Electric vehicle wireless charging structure

Wireless charging technology for electric vehicles transmits electric energy to the receiving end of the vehicle within a certain range on the ground in the form of a high-frequency alternating magnetic field through the power rail buried in the ground, and then powers the on-board energy storage device, which allows electric vehicles to carry a small number of battery packs, extending their range, and making power replenishment safer and more convenient. The main parameter indicators of dynamic wireless power supply technology include power transmission distance, power, efficiency, lateral displacement adaptability of the coupling mechanism, electromagnetic compatibility, etc. Therefore, the development of a dynamic wireless power supply system with high power, high efficiency, strong lateral displacement adaptability, low electromagnetic radiation, and moderate cost has become the main research hotspot of major research institutions at home and abroad.

In order to save energy and reduce environmental pollution, electric vehicles have been vigorously promoted by countries around the world. Due to the limitations of battery capacity and charging infrastructure, charging has become the main bottleneck in the development of electric vehicles. Wireless charging technology has gradually developed into the main way to charge electric vehicles because it can solve the interface limitations and safety issues faced by traditional conductive charging. However, static wireless charging and wired charging also have the same problems as frequent charging, short driving range, large battery usage and high cost. Especially for public buses such as electric buses, their continuous driving capacity is particularly important. In this context, dynamic wireless charging technology for electric vehicles came into being, providing real-time energy supply for electric vehicles in a non-contact manner. However, with the deepening of research, many key issues and bottlenecks need to be solved, such as high-performance magnetic coupling mechanism design issues, electromagnetic compatibility issues, and robust control of energy transmission. The solution of these problems has a guiding role in the development of dynamic wireless power supply technology. The core of low-carbon economy is the application of new energy technology and energy-saving and emission reduction technology. Electric vehicles can better solve the problems of motor vehicle emission pollution and energy shortage, and are a strategic emerging industry in my country. As an important prerequisite and foundation for the large-scale promotion and application of electric vehicles, the construction of electric vehicle charging and swapping facilities has attracted widespread attention from all parties. The development of the new energy industry, especially the rapid growth of pure electric vehicles, will inevitably put forward higher requirements for the diversification and convenience of electric vehicle charging methods. As an emerging technology, wireless charging technology is mainly used in the charging of low-power devices such as mobile phones, computers, and walkmans. It is still a new concept in the field of electric vehicles [1]. As wireless charging technology matures, electric vehicles will be the most promising market for wireless charging devices.

Researchers from the Energy Storage Thermal Management Research Institute wrote and published this full article on "Popular Science and Solutions for Wireless Charging Technology"

Among the charging methods for new energy vehicles, the most commonly heard are fast charging and super charging. Both are DC charging piles, but whether it is AC or DC, the conversion principle is the same. This article will provide some scientific knowledge about charging technology and establish the concept of charging technology.

Charging Technology Popularization

PFC

PFC is a circuit that improves the "power factor". In short, the function of PFC is to stabilize and regulate the output voltage, so that a highly stable output voltage can be obtained (the output does not change with input voltage fluctuations). Before understanding PFC, you should first know the AC power.

Active power (P), unit: W;

Reactive power (Q), unit: Var;

Apparent power (S), unit: VA.

Power factor = cosθ of angle, between 1 and -1

P/S=cosθ

By adjusting the AC input current waveform, reducing the voltage and current phase difference, suppressing harmonic current, and making cosθ close to 1.

LLC

LLC is a series-parallel resonant circuit with a resonant inductor Lr, an excitation inductor Lm and a resonant capacitor Cr.

"Resonance" actually means resonance.

The resonant circuit uses inductance (L) and capacitance (C) to complete the resonance of the circuit. The impedance of the AC circuit is not a constant and changes with the frequency.

Impedance (Z)

Impedance is a measure of the resistance of a circuit to electric current. In an AC circuit, it is divided into a real part and an imaginary part. The real part is resistance and the imaginary part is reactance (active power is real and reactive power is imaginary).

