Overview of the application of power electronic technology in hybrid electric vehicles

Overview of the application of power electronic technology in hybrid electric vehicles

Abstract: The article summarizes the development and basic structure of hybrid electric vehicles. On this basis, combined with the latest generation of hybrid electric vehicles Prius THS â…¡ from Toyota Motor Corporation, it introduces the specific application of power electronics technology in hybrid electric vehicles. . Finally, combined with the actual situation of hybrid electric vehicles, the problems that need to be solved are put forward.

Keywords: electric vehicle hybrid power electronics AC drive inverter

1 Introduction

Power electronics technology is a discipline that studies the power conversion and control of power semiconductor devices. It is an emerging discipline that emerges from the intersection of electronics, power semiconductor devices and control. Its research content is very extensive, mainly including power semiconductor devices, magnetic materials, power electronic circuits, control integrated circuits and power conversion devices composed of it. At present, the main directions of power electronics research are:

(1) Design, test, model analysis, process and simulation of power semiconductor devices;

(2) Circuit topology, modeling, simulation, control and application of power switching converters;

(3) Electric power inverter technology and its application in industrial fields such as electric drive and electric power system.

Electric vehicles (EVs), as clean, efficient and sustainable vehicles, are of great significance for improving air quality and protecting the environment, as well as providing solutions for increasingly serious oil charter flights; at the same time, electric vehicles A new field of application, covering all the conversions of DC / DC and DC / AC, is a field of application with very high practical value.

2. Introduction to Hybrid Electric Vehicles

At present, the world automobile industry is in the development period of technological revolution and major industrial adjustment. Safety, environmental protection, energy saving and intelligence have become major issues of common concern to the automobile industry. In order to make human society and the automobile industry continue to develop, countries around the world, especially developed countries and some developing countries, are studying various new technologies to improve the coordination between cars and the environment.

As the main direction of the transformation and development of the automotive industry in the 21st century, electric vehicles have now transitioned from the experimental development test stage to the commercial trial production stage. Many well-known automobile manufacturers in the world have launched high-tech safety or environmentally friendly model concept cars. The purpose is to guide the world's automotive technology trends.

2.1 Characteristics and development of various types of electric vehicles

According to the different power sources used, electric vehicles can be roughly divided into three categories: electric storage electric vehicles or pure electric vehicles (Battery Electric Vehicle), hydrogen-based fuel cell electric vehicles (Fuel Cell Electric Vehicle) and hybrid electric vehicles Automotive (Hybrid Electric Vehicle).

Pure electric vehicles are powered solely by batteries, but the performance and price of power batteries have not yet achieved major breakthroughs. Therefore, the development of pure electric vehicles has not reached the intended purpose;

Fuel cell electric vehicles have unparalleled advantages such as high energy conversion rate, non-polluting environment, and service life. However, due to the lack of major breakthroughs in fuel cell technology and research, the development of fuel cell electric vehicles has also been limited.

A hybrid electric vehicle is a new type of vehicle that uses both an electric motor and an engine as its power device. The advanced control system allows the two power devices to coordinate organically to achieve optimal energy distribution, low energy consumption, low pollution, and high automation. Since 1995, major automobile manufacturers in the world have shifted their research focus to the research and development of hybrid electric vehicles. Major automobile companies in Japan, the United States, and Germany have all developed hybrid vehicles including cars, vans, and trucks. electric car.

Take Toyota Motor Corporation, which is the forefront of research and development of hybrid electric vehicles, as an example. The hybrid electric vehicle developed has reached practical level. Since the Prius, the world's first mass-produced hybrid electric vehicle launched in 1997, its Later, in 2002, the hybrid van was launched. The hybrid system of the car uses the electric four-wheel drive and four-wheel drive force / braking force integrated control system that were first mass-produced in the century. In 2003, Toyota launched a new generation of Prius, also known as "the new era Toyota hybrid system-THS â…¡" (see Figure 1), energy saving effect can reach 100km fuel consumption less than 3L. Since 2004, Toyota has introduced a new Lexus RX luxury hybrid sedan to the European market. Toyota plans to use all gasoline-electric hybrid engines in 2012 to improve fuel economy and reduce emissions.

2.2 Classification and characteristics of hybrid electric vehicles

According to different combinations of engine and electric motor, hybrid electric vehicles can be divided into three categories: series, parallel and hybrid, the basic structure shown in Figure 2.

Figure 3 shows the power distribution of the motor and engine in different hybrid types:

In the series hybrid power system, the engine drives the generator, and the electric energy is used to drive the wheels by the electric motor. That is, all the kinetic energy emitted by the engine must first be converted into electrical energy, and this electrical energy is used to drive the vehicle.

