RS-485EVALBOARD4

DESIGN NOTE RS-485 Port Protection Evaluation Board 4 INTRODUCTION This evaluation board serves as an aid in evaluating circuit protection solutions for RS-485 serial ports. It uses a Bourns® TBU® High-Speed Protector (HSP), a Gas Discharge Tube (GDT) Surge Arrestor and a Transient Voltage Suppressor (TVS) that are designed to meet the required industry standards on RS-485 port interfaces. The recommended Bourns® TBU® HSP solution offers enhanced performance features over competing technologies. These performance features can help engineers improve the surge and transient protection level on RS-485 ports, while allowing them to place the entire circuit protection solution into a smaller PCB area as compared to alternative solutions. The Bourns® RS-485 Evaluation Board 4 measures 35 mm x 25 mm x 0.85 mm, and is manufactured using an FR-4 PCB with nickel-gold pad plating on the top and bottom sides. J1 Bourns’ three previous versions of the RS-485 Evaluation Boards (EVB1, 2 and 3) used the Bourns® Model TBU-CA, which is a single-channel device. Because the RS-485 interface has two lines, this new evaluation board provides further PCB area reduction by utilizing the new Bourns® Model TBU-DF HSP, which is a dual-channel device. This latest evaluation board is the smallest RS-485 evaluation board offering a more compact solution helping designers save valuable PCB real estate. HOW TO CONNECT THE EVALUATION BOARD FOR TEST SET-UP • Connect J1A and J1B to the exposed lines. • Connect J2A and J2B to the RS-485 IC device. J2 GND GND TISP1 TBU® HSP A A TVS B B TISP2 GND GND Figure 1 RS-485 Evaluation Board 4 Schematic Figure 2 Table 1 RS-485 Evaluation Board 4 Bill of Materials RS-485 Evaluation Board 4 Top Side Layout No. Part Number Qty. Description Product 1 2 3 TBU-DF085-300-WH 2030-42T-SM-RPLF CDSOT23-SM712 1 1 1 850 V 300 mA Dual Channel Bidirectional TBU® HSP Fast Acting, 3-Electrode 420 V Gas Discharge Tube SOT23 12 V Dual Bidirectional TVS Diode TBU® HSP GDT TVS Diode The default configuration of this board uses a TBU® HSP, a GDT and a TVS diode. The board allows different configurations: • One Model 2030-42T-SM-RPLF could be replaced by two Model TISP4350J3BJR-S • One Model 2030-42T-SM-RPLF could be replaced by two Model TISP4500H3BJR-S • One Model 2030-42T-SM-RPLF could be replaced by one Model 2036-07-SM-RPLF www.bourns.com 8/28 • e/K1901A DESIGN NOTE RS-485 Port Protection Evaluation Board 4 RS-485 EVALBOARD 4 CONFIGURATION Protection of RS-485 ports are typically required in three scenarios. The first scenario is for exposed and harsh environments, such as outdoor installations where induced lightning surges are a threat. Customers for these types of applications are familiar with the ITU-T K.20/21/44 recommendations (specifying the 10/700 µs voltage, surge) or with the Telcordia GR-1089-CORE or IEC 61000-4-5 standard (specifying the CWG 8/20 µs current, 1.2/50 µs voltage surges). The third scenario protects against installation errors and faults. For example, 12 VDC or 24 VDC lines are frequently run together with the signal lines. The cable runs may include other exposed lines that can induce lightning surges onto RS-485 ports. In addition, there is the risk of deliberate and malicious attacks on RS-485 ports by unauthorized users. All of these can typically be taken care of with a protection solution that meets the higher protection levels offered in scenarios one and two. The second scenario accommodates long cable runs where multiple lines (data and AC power) are used in the same trucking or cabling. During a fault incidence, the AC power lines may come in contact with the signal lines. These applications require 230 VAC tests specified in ITU-T K.20/21/44, or 120 VAC tests specified in Telcordia GR-1089-CORE. There are applications with lower voltage application/installation threats, but where the standardized 120 VAC or 230 VAC tests are still used to test the robustness of the protection solution. This evaluation board brief will highlight options for two levels of lightning surges that use a TISP® Thyristor Surge Protector for the lower surge level and a GDT for the higher surge level. By varying the chosen voltage, this brief will also demonstrate that AC power cross up to 120 VAC or 230 VAC can be accommodated by a Bourns® Model TBU-DF based RS-485 protection solution. Table 2 Bourns® TBU® High-Speed Protector (HSP) and Overvoltage Protector (OVP) Combination VBO (V) TBU® HSP Part Number 275 350 TBU-DF055-300-WH 360 850 TBU-DF085-300-WH 200 A, 5/310 µs 350 500 TBU-DF055-300-WH 10,000 A, 8/20 µs 2,000 A, 5/310 µs 75 750 TBU-DF085-300-WH OVP Part Number OVP Surge Rating TISP4350J3BJR-S 2030-42T-SM 800 A, 8/20 µs 350 A, 5/310 µs 500 A, 8/20 µs 150 A, 5/310 µs TISP4500H3BJR-S 2036-07-SM VDRM (V) (1000 V/µs) Surge Capability IEC61000-4-5, Level 4, 4 kV 1.2/50 μs, 42 Ohm ITU K.20/21/45 Enhanced Lighting, 6 kV 10/700 μs, 40 Ohm AC Power Cross Capability 120 VAC 230 VAC 230 VAC – Overvoltage Protector (OVP) Selection: • Breakover voltage (VBO) or impulse breakdown voltage (Vimp) of the overvoltage protector (OVP) should be below the maximum Peak Impulse Voltage (Vimp) of the TBU® HSP. • VBO or Vimp of the OVP should be above the maximum voltage of AC power cross (peak of VAC). SURGE CAPABILITY TISP1 TBU® HSP TVS RS-485 TISP2 Surge Generator 1.2/50 µs, 4 kV Figure 3 RS-485 Port Protection Evaluation Board Surge Test Setup www.bourns.com 8/28 • e/K1901A DESIGN NOTE RS-485 Port Protection Evaluation Board 4 Below are graphs of different surge tests performed on Bourns' RS-485 Evaluation Board 4 based on the various standards requirements. Table 3 Test Results for Bourns® TBU® High-Speed Protector (HSP) and Overvoltage Protector (OVP) Combination TBU® HSP & OVP Combination Products 1 Bourns®Model TBU-DF055-300-WH 2 Bourns® Model TISP4350J3BJR-S Test Results 1 Bourns® Model CDSOT23-SM712 Surge Capability: IEC61000-4-5 Level 4, 4 kV 1.2/50 µs • Upon application of the 4 kV (95.2 A) surge (Channel 2), the transient current flowing through TBU-DF055-300-WH increases to trigger TBU-DF055-300-WH quickly (