Design of high-power electric vehicle charger

Pure electric vehicles use lithium batteries as the power source. When they are fully charged, they use electric power to promote their cars. Unlike gasoline engine cars, which need to add gasoline, pure electric vehicles charge them through an external power source after the power is consumed, usually with a single mileage of 100 to 200 kilometers. Compared with traditional cars, pure electric vehicles have an unparalleled advantage in terms of cost of use. They consume about 15 kWh per 100 kilometers and cost 8 yuan, which is only 1/10 of the cost of a gasoline engine car. At present, the country has begun to carry out demonstration and promotion of electric vehicles and new energy vehicles. Electric vehicle charging stations are one of the main links, and must be coordinated with other areas of electric vehicles.

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Charging mode The electric vehicle energy supply system is mainly composed of a power supply system, a charging system and a power storage battery. In addition, it also includes charging monitoring, battery management and smoke alarm monitoring. The charger is an important part of the charging system. Charging stations charge cars in three ways: normal charging, fast charging, and battery replacement. Ordinary charging is mostly AC charging. For AC chargers with a capacity not exceeding 5 kW, the input is single-phase AC with rated voltage of 220V and 50Hz. For AC chargers with capacity greater than 5kW, the input is three-phase AC with rated line voltage of 380V and 50Hz. . Plug the AC plug directly into the charging interface of the electric car, and the charging time takes about 4-8 hours. The fast charging is mostly DC charging. The DC charger input is three-phase AC with 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 rated current, the power factor should be greater than 0.85 and the efficiency should be no less than 90%.

The charger should be able to ensure that the power battery cell voltage, temperature and current do not exceed the allowable value during charging. The charger should have anti-output short circuit and anti-reverse function. The charger can charge at least one of the following three types of power batteries: lithium ion batteries, lead acid batteries, and nickel metal hydride batteries.

The power battery pack charging mode adopts a "constant current-constant voltage" two-stage charging mode. At the beginning of charging, constant current charging is generally performed using an optimum charging ratio (0.3 C for a lithium ion battery). (C is the capacity of the battery, such as C=800mAh, the charging rate of 1C is 800mA.) At this stage, because the electromotive force of the battery is low, even if the charging voltage of the battery is not high, the charging current of the battery will be large, and it must be Limit the charging current. Therefore, the charging at this stage is called "constant current" charging, and the charging current is kept at the current limiting value. With the continuation of charging, the battery electromotive force continues to rise, and the charging voltage continues to rise. Constant voltage charging is maintained when the battery voltage rises to the highest allowable charging voltage. At this stage, as the battery electromotive force continues to rise and the charging voltage remains unchanged, the charging flow of the battery is in a hyperbolic trend and continues to drop to zero. However, during the actual charging process, when the charging current is reduced to 0.015C, it means that the charging can be stopped when the charging is full. The charging at this stage is called “constant voltage” charging, and the charging voltage at this stage: U=E+IR is a constant voltage value. This is the basic requirement for the charging mode of the lithium-ion battery pack. In addition, the charging system must also have automatic adjustment of charging parameters, automatic control and automatic protection. Especially in the constant voltage charging phase, if the charging voltage of the single cell 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 passing. Pressure 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 cells connected in series in the battery pack)

According to the charging characteristics of the battery and the charging requirements of the electric vehicle battery pack, the commonly used charging device is a charger, which can be classified into a DC charger and a pulse charger. The DC charger is to isolate the regulated output DC power supply after the grid power supply is rectified and filtered, and supplies the power battery pack for charging. The most widely used DC charger is the high frequency switching power supply charger. The utility model has the advantages of small volume, light weight, reliable operation, high efficiency, high power factor, strong adaptability of the power grid, small power and large size, and easy realization of intelligence. The pulse charger can reduce the polarization phenomenon caused by the battery during charging, thereby improving the charging efficiency of the battery, reducing the charging time, and achieving fast charging, but the pulse charger technology needs further study.

Electric vehicles have long charging time, and charging is difficult as a problem in the promotion and application of electric vehicles. Take a large lithium-powered electric bus as an example, with a battery capacity of 700Ah. The maximum charging current is 210A (equivalent to a charging rate of 0.3C for 700AH battery capacity), and the maximum charging voltage is 700V (equivalent to 165 series voltages of lithium battery cells with a maximum charging voltage of 4.2V). The maximum output power of the charger is required. 245kW. It takes at least 3 hours to charge the electric car according to the optimal charging requirements. Therefore, the charging method of an electric vehicle cannot be charged like a fuel car refueling at a gas station. If the battery is fully charged in 20 minutes, it must be charged at least with a charging rate of 3C, which is possible for a lithium iron phosphate lithium ion battery.

In summary, the charging of electric vehicles is still based on ordinary charging, supplemented by fast charging. For electric buses, the charging station is located in the bus terminal. Use the trough to charge after work in the evening, 5 to 6 hours. For vehicles that run all day, when the driving range is insufficient, the intermediate rest time can be used for supplementary charging. The number and capacity of the chargers are based on the size of the fleet and the charging stations are managed by the fleet. For example, 12 large lithium-powered electric buses require 12 chargers. For fast charging, 6 chargers can be used for parallel charging, the maximum output power is 1470kW, and the maximum charging current is 2100A (equivalent to the 3C charging rate of 700AH battery). Or use 8 chargers to charge 8 electric vehicles in a normal way. Each output has a maximum charging voltage of 700V and a maximum charging current of 500A (equivalent to a charging rate of 0.7C for 700AH battery). The fast charging mode of 1C~3C has been discussed and applied, but it should be ensured under the premise of battery safety and service life. According to the maximum power configuration of the above charger, the effective total power of the power transformer is about 3000 kW or more.

