Use LCC Compensation scheme for radio power transmission

Chao Qing TsinghuaJoking

▌01 Coil compensation circuit


1. Design background

In wireless magnetic resonance power transmission system , Because there is often a large gap between the transmitting coil and the receiving coil , Or not aligned , The mutual inductance between two coils is often very low . Usually less than 0.3. In this case National college students intelligent car energy saving group [1] It's going to be worse in the game . As the model car runs onto the transmitting coil , Rely on simple photoelectric or magnetic positioning , The receiving coil on the car model is often difficult to align with the center of the transmitting coil .
In order to avoid the obstruction of inductance caused by magnetic flux leakage to power transmission , It is often necessary to use capacitors to compensate the transmitting and receiving coils . The simple capacitor series compensation was tested a few days ago , You can get 50W Transmission power , Efficiency in 75% about . Although the design of series compensation circuit is simple , But there is instability in the sending system . Especially when the load fluctuates greatly , It will cause the current in the transmitting coil to fluctuate greatly .
In order to adapt to load fluctuations , Always use LCC Circuit compensation form . It can be used when the load changes , Keep the current in the transmitting coil constant , So the stability of the system is improved .

2.LCC Compensation plan

LCC Circuit compensation refers to adding three compensation devices to the original transmitting coil , They make up a T Type of circuit network :

  • T The left side branch of the model : Series compensation inductor Lp
  • T On the right side : Series compensation capacitor Cps
  • T The lower branch of the model : Parallel compensation capacitor Cpp
    The transmitting and receiving coils are symmetrical LCC Compensation plan .
     Use LCC Compensation scheme for radio power transmission
    ▲ use LCC Wireless transmitting and receiving circuit for supplement

    3. symmetry T Type compensation circuit

    Compared with the original series compensation , There is only one compensation capacitor parameter , In the design, the parameters of the compensation capacitor can be obtained only considering the resonant frequency of the circuit .
    use LCC Compensation plan , The parameters of each side compensation network become three parameters :Lp,Cps,Cpp. This makes circuit design complicated .
    To simplify the design , It's often symmetrical below T The circuit is designed on the basis of network . In the load Z0 And power Ui Between , Two are used jX( inductance ) And a -jX( capacitance ) Formed a T Type compensation network . The reactance amplitudes of the three devices are the same at the operating frequency . So this circuit has only one parameter in the design process X, So the design process is simple .
     Use LCC Compensation scheme for radio power transmission
    ▲ Symmetrical ladder circuit structure
    One of the most important characteristics of this circuit , It's the load Z0 The working current of I0 It's a constant value :
     Use LCC Compensation scheme for radio power transmission
    It's related to the load Z0 It doesn't matter. . If the load Z0 It is the reflection resistance of the corresponding secondary side in the corresponding transmitting coil , This also means that the current in the transmitting coil I0 It doesn't change with the load , This keeps the system stable .
    If the receiving coil has made good capacitance compensation , The load of the corresponding coil is assumed to be RL, So by coupling the transmit and receive coils , The reflection resistance corresponding to the transmitting coil : Use LCC Compensation scheme for radio power transmission
    therefore , Regardless of the actual load RL The change of , Or the mutual inductance between the sending and receiving coils M The change of , Reflected in the transmitting coil, the corresponding reflection impedance is changed .

    ▌02 LCC Compensation network parameter design


Based on The output part of wireless charging system adopts LCC Overview of topology research [2] The design of the project in LCC Network parameters of .
 Use LCC Compensation scheme for radio power transmission
▲ Sending and receiving coils
Sending and receiving coil parameters :

  • Inductance :29 Weiheng ;
  • Mutual inductance : At a distance 3 Centimeter hour , The mutual inductance is 9.5 Weiheng ;

