Wireless charging

Chao Qing TsinghuaJoking

 Wireless charging

Nowadays, the innovation of energy application continues to increase in academia and power industry , The goal is to make full use of electricity when it comes , Renewable energy is the most eye-catching , For example, solar panels 、 Wind power and other natural resources , They are converted into electricity and used for different purposes , For example, supporting the operation of modern buildings 、 Drive mobile devices . Sometimes this energy is converted into “ Energy harvesting ”.
There is a common goal in electric energy application technology , That's the convenience of energy applications , That is, there is enough energy to provide when needed . Especially for mobile devices , It's important to reduce dependence on the grid , Eliminating the power supply cable on the equipment can improve the convenience of equipment application . therefore , Use wireless charging ( Also known as contactless power conversion :Contactless Power Transfer) Technology can solve the problem of equipment power supply , It improves the comfort and convenience of equipment application , In particular, there is no longer the possibility of electric shock .
Wireless charging is very convenient in the application of electric vehicles , Including electric cars 、 Electric bikes, etc . They can be recharged by wireless charging when parking , It can even use wireless charging to harvest power during operation . Compared to traditional cable charging , The charging efficiency of wireless charging will be lower , The reason is due to the leakage of the magnetic field between the transmitting and receiving radio coupling coils .
In the 16th National College Students' intelligent car competition , One of the competition topics is energy-saving beacon group . It is based on wireless charging technology , It is required to design a circuit that can harvest electric energy efficiently 、 The efficient intelligent car completes the traversal between beacons under the guidance of wireless magnetic field .
In order to improve the rhythm of the game , Reduce the stopping time of wireless energy-saving car model , The power of radio beacon in radio output stage can reach 100W, In this way, the power supply of the car model can be completed in a few seconds , Make it run to the next beacon .
In this paper, some experimental contents related to wireless charging sending and receiving are given below .
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The frame of wireless transmitting and receiving circuit used in the experiment is shown in the figure below . Use MOS The half bridge circuit drives the coil which converts DC power into AC signal L1, The receiving coil of the accessory L2 The magnetic field is converted into electric energy by electromagnetic coupling , And the AC voltage is converted to DC power again through the rectifier bridge .
Series capacitors C1,C2 Respectively with L1,L2 Form a series resonance , Counteract the effect of magnetic flux leakage on energy transfer .
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▲ The framework of wireless energy transmitting and receiving circuit
Signal frequency is an important parameter of the system in radio power transmission . The higher the frequency, the smaller the size of the coil , But at the same time, it will also increase the loss of power conversion circuit . Because the radio power transmission in the smart car competition also shoulders the function of electromagnetic navigation , Therefore, appropriately increasing the oscillation frequency can reduce the size of the receiving coil and the volume of the navigation coil . In the following experiment , The design oscillation frequency is about 100kHz about .
In order to reduce the skin effect of wire under high frequency signal , use 200 The coupling coil is made of two strands of yarn . Here are two identical coils made , The number of turns is 9 Turn , The coil diameter is about 17 centimeter . One for power transmission , There's one for power reception .

  • The parameters of the coil :

  • inductance :L0=29μH, resistance :R0=0.086Ω
    Two coils L1,L2 There is electromagnetic mutual inductance between them M, It has something to do with the shape of the two coils and the distance between them .
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    ▲ The slide rail drives the coil away from
    By applying an AC signal to one of the coils , The electromagnetic mutual inductance between two coils can be obtained by measuring the induced electromotive force in another coil M, The mutual inductance between coils can be calculated : Wireless charging
    among ,L1,L2 It's the inductance of each coil . The following shows how the mutual inductance between coils decreases as the distance increases .
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    ▲ Mutual inductance at different distances
    Use two 47nF The capacitors are connected in parallel , As C1,C2, According to the inductance of the coil , It can be calculated that the resonant frequency is about :
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    By rapid plate making , We can build an experimental system , Test the radio power transmission effect . In the experiment , The load resistance attached to the rectifier bridge of the receiving coil is a 100W Of ,0~20Ω Adjustable resistance of .
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    ▲ Test circuit
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    First of all, under different load resistances , The transmission power and efficiency of the system .
  • The test conditions are :
  • Working voltage :U1=24V
    The distance between the two coils :3.5 centimeter
    The following table shows that under different load resistances , Wireless transmission power and efficiency :
     Wireless charging

As can be seen from the above figure , The higher the load resistance is , The greater the output power and efficiency . This is different from the power characteristics provided by ordinary DC power supply . In order to effectively charge the received electric energy into the energy storage capacitor of the smart car , Effective power conversion is needed .
In the previous analysis , Both the transmitting and receiving coils need to work in series resonance . If the inductance in the circuit 、 The capacitor does not resonate at the operating frequency , Then the transmission power will drop .
The figure below shows the circuit operating at different frequencies , The actual measured transmission power 、 Receiving output power and transmission efficiency curve .
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▲ Different frequencies and power and efficiency
You can see that the system is only near the resonant frequency , The power transmitted on the same load will be very large .
When both input and output are series resonant , The transmission efficiency formula of the system is :
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The circuit resistance on the primary side and the negative side R1,R2 When they are all small , The transmission efficiency of the system is basically a constant .
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▲ The experimental part of the circuit
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In front of the verification of a design and production of radio power transmission system in 100W Transmission function and efficiency within the scope of , The receiving coil is tuned in 100kHz Left and right 80% About the transmission efficiency . This receiving coil uses 200 It's made of two strands of yarn , For the energy-saving beacon group, the size of the car model will be slightly larger . So , Need to be on receiving coil at receiving power 、 weight 、 Size and other aspects of comprehensive optimization .

Click on the “ Read the original ” You can refer to the specific data of the experiment .