Abstract : This paper mainly introduces the design of a 2.2kW general-purpose frequency converter using the PM25RL1A120 IPM module. The design focuses on the main circuit, IGBT drive and fault handling circuits, and switching power supply. The frequency converter designed using this scheme has advantages such as compact structure, small size, low harmonic content, and high reliability, meeting the control needs of three-phase asynchronous motors in the market.
Keywords : PM25RL1A120; frequency converter; main circuit; switching power supply
1. Preface
With the rapid development of science and technology such as power electronics and computers, frequency converters have received increasing attention and are being used in industries such as power, metallurgy, textiles, steel, hoisting, and printing. They have become one of the key technologies for energy conservation, improving production efficiency, enhancing product quality, optimizing processes, rationalizing equipment, reducing equipment maintenance workload, and improving people's living environment. Frequency converter technology is one of the most common and needed new technologies closely related to people's daily lives, and therefore, it is also one of the fastest-growing areas of technological innovation.
One of the core technologies of frequency converters is power electronics, most notably the switching devices in the inverter section. Currently, the most advanced technologies are still controlled by a few semiconductor device manufacturers in Europe, America, and Japan. Since the introduction of Insulated Gate Transistors (IGBTs), they have quickly become the dominant devices in small and medium power electronic equipment, and their voltage, capacity, and switching frequency performance continue to improve. Intelligent Power Modules (IPMs) are advanced power switching devices that integrate multiple IGBTs. They possess the advantages of high-power transistors (GTRs): low saturation voltage, high current density, and high withstand voltage, as well as the advantages of field-effect transistors (MOSFETs): high input impedance, high switching frequency, and low drive power. In addition to these advantages, IPM modules also integrate various control, detection, and protection circuits, making them more convenient to use and facilitating product simplification and high integration. Using IPMs as power conversion devices in frequency converters reduces system development time and significantly reduces the size of the frequency converter. Furthermore, it enhances system reliability, adapting to the current development trend of power electronics products—miniaturization and functional integration—leading to the increasingly widespread application of IPM modules in the field of power electronics.
This article introduces the hardware circuit design of a 2.2kW general-purpose frequency converter. The design uses the PM25RL1Al20 IPM module as a switching device. It also details the main circuit design and the switching power supply design. This design scheme, integrating the IPM module and switching power supply into the frequency converter, results in a highly integrated and compact product. Furthermore, the comprehensive protection functions inherent in the IPM module significantly improve the product's reliability, enhance the frequency converter's market competitiveness, and bring substantial economic benefits to the company.
2. Introduction to PM25RL1A120 [1]
The PM25RL1A120 is a newly launched intelligent power module from Mitsubishi Electric Corporation of Japan. It has a rated current of 25A, a rated voltage of 1200V, and a maximum switching frequency of 20kHz, meeting the design requirements of most power electronic devices. The internal circuit diagram of this module is shown in Figure 1.
Figure 1 Internal circuit diagram of PM25RL1A120
The system block diagram of this module shows that the PM25RL1Al20 has a total of 7 IGBTs, 6 of which control the IGBT's on/off state, forming an alternating three-phase current. The remaining IGBT is used for the inverter's braking circuit. PN is the DC voltage input terminal, which in practical applications is connected to the DC voltage (DC 540V) rectified by the three-phase rectifier bridge. U, V, and W are the IPM output terminals, providing AC power to the motor. Terminal B is used to connect an external braking resistor. When the bus voltage exceeds a certain value, the IGBT turns on, and the increased bus voltage is released through the braking resistor. Four power supplies: VUP1-Vupc, VVP1-VVPC, VWP1-VWPC, and VN1-VNC provide operating power to the driver chip of each IGBT, with a rated value of 15V. UP, VP, WP, UN, VN, and WN are the control terminals for controlling the switching of each IGBT. When the voltage values of these signals are less than or equal to 0.8V, the IGBT is in the on state; when they are greater than or equal to 9V, the IGBT is off. UFO, VFO, WFO, and FO are the error output terminals when the IPM module experiences a fault alarm. In actual design work, to ensure the safer and more reliable operation of the IPM module, all fault signals are processed and sent to the host computer. The host computer uses the high and low level changes of these fault signals to determine what kind of fault has occurred in the module, such as overheating, short circuit, or overcurrent. According to the PM25RL1Al20 datasheet, the correct basis for determining the type of fault is the duration of the FO signal level change. When the module experiences an overheating alarm, the FO signal remains valid as long as the module's temperature exceeds the allowable value until the temperature drops below the reset value, which typically takes several seconds. For short circuit and overcurrent faults, the typical duration of the FO signal is 2.2 ms.
