Design of parallel inverter power supply based on LF2407A-EEWORLD

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1 Introduction

The parallel operation of inverter power supplies can not only realize power synthesis, but also easily form a modular, redundant, and intelligent distributed high-power power supply network system, which is an important direction for the development of power supply technology. Fieldbus control technology can easily build a distributed inverter power supply local control network, allowing the AC power supply system to develop from traditional centralized power supply to distributed power supply and even intelligent power supply system power supply mode, which is an effective means to achieve intelligent control of the system. This article introduces a parallel inverter power supply based on CAN fieldbus, making full use of the internal resources of TI's TMS320LF2407A DSP chip, acquiring and parsing field control data from each parallel module through the embedded CAN bus, responding to strong real-time operations on site, and realizing scheduling and monitoring of module work.

2 Inverter power parallel technology

The key issue in realizing the parallel operation of inverter power supplies is that each inverter power supply must share the load current, that is, to realize the current-sharing operation of the inverter power supply. Since the parallel operation of the inverter power supply modules constitutes an AC power supply system, and the output of each module is an AC signal, the parallel operation between them is much more complicated than the parallel operation of DC power supplies. The parallel operation of inverter power supplies needs to meet five conditions [1], namely, the same voltage, frequency, waveform, phase and phase sequence. Only in this way can the circulating current be eliminated, the load power be evenly distributed, the best operating state be achieved, and the parallel operation of the inverter power supply can be truly realized. The difference in the amplitude and phase of the power supply voltage will cause the current difference of the inverter power supply. Therefore, synchronous operation and consistent current output become the fundamental problems to be solved in realizing the parallel control of the inverter power supply. This design uses the CAN bus embedded in the TMS320LF2407A DSP chip to realize the synchronous operation and current sharing of the parallel inverter power supply.

3 CAN bus design based on DSP

The hardware design structure block diagram of the DSP-controlled inverter power parallel control based on the CAN bus is shown in Figure 3-1. The DSP compares the feedback value obtained by sampling with the given value, controls the output PWM pulse, drives the inverter bridge power switch, and outputs a sinusoidal voltage with adjustable frequency, amplitude, and phase. The CAN controller receives commands from the CAN bus to control the parallel inverter power module.

Figure 3-1 System hardware structure

3.1 CAN bus hardware design

The DSP chip TMS320LF2407A has a built-in CAN controller. CAN bus communication plays an important role in the implementation of the inverter parallel connection in this paper. It provides synchronization signals and data transmission of current set values for parallel operation. When interfacing with the CAN bus, a CAN bus receiver must be used [2]. The DSP chip TMS320LF2407A is powered by a 3.3V power supply. TI has designed three types of CAN bus transceivers specifically for the 3.3V powered 240x series DSP, namely SN65HVD230, SN65HVD231 and SN65HVD232. They all use a 3.3V power supply. A maximum of 120 nodes can be connected to the bus. When the temperature rises to a certain value, the bus is automatically shut down. The power-on or removal of a node has no effect on other nodes on the bus. This series of devices supports a transmission rate of up to 1Mbps. In this system, SN65HVD232 is selected as the CAN receiver. Its pin distribution is shown in Figure 3-2. Pin 1 (D) is used to send data and pin 4 (R) is used to receive data, and they should be connected to CANTX/IOP6 and CANTX/IOP7 of the DSP chip respectively.

In order to further improve the anti-interference ability of the system, a high-speed optocoupler 6N137 is used between the CAN communications of the two DSPs to form an isolation circuit as shown in Figures 3-3 and 3-4. Using the 6N137 high-speed optocoupler, the CAN network speed can reach the same speed as when driven by a resistor network [3]. The CANTX in the data transmission isolation diagram and the CANRX in the data reception isolation circuit are both pins on the DSP. CANTX is the CAN data transmission pin, and CANRX is the CAN data reception pin. R1 in the circuit is a current limiting resistor, and the input current of the optocoupler 6N137 cannot be less than 7.5mA; resistor R2 is a pull-up resistor; resistor R3 is the matching resistor at both ends of the CAN bus network. Here, a 120Ω resistor is selected [4]. Matching resistors play a very important role. Ignoring them will greatly reduce the anti-interference ability of the system data communication or even make it unable to work normally.

3.2 CAN bus software design

This design adopts the master-slave control strategy of competing for mastership to realize the communication of the parallel inverter power supply system. The synchronization signal and current instantaneous amplitude reference of all slave modules are taken from the master module. In order to meet the requirements of parallel redundancy and avoid the paralysis of the entire system due to the failure of the master module, the master module is dynamically established. At regular intervals, each module must broadcast a "compete for mastership" request. If the master module has not yet been established, or the established master module fails, the module that first sends the "compete for mastership" request can become the new master module. Using the principle of competing for mastership and master-slave control, when a master module fails, other slave modules compete to become the master module and replace the original master module. This mechanism can ensure that the entire system will not be paralyzed due to the paralysis of a communication master node, causing the entire inverter module communication system to be paralyzed [5].

The programming of the CAN controller of TMS320LF2407A mainly uses several mailboxes to receive and send data. There are 6 mailboxes configured, 2 for receiving (0, 1), 2 for sending (4, 5), and 2 for configurable (2, 3); the data length of each mailbox is 8B. Set mailbox 3 as a non-broadcast mailbox for sending, mailbox 2 as a mailbox for receiving non-broadcast information, mailbox 4 as a mailbox for sending broadcasts, and mailbox 0 as a mailbox for receiving broadcasts. The entire communication system is mainly divided into a master node and multiple sub-nodes. Among them, mailbox 4 regularly sends a master request to the bus in the form of broadcast at a certain time to detect whether the master node is working normally. The function of mailbox 0 is to receive and distinguish these two types of information, and judge its own status to decide whether to use mailbox 4 to send an anti-master request. The main function of mailbox 2 is to receive the control information of the master node and notify mailbox 3 to send the response information of the response. The description of the information sending process is given in Figure 3-5.

4 Experiments

According to the control scheme of parallel connection of inverter power supply of CAN field bus proposed in this paper, parallel connection experiment was carried out on two inverter power supplies. Figure 4-1 shows the output voltage of two parallel inverter power supplies, with a voltage amplitude of 100V/grid, and Figure 4-2 shows the output current of two parallel inverter power supplies, with a current amplitude of 10A/grid, indicating that the current sharing effect is very good. Figure 4-3 shows the synchronization signal of the slave module tracking the master module, and Figure 4-4 shows the signal sent by the CAN bus of the master module and the signal received by the CAN bus of the slave module. The signal is not lost during the transmission process, and the consistency is very good, achieving the expected effect.

5 Conclusion

The design of this paper utilizes the built-in CAN bus control technology of LF2407A to conveniently build a distributed inverter local control network, which not only simplifies the inverter parallel connection method, but also provides stable and reliable data communication for each module and supports the hot pluggability of the inverter. The system has the advantages of simple structure, convenient expansion and high reliability.

Keywords：LF2407A Reference address：Design of parallel inverter power supply based on LF2407A

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