Abstract : With increasing environmental awareness, "green" electric vehicles have received widespread attention. This paper introduces the features, comprehensive protection functions, and application of the fifth-generation L-type intelligent power module manufactured by Mitsubishi Electric Corporation of Japan in the design of electric vehicle drive systems. Actual testing shows that this device fully meets the requirements for use in electric vehicle drive systems and is worthy of widespread adoption.
Keywords : Mitsubishi; intelligent power module; electric vehicle; drive system; design
1 Introduction
With the increasing scarcity of oil resources and environmental degradation, government departments have successively introduced various policies to encourage enterprises to develop new energy industries. Electric vehicles, due to their advantages such as zero emissions, high efficiency, high cost performance, and safety and reliability, have become ideal "green" motor vehicles and have received widespread attention, which has promoted the rapid development of the electric vehicle industry. The core components of an electric vehicle mainly consist of three parts: the drive system, the motor, and the battery. Among them, the power switching devices in the drive system are the key to improving efficiency, reducing power consumption, extending motor life, and improving system reliability [1].
With the rapid development of computer technology, power electronics technology, semiconductor technology and automatic control theory, the core power conversion devices of the drive system are also constantly being upgraded. At present, the inverter part of the drive system still adopts the sinusoidal pulse width modulation (SPWM) control theory. Traditional bipolar transistors are not suitable for high-frequency SPWM control due to their low switching frequency. Moreover, since the control method adopts pulse amplitude modulation (PAM), the output of the inverter contains a large number of harmonic components, and the motor is easily damaged. The insulated gate field-effect transistor (IGBT) has the advantages of high current density, low saturation voltage and high voltage resistance of high power transistor (GTR), as well as the advantages of high input impedance, high switching frequency and low drive power of field-effect transistor (MOSFET). In addition, it has high input impedance and small dead time [2]. It has replaced MOSFET and GTR and become the most important power conversion device. It is widely used in inverters, solar energy, battery (EPS) systems and AC servo drives. This article introduces the application of the fifth-generation L-type IPM module produced by Mitsubishi Corporation of Japan in electric vehicles. This module integrates IGBT chips and various drive protection circuits, which simplifies the design and debugging of the drive system, ensures product reliability, and meets the needs of mass production.
2. Introduction to IPM Smart Modules
Intelligent Power Module (IPM) is an abbreviation for Intelligent Power Module. The appearance of Mitsubishi's fifth-generation L-shaped IPM module is shown in Figure 1.
Figure 1. Outline drawing of Mitsubishi L-type IPM
The IPM module uses a trench gate bipolar transistor (CSTBT) chip with full gate carrier storage, an advanced power switching device that achieves extremely low module voltage drop, low loss, high efficiency, and excellent power saving. It is ideally suited for various power conversion devices with switching frequencies below 20kHz. The IPM module integrates a current sensor for monitoring power.
The output current value of the device is used to achieve perfect overcurrent and load short-circuit protection. In order to further improve the performance and enhance the stability of the module, it also integrates temperature detection and undervoltage protection circuits to give it sufficient self-protection capability. Even if there is a fault such as insufficient control voltage or overheating, the IPM will issue a blocking signal to protect the IPM from damage. The internal circuit principle block diagram of the IPM is shown in Figure 2 [2].
Figure 2. Block diagram of the internal circuit of IPM
3. IPM's comprehensive protection features
The IPM integrates logic, control, detection and protection circuits to prevent malfunctions caused by external interference and damage to IGBT chips caused by overcurrent or short circuit. In order to detect the output current of IGBT in real time, many current sensors are placed inside the IPM module. The output signals of these sensors are directly fed back to the internal processing circuit for various fault judgments. The IPM integrates various fault protection circuits, including overcurrent protection, short circuit protection and undervoltage protection. When the IPM is in an abnormal state, as long as one of the above protection functions sends a signal, the output of IGBT will be immediately shut off. At the same time, the IPM sends a fault signal Fo so that the host computer can make timely processing. These functions integrated inside the IPM are very convenient to use and greatly reduce the system development time. At the same time, they improve the self-protection capability under fault and greatly enhance the reliability of the system [3].
3.1 Overcurrent Protection
When the IPM detects that the IGBT's output current exceeds the overcurrent threshold and the duration exceeds the overcurrent delay time, the IPM will block the IGBT's output and output an alarm signal. For the latest generation of IPM modules, the overcurrent delay time (toff) is 10μs. When the current sensor detects an instantaneous pulse current with a duration of less than 10μs, even if the amplitude exceeds the overcurrent threshold, the control circuit will not issue an alarm signal. To more efficiently protect the IGBT, when an overcurrent fault occurs, the system slowly shuts down the IGBT. This avoids the surge voltage of large current generated during shutdown, thereby improving the reliability of IGBT operation. This current detection and processing technology can detect overcurrent faults in various loads, including short circuits to ground in motors.
3.2 Short Circuit Protection
If a short circuit occurs in the load or a control system malfunction causes a direct connection between the upper and lower arms, the IPM module integrates a state-of-the-art current sensor and short-circuit protection circuit. This circuit has an extremely fast response time, less than 100ns, and can monitor the IGBT output current in real time. Upon the occurrence of the aforementioned short circuit, the IPM immediately shuts off the IGBT output and sends a fault code to the host computer for processing. In Mitsubishi's latest fifth-generation IPM, various technologies are employed to prevent damage to the IGBTs due to short circuits. The most basic and crucial technology is that the IGBT must be turned off within 10μs in the event of a short circuit. Therefore, the IGBT's drive and control circuits (including the snubber circuit) are specially designed to meet the requirements of short-circuit protection. In particular, the technology of turning off the IGBT by detecting the voltage VGE between the gate and emitter ensures timely short-circuit protection while reducing the electrical and thermal stress on the IGBT. A brief introduction to this technology follows.
