Abstract: This paper details the design process of a network frequency converter based on an embedded system. The paper is divided into three parts: 1. Hardware design and implementation of the SVPWM frequency converter. The inverter circuit in the main circuit of the system is constructed using the intelligent power module PM20CSJ060, and the ARM microcontroller LPC2292 is used as the control core of the system control circuit; 2. Hardware development of the Ethernet interface. The Ethernet hardware interface circuit is completed using the network card chip RTL8019AS; 3. Software development of the frequency converter. The overall system software based on μC/OS-II is completed. Keywords: Frequency converter; Embedded system; Ethernet 1 Introduction With the development of modern control theory, power electronics technology, computer control technology, and sensor technology, a revolution is underway in the entire drive field. Breakthrough progress has been made in the speed regulation theory of AC motors, and AC drives replacing DC drives has become an irreversible trend. Frequency converters, with their significant energy savings, strong overload capacity, high speed regulation accuracy, fast response speed, comprehensive protection functions, and convenient use and maintenance, will be increasingly widely used in the AC drive field. This paper studies the design process of a network frequency converter based on an embedded system. 2. Hardware Design of the Frequency Converter 2.1 Main Circuit Design Frequency converters can be classified into AC-AC, AC-DC-AC, voltage-type, and current-type frequency converters based on their main circuit design, each with its own characteristics. The frequency converter designed in this paper belongs to the AC-DC-AC voltage type. Its main circuit consists of a three-phase full-wave rectifier, capacitor filter, and intelligent power module PM20CSJ060, as shown in Figure 1. PM20CSJ060 integrates six IGBTs, gate drive circuits, undervoltage, overcurrent, overheat, and short-circuit protection circuits, as well as a fault signal output circuit. P and N are the positive and negative terminals of the DC input, respectively; U, V, and W are the three-phase AC voltage output terminals; VUP1～VUPC, VVP1～VVPC, VWP1～VWPC, and VN1～VNC are four independent drive power supplies. The first three supply power to the three upper bridge arm components (U, V, and W), respectively, while the fourth supply power to the three lower bridge arm components and the braking circuit components; UP, VP, WP, UN, VN, and WN are the base drive input signals for the six IGBTs, all of which are active low-level signals and are opto-isolated from the external control circuit; F0 is the output signal of the fault detection circuit within the IPM module. When it is low, it indicates that the module has experienced one of the following faults: overcurrent, short circuit, undervoltage, or overheating. It only provides an indication signal to the external control circuit. Even if the external control circuit does not take any action, the module will block the base drive signal through its self-protection circuit, thus protecting itself. Because the PM20CSJ060 has a self-protection function, it is not necessary to provide overcurrent, overvoltage, and overheat protection circuits for all IGBTs in the entire system. Figure 1. Inverter Main Circuit 2.2 Control Circuit Design The inverter control circuit uses the ARM microcontroller LPC2292 as its control core and mainly consists of a power supply circuit, AC voltage and current detection circuit, DC voltage detection circuit, fault detection and handling circuit, PWM pulse output circuit, LCD display, and keyboard input circuit. 1. Power Supply Circuit In addition to the +15V power supplies for the four IGBT drivers, the LPC2292 microcontroller itself also requires a power supply. Its CPU core requires +1.8V, and the I/O ports require +3.3V. Therefore, the control circuit needs three types of power supplies. The four +15V power supplies are implemented using four LM7815 three-terminal regulators; while the +1.8V and +3.3V power supplies are implemented using LM7805 three-terminal regulators and an LDO chip (low dropout voltage regulator chip). 