Design of High Power Electric Vehicle Charger

Pure electric vehicles use lithium batteries as the power source. After fully charged, they use electricity to do work to propel the car. Unlike gasoline engine vehicles that need to add gasoline, pure electric vehicles charge them through an external power supply after the power is exhausted, and usually have a single mileage of 100 to 200 kilometers. Compared with traditional cars, pure electric vehicles have an incomparable advantage in cost of use. They consume about 15 kilowatt-hours of electricity per 100 kilometers and are of great benefit to environmental protection. At present, the country has embarked on the demonstration and promotion of electric vehicles and new energy vehicles, and the electric vehicle charging station is one of the main links, which must achieve common and coordinated development with other fields of electric vehicles.

Charging Mode
The electric vehicle energy supply system is mainly composed of a power supply system, a charging system, and a power battery. In addition, it also includes charging monitoring, battery management, and smoke alarm monitoring. The charger is an important part of the charging system. There are generally three ways to charge a car at a charging station: normal charging, fast charging, and battery replacement. Ordinary charging is mostly AC charging. For an AC charger with a capacity not exceeding 5kW, the input is a single-phase AC with a rated voltage of 220V and 50Hz; for an AC charger with a capacity greater than 5kW, the input is a three-phase AC with a rated line voltage of 380V and 50Hz. . Insert the AC plug directly into the charging port of the electric vehicle, and the charging time will take about 4 to 8 hours. Fast charging is mostly DC charging. The input of a DC charger is a three-phase AC with a rated line voltage of 380V and 50Hz. The output voltage generally does not exceed 700V, and the output current generally does not exceed 700A. The output voltage of the AC input isolated AC/DC charger is 50% to 100% of the rated voltage, and when the output current is the rated current, the power factor should be greater than 0.85, and the efficiency should not be less than 90%.

The charger should be able to ensure that the voltage, temperature, and current of the power battery unit do not exceed the allowable value during the charging process. The charger should have anti-output short-circuit and anti-reverse functions. The charger can charge at least one of the following types of power storage batteries: lithium-ion, lead-acid, and nickel-metal hydride.
The charging mode of the power battery pack adopts the "constant current and constant voltage" two-stage charging mode. At the beginning of charging, the optimal charging rate (0.3C for lithium-ion batteries) is generally used for constant current charging. (C is the capacity of the battery, such as C="800mAh", 1C charging rate means the charging current is 800mA) At this stage, due to the low electromotive force of the battery, even if the charging voltage of the battery is not high, the charging current of the battery will be very large, and the charging current must be limited. Therefore, this stage of charging is called "constant current" charging, and the charging current is kept at the current limit value. With the continuation of charging, the electromotive force of the battery continues to rise, and the charging voltage also continues to rise. When the battery voltage rises to the maximum allowable charging voltage, keep constant voltage charging. At this stage, because the electromotive force of the battery is still rising, while the charging voltage remains unchanged, the charging current of the battery keeps decreasing in a hyperbolic trend until it drops to zero. But in the actual charging process, when the charging current decreases to 0.015C, it means that the charging is full and the charging can be stopped. The charging at this stage is called "constant voltage" charging, and the charging voltage at this stage: U=E+IR is the constant voltage value. This is the basic requirement for the charging mode of the lithium-ion power battery pack. In addition, the charging system must also have the functions of automatic adjustment of charging parameters, automatic control, and automatic protection. Especially in the constant voltage charging stage, if the charging voltage of a single battery exceeds the allowable charging voltage, the charger should be able to automatically reduce the charging voltage and current so that the charging voltage of the battery does not exceed the allowable charging voltage, preventing the battery from being overcharged. Voltage charging. The charging process and the changes in charging voltage and current are shown in Figure 1.

Figure 1 charging curve (n is the number of single cells connected in series in the battery pack)

According to the charging characteristics of the battery and the charging requirements of the electric vehicle power battery pack, the commonly used charging equipment is a charger, which can be divided into two types: DC charger and pulse charger. The DC charger is to rectify and filters the grid power supply and then isolate and stabilizes the output DC power supply to supply the power battery pack for charging. The most widely used DC charger is the high-frequency switching power supply charger. It has the advantages of small size, lightweight, reliable operation, high efficiency, high power factor, strong grid adaptability, and power.