1) Reduce short-circuit current
By gradually reducing the gate voltage using technology, as the IGBT turns off, the short-circuit current decreases as the channel resistance gradually increases, and di/dt also decreases until the IGBT is completely turned off. This technology reduces the peak voltage during turn-off.
2) Gate and emitter voltage (VGE)
The magnitude of the VGE voltage depends on the short-circuit current when a short circuit occurs, and this value is entirely dependent on the dv/dt of the gate and collector. To reduce the impact of dv/dt on the IGBT drive signal, a common method is to stabilize the voltage value by connecting a Zener diode in parallel, as shown in Figure 3.
Figure 3. Schematic diagram of VGE clamping circuit
To prevent interference from causing malfunctions in the IGBT drive signal, the clamping diode must be an ultra-fast recovery diode, and it must be directly connected to the signal terminals of the IGBT module during circuit design. In some low-power IPM modules, to ensure stable and reliable IGBT drive voltage, dual 16-18V Zener diodes are added near the gate and emitter of the IGBT to clamp the voltage spikes coupled to the gate caused by dv/dt.
3) Shorten short-circuit time
Since a large amount of heat is generated during a short circuit, the duration of the short circuit needs to be reduced in order to reduce the thermal stress on the module. However, shortening the time will lead to an increase in di/dt. Therefore, the two techniques mentioned above must be used to avoid the increase in thermal stress on the IGBT caused by shortening the short circuit time.
3.3 Undervoltage alarm
The normal operating voltage of the IPM internal control circuit is 15V. However, if this voltage is lower than the undervoltage shutdown threshold of 12.5V and the duration exceeds toff=10ms, the IPM module will block the output and issue a fault signal. When the power supply voltage is higher than the undervoltage reset value, the system resumes normal operation. Because interference can cause voltage spikes in the power supply, but if the duration is less than 10ms, the undervoltage detection circuit will ignore the spikes, and the control circuit will still operate normally.
3.4 Overheat protection
To prevent the IGBT temperature from exceeding the overheating setpoint, a temperature sensor is installed near the IGBT. When the temperature sensor detects that the IGBT temperature exceeds the setpoint, the control circuit integrated within the IPM sends a cutoff signal to block the gate drive and simultaneously issues a fault signal, indicating an overheating alarm. This continues until the temperature returns to normal. When the temperature drops below the reset temperature and the control level is high, the power device will return to normal operation upon the next low-level signal input.
4. Application of Mitsubishi IPM in Electric Vehicle Drive System Design
Electric drive and control system is the core of electric vehicle, and the efficiency, current output capability and reliability index of drive system are the most important factors [4]. Mitsubishi's latest generation of intelligent power modules has the following characteristics: small size, high conversion efficiency, high carrier frequency, perfect protection function, high reliability, simple control, and convenient connection with host computer. It meets the design input requirements of the control system in electric vehicle. Therefore, Mitsubishi's latest generation IPM module PM50CL1A060 is selected for the development of electric vehicle control system. When using this intelligent power module for system design, the following factors need to be considered from the perspective of performance and reliability index analysis.
1) To prevent conducted interference, a high-speed optocoupler A3120 should be used for isolation between the host computer and the IPM module.
2) In order to ensure that the drive system has sufficient overload capacity to meet the vehicle's starting and overtaking requirements, the current value of the IPM should be determined according to the multiple of the motor's rated current value [4].
3) Reasonable thermal design is required to ensure that the operating temperature of the IGBT is always less than the maximum junction temperature (150℃).
5 Conclusion
Using a professional controller test platform, various performance tests were conducted on the electric vehicle drive system. The test results showed that the system fully met the design requirements. Because the latest generation of IPM modules is used, various protection functions are more complete and have high reliability indicators. At the same time, the new generation of IGBT chips used in IPM reduce the overall power consumption of the system and improve system efficiency. In addition, the IPM module is easy to install and has strong maintainability. The above features make it suitable for a variety of applications, including electric vehicles. This solution has good promotion significance [3].
References:
[1] Du Yuren, Zhang Zhenghua. Design of intelligent control system for variable frequency electric light vehicle [J]. Modern Electronics Technology, 2000(11): 39-41.
[2] Qiu Zhijian, He Ligao, Deng Zhiquan. Design of an integrated low-power inverter based on fourth-generation IPM [J]. Machine Tool Electrical Appliances. 2003(10):32-34.
[3] Li Congfei, Chen Fan, Liu Dichun. Research on several problems in the use of IPM [J]. Electrical Application, 2005(10): 24-26.
[4] Dou Guozhen. Research and design of AC drive system for energy-saving electric vehicles [J]. Electrical Machines and Appliances, 2003(8):42-46.
Author : Ding Yunfei, born in 1978, is a senior engineer with a Bachelor of Engineering degree. He has been engaged in the research and development project management and quality management of high-end CNC machine tools, all-digital bus CNC systems and servo drives.
