1. Common means of communication

It is mentioned in the previous digital power supply and analog power supply that in order to better realize the management and control of digital power supply, digital power supply needs to have communication functions. Through the upper computer software, engineers can set power parameters and control power status. However, because the TTL level output of the digital power control core is inconsistent with the level standard definition when communicating with peripheral devices, the level conversion chip is needed to realize the data exchange between them. The communication methods commonly used in digital power supply include RS485, RS232, CAN, TCP/IP and I2C.

(1) Classification of communication methods

Communication mode is divided into serial communication and parallel communication according to data transmission mode.

Parallel communication: The transmission is carried out in bytes or multiples of bytes. The transmission speed is fast but the cost is high over long distances. It is suitable for short distance, large number and fast information exchange scenarios.

Serial communication: also known as point-to-point communication, the communication sends one data at a time, the line is low in cost, easy to expand, suitable for long-distance transmission, is the most commonly used communication method.

According to different classification methods, serial communication can be divided into the following types:

1, according to the direction of communication is divided into simplex communication, half duplex communication and full duplex communication.

① In simplex communication, information can only be transmitted one-way, and the receiving and sending ends are fixed.

2 Half duplex communication information can be bidirectional transmission, but the sending and receiving can not be carried out at the same time, the communication receiving and sending end is variable, such as the walkie-talkie is a typical half duplex communication mode.

③ Full-duplex communication allows data to be transmitted in two directions at the same time, a transmitter and receiver are set up at each end, and two data lines are configured for signal transmission.

2, according to the data synchronization method can be divided into synchronous communication and asynchronous communication.

① Synchronous communication requires transceiver clock The frequency is consistent, and the information frame consists of synchronization characters, data characters, and check characters (CRC).

(2) Asynchronous communication does not require the receiver and sender clock synchronization, the sending time interval is uncertain, and the byte should be added to the start bit and the stop bit during transmission, so that the receiving end can correctly receive information.

(2) Comparison of communication methods

The common communication methods in digital power supply are compared here.

2. Introduction of communication circuit

(1) RS232

RS-232 uses negative logic level, "0" level voltage range is 3~15V, "1" level voltage range is -15~-3V, 9 pin is the current mainstream interface form.

RS-232 level switching circuits are divided into two types: isolated and non-isolated.

The non-isolated circuit can be implemented by triode or non-isolated level conversion chip (such as ADI MAX232ESE, ADM232AARNZ, ti MAX3232IDR, etc.). The isolated level conversion chips include RSM232, ADI ADM3251EARWZ and Maxliner SP3232EEY.

This paper provides a typical triode level switching circuit and RSM232 interface connection circuit diagram.

Analysis of the working principle of the triode level conversion circuit:

• Through the action of diode D1 and capacitor C7, the voltage at point A is maintained at -3V~-15V.

• When TXD=1, Q3 is cut off, PCRXD voltage is equal to PCTXD voltage, PCRXD=1;

• When TXD=0 and Q3 is on, the PCRXD voltage is about +5V and PCRXD=0.

• When PCTXD=1, Q4 is cut off, RXD voltage is about 5V, RXD=1;

• When PCTXD=0, Q4 is on, RXD voltage is 0, RXD=0.

• D2 is to prevent the BE reverse breakdown of Q4.

(2) RS485

RS-485 standard makes up for the shortcomings of short communication distance and low rate of RS-232. The data signal adopts differential transmission mode and has strong anti-interference ability. RS-485 uses A pair of twisted pairs (A and B) for data transmission, indicating "0" when the voltage difference between AB lines is in the range of -6 to -2V, and "1" when the voltage difference between AB lines is in the range of +2 to +6V. At the same time, RS485 needs to be connected to a matching resistor at the farthest end of the transmission cable. Its resistance value should be equal to the characteristic impedance of the transmission cable. The connection diagram is as follows.

RS485 level conversion chip is divided into isolated and non-isolated two kinds.

Typical non-isolated chips are the MAX3485, ADI's MAX13487EESA+T, and TI's SN75176BDR. Commonly used isolation chips are RSM3485PHT, TI ISO3082DWR, and ISL32705E.

(3) CAN

CAN bus communication adopts the form of differential signal for data transmission. Signal transmission lines are divided into CAN_H and CAN_L. The logic "0" on the bus indicates dominance, and the differential voltage difference is about 2V(CAN_H=3.5V,CAN_L=1.5V); The logic "1" indicates recessive, and the differential voltage is 0V9CAN_H=2.5V,CAN_L=2.5V). CAN adopts the form of data frame for data transmission. The standard CAN data frame structure is shown in the figure below.

CAN communication interface circuit can be divided into isolated and non-isolated two kinds.

The non-isolated circuit is to connect the CAN interface of the control core directly to the TX and RX of the driver IC, and there is no electrical isolation between the nodes. In order to ensure the communication stability of bus network, CAN communication interface usually adopts isolation structure. Isolation circuits CAN be implemented using discrete devices (such as optocouplings) or integrated devices (isolated CAN transceivers).

Commonly used CAN communication transceiver chips are NXP TJA1050T, the United States microchip MCP2551T and TI company ISO1050.

Take ISO1050 as an example. The electrical isolation structure is integrated inside the chip, and the isolation transformer is used to isolate the power supply Vcc1 and Vcc2 on both sides of the chip to ensure that the chip can play an effective isolation role. The circuit is connected with a decoupling capacitor between the power end and the ground end of the chip to reduce interference, and the TVS diode in parallel between the CAN_H and CAN_L ends and the ground plays a fast voltage protection role.

(4) TCP/IP

TCP/IP protocol is divided into four layers: link layer, network layer, transport layer and application layer.

The application layer contains http and ftp protocols, and the transport layer contains TCP and UDP protocols. The network layer contains the IP protocol, adding IP addresses and other data to the data to determine the destination of transmission. The data link layer adds the Ethernet protocol header to the data and performs CRC coding to prepare for the final data transmission.

TCP/IP communication can be implemented by Ethernet protocol stack chips embedded with TCP/IP protocol (such as W5500, CH395, WT8266-S3) or switches, which are often used in digital power supplies at present.

The communication principle and circuit implementation of common communication methods of digital power supply are introduced here. After talking about sampling circuit, driving circuit, communication circuit, then our digital power peripheral circuit course will come to an end. Next is the long-awaited topology, the small edition will introduce the common circuit topology of digital power supply one by one, and the next phase we will first explain the topology composition and working principle of the phase-shifted full bridge power supply.