EMC Compliant RS-485 Transceiver Protection Circuits
In these real systems, lightning strikes, power source fluctuations, inductive switching, and electrostatic discharge can cause damage to communications ports by generating large transient voltages. Designers must ensure that equipment does not just work in ideal conditions, but it must also work in real-world conditions. To ensure that these designs can work in electrically harsh environments, EMC regulations must be met.
Many EMC problems are not simple or obvious and must be considered at the start of the product development cycle. Proper solutions and protection circuits must be part of the total design effort and not left to the last minute. The protection circuits must incorporate the input and output structure of the specific transceiver manufacturer as part of the design.
The IEC 61000 specifications define the set of EMC immunity requirements. Within this set of specifications, designers must be concerned with the following three types of high voltage transients for data communication lines:
- IEC 61000-4-2 electrostatic discharge (ESD)
- IEC 61000-4-4 electrical fast transients (EFT)
- IEC 61000-4-5 surge immunity
ESD and EFT have similar rise times, pulse widths, and energy levels. The surge transient has longer rise times and pulse widths; as a result, the surge transient energy can be three to four orders of magnitude larger than the energy in an ESD or EFT transient. Due to the similarities between ESD and EFT transients, the design of the circuit protection can be similar. However, due to the high energy associated with surge transients, they must be dealt with differently.
Each solution protects data ports to ESD voltages of 8 kV contact and 15 kV air discharge, and EFT voltages of 2 kV. The different solutions provide an increased level of surge protection up to 6 kV. Protection levels for the circuits are summarized in Table 1.
Table 1. Protection Levels Offered by Each of the Three Protection Circuits in Figure 1.
|Protection Scheme||ESD||EFT (kV)||Surge (kV)|
|1. TVS|| 8 kV contact,
15 kV air discharge
|2. TVS/TBU/TISP|| 8 kV contact,
15 kV air discharge
|3. TVS/TBU/GDT|| 8 kV contact,
15 kV air discharge
Figure 2 shows a photo of the EVAL-CN0313-SDPZ board. There are three ADM3485E devices on the board, one for each protection scheme. Each protection scheme provides ESD and EFT protection as described and increasing levels of surge protection.
A complete design support package for the EVAL-CN0313-SDPZ board including schematics, layout files, and bill of materials can be found at www.analog.com/CN0313-DesignSupport.
The ADM3485E is a 3.3 V, low power data transceiver suitable for half-duplex communication on multipoint transmission lines. It has a data rate up to 12 Mbps with a common-mode range on the bus pins (A and B) of −7 V to +12 V. Data transmits on the DI pin, and it is received on the RO pin. Both the driver and receiver outputs can be enabled or disabled, that is, put into a high impedance state, by changing the logic levels on the DE and RE pins, respectively.
Power and ground are connected via a screw-wire connector (VCC and GND). This connector supplies all three ADM3485E devices.
Logic inputs DE and RE are set using LK1 to LK6. For each ADM3485E, LK2, LK4, and LK6 relate to DE and LK1, LK3, and LK5 relate to RE. For each link, Position A connects the logic pin to VCC, Position B connects the logic pin to GND, and Position C connects the logic pin to the four terminal, side screw-wire connector. The input DI and output RO pins are connected directly to the four terminal screw connector.
The EVAL-CN0313-SDPZ is also compatible with the Analog Devices, Inc., ezLINX™ board (EZLINX-IIIDE-EBZ) and the system development platform (EVAL-SDP-CB1Z). Connector J8 links the UART and GPIO interfaces on the SDP or the ezLINX board to the logic I/O of the ADM3485E devices. The I/O connections and jumper configurations are shown in Table 2.
Table 2. ezLINX and SDP I/O Connections and Jumper Configurations
|ADM3485E||I/O Pin||SDP/ezLINX Connector||Selection|
The ADM3485E transmitter and receiver share the same differential bus pins (A and B). The protection circuitry is used to protect these bus pins.
In the first protection circuit, shown in Figure 1 as TVS, it uses one component, the Bourns CDSOT23-SM712. This is the transient voltage suppressor (TVS) array shown on the EVAL-CN0313-SDPZ. It consists of two bidirectional TVS diodes optimized to protect RS-485 systems with minimal overstress while allowing the full range of the RS-485 signal and common-mode excursions. The TVS has high impedance to ground in normal operating conditions. When an overvoltage occurs, the TVS enters into avalanche breakdown mode and clamps the pin voltage to a safe predetermined level. It then diverts the transient current away from the ADM3485E to ground.
This protection scheme offers up to 8 kV contact and15 kV air discharge ESD, 2 kV EFT, and 1 kV surge protection.
As described in the CDSOT23-SM712 data sheet, the part is designed specifically for RS-485 devices. The next two protection schemes add to the CDSOT23-SM712 to provide higher levels of circuit protection against surge.
In the second scheme, shown in Figure 1 as TVS/TBU/TISP, the CDSOT23-SM712 TVS provides secondary protection, and Bourns TISP4240M3BJR-S provides the primary protection. The TISP4240M3BJR-S is a totally integrated surge protector (TISP). The TISP is a solid-state thyristor. When its predetermined voltage is exceeded, the TISP provides a low impedance path to ground, diverting a majority of the transient energy away from the ADM3485E.
Bourns TBU-CA065-200-WH transient blocking unit (TBU) is a nonlinear overcurrent protection device between the primary and secondary protection devices that ensures coordination occurs. The TBU is an overcurrent blocking device that becomes an open circuit at a predefined current. In blocking mode, TBU has very high impedance to block transient energy. This protection scheme offers up to 8 kV contact and15 kV air discharge ESD, 2 kV EFT, and 4 kV surge protection.
The third protection scheme, shown in Figure 1 as TVS/TBU/ GDT, operates in a similar fashion to Protection Scheme 2. In this scheme, a gas discharge tube (GDT) is used instead of the TISP. The GDT protects to higher overvoltages and overcurrