Design of high-power electric vehicle charger

Pure electric vehicles use lithium batteries as their power source. After being fully charged, they use electricity to propel the vehicle. Unlike gasoline-powered vehicles that need to be recharged with gasoline, pure electric vehicles are recharged through an external power source after the power is exhausted. Usually, the single driving range is 100 to 200 kilometers. Compared with traditional vehicles, pure electric vehicles have an incomparable advantage in terms of use cost. They consume about 15 kWh of electricity per 100 kilometers and cost 8 yuan, which is only 1/10 of the cost of gasoline-powered vehicles. 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 developed in coordination with other areas of electric vehicles.

Charging Mode

The energy supply system of electric vehicles is mainly composed of power supply system, charging system and 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 for charging cars at charging stations: ordinary charging, fast charging and battery replacement. Ordinary charging is mostly AC charging. For AC chargers with a capacity not exceeding 5kW, the input is a single-phase AC with a rated voltage of 220V and 50Hz. For AC chargers with a capacity greater than 5kW, the input is a three-phase AC with a rated line voltage of 380V and 50Hz. Plug the AC plug directly into the charging port of the electric vehicle, and the charging time takes about 4 to 8 hours. Fast charging is mostly DC charging. The input of the 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. When the output voltage of the AC input isolated AC/DC charger is 50% to 100% of the rated voltage and the output current is the 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 voltage, temperature and current of the power battery cells do not exceed the allowable values ​​during the charging process. The charger should have the function of preventing output short circuit and reverse connection. The charger can charge at least one of the following three types of power batteries: lithium-ion battery, lead-acid battery, and nickel-metal hydride battery.

The power battery pack charging mode adopts a two-stage charging mode of "constant current-constant voltage". 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 battery charging voltage is not high, the charging current of the battery will be very large, and the charging current must be limited. Therefore, the charging at this stage is called "constant current" charging, and the charging current is maintained at the current limit value. As the charging continues, the battery electromotive force continues to rise, and the charging voltage continues to rise. When the battery voltage rises to the maximum allowable charging voltage, constant voltage charging is maintained. At this stage, since the battery electromotive force is still rising, and the charging voltage remains unchanged, the battery charging current continues to decline in a hyperbolic trend until it drops to zero. However, in the actual charging process, when the charging current decreases to 0.015C, it means that the charge is full and the charging can be stopped. This stage of charging is called "constant voltage" charging. The charging voltage at this stage is: U=E+IR, which is the constant voltage value. This is the basic requirement for the charging mode of lithium-ion power battery packs. 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 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, thereby preventing the battery from being over-charged. 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 the charger, which can be divided into two categories: DC charger and pulse charger. The DC charger is to isolate and stabilize the output DC power supply after rectification and filtering of the grid power supply, and supply it to the power battery pack for charging. At present, the most commonly used DC charger is the high-frequency switching power supply charger. It has the advantages of small size, light weight, reliable operation, high efficiency, high power factor, strong grid adaptability, small or large power, and easy to realize intelligentization. The pulse charger can reduce the polarization phenomenon generated by 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 long charging time and difficulty of electric vehicles are 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 0.3C charging rate of a 700AH battery capacity), and the maximum charging voltage is 700V (equivalent to the series voltage of 165 lithium battery cells with a maximum charging voltage of about 4.2V), so the maximum output power of the charger is 245kW. It takes at least 3 hours to charge an electric vehicle according to the optimal charging requirements. Therefore, electric vehicles cannot be charged like fuel vehicles at gas stations. If you want to quickly charge it in 20 minutes, you must use at least a 3C charging rate, which is possible for lithium iron phosphate lithium-ion batteries.

In summary, 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 set up in the bus terminal. After get off work in the evening, the low-peak charging time is used for 5 to 6 hours. For vehicles running all day, when the driving range is insufficient, they can be supplemented by rest time. The number and capacity of chargers depend on the size of the fleet, and the charging station is managed by the fleet. For example, 12 large lithium-powered electric buses require 12 chargers. When charging quickly, 6 chargers can be charged 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 a maximum output voltage of 700V and a maximum charging current of 500A (equivalent to a charging rate of 0.7C for a 700AH battery). The 1C to 3C fast charging mode has been explored for application, but it should be carried out under the premise of battery safety and service life. According to the maximum power configuration of the above charger, the total effective power of the power transformer is approximately over 3000kW.

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At present, major automobile manufacturers have developed and produced hybrid electric vehicles and pure electric vehicles. Taking BYD E6 pure electric vehicle as an example, the battery type is lithium iron cobalt phosphate battery, with a battery capacity of 200Ah, a 3C charging current of 600A, and a nominal voltage of 316.8V (equivalent to the voltage of 96 lithium iron cobalt phosphate battery cells with a charging voltage of about 3.3V in series). 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 fuel vehicles.

System Structure

The input of the high-power electric vehicle charger is a three-phase AC with a rated line voltage of 380V and 50Hz, and the output rated voltage is 700V and the rated current is 600A. The system adopts a 19-inch standard rack with a compact structure, reasonable layout and beautiful appearance. The overall dimensions are: height × width × depth is 2200mm × 600mm × 600mm. 60 modules are connected in parallel, each module is 10A/700V, and the module dimensions are: height × width × depth is 133mm × 425mm × 270mm, 15 layers and 4 rows, divided into four cabinets for placement. The four cabinets can be transported separately and arranged compactly on the left and right when in use. The front and rear doors of the rack are double-doored for easy maintenance. The power supply line and bus output positions are both input at the bottom. The power input circuit breaker and the monitoring unit touch screen are installed in the front of the host intermediate control cabinet. The schematic diagram of the charger control structure is shown in Figure 2.

Figure 2 Schematic diagram of charger control structure

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 AC input is filtered and rectified by a three-phase bridge uncontrolled rectifier circuit. After the power factor correction pre-stabilization of 800V, it is filtered and output DC 700V for charging the power battery through a high-frequency DC/DC half-bridge power converter. After analysis and calculation, the transformer uses a double E65 core and the primary coil has 12 turns. According to the maximum output voltage of 700V, the minimum input voltage of 780V, and the maximum duty cycle of 0.95, the number of secondary winding turns N2 can be obtained, N2=(12/780)×(700/0.95)=11.33. Considering factors such as leakage inductance and secondary rectifier voltage drop, N2 is taken as 12 turns.

Figure 3 Schematic diagram of charger power supply

Since the electric vehicle charger is a nonlinear load, it will generate harmonics, which is a pollution to the power grid. Effective measures must be taken, such as power factor correction or reactive power compensation technology, to limit the total harmonics of the electric vehicle charger entering the power grid. In order to improve the power factor and reduce the input power grid harmonics, an active power factor correction circuit is used, as shown in Figure 4. It uses a three-phase three-switch three-level BOOST circuit, works in continuous mode, and the switch uses 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 capacitor midpoint is approximately the same as the potential of the midpoint of the power grid, so the current on the corresponding phase can be controlled by the bidirectional switches S1, S2, and S3. When the switch is closed, the current amplitude on the corresponding phase increases, and when the switch is opened, 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, thereby achieving current control. The control circuit uses three control chips UC3854A. The phase voltage provides synchronization signals and pre-correction signals to UC3854A through a three-phase isolation transformer. The current feedback uses a Hall current transformer to control three switches respectively, forming a multi-closed loop system with three current feedback inner loops and one voltage feedback outer loop. The advantage of this circuit is that it has a simple structure and only requires one power switch per phase. It has a three-level characteristic with small harmonic current and small voltage and current stress on the switch tube. No neutral line is required, no third harmonic, and the power factor is very high at full load. The switch stress is small, the turn-off voltage is low, the switch loss is low, and the common-mode EMI is low.

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

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The DC/DC power converter adopts a half-bridge circuit topology, with fewer power devices, simple control and high reliability. As shown in Figure 5, the MOSFET and IGBT parallel connection technology is used to fully utilize the advantages of MOSFET's fast switching speed and IGBT's low on-state voltage drop. Measures are taken in the circuit to delay the MOSFET's turn-off time by a certain time compared to the IGBT, which greatly reduces the current tail of the IGBT, reduces the switch on-state loss, improves efficiency and reliability, and enables the output power of the half-bridge circuit to achieve 7kW. 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, full-bridge rectification has a high transformer utilization rate and is more suitable for this occasion.

Figure 5 MOSFET/IGBT parallel combination switch circuit

Figure 6 PWM forced current sharing method working block diagram

The system adopts PWM forced current sharing method, and the working block diagram is shown in Figure 6. This is an improved method that combines 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, and the obtained PWM signal is used to control 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 optical coupler to adjust the module reference voltage, thereby changing the output voltage, adjusting the output current, and achieving current sharing. In this way, each module is equivalent to a voltage-controlled current source. This current sharing method has high accuracy, good dynamic response, and can control many modules, which 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 fault 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.