At present, major auto manufacturers have developed oil-electric hybrid vehicles and pure electric vehicles. Take BYD E6 pure electric vehicle as an example, the battery type is lithium iron cobalt lithium battery, the battery capacity is 200Ah, the charging current of 3C is 600A, the nominal voltage is 316.8V (equivalent to 96 lithium iron phosphate cobalt with a charging voltage of 3.3V) Battery cell series voltage). The charger's output power is 192kW. The fast charging time is 80% full for 15 minutes. The energy consumption per 100 kilometers is about 21.5 kWh, which is equivalent to the consumer price of 1/3 to 1/4 of the fuel vehicle.

System structure The input of high-power electric vehicle charger is three-phase AC with rated line voltage of 380V and 50Hz. The rated voltage is 700V and the rated current is 600A. The system adopts 19" standard frame, which is compact in structure, reasonable in layout and beautiful in appearance. Dimensions: height × width × depth is 2200mm × 600mm × 600mm. 60 modules are connected in parallel, each module is 10A/700V, module size : Height × width × depth 133mm × 425mm × 270mm, 15 layers and 4 columns, divided into four cabinets, four cabinets can be transported separately, compactly arranged in the left and right. The front and rear doors of the rack are double-opened, convenient Maintenance. The power input line and bus output position are all input at the bottom. The power input circuit breaker and the monitoring unit touch screen are installed in the front part of the middle control cabinet of the main unit. The control structure diagram of the charger is shown in Figure 2.

Figure 2 Schematic diagram of the control structure of the charger

Switching power supply main circuit design 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 type uncontrollable rectifier circuit filters and rectifies the three-phase AC input, and the power factor correction pre-regulation After 800V, the high-frequency DC/DC half-bridge power converter, the filtered output DC 700V is used to charge the power battery. After analysis and calculation, the transformer adopts double E65 magnetic core and the primary coil is 12匝. According to the output voltage of up to 700V, the input voltage is minimum 780V, and the maximum duty ratio is 0.95, the number of secondary winding turns N2, N2=(12/780) can be obtained. ×(700/0.95)=11.33, taking N2 as 12匝 considering factors such as leakage inductance and secondary rectification voltage drop.

Figure 3 Block diagram of the charger power supply

Since the electric vehicle charger is a non-linear load, harmonics are generated, which is a pollution to the power grid. Effective measures must be taken, such as power factor correction or reactive power compensation, to limit the total amount of harmonics that the electric vehicle charger enters the grid. In order to improve the power factor and reduce the input network harmonics, an active power factor correction circuit is used, as shown in FIG. It adopts a three-phase three-switch three-level BOOST circuit, which operates in continuous mode. The switch uses a two-way switch composed of two MOSFETs. In the figure, switches S1, S2, and S3 are bidirectional switches. Due to the symmetry of the circuit, the capacitance midpoint potential VM is approximately the same as the potential at the midpoint of the grid, so that the currents on the corresponding phases can be separately controlled by the bidirectional switches S1, S2, S3. When the switch is closed, the amplitude of the current on the corresponding phase increases. When the switch is turned off, the diode on the corresponding bridge arm is turned on (the current is positive, the upper arm diode is turned on; when the current is negative, the lower arm diode is turned on). The current on the Boost inductor is reduced by the output voltage to achieve current control. The control circuit uses three control chips UC3854A. The phase voltage provides synchronous signals and pre-correction signals to the UC3854A through a three-phase isolation transformer. The current feedback uses a Hall current transformer to control three switches to form three current feedback inner loops. A multi-closed loop system with a voltage feedback outer loop. The advantage of this circuit is that it is simple in structure and requires only one power switch per phase. The three-level characteristic has a small harmonic current, and the switching tube voltage and current stress is small. No need for a neutral line, no third harmonics, high power factor at full load. The switching stress is small, the shutdown voltage is reduced, the switching loss is low, and the common mode EMI is low.

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

The DC/DC power converter adopts a half-bridge circuit topology with few power components, simple control and high reliability. As shown in Figure 5, the MOSFET and IGBT parallel technology makes full use of the advantages of fast MOSFET switching speed and reduced IGBT conduction voltage. Taking measures on the circuit, the turn-off time of the MOSFET is delayed by a certain time than 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 mode adopted on the output side is half-wave rectification, center-tap full-wave rectification and full-bridge rectification. Due to the high output voltage, full-bridge rectification has a high utilization rate of the transformer and is suitable for this application.

Figure 5 MOSFET / IGBT parallel combination switch circuit

Figure 6 PWM forced current sharing method block diagram

The system uses 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. The working principle is to compare the system bus voltage Us with the system reference voltage Ur to generate the error voltage Ue, and use the error voltage to control the PWM modulator. The PWM signal controls the current of each module. The current requirement signal of each module is the same. The PWM signal is compared with the output current of the module through the optocoupler to adjust the module reference voltage, thereby changing the output voltage and adjusting the output current to achieve current sharing. Thus, each module is equivalent to a voltage controlled current source. The current sharing mode has high precision, good dynamic response, and many control modules, and can easily form a redundant system. Forced current sharing depends on a certain module. If the module fails, it cannot be evenly flowed, so the module fault exit function must be designed. In forced current sharing, the number of system modules can reach 100. Even if the module voltage difference is large, no adjustment is needed after the parameters are set. The current sharing accuracy is better than 1%, the load response is fast, and there is no oscillation phenomenon to meet the application requirements.