Design of Power Supply Monitoring System Based on CAN Communication

introduction

One of the development directions of power supply technology is to use power module parallel technology to achieve power synthesis and form a modular, intelligent distributed high-power power supply system. In order to coordinate the work of the parallel modules, reliable monitoring of the distributed power supply system is one of the hot spots in the development of power supply technology. The problems with the current commonly used approach to distributed power supply monitoring are mainly low digitization, low speed, low accuracy and reliability, etc. However, power supply control is very important in industrial control.

1 Overall design of power supply monitoring system

Traditional power supply system parallel systems mostly use analog methods to achieve current sharing between modules, but there are some common shortcomings: there must be a current sharing control bus, and a special current sharing controller needs to be added. Moreover, the current sharing bus is an analog level signal line with weak anti-interference ability; it is difficult to ensure the consistency of the modulation frequency of the power module. At the same time, when multiple sub-parallel power supply units form a distributed power supply system, it is of great significance to conduct real-time, accurate and unified monitoring and scheduling of the entire system. The prerequisite for power supply monitoring is to be able to communicate between each sub-unit and the monitoring unit. This system design uses the CAN bus, which is often used in industrial occasions and has strong self-adaptation, self-protection function and strong anti-interference ability.

The system consists of 8 control units and 1 central monitoring unit. The control unit and the monitoring unit transmit data via the CAN bus, and each unit is a CAN node. The core processor of each unit is the TMS320F2812 DSP chip of r11. Each control unit has a corresponding user board. The control unit and the panel complete the power parameter acquisition of each layer of power modules, the current reference output, the "remote/local acquisition" and "remote, local adjustment" switching, and the panel status information display: the central monitoring unit completes the real-time summary display of the data collected by the control units of each layer and the "remote adjustment" of the power modules of each layer. The overall structure of the system is shown in Figure 1.

Figure 1 System control unit structure diagram

2 System Central Monitoring Unit and Peripheral Circuit Design

The central monitoring unit can realize the summary display of module information and the "remote adjustment" of the power supply of each layer. The data collected and processed by the control units of each layer are transmitted to the central monitoring unit via the CAN bus. This information can be displayed in pages on the OLED screen, and the keyboard operation can be accepted to freely select the information to be displayed. The "remote adjustment" value is also obtained through keyboard input. The processor selected for this system is TI's TMS320F2812, which is a 32-bit fixed-point digital signal processor with a processing speed of up to 150MIPS. The processor also integrates 128KB of Flash memory and a 128-bit password protection mechanism, which greatly improves the flexibility of the application. At the same time, a 16-channel high-performance 12-bit ADC unit is integrated on the chip, and two sample-and-hold circuits are provided, which can be

In order to realize the synchronous sampling of dual-channel signals, the structure diagram of the central monitoring unit is shown in Figure 2.

Figure 2 Structure diagram of central monitoring unit

2.1 ADC module circuit design

The ADC module inside TMS320F2812 is a 12-bit pipelined analog-to-digital converter. The analog circuit of the analog-to-digital conversion unit includes forward analog multiplexers (MUXs), sample/hold (S/H) circuits, conversion cores, voltage references, and other analog auxiliary circuits. The digital circuit of the analog-to-digital conversion unit includes a programmable conversion sequencer, result registers, interfaces with analog circuits, interfaces with chip peripheral buses, and interfaces with other on-chip modules. The wiring of the ADC module is shown in Figure 3.

Figure 3 ADC module wiring diagram

2.2 eCAN module circuit design

CAN bus is a multi-master serial communication mode with a high level of security, which can effectively support distributed timely control. CAN bus has strong anti-interference ability and can work reliably in strong noise interference and harsh working environment. The CAN controller of TMS320F2812 provides a complete CAN protocol for DSP, reducing the processor overhead during communication. The eCAN module is mainly composed of CAN protocol kernel (CPK) and message controller. The CAN protocol kernel mainly completes two functions: decoding the message received on the CAN bus according to the CAN protocol and sending the decoded message to the receiving buffer; another function of the CAN protocol kernel is to transmit messages on the CAN bus according to the CAN protocol. The message controller determines the message received by the CAN protocol kernel and decides whether to keep it for CPU or discard it. The message controller also sends the next message to the CAN protocol kernel (CPK) according to the polarity of the message. In the CAN communication circuit, Rx and Tx are the GPIOF7 and GPIOF6 pins of the DSP respectively. At the initial power-on, they default to general I/O pins, and are configured by software to receive and send data for the eCAN module.