The parallel hybrid system uses an engine and an electric motor to drive the wheels. The two power sources are used according to the situation. Because the power sources are parallel, they are called parallel hybrid systems.

The hybrid type is also called series-parallel type, which can maximize the respective advantages of series type and parallel type. Toyota Prius series hybrid power system adopts this working method. When working, the power distributor is used to distribute the power of the engine: on the one hand, it directly drives the wheels, and on the other hand, it controls the power generation autonomously. Since electric motors are to be driven by electric energy, the use ratio of electric motors is increased compared with the parallel type.

3. Power electronic technology and devices commonly used in HEV

This paper combines Toyota's new generation hybrid power system THS â…¡ to specifically study the application of power electronics technology in HEV. The vehicle electrical drive system of THSâ…¡ (see Figure 4) is mainly composed of high-efficiency engine using AtkinSon cycle, permanent magnet AC synchronous motor, generator, power distribution device, high-performance nickel metal hydride (NI-MH) battery, control management unit And the production of DC-DC converters and other related inverter components.

The high-voltage power supply circuit, various inverters, and auxiliary DC-DC converters for 14V batteries form a power control unit (see Figure 5). The unit integrates a DSP controller, drive and protection circuit, DC voltage stabilizing capacitor, semiconductor, Insulator, sensor, liquid cooling circuit and CAN bus interface to communicate with the car.

3.1 Inverter unit for motor / generator

In the Prius THS â…¡ main drive system, the three-phase voltage inverters (50kW and 30kW power) used by the motor and generator are integrated into a module (as shown in Figure 6, the electrical structure of the inverter is shown in Figure 6). 7), the maximum power supply voltage of the DC bus is set to 500V. The power device is a commercial IGBT (850V / 200A) with anti-parallel freewheeling diode. This power class IGBT has the ability to withstand a maximum 500V back pressure, as well as other capabilities such as avalanche breakdown and transient short circuit.

Each bridge arm of the inverter for motor is composed of two IGBT modules and diode modules connected in parallel. The area of ​​each IGBT chip is 133mm2 (13.7mm × 9.7mm), and the emitter uses a 5μm thick aluminum film; while the area of ​​each diode chip is 90mm2 (8.2mm × 11mm).

At present, electric vehicles generally use PWM-controlled voltage inverters, which have the characteristics of simple circuits and high efficiency. At the same time, PWM inverters show the following development trends:

(1) The IGBT device is usually used, the working frequency is high, and the low-frequency harmonic component and the current impact of starting are reduced. The highest switching frequency currently applied in foreign countries has reached 20kHz;

(2) The rated frequency of the motor has been increased accordingly, and the speed adjustment range has been expanded. While better meeting the operating requirements, the size and weight of the motor have been reduced, and the power ratio has been improved. At present, the highest rated frequency of special motors for electric vehicles abroad has reached 500Hz;

(3) The computer control system with DSP as the core can achieve reliable vector control and calculation. The motor can achieve fast constant torque starting and weak magnetic high-speed operation. This control system is stable, the current impact is small, and the control efficiency is high.

In addition to the above traditional PWM control technology, recently, a resonant DC link converter and a high-frequency resonant AC link converter have appeared. Resonant converters with zero-voltage or zero-current switching technology have the advantages of low switching loss, small electromagnetic interference, low noise, high power density and high reliability, which have aroused wide interest from researchers.

At present, the commonly used electronic switching devices used in power converters are mainly GTO, BJT, MOSFET, IGBT and MCT. Due to the combination of the characteristics of IGBT and BJT, the high impedance voltage controlled gate can significantly reduce the gate drive Power, so that the gate drive circuit can be integrated; and the extremely short switching time of the IGBT can make the system have a quick response capability, and reduce the switching loss and reduce the noise, so the IGBT is a good switching device. MCT is also a potential device of choice. Although the current MCT ratings still need to be improved; however, because MCT has a low pressure drop, with the improvement of MCT's new manufacturing process and the use of new materials, the future MCT is in There will be good application prospects in electric vehicles.

3.2DC-DC boost converter unit

In THS, the battery is directly connected to the motor and generator through the inverter (see Figure 8); while in THSâ…¡, the voltage output by the battery pack is first boosted by the DC-DC boost converter, and then connected to the inverter The inverter is connected, so the DC bus voltage of the inverter is increased from 220V of the original THS to the current 500V.

Figure 9 is a schematic diagram of energy exchange in the THSâ…¡ system. The power of the generator in Figure 9 is 30kW, and the instantaneous power of the battery pack is 20kW. The two together provide energy for a 50kW motor; The design is 20kW.

