Power Isolation and Signal Isolation Solutions for DC Turnout Indication Systems

　　Background introduction: The turnout indication system is a device that detects the direction of the switch machine and the operating current. It is an important part of the centralized signal monitoring system and the guarantee of safe railway operation. This article will share the application of power isolation and signal isolation solutions in the DC turnout indication system.

　　Figure 1. DC turnout display system block diagram

　　Recommended solution:

　　1. Power Solution

　　According to the provisions of the "Signal Centralized Monitoring System Collection Technical Plan and Construction Process Requirements":

　　1) The collector housing and package of the DC turnout indication system shall adopt the railway signal special relay housing and base specified by the Ministry of Railways, which shall comply with the standard GB/T 7417-2001; each collector shall collect 4 groups of DC switch turnout indication voltages, and each group of DC switch shall be indicated by 2 groups of indication voltages, which respectively indicate the positioning acquisition signal and the reverse position acquisition signal;

　　2) There is one terminal between each group of turnout sampling lines, and the withstand voltage is AC4000V or above; the distance between the pads of the input lines of different groups of turnouts is greater than 6mm. If the safety gap is not met, an isolation groove with a width greater than 1mm needs to be opened;

　　3) The withstand voltage between the terminals, between the input and output, and between the input and power supply is higher than DC2500V. For strong currents exceeding AC3000V and currents greater than 200mA between the terminals, the fuse between the high and low ends of the input circuit is disconnected, showing an open circuit state to the outside; the wiring areas of the high-voltage part and the low-voltage part are strictly separated, indicating that the wiring interval of the voltage input part is greater than 6mm.

　　The isolated power supply for the DC voltage display part of the collector must have the following characteristics:

　　1) Small package size reduces board space requirements, leaving more space for input isolation circuits, ensuring a safe distance between pads and improving overall product safety;

　　2) High withstand voltage characteristics, meeting the 2500V isolation characteristic requirements, can prevent high voltage shocks such as induction lightning, locomotive crossing the track, etc. from being connected to the subsequent measurement circuit, thus improving the module's ability to withstand high voltage;

　　3) Low power consumption, high efficiency requirements, 8 groups of power supplies working together can significantly reduce the temperature rise of the product and can reduce the power requirements of the bus power supply;

　　In response to this characteristic requirement, Mornsun launched E_T-W2 & F_T-W2 0.25W, constant voltage input, 3000VDC isolated non-regulated positive and negative dual-channel/single-channel output DC-DC module power supply series products to solve the problem of power supply selection in voltage collection of DC turnout indication system;

　　This power supply series has the following features:

　　● Small size: 15.24mm*7.5mm*6.5mm;

　　● SMD packaging is more suitable for automatic patch production;

　　● Isolation voltage 3000VDC, meeting the railway standard requirements for turnout data collection systems;

　　● Operating temperature range: -40℃～+85℃, meeting harsh outdoor working conditions;

　　● No need for external components, reducing PCB area and flexible application;

　　Figure 2. Schematic diagram of F_T-W2

　　● International standard pinout;

　　Generally, the bus input voltage of the switch indication collector is 12V, and the switch indication voltage is generally 0-5V or 0-10V. The signal conversion part can use the module power supply F1212T-W2 product to meet the design requirements; [page]

　　For the bus input power supply of the acquisition box of the turnout indication system, since each turnout can only work in one of the working states, i.e., there are only 4 groups of F1212T-W2 working, the DC-DC 3W product is selected to meet the design requirements. Similarly, this bus input power supply must have a small volume packaging requirement, leaving enough space for the input isolation circuit; Golden Sun WRF_S-3WR2 product can meet the design requirements;

　　The traditional RS485 communication solution uses optocoupler isolation and DCDC isolation self-built solutions. Because it occupies a large area, it has limitations in the application of the acquisition box of the DC turnout indication system. Therefore, it is recommended to use the Golden Sun RS485 isolation transceiver module. This module is an interface isolation transceiver module that integrates transceiver chip, isolation chip and DC/DC isolation power supply, which can completely replace the traditional optocoupler isolation solution.

　　Figure 3. Schematic diagram of TDXXXD485H

　　● Magnetic coupling isolation technology replaces traditional optical coupling isolation technology, which is more reliable;

　　● Modular packaging can reduce the PCB area and is more suitable for collector applications;

　　● Isolation voltage 2500VDC, meeting the requirements of railway standards for communication isolation;

　　● Operating temperature range: -40℃～+85℃, meeting the working conditions in harsh outdoor environments;

　　● The internal pins integrate anti-static modules to reduce PCB area and provide flexible applications;

　　3. EMC circuit solutions

　　1) EMC circuit protection of turnout voltage input terminal

　　According to the "Technical Scheme and Construction Process Requirements for the Centralized Signal Monitoring System", the EMC protection circuit of the turnout voltage input terminal must be connected across a 2W metal film high-voltage resistor in each set of turnout input and output parts to prevent the high-voltage part from being connected in series with the subsequent circuit. This high-voltage resistor can significantly reduce this high voltage. In the signal conversion part, it is necessary to connect a TVS tube in series. On the one hand, it can prevent the output end of the DCDC power supply from being damaged by the high-voltage input part; on the other hand, it can protect the signal conversion part from abnormal operation due to the instantaneous high-voltage connection.

　　2) EMC circuit protection of the collector's bus power supply

　　Because the bus power supply of the collector is already in the secondary power supply, the primary power supply side has done a good job of protecting against surges and group pulses. Generally, it is only necessary to connect an electrolytic capacitor of about 47uF-100uF and an SMBJ TVS tube in parallel at the input end of the bus power supply to meet the design requirements;

　　3) EMC circuit protection of 485 communication part

　　EMC recommended circuit for 485 module

　　Figure 4.485 module recommended circuit

　　The specific recommended component parameters are as follows:

　　By applying this recommended circuit, the EMS port capability of the signal port can reach:

　　1). Electrostatic discharge IEC/EN61000-4-2 Contact ±4KV

　　2) Pulse group immunity IEC/EN61000-4-4 Signal port ±1KV

　　3). Surge immunity IEC/EN61000-4-5 Signal port ±4KV/±6KV