It can be small or large, and it is easy to realize the advantages of intelligence. The pulse charger can reduce the polarization phenomenon of the battery during charging, thereby improving the charging efficiency of the battery, reducing the charging time, and realizing fast charging, but the pulse charger technology needs further research.
The charging time of electric vehicles is long, and the difficulty of charging is one of the problems in the popularization and application of electric vehicles. Take a large lithium-powered electric bus as an example, equipped with a battery capacity of 700Ah. The maximum charging current is 210A (equivalent to 0.3C charging rate of 700AH battery capacity), and the maximum charging voltage is 700V (equivalent to the voltage of 165 lithium battery cells with a maximum charging voltage of about 4.2V in series), so the maximum output power of the charger is 245kW. According to the optimal charging requirements, the charging time for electric vehicles needs at least 3 hours. Therefore, electric vehicles cannot be charged in the same way as fuel vehicles are refueled at gas stations. If it is fully charged in 20 minutes, it must be charged at least with a charging rate of 3C, which is possible for lithium iron phosphate lithium-ion batteries.
To sum up, the charging of electric vehicles still adopts the charging method of ordinary charging as the main method and fast supplementary charging as the auxiliary method.
For electric buses, the charging station is located in the bus terminal. After getting off work in the evening, use the low valley to charge, the time is 5-6 hours. For vehicles that run all day, when the mileage is not enough, you can use the rest time in the middle to recharge. The number and capacity of chargers are determined according to the size of the fleet, and the charging stations are managed by the fleet. For example, 12 large lithium-powered electric buses need 12 chargers. During fast charging, 6 chargers can be used to charge in parallel, with a maximum output power of 1470kW and a maximum charging current of 2100A (equivalent to the 3C charging rate of a 700AH battery), or 8 chargers can be used to charge 8 electric vehicles at ordinary times, each with the highest output The charging voltage is 700V, the maximum charging current is 500A (equivalent to the charging rate of 0.7C for a 700AH battery), and the fast charging mode of 1C~3C has been discussed and applied, but it should be carried out under the premise of battery safety and service life. According to the above maximum power configuration of the charger, the effective total power of the power transformer is about 3000kW or more.
At present, major automobile manufacturers have developed oil-electric hybrid vehicles and pure electric vehicles. Take the BYD E6 pure electric vehicle as an example, the battery type is a lithium iron cobalt phosphate battery, the configured battery capacity is 200Ah, the charging current of 3C is 600A, and the nominal voltage is 316.8V (equivalent to 96 lithium iron cobalt phosphate batteries with a charging voltage of about 3.3V The output power of the charger is 192kW. The fast charging time is 15 minutes to fully charge 80%. The energy consumption per 100 kilometers is about 21.5 kWh, which is equivalent to 1/3 to 1/4 of the consumer price of a fuel vehicle.
system structure
The input of the high-power electric vehicle charger is a three-phase alternating current with a rated line voltage of 380V and 50Hz, an output rated voltage of 700V and a rated current of 600A. The system adopts a 19" standard rack, with a compact structure, reasonable layout, and beautiful appearance.
Dimensions: height x width x depth is 2200mmx 600mmx 600mm. 60 modules are connected in parallel, each module is 10A/700V, the module size: height x width x depth is 133mmx425mmx 270mm, 15 floors, and 4 columns, placed in four cabinets, the four cabinets can be transported separately, and they are arranged compactly when in use. The front door and the back door of the rack are both double doors, which are convenient for inspection and maintenance. Both power inlet and busbar outlet locations are entered at the bottom. The power input circuit breaker and the touch screen of the monitoring unit are installed at the front of the central control cabinet of the main engine. The schematic diagram of the charger control structure is shown in Figure 2.

Figure 2 Schematic diagram of charger control structure

Main Circuit Design of Switching Power Supply
The principle block diagram of the high-power high-frequency switching power supply used in the electric vehicle charger is shown in Figure 3. The three-phase bridge uncontrollable rectifier circuit filters and rectifies the three-phase AC input. DC/DC half-bridge power converter, filter output DC 700V to charge the power battery. After analysis and calculation, the transformer adopts double E65 magnetic cores, and the primary coil has 12 turns. According to the highest output voltage of 700V, the lowest input voltage of 780V, and the maximum duty cycle of 0.95, the number of turns of the secondary winding N2 can be obtained, N2=(12/780) x(700/0.95)=11.33, considering factors such as leakage inductance and secondary rectification voltage drop, take N2 as 12 turns.