This system has the following advantages:

(1) Since the maximum output power capability of the motor is proportional to the DC bus voltage, compared with the 202V power supply condition of the original THS system, the motor in the THSâ…¡ system is powered by 500V without increasing the drive current. The maximum output power and torque output capacity is 2.5 times that of the original THS system; in addition, the same size motor can also avoid higher power output;

(2) Since the DC bus supply voltage variable system is used, THSâ…¡ can freely adjust the DC bus supply voltage according to the actual needs of the motor and generator, so as to select the optimal power supply voltage and reduce the switching losses of the inverter and Energy saving purpose of copper loss of motor;

(3) For a battery pack with a certain supply voltage, the output voltage of the step-up transformer can be adjusted to meet the actual needs of the motor and generator, so to a certain extent, the number of batteries used can be reduced To reduce the quality of the vehicle.

Each branch of the DC-DC boost converter shown in Fig. 9 has two IGBT modules and freewheeling diode modules connected in parallel, where the area of ​​each IGBT chip is 255mm2 (15mm × 15mm), and each freewheeling diode chip The area is 117mm2 (13mm × 9mm). The circuit topology shown in Fig. 9 can ensure the instantaneous conversion of the charging and discharging of the battery without interrupting the normal operation of the system. Due to the role of the DC-DC boost converter, the system voltage on the main capacitor is different from the output voltage of the battery pack, thereby ensuring that the motor and generator operate at high voltage without being affected by the low voltage of the battery pack Limitation of output capacity.

3.3DC-DC step-down converter unit

Generally, all kinds of electrical equipment in automobiles are powered by 14V battery packs (rated voltage is 12V). Prius also chooses 14V battery packs as the power supply for vehicle electrical equipment such as control computers, lights, brakes, etc., and charges the battery The work is completed by DC 220V through a DC-DC step-down converter. The circuit diagram of the converter is shown in Figure 10. The capacity of the converter is 1.4kW (100A / 14V), and the power device uses a voltage-controlled commercial MOSFET (500V / 20A), and the area of ​​each MOSFET chip is 49mm2 (7mm × 7mm).

3.4 Inverter units for other AC equipment

The Prius THSⅡ air-conditioning system uses a motor-driven air compressor to replace the traditional air compressor driven by the engine. In order to drive the motor for air compressor, a small power inverter (DC202V, 1.6kW) is designed. The power device is a commercial IGBT with anti-parallel freewheeling diode (600V / 30A), where the area of ​​each IGBT chip is 22.1mm2 (4.7mm × 4.7mm), and the area of ​​each freewheeling diode chip is 9mm2 (3mm × 3mm).

4HEV requirements for power electronics technology

Restricted by the actual operating conditions, it is required that the power electronic technology and devices for hybrid electric vehicles should have the characteristics of low cost, small size, large specific power, and easy installation. In addition, the following technical details need to be considered:

(1) Sealing problem of power electronic devices

All kinds of power electronic devices for vehicles must be effectively sealed to withstand the effects of temperature and vibration, and can prevent the intrusion of various automotive liquids.

(2) Electromagnetic compatibility / electromagnetic interference (EMC / EMI)

Hybrid electric vehicles are a relatively small space, which contains various control chips and weak circuits, so when designing on-board power electronic devices, in order to eliminate the hidden dangers of future accidents, it is necessary to study and solve EMC / EMI problem.

(3) Utilization of DC bus voltage

The voltage of the energy storage system of a hybrid electric vehicle is variable. The size of the voltage depends on the actual load of the vehicle, the operating conditions (electric or power generation), and whether the motor is operated in a weak magnetic field. The typical bus voltage fluctuation range is -30% ~ + 25% of the nominal value. Therefore, how to make full use of the DC bus voltage in the case of frequent changes in automobile operating conditions has become a problem that the control strategy designer needs to solve.

(4) Control problems of power electronic devices

"High switching frequency" and "high sampling rate" are currently commonly used in power electronic devices and AC drive systems of hybrid electric vehicles. Objectively, "double high" requires high-precision encoders and solvers, so this means When a wide temperature gradient and saturation state appear in the motor, how to reduce the parameter sensitivity to meet the control requirements.

5 Conclusion

This article combines Toyota Motor Corporation's latest generation hybrid electric vehicle Prius THSâ…¡, summarizes the application of power electronics technology in hybrid electric vehicles, and proposes technical problems that need to be considered and solved.

With the development of power electronic technology, microelectronic technology and control technology, digital AC drive systems are widely used in commercial electric vehicles; and the development of hybrid electric vehicles using AC motor drive systems has been developed by the automotive industry in a sustainable manner. One of the important ways. With the increase in human requirements for the living environment, the awareness of rational use of energy has increased. As a modern vehicle with low pollution and high efficiency, hybrid electric vehicles will have a comprehensive development and application.

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