2. AC Current and Voltage Detection Circuit The AC current detection for each phase uses the TA17 series current transformer TA17-04. An operational amplifier circuit converts the current signal output from the transformer into a corresponding voltage signal for sampling by the microcontroller. Figure 2(a) shows the current detection circuit for phase A. The input current range of TA17-04 is 0–40A, and the output current range is 0–20mA. Since the microcontroller's sampling voltage range is 0–3V, a feedback resistor Rf1 = 150Ω is chosen. Additionally, the capacitor Cr and adjustable resistor r1 in the figure are used to compensate for phase shift. (a) Phase Current Detection Circuit (b) Phase Voltage Detection Circuit Figure 2 AC Current and Voltage Detection Circuit 3. DC Voltage Detection Circuit DC voltage detection involves taking the voltage across the filter capacitor, dividing it by resistors to convert it into a 0–5V voltage signal, then adjusting it to a 0–3V voltage signal via a linear optocoupler 6N138, and outputting it through a voltage follower for sampling by the microcontroller's A/D channel. 4. Fault Detection and Handling Circuit: The PM20CSJ060 has self-protection functionality. When overcurrent, undervoltage, short circuit, or overheating occurs, the IMP's gate drive unit will cut off the current and output a fault signal (FO). When any upper arm of the U, V, or W phase fails, a fault signal will also be output from the corresponding output terminal. Additionally, the system's overvoltage/undervoltage protection circuit has two fault output terminals. All these fault signals are active low, so we can AND these fault output terminals together using an AND gate and then send them to the microcontroller's interrupt port. However, some ports require optocoupler isolation before ANDing. 5. PWM Pulse Output Circuit: The PWM pulses driving the six IGBTs inside the IPM are first output from the LPC2292's internal PWM pulse width modulator, then sent to the IMP's six pulse input terminals after optocoupler isolation. 2.3 Protection Circuit Design Although the PM20CSJ060 has self-protection functions such as overcurrent, undervoltage, short circuit, and overheating, to improve system reliability and better protect the IGBTs, we added a fast and accurate protection circuit to prevent damage to the system caused by various faults. 1. Undervoltage/Overvoltage Protection Circuit Because the IGBT's withstand voltage between the collector and emitter and its ability to withstand reverse voltage drop are limited, and the mains voltage fluctuations are very large, this can lead to DC circuit overvoltage or undervoltage. Therefore, a DC voltage undervoltage/overvoltage protection circuit is required to protect the IGBTs and other components from damage. The system's undervoltage/overvoltage protection circuit is shown in Figure 3. The 6N138 in the diagram is a linear opto-isolator. Its output voltage signal is proportional to the DC circuit voltage. When the DC circuit voltage is too low, a lower voltage is output from the VO terminal of the 6N138. This voltage is compared with the critical undervoltage value; if the difference is less, a low-level undervoltage fault signal is output after comparison by the comparator LM393. Conversely, when the DC circuit voltage is too high, a higher voltage is output from the VO terminal of the 6N138. This voltage is compared with the critical overvoltage value; if the difference is greater, a low-level overvoltage fault signal is also output after comparison by the comparator LM393. These two signals are sent to the microcontroller's interrupt port via an AND gate. The microcontroller responds to the interrupt and performs the corresponding processing, thus providing undervoltage/overvoltage protection. Figure 3 shows the undervoltage/overvoltage protection circuit. 2. Current-limiting start-up protection circuit: This circuit is used to prevent damage to the rectifier diode due to excessive capacitor charging current during motor start-up. When the motor starts, the starting current is very large. To protect the rectifier diodes, a current-limiting resistor R1 is connected in series in the main circuit. After a 15-second timer, the microcontroller controls the relay to close the normally open contact, short-circuiting the current-limiting resistor R1, ending the current-limiting starting process, and entering normal operation. 3. Pump-up Voltage Protection Circuit: When the motor load enters braking mode, the feedback current will charge the intermediate DC circuit capacitor, causing the DC voltage to rise, generating the so-called pump-up voltage. If this voltage is not limited, it will cause permanent damage to the IGBT. The generation of pump-up voltage is an unavoidable phenomenon during motor braking; therefore, an energy release path must be provided for the braking process. 