## 1. Design background

In the wireless magnetic resonance power transmission system, the mutual inductance between the transmitting coil and the receiving coil is often very low due to the large spacing or no alignment between the transmitting coil and the receiving coil. It is usually less than 0.3. This situation will be worse in the National College Students’ smart car energy saving Group [1] competition. Because the vehicle model runs to the transmitting coil and relies on simple photoelectric or magnetic field positioning, the receiving coil on the vehicle model is often difficult to align with the center of the transmitting coil.

In order to avoid the obstruction of inductance to power transmission caused by magnetic flux leakage, it is often necessary to use capacitance to compensate the sending and receiving coils. A few days ago, a simple capacitor series compensation was tested, which can obtain 50W transmission power and the efficiency is about 75%. Although the design of series compensation circuit is simple, there is instability in the transmission system. Especially when the load fluctuates greatly, the current in the transmitting coil will fluctuate greatly.

In order to adapt to the fluctuation of load, LCC circuit compensation is often used. It can keep the current in the transmitting coil constant when the load changes, so as to improve the stability of the system.

## 2. LCC compensation scheme

LCC circuit compensation means that three compensation devices are added to the original transmitting coil to form a T-shaped circuit network

T-type left branch: series compensation inductance LP

T-type right branch: series compensation capacitor CPS

T-type lower branch: parallel compensation capacitor CPP

Symmetrical LCC compensation scheme is adopted for transmitting and receiving coils.

Wireless transmitting and receiving circuit supplemented by LCC

3. Symmetrical T-type compensation circuit

Compared with the original series compensation, there is only one compensation capacitor parameter, so the parameters of the compensation capacitor can be calculated only considering the resonant frequency of the circuit.

Using LCC compensation scheme, the parameters of each side compensation network become three parameters: LP, CPS, CPP. This makes the circuit design more complicated.

In order to simplify the design, circuits are often designed on the basis of T-shaped network. Two JX (inductors) and one – JX (capacitors) are used to form a T-shaped compensation network between load Z0 and power UI. The reactance amplitudes of the three devices are the same at the operating frequency. Therefore, the circuit has only one parameter x in the design process, so the design process is simple.

Symmetrical ladder circuit structure

The most important characteristic of this circuit is that the working current I0 of load Z0 is a constant value

It has nothing to do with load Z0. If the load Z0 is the reflection resistance of the corresponding secondary side in the corresponding transmitting coil, it also means that the current I0 in the transmitting coil will not change with the change of the load, which makes the system stable.

If the receiving coil has a good capacitance compensation and the load of the corresponding coil is assumed to be RL, then through the coupling of the transmitting coil and the receiving coil, the reflection resistance corresponding to the transmitting coil is:

Therefore, no matter the change of the actual load RL or the change of the mutual inductance m between the transmitting coil and the receiving coil, the corresponding reflected impedance is changed in the transmitting coil.

Parameter design of 02lcc compensation network

The network parameters of LCC are designed according to the scheme in [2].

Transmit and receive coils

Transmit and receive coil parameters:

Inductance: 29 μ Heng;

Mutual inductance: when the distance is 3 cm, the mutual inductance is 9.5 μ Heng;

1. Design conditions

(1) Output load

Assume resistance load RL = 10 Ω。 After full bridge rectification, what is the equivalent load impedance according to full bridge rectification[ 3] The impedance before full bridge rectifier is about:

Suppose the working frequency: F0 = 95khz.

Reflection resistance of primary side:

Square wave and corresponding fundamental peak

Primary LCC compensation structure

2. Calculation results

According to the value of I0 calculated above, the parameters of LCC compensation device can be calculated separately

LCC compensation parameters after calculation:

3. Error impact analysis

In the actual experiment, there are corresponding differences between the design parameters and inductance L1, capacitance CPP and CPS

Only inductors and capacitors of specifications can be made in series and parallel. Therefore, they can only take values close to the design;

Meet ZVS (zero voltage switch) condition: the inverter needs to present inductive condition.

In applying LCC compensation network to dynamic wireless charging system [4], the expression of network parameters in the state of deviation from the actual symmetry is given. Based on the LCC compensation lower branch XP,

The figure below shows U1 = 300V, XP = 12 ohm, and rref = 8 Ω In this case, it’s different α，β The effect on the current.