Figure 3 The functional block diagram of the charger power supply

Since the electric vehicle charger is a nonlinear load, it will generate harmonics, which is a kind of pollution to the power grid. Effective measures must be taken, such as power factor correction or reactive power compensation and other technologies, to limit the total harmonics of electric vehicle chargers entering the grid. In order to improve the power factor and reduce the input grid harmonics, an active power factor correction circuit is used, as shown in Figure 4. It adopts a three-phase three-switch three-level BOOST circuit, works in continuous mode, and the switch adopts a bidirectional switch composed of two MOSFETs. In the figure, switches S1, S2, and S3 are bidirectional switches. Due to the symmetry of the circuit, the potential VM of the midpoint of the capacitor is approximately the same as the potential of the midpoint of the grid, so the currents on the corresponding phases can be controlled respectively through the bidirectional switches S1, S2, and S3. When the switch is turned on, the current amplitude on the corresponding phase increases, and when the switch is turned off, the diode on the corresponding bridge arm is turned on (when the current is positive, the upper arm diode is turned on; when the current is negative, the lower arm diode is turned on). Under the action of the output voltage, the current on the Boost inductor decreases, so as to realize the control of the current. Its control circuit uses three control chips UC3854A, the phase voltage provides synchronous signals and pre-correction signals to UC3854A through three-phase isolation transformers, and the current feedback uses Hall current transformers to control three switches respectively, forming three current feedback inner loops and A multi-closed-loop system with a voltage feedback outer loop. The advantage of this circuit is that its structure is simple, and only one power switch is needed for each phase. With three-level characteristics, the harmonic current is small, and the voltage and current stress of the switching tube are small. There is no need for a neutral line, no third harmonic, and a high power factor at full load: small switching stress, low turn-off voltage, low switching loss, and low common-mode EMI.

Figure 4 Three-phase three-switch three-level APFC circuit topology

The DC/DC power converter adopts a half-bridge circuit topology, fewer powerful devices, simple control, and high reliability. As shown in Figure 5, the parallel connection technology of MOSFET and IGBT is adopted to make full use of the advantages of the fast switching speed of MOSFET and the low turn-on voltage of IGBT. Measures are taken on the circuit to make the turn-off time of the MOSFET delayed by a certain time compared with the IGBT, which greatly reduces the current tailing of the IGBT, reduces the on-state loss of the switch, improves the efficiency and reliability, and makes the output power of the half-bridge circuit 7kW can be achieved. The rectification methods used on the output side include half-wave rectification, center-tapped full-wave rectification, and full-bridge rectification. Due to the high output voltage and the high utilization rate of the transformer for full-bridge rectification, it is more suitable for this occasion.

Figure 5 MOSFET/IGBT parallel combined switch circuit

Figure 6 PWM forced current sharing method working block diagram

The system adopts the PWM forced current sharing method, and the working block diagram is shown in Figure 6. This is an improved method combining system voltage control and forced current sharing. Its working principle is to compare the system bus voltage Us with the system reference voltage Ur to generate an error voltage Ue and use the error voltage to control the PWM modulator. The obtained PWM signal controls the current per module. The current requirement signal of each module is the same, and the PWM signal is compared with the output current of the module through the optocoupler to adjust the reference voltage of the module, thereby changing the output voltage, adjusting the output current, and realizing current sharing. In this way, each module acts as a voltage-controlled current source. This current sharing method has high precision, good dynamic response, many controllable modules, and can easily form a redundant system. Forced current sharing depends on a certain module. If the module fails, the current cannot be shared, so the module failure exit function must be designed. In forced current sharing, the number of system modules can reach 100. Even if the module voltages differ greatly, no adjustment is required after the parameters are set. The current sharing accuracy is better than 1%, the load response is fast, and there is no oscillation phenomenon, which meets the application needs.

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