3. Hardware Design of the Inverter Ethernet Interface Circuit From a hardware perspective, the Ethernet interface circuit mainly consists of two parts: the MAC controller and the physical layer interface (PHY). Common Ethernet interface chips, such as RTL8019, RTL8029, RTL8039, and CS8900, also primarily include these two parts in their internal structure. In designing the Ethernet interface circuit, this paper uses the RTL8019AS as the Ethernet interface chip. The circuit diagram of the interface circuit is shown in Figure 4, where FC-518LS is the network isolation transformer. As mentioned earlier, the inverter control circuit design utilizes the LPC2292 microprocessor to implement the overall system control function. As shown in Figure 4, the LPC2292 microprocessor is also used to implement the Ethernet interface function of this inverter. This means that in addition to generating the SVPWM waveform, the LPC2292 is also responsible for data exchange with the external network. Regarding the network, the LPC2292 mainly monitors the RTL8019AS network card chip, sending data to or receiving data from the Ethernet, and then connecting to the Internet via Ethernet, thus realizing a true embedded TCP/IP device. Figure 4: Ethernet Interface Circuit Diagram. Figure 4: System Overall Software Design Based on μC/OS-II . μC/OS-II is a preemptive, multi-tasking real-time operating system that can manage 64 tasks. Besides 8 system tasks, the application can have up to 56 tasks. When using μC/OS-II to implement the software design of a system, the entire system is typically divided into several parts, each treated as a separate task. Under the unified management of μC/OS-II, the work of each part is coordinated to achieve the overall system software design requirements. This paper adopts the μC/OS-II framework in its software design, modularizing the system software and dividing it into multiple tasks to complete together. In typical 32-bit ARM application systems, most software is programmed in C language and uses an embedded operating system as the development platform, which greatly improves development efficiency and software performance. To perform system initialization, using an assembly file as startup code is a common practice. This allows for stack initialization, system variable initialization, interrupt system initialization, I/O initialization, address remapping, and other operations. The startup code is a piece of code executed after chip reset and before entering the C language's main() function; its main purpose is to provide the basic runtime environment for running C language programs. The inverter designed in this paper uses the ARM microcontroller LPC2292 as its control core. According to the functional requirements of the system, it mainly performs the following tasks: (1) Keyboard setting (Task1): Some parameters can be set through the keyboard, such as motor operating frequency, system time, etc. (2) Pulse generation (Task2): Generate SVPWM waveform to drive IGBT. (3) Data sampling and processing (Task3): Sample the DC side voltage and current, AC side voltage and current, etc. of the inverter and perform corresponding algorithm processing. (4) Listening service (Task4): Listen to the service port. When the client requests a connection, provide the client with a pre-designed webpage containing some real-time data of the system; or when the client PINGs the local WEB server, make an echo response. (5) Fault handling (Task5): When a fault occurs, perform corresponding processing according to the fault type. (6) LCD display (Task6): Display some system parameters. Therefore, the functions to be implemented by LPC2292 are divided into six parts, each implemented using a task in μC/OS-II. These six tasks are Task1, Task2, Task3, Task4, Tasks, and Task6. The author's innovation lies in using an embedded system to design and implement the frequency converter, resulting in high reliability, strong performance, and good real-time capabilities. The integration of an Ethernet interface into the frequency converter simplifies remote control and monitoring of the web-based frequency converter. References [1] Lü Ting, Shi Hongmei. Principles and Applications of Frequency Conversion Technology [M]. Machinery Industry Press, 2004 [2] Zhang Yanbin. SPWM Frequency Conversion Speed ​​Regulation Application Technology (Second Edition) [M]. Machinery Industry Press, 2004 [3] Yi Xudong. Application of μC/OS-II in Embedded Systems [D], 2003 [4] Chen Guocheng. New Power Electronic Conversion Technology [M]. China Electric Power Press, 2004 [5] Fang Yuming, Hang Bolin. Control of AC-AC Frequency Converter Based on Embedded System [J]. Microcomputer Information, 2007, 6-2: 15-16