This system uses the SN65HVD2303.3V CAN transceiver produced by Texas Instruments. The transceiver has the characteristics of differential transceiver capability, slope control, wide range common mode interference, electromagnetic interference (EMI) capability, high input impedance and up to 120 CAN nodes. SN65HVD230 has three different working modes: high speed, slope and wait. Its working mode control can be achieved by setting the RS control pin.

In this design, considering the real-time display and control of information, the large amount of data transmitted and the short communication distance, the high-speed mode is adopted to improve the real-time display and control capabilities of information. The RS pin is grounded. Since each control unit is electrically isolated, the control unit and the CAN communication part are isolated by optical isolation. For level matching, LVTTL/LVCMOS compatible high-speed optical isolation is adopted. This design uses Agilent's HCPL-260L.

2.3 DAC Circuit Design

In this design, the DAC uses ADI's DAC8562. This is a 12-bit parallel DAC powered by +5V. Since the external power supply of this system is also 5V, and the 12-bit accuracy is sufficient to meet the system requirements, this DAC is used. The connection circuit of AC8562 and DSP is shown in Figure 4:

Figure 4 DAC connection circuit diagram

2.4 OLED display circuit design

The information display of the central monitoring unit adopts OLED screen. Organic light-emitting display OLED is a new generation of flat panel display technology that is more advanced than liquid crystal display technology. It is recognized by the industry as the next generation display technology with the most development prospects. This design adopts the latest product of Visionox, VGGl2864E-S001. This is a 128 x 64 line dot matrix OLED monochrome, character, and graphic display module. The module has a built-in 64 x 64 display data RAM, in which each bit of data corresponds to the light and dark state of a point on the OLED screen. It has the characteristics of 8-bit line data interface, read and write operation timing, and simple interface circuit.

There are two ways to access the OLED module: direct access and indirect access. In the direct access mode, the read and write enable signal E of the OLED is generated by the combination of XRD and XWE of the DSP. In the indirect access mode, all control signals including E are connected to the I/O port of the DSP, and the read and write operation timing of the OLED is simulated by software. In the design, the two operation modes are comprehensively considered, and the control signals of XRD, XWE, DSP I/O port and OLED are all led to the CPLD to facilitate future selection and debugging. The connection circuit diagram of OLED and DSP is shown in Figure 5.

Figure 5 Connection circuit between OLED and DSP

3 System Software Design

The program starts to initialize the GPIO, ADC, ECAN and other peripherals and enters an infinite loop. In the loop, the program starts each peripheral and performs the corresponding functions. The program flow chart is shown in Figure 6.

Figure 6 Control unit software flow chart

The design of CAN bus communication software includes the formulation and implementation of application layer protocol, the clarification of the functions of each node and the data to be interacted with each other, the specification of the exact meaning of each bit of data and the response processing to be done. The design of CAN node software is mainly divided into three parts, namely, initialization of CAN module, data sending program, and receiving program. The CAN module must be initialized before use. First, set CANTX and CANRX as CAN communication pins. In the standard T operation mode, the CAN module works in initialization mode by setting CCR (CANMC.12) to 1. Wait until CCL (CANLS.4) is 1 before executing the initialization operation. The initialization operation must first set the bit time configuration register CANBTC. The CANBTC setting determines the baud rate of the bus transmission signal, which is the core configuration issue of the CAN controller module. The CAN initialization configuration flow chart is shown in Figure 7.

Figure 7 CAN module initialization configuration

4 Conclusion

Innovation of this article: In the current era when digital power supply monitoring is becoming increasingly popular, this article proposes a monitoring system for distributed multi-module power supplies based on TMS320F2812DSP. This monitoring system uses stable, safe, and high-speed CAN communication technology as the communication method between modules of this system. Considering the need for system expansion, a processing chip with large storage space is selected to facilitate the expansion of system programs; at the same time, when selecting chips, try to consider processing chips and peripheral functional circuits with high integration and high reliability that meet system functional requirements. This system focuses on practical utility, and in actual industrial control, the system works stably.