Reactance

Reactance is divided into capacitive reactance caused by capacitors and inductive reactance caused by inductors, both of which change with the current frequency of the circuit, but resistance does not.

*Capacitive reactance, capacitive reactance = 1/2πfC, it increases at low frequencies and decreases at high frequencies. The frequency of direct current is 0, and the capacitive reactance is infinite. Therefore, the capacitor is AC and resists direct current, and the voltage of the capacitor cannot change suddenly.

*Inductive reactance, inductive reactance = 2πfL, which is opposite to capacitive reactance. It becomes smaller at low frequencies and becomes larger at high frequencies. The frequency of DC power is 0, and the inductive reactance is 0, so the inductor passes DC power but blocks AC power. The current in the inductor cannot change suddenly.

When capacitive reactance = inductive reactance, reactive power is 0, and only active power is considered. By utilizing the characteristics of capacitors and inductors, the DC is adjusted to the target waveform by controlling the switching frequency/adjustment frequency, thereby achieving constant voltage.

▲ DC characteristic curve of the designed LLC circuit drawn by FHA

Zero voltage switching (ZVS), the voltage is 0 when switching

Zero current switching (ZCS), the current is 0 during switching

Wireless charging

There are four wireless charging methods:

Electromagnetic induction is the main wireless charging method, but it is not suitable for new energy vehicles because the transmission distance is too short (a few centimeters at most);

· Magnetic field resonance, also known as magnetic coupling resonance, meets the distance, power and conversion efficiency standards for electric vehicle wireless charging, using a resonant circuit;

Electric field coupling has fewer restrictions on electrode shape and material, and the electrodes can be thinner. It does not require precise positioning like electromagnetic induction, has more freedom in positioning and generates less heat. However, its disadvantage is the same as electromagnetic induction, that is, the distance is too short.

Radio waves have the longest range, but the conversion efficiency is too low.

The magnetic field resonance type is different from the LLC used in charging piles. There are several resonant circuit options for wireless charging:

Q1 to Q4 are four primary (transmitter side) MOSFETs; D1 to D4 are secondary (receiver side) rectifier diodes. The charging principle is the same as that of the charging pile, but the choice of resonant circuit is different.

(I) Resonant circuit type

1. S serial circuit

The capacitor and inductor are connected in series: on the primary side, they can be directly connected to the voltage source inverter, with low input impedance, small loss, and easy voltage feedback regulation; on the secondary side, they have characteristics similar to a constant voltage source (stable output voltage).

2. P parallel circuit

Capacitors and inductors are connected in parallel: the primary side needs a current source to supply power, is easily disturbed, and has few practical applications; the secondary side has characteristics similar to a constant current source (stable output current). Therefore, both sides of SS are serial circuits, and both sides of PP are parallel circuits. SP is the primary side in parallel and the secondary side in series, and PS is the primary side in series and the secondary side in parallel.

3. LCL constant current source

It has high power factor and harmonic/filtering capabilities at light loads.

Other resonant circuit types are based on the expansion of the above three circuit topologies, optimizing the stability conditions, input impedance and system transmission.

(II) Coil coupling structure

1. Toroidal coil

It is easy to wind, and has low iron loss, magnetic conductor and wire loss. The disadvantage is poor coupling.

2. Figure 8 coil

The two toroidal coils are connected in series in opposite directions, generating opposite magnetic fields. Similar to the toroidal coil, the coupling coefficient and loss are between the toroidal and toroidal coils.

3. Spiral tube coil

The typical copper-clad iron structure has the advantages of concentrated magnetic lines and high coupling coefficient, but the disadvantages are high iron loss and copper loss.

Wireless technology sharing from new energy vehicle companies

NIO

The performance of the coil coupling structure is an important factor affecting magnetic field resonance power transmission.

NIO optimizes the circuit topology of the DDQ coil, focusing on improving the coupling coefficient. The ratio of the actual mutual inductance between the two inductor components to the maximum mutual inductance is between 1 and -1.