α，β Influence on the current of coupling coil

4. Making LCC compensation network

(1) Making inductance LP

Main inductance: LP = 4.56uh

The original inductance framework

Ring skeleton parameters:

α，β Size: 32mm × 20mm × 11mm

α，β Turns: N1 = 42

α，β Inductance: L1 = 203.6uh;

According to LP requirements, the number of turns to be made is as follows:

Six turns of inductor is wound by litz wire, and the measured inductance is L = 5.895uh.

Wound 6 turns inductor

(2) Making CPP, CPS

Using 0.22uf capacitor, CPP and CPS are made by series and parallel connection.

Capacitors made

Two capacitors are connected in series to make CPS. Cps=0.11uH。

Three capacitors are connected in parallel to make CPP. Cpp=0.66uH。

(3) Compensation network module

The LCC compensation network circuit is made by using the simple experimental circuit of sticking copper foil [5].

The LCC compensation module is made

LCC network parameters:

○Lp=5.901uH

○Cpp=650.2nF

○Cps=104.8nF

Coupling test bench connected together

LCC compensation network of transmitting coil (left) full bridge rectification of receiving coil (right)

03 experimental test

## 1. No load test

Move the receiving coil away, and only measure the working condition of the transmitting coil under no-load.

Measurement of transmitting coil under no load

The following shows the change of the working current of the transmitting circuit under different working frequencies. It can be seen that at the designed operating frequency of about 95khz, the working current of the system is the smallest, only about 60mA.

No load current at different frequencies

If it is a simple series compensation, the working current will reach the maximum when the transmitting coil is no-load. At this time, the power consumption of the system is also the largest, which is consumed in the driving circuit and working coil.

After LCC compensation, the situation is opposite. Under no-load condition, the working current of the system automatically reaches the minimum. Therefore, there is no need for additional current control of the system.

## 2. Loading test

The receiving coil is aligned with the transmitting coil, and after full bridge rectification, two 50w30 ohm cement resistors are connected in parallel, and the load resistance is 15 ohm.

15 ohm load

The input power, output power and power conversion efficiency of the system at different frequencies are given as follows:

Conversion efficiency and power at different frequencies

It can be seen that the conversion efficiency of the system reaches the highest when the design working frequency is 95khz. But at 105 kHz, the output power of the system is the highest.

## 3. Full load test

According to the previous design, the full load working condition of the system. When the load is 10 ohm (three 50W, 30 ohm cement resistors are connected in parallel) and the driving bridge voltage is 24 V, the output power should be about 50 W. Here are the results of the measurements:

Working voltage of power supply: VBUS = 24 V

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%

The test results show that the working conditions of the system basically meet the design requirements.

The figure below shows the temperature distribution of LCC compensation circuit and receiving circuit after working for a period of time. It can be seen that the series compensation inductor LP has a great temperature rise, which consumes a certain amount of power. In the output circuit, the temperature of full bridge rectifier also increases.

Steady state temperature distribution

4. Current in coil

The former design LCC compensation circuit parameters, based on the principle that the symmetrical T-shaped circuit will make the current of the sending coil keep constant. The current clamp is used to measure the current of the transmitting coil under full load and no load.

The figure below shows the current waveform (cyan) of the transmitting coil when the system is in no load.

Coil driving voltage and current in coil

The figure below shows the current waveform (cyan) in the transmitting coil when the system is working at full load. Compared with no-load and full load, it can be seen that the current amplitude in the transmitting coil is basically constant.

Coil driving voltage and current in coil

Compared with the above measurement results, we can see that the current in the coil is basically constant.

**conclusion**

This paper discusses the design of LCC compensation network parameters based on symmetric T-shaped network. Under the condition of half bridge driving voltage of 24 V and output 50 W on 10 ohm resistance load, the LCC parameters are designed and verified by experiment

The power output of the system reaches 48.89w;

The transmission efficiency is 76%;

Working at 95khz, the no-load current of the system is 60mA, and it can adapt to the situation that the receiving load changes dramatically without any control of the main control circuit.

In order to further improve the efficiency of the system, it is necessary to optimize the production of series inductor LP in LCC. High frequency, anti saturation magnetic ring is used to 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 topology 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］ApplyingLCCcompenstationNetworktoDynamicWirelessChargingSystem： https://ieeexplore.ieee.org/stamp/stamp.jsp ？ tp=&arnumber=7405298

(5) making a simple experiment circuit for pasting copper foil https://zhuoqing.blog.csdn.net/article/details/112150112

Editor: hfy