3. Hardware circuit design of the frequency converter
The design scheme of this inverter adopts the traditional AC-DC-AC circuit [2] [3]. Therefore, its hardware circuit design is mainly divided into three parts: three-phase bridge rectifier circuit, inverter circuit and switching power supply circuit responsible for the normal operation of all devices in the inverter. The three-phase bridge rectifier circuit rectifies the power frequency voltage input from the grid into DC voltage through an uncontrollable three-phase rectifier bridge. Since the rectified voltage is a pulsating voltage, a large-capacity electrolytic capacitor is required for filtering, and then a smooth DC voltage is provided to the inverter unit. The switching power supply provides power to the devices in the inverter, such as the 4-channel 15V voltage responsible for the operation of the IPM and the 5V voltage required for the normal operation of the host computer, and the ±15V power supply required by the analog circuit. The inverter circuit composed of PM25RL1Al20 is the core of the entire circuit design. Its design directly affects the performance and reliability of the entire inverter. In this paper, the design of its control circuit is mainly described.
Figure 2 shows the electrical schematic of the inverter's rectifier circuit. The left terminal receives a three-phase 380V, 50Hz AC input. Z1, Z2, and Z3 are three varistors connected between phases to absorb surge voltages from the power grid, preventing damage to the inverter from large surge voltages. C1, C2, C3, and C4 are X and Y capacitors, used for differential-mode and common-mode interference filtering. The EMC-treated three-phase AC input power is rectified into DC voltage by the rectifier bridge, filtered by six large-capacity electrolytic capacitors (C5-C10) to become a smooth DC voltage, and then absorbed by non-inductive capacitors to absorb high-frequency components before serving as the input power for the entire inverter unit and switching power supply. Since the inverter's AC input voltage is 380V, the rectified bus voltage is 540V DC. During motor braking, the bus voltage rises, reaching a maximum of 800V DC. Commonly available capacitors typically have a voltage rating of 400V or 450V. Considering the safe use of the capacitors, these six electrolytic capacitors need to be connected in series in pairs to increase their voltage rating. Due to differences in manufacturing processes, the capacitance and insulation resistance of each electrolytic capacitor vary, resulting in unequal voltage values at each capacitor terminal. Therefore, voltage equalization resistors are used to eliminate this variation, utilizing the voltage divider principle to ensure equal voltage across each capacitor. These resistors also serve to rapidly discharge the electrolytic capacitors when the inverter is powered off. The four power-on buffer resistors are NTC resistors with a negative temperature coefficient, designed to limit the inverter's current during startup. Because the electrolytic capacitors C5-C10 have large capacitances, they are short-circuited at the moment the inverter is powered on, resulting in a very large charging current. By adding these starting resistors, the current in the circuit is ensured to rise slowly, preventing damage to the electrolytic capacitors from prolonged current surges. Once the current reaches a set value, these resistors are short-circuited via relays to prevent additional power loss caused by prolonged current flow through the resistors. The control signals for these relays come from the host computer. After a certain power-on delay, the host computer sends a signal to activate the relays and short-circuit these resistors. In this design, high-power NTC thermistors were chosen. At the moment the inverter is powered on, the initial resistance of the NTC thermistors is large, suppressing excessive surge current in the rectifier circuit, thus protecting the electrolytic capacitors and inverter unit from the impact of surge current. In traditional designs, high-power cement resistors are typically chosen as current-limiting resistors. However, when using cement resistors, heat dissipation must be considered. If the heat dissipation is poor, the power resistor may burn out due to temperature rise, which in turn can damage the relay. In contrast, the NTC used in this circuit ensures that when the circuit is in normal operation, the thermistor's temperature rises due to the current flowing through it, causing the resistance value to drop to a very low level. Even if the relay is damaged, it will not affect the normal operation of the circuit.