    1. Design conditions

    (1) Output load

    Suppose a resistive load RL=10Ω. After full bridge rectification , according to What is the equivalent load impedance of full bridge rectifier ?[3] Discuss , The impedance before rectifying the full bridge is about :
     Use LCC Compensation scheme for radio power transmission
    Suppose the operating frequency :f0=95kHz.
    The reflection resistance of the primary side :
     Use LCC Compensation scheme for radio power transmission
    Suppose the output power : Use LCC Compensation scheme for radio power transmission .
    (2) Input power
    Working voltage : Use LCC Compensation scheme for radio power transmission .
    The effective value of the corresponding fundamental wave :
     Use LCC Compensation scheme for radio power transmission
     Use LCC Compensation scheme for radio power transmission
    ▲ Square wave and the corresponding peak value of fundamental wave
    Let's assume that the efficiency from the primary side to the load is : Use LCC Compensation scheme for radio power transmission .
    So the reflected power of the power supply is :
     Use LCC Compensation scheme for radio power transmission
    The current flowing through the reflecting resistor :
     Use LCC Compensation scheme for radio power transmission
     Use LCC Compensation scheme for radio power transmission
    ▲ The original side LCC Compensation structure

    2. The result of the calculation is


According to the previous calculation I0 Size , It can be divided into two parts LCC Parameters of compensation device :
 Use LCC Compensation scheme for radio power transmission
 Use LCC Compensation scheme for radio power transmission
 Use LCC Compensation scheme for radio power transmission
 Use LCC Compensation scheme for radio power transmission
After calculation LCC Compensation parameters :

 Use LCC Compensation scheme for radio power transmission

3. Error impact analysis

In a practical experiment , Because of the related inductance L1, capacitance Cpp,Cps There will be corresponding differences with the design parameters , The main reasons include :

  • It can only pass through the inductance of specifications 、 Capacitors are made in series and parallel . So they can only take values close to the design ;
  • Satisfy ZVS(Zero Voltage Switch) Conditions : The inverter needs to present inductive conditions .
    stay Applying LCC compenstation Network to Dynamic Wireless Charging System[4] The expression of the network parameters in the state of deviation from the actual symmetry is given . With LCC Compensate the lower branch Xp Based on , The left-hand deviation ratio is defined as  Use LCC Compensation scheme for radio power transmission ; The right-hand deviation ratio is defined as : Use LCC Compensation scheme for radio power transmission , Then the current flowing through the coupling coil is expressed as :
     Use LCC Compensation scheme for radio power transmission
    You can see from the formula , Be equal to 1 when , The current flowing through the coupling coil is a constant : Use LCC Compensation scheme for radio power transmission .
    The image below shows U1=300V,Xp=12 ohm ,Rref=8Ω Under the circumstances , Different α,β The effect on the current .
     Use LCC Compensation scheme for radio power transmission
    ▲ α,β The effect on the coupling coil current

    4. Make LCC Compensation network

    (1) Making inductors Lp

    The main production of inductance :Lp=4.56uH
     Use LCC Compensation scheme for radio power transmission
    ▲ The original inductance skeleton
    Ring skeleton parameters :

  • Size :32mm×20mm×11mm
  • Turns :N1=42
  • inductance :L1=203.6uH;
    according to Lp requirement , The number of turns to be made is :
     Use LCC Compensation scheme for radio power transmission
    utilize Litz Wire wound 6 Turn inductance , Measuring inductance :L=5.895uH.
     Use LCC Compensation scheme for radio power transmission
    ▲ Wound 6 Turn inductance

    (2) Make Cpp,Cps

    Use 0.22uF The capacitors are made in series and parallel Cpp,Cps.
     Use LCC Compensation scheme for radio power transmission
    ▲ Made of capacitors

  • Two capacitors are made in series Cps.Cps=0.11uH.
  • Three capacitors are made in parallel Cpp.Cpp=0.66uH.

    (3) Compensation network module

    utilize Paste copper foil simple experimental circuit production [5] Make LCC Compensation network circuit .
     Use LCC Compensation scheme for radio power transmission
    ▲ To make the LCC Compensation module

    LCC Network parameters :

  • Lp=5.901uH
  • Cpp=650.2nF
  • Cps=104.8nF

 Use LCC Compensation scheme for radio power transmission
▲ A coupled test bench connected together
 Use LCC Compensation scheme for radio power transmission
▲ Sending coil's LCC Compensation network ( Left ) Full bridge rectification of receiving coil ( Right )

▌03 Experimental tests


1. No load test

Move the receiving coil away , Only measure the working condition of the transmitting coil under no load .
 Use LCC Compensation scheme for radio power transmission
▲ The transmitting coil is measured under no load
The following shows that at different operating frequencies , The change of working current of transmitting circuit . You can see that at the designed operating frequency point 95kHz about , The working current of the system is the minimum , Only 60mA about .
 Use LCC Compensation scheme for radio power transmission
▲ No load current at different frequencies
If it's a simple series compensation , When the transmitting coil is empty , The working current will reach the maximum . At this time, the power consumption of the system is also the largest , All this power is consumed in the driving circuit and the working coil .
after LCC compensate , On the contrary , Under no load , The working current of the system reaches the minimum automatically . So there is no need for additional current control of the system .

