bright, and the output stage is continuous. Work, suitable for maintenance or work under weak loads.
2. Receiving part
In fact, any device with a receiving coil can be used as a receiving circuit. Here is just one example. It can measure the power of the receiving part, and can also adjust the resonant state of the circuit to make it the most sensitive. The circuit is very simple, so I won't go into details. The circuit is shown in Figure 2b:
If the receiving distance and the conversion efficiency of the detection system are not tested, the circuit of Figure 2b may not be installed.
Part selection
The efficiency of wireless power supply is related to the working state of the transmitting stage, and also has a very close relationship with the quality of the coil that emits electromagnetic energy. Therefore, the transmitting coil L1 uses 36&TImes; Φ0.1mm Litz wire, which is wound around the disc. Skeleton on the spider web. See Figure 3:
The skeleton of the coil is made of a disc, and in order to avoid eddy current loss, the metal plating on the disc should be removed. The inner diameter of the skeleton was 66 mm, and it was wound with a strand of 36 strands of Φ0.1. The receiving coil in Fig. 2b is also wound into a spider web, and is wound with 8 strands of Φ0.1 multi-strand enamelled wire.
The high-frequency output stage uses high-power FETs, such as the IRF series of 634, 630, or other VMOS tubes with a withstand voltage of 200V, a current of 5A, and a maximum loss power of more than 20W. Use a radiator with a large enough area when using it.
Resonant capacitor C4 requires a capacitor capacitor with a withstand voltage of 200V or higher; C11, C12, C13, and C14 require a withstand voltage of 35V or higher. Timing capacitors C5 and C10 require accurate capacitance and low leakage. It is best to use tantalum capacitors. If there is no tantalum capacitor, aluminum electrolysis with a withstand voltage of 25V or higher should be used. C7, C8, C11, and C12 should be soldered close to IC4.
There are no special requirements for other components.
Debugging and installation
First adjust the launch part. Input 24V DC power supply, when the debugging switch is in the off state, the red light of the LED will flash in about 1 second period, which indicates that IC2 is working normally. Close the debug switch SA in Figure 2a so that the oscillating portion operates continuously, and the red light will remain lit. Check whether the DC operating point of each point is normal. At this time, the current of the whole machine is about 50mA, and the drain current of VT3 is about 20mA. If the voltages at 24V and 9V are normal, the oscilloscope can be used to check the waveform of each key point.
When the waveform of each point is basically normal, use a 2200pF polyacrylic capacitor (withstand voltage of 250V) and a variable of 1000pF (multiple connected variable parallel connection), carefully adjust the variable to minimize the current of the whole machine. Measure the total capacitance in parallel, replace one or several fixed equivalent capacitors in parallel with the original fixed and variable capacitors, and solder them on the board.
Disconnect the maintenance switch SA to operate the oscillator in an intermittent state; rotate the Rp to the maximum to make the LED green light, gradually reduce the Rp, so that the green light of the LED is extinguished, and the red light just flashes. At this time, the whole machine current swings between 10 and 20 mA.
When the receiving portion is close to the transmitting coil and the load switch SA of the receiving portion is disconnected, the indicator LED on the receiver will flash in synchronization with the LED of the transmitting portion. Similarly, a fixed capacitor of about 0.01 μF is connected to both ends of the resonant coil, and the distance between the transmitting coil and the receiving coil is gradually opened, and the size of the resonant capacitor is appropriately increased or decreased to make the indicator light brightest. After adjustment, fix the capacitor and solder it on the board. If you observe the waveform of the resonant tank with an oscilloscope, you should see a sine wave with the same frequency as the transmitting coil.
Close the load switch SA (Fig. 2b) and place the receiving coil at 5~10mm directly above the transmitting coil. At this time, the green light of the two-color LED of the transmitting part will automatically light up, indicating that the transmitting part has detected the load and works. In a continuous state. If the receiving coil is removed again, the green light will automatically go out and the red light will flash again, indicating that the detection function of the smart part is normal. If the detection function is not normal, you should carefully adjust the Rp.
When the circuit detects the load, the total current of the transmitting part is about 200~300mA, which varies depending on the load.
Wireless power supply desk lamp
The circuit is basically the same as Figure 2b except that the 50Ω resistor as a dummy load is replaced by four high-power LEDs in series. The receiving coil is made of a disk-shaped coil with an enamel wire of Φ1.2 mm or thicker, and has an inner diameter of 67 mm around 8 turns, and a lead wire of 25 cm at each end, which will serve as a stand for the small table lamp in the future.
Find a disc, remove the metal coating, drill two holes in the appropriate place, thread the coiled leads through the holes in the disc, and bond the coil to the disc.
Mount 4 1W LEDs on a self-contained heat sink.
Also find a small disc with a diameter of 80mm (no need to remove the film) and a plastic bottle cap of appropriate size as the raw material for the lampshade for making the lamp.
The production process is shown in the series below.
summary
1. From the waveform of each point of the FET, the working state of the output stage is not very ideal, especially the high-order harmonics of the source voltage waveform, which obviously reduces the conversion efficiency of the circuit. The reason should be due to the excessive waveform of the gate drive voltage. If the gate waveform is changed to a narrower pulse, the conduction angle of the FET is made smaller, and the estimated efficiency is improved.
2. Due to the rush of production time, the 7809 analog voltage regulator circuit is directly used from 24V to 9V. The efficiency is obviously very low. If the switch regulator power supply is used, the efficiency of the drive part can be greatly improved, and the circuit can be operated intermittently. More power saving.
3. The device is designed to simulate a desktop wireless power supply system, so the distance between the transmitting coil and the receiving is greater than 12 mm, which also causes a significant drop in conversion efficiency, because the greater the distance between the two coils, the lower the efficiency.
4. The wireless desk lamp is only an application of wireless power supply, and the receiving part can also be used in the fields of wireless charging, underwater LED light or rotating LED graphic display.
5. Because the off-the-shelf wireless power supply module VOX330 is not easy to buy, the power supply uses 24V DC. If you have a high-voltage wireless power supply module at hand, it will be more practical to use 220V AC directly.
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