Figure 2. Rectifier circuit design of the frequency converter
Figure 3 is the electrical schematic diagram of IPM drive control [1]. The design level of this control circuit directly determines whether the frequency converter can work stably and reliably. Here, we take the control of one of the IGBTs as an example to explain the principle of IGBT conduction and turn-off. The operation of other IGBTs is the same. According to the IPM chip manual, IGBTs are turned on at low level and turned off at high level. Therefore, the control signal needs to be pulled up to high level to ensure that each IGBT is in the off state when the frequency converter has no output. At the same time, in order to prevent the IGBT from malfunctioning due to voltage spikes in the power supply, the power supply of each IGBT needs to be filtered. In addition, when laying out the PCB, we must pay attention to placing these filter capacitors as close as possible to the isolation optocoupler.
Figure 3 Electrical schematic diagram of IPM drive control
In the control circuit, the high-speed optocoupler TLP559 is selected as the isolation device [1] to isolate the weak electrical signal sent by the host computer from the strong electrical signal of the inverter section of the frequency converter. The UPC, UNC, VPC, VNC, WPC, and WNC sent by the control board are controlled to turn on and off the IGBT after being isolated by the optocoupler. The four error signals UFO, VFO, WFO, and FO come from the output signals of the upper and lower bridge arms of the IPM module. Since the processing method of these signals is the same, only one picture is listed for the principle explanation, see Figure 4. The fault signal sent by the IPM is a normal signal, so PS2252L-1 can be selected as the isolation device. The output signal IPMFO after being isolated by the optocoupler is sent to the host computer for processing. When the signal level changes, the host computer considers that the frequency converter has alarmed after processing. At the same time as issuing the alarm information, the output of PWM is blocked to prevent damage to the frequency converter.
Figure 4. Schematic diagram of IPM fault handling principle
Figure 5 shows the switching power supply circuit that ensures the operation of the frequency converter [4]. In this design, the most mature switching power supply design scheme is selected. The switching power supply circuit mainly consists of the following parts: high-frequency switching transformer, high-performance fixed-frequency current mode controller UC3844, and MOSFET K1317. Its main function is to convert the output DC voltage of the three-phase rectifier bridge through the switching transformer to provide power to the host computer, peripheral control circuit and IPM module. According to the design input requirements of the frequency converter, the following power supplies are required:
Voltage Application | voltage value | Voltage fluctuation range |
host computer | +5V | ±5% |
IPM Power | +15V, 4 groups | ±5% |
Analog quantity | ±15V | ±5% |
Switch output | +24V | ±10% |
To ensure that the design specifications of the switching power supply meet the usage requirements, the electrolytic capacitors selected in the circuit must be power supply-specific products to reduce the ripple voltage of all output voltages and reduce the interference of the power supply to other electronic components in the circuit.
Figure 5 Schematic diagram of switching power supply
In addition, this article only introduces the schematic design of the main circuit of the inverter, the switching power supply, and the IGBT drive circuit. After completing the schematic design, the corresponding PCB design is also required. This part is not the main content of this article. However, we should note that the three-phase rectifier bridge, IPM PM25RL1Al20 and the MOFET tube used in the switching power supply are all power devices with high heat generation. Therefore, the heat dissipation problem of the inverter must be considered when laying out the PCB. In practical applications, these devices must be effectively cooled. Installing heat sinks is an effective and economical way. At the same time, thermal silicone should be evenly applied between the heat sink and the device to ensure that the temperature rise of the device is within the required range [2].
4. Conclusion
This paper designs a 2.2kW general-purpose frequency converter using Mitsubishi's latest generation Intelligent Power Module (IPM) PM25RL1Al20. The main circuit of this frequency converter adopts a traditional AC-DC-AC frequency conversion circuit, and the power supply uses a high-frequency switching power supply. The frequency converter designed using this scheme has the advantages of compact structure, small size, low harmonic content, and high reliability. It fully meets the drive requirements of various three-phase AC asynchronous motors widely used in the market. Since its launch, this product has achieved a good market response, and its market share is continuously increasing.
References
[1]http://www.mitsubishielectric.com/cn/semiconductors/content/product/powermod/powmod/intelligentpmod/l1/pm25rl1a120_e.pdf
[2] Huang Lipei, Zhang Xue. Inverter Application Technology and Motor Speed Regulation. People's Posts and Telecommunications Press, 1995.
[3] Xu Hai, Shi Lichun. Principles and Applications of Frequency Converters. Tsinghua University Press, 2010.
[4] (US) Prisman, Murray, Wang Zhiqiang. Switching Power Supply Design. Beijing: Electronic Industry Press, 2010.
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.