2. Load test

Align the receive coil with the transmit coil , And connect two after full bridge rectification 50W30 Ohm of cement resistance in parallel , The load resistance is 15 ohm .
 Use LCC Compensation scheme for radio power transmission
▲ 15 Ohmic load
Here's how it works at different frequencies , The input power of the system 、 Output power and power conversion efficiency :
 Use LCC Compensation scheme for radio power transmission
▲ Conversion efficiency and power at different frequencies
It can be seen that , In designing the operating frequency 95kHz when , The conversion efficiency of the system is the highest . But in 105kHz when , The output power of the system is the highest .

3. Full load experiment

According to the previous design, the system is fully loaded . When the load is 10 ohm ( By three 50W,30 Ohm of cement resistance in parallel ), The drive axle voltage is 24V when , The output power should be 50W about . Here is the result of the measurement :

  • Working voltage of power supply :: Vbus=24V
  • Rectifier bridge output voltage :Vout=22.11V, Output power :48.89W
  • Power supply current Ibus=2.66A, System input power :64.32W
  • System efficiency :76.0%
    It can be seen from the test results that , The working conditions of the system basically meet the design requirements .
    The figure below shows after a period of work ,LCC Temperature distribution of compensation circuit and receiving circuit . You can see the series compensation inductor Lp There's a big temperature rise , It loses a certain amount of power . In the output circuit , The temperature of full bridge rectifier also increases .
     Use LCC Compensation scheme for radio power transmission
    ▲ Steady state temperature profiles

    4. The current in the coil

    The front design LCC Compensation circuit parameters , It's based on the principle of symmetry T The current of the transmitting coil will be kept constant by the type a circuit . The current clamp is used to measure the current under full load and no load respectively , The current of the transmitting coil .
     Use LCC Compensation scheme for radio power transmission
    ▲ Measure the current in the coil
    The figure below shows the system under no load , The current waveform in the transmitting coil ( Cyan ).
     Use LCC Compensation scheme for radio power transmission
    ▲ The coil drive voltage and the current in the coil
    The figure below shows the system working at full load , The current waveform in the transmitting coil ( Cyan ). Compare empty and full load , It can be seen that the current amplitude in the transmitting coil is basically constant .
     Use LCC Compensation scheme for radio power transmission
    ▲ The coil drive voltage and the current in the coil
    Compare the above measurements , It can be seen that the current in the coil is basically constant .

    ▌ Conclusion


In this paper, based on symmetry T Type network design LCC Compensation network parameters . And the half bridge driving voltage 24V Under the circumstances , stay 10 Output on ohm resistance load 50W Design under different conditions LCC The parameters are tested by experiments , Verified :

  • The power output of the system has reached 48.89W;
  • The transmission efficiency is 76%;
    Working in 95kHz Next , The no-load current of the system is 60mA, There is no need for any control of the main control circuit , It can adapt to the situation that the receiving load changes dramatically .
    In order to further improve the efficiency of the system , Need to be right LCC Medium series inductance Lp To optimize the production of . High frequency , Anti saturation magnetic ring production , Reduce the loss of compensation circuit .

    Reference material

[1]
National college students intelligent car energy saving group : https://zhuoqing.blog.csdn.net/article/details/110253008
[2]
The output part of wireless charging system adopts LCC Overview of topology research : https://zhuoqing.blog.csdn.net/article/details/113770750
[3]
What is the equivalent load impedance of full bridge rectifier ?: https://zhuoqing.blog.csdn.net/article/details/113777100
[4]
Applying LCC compenstation Network to Dynamic Wireless Charging System: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7405298
[5]
Paste copper foil simple experimental circuit production : https://zhuoqing.blog.csdn.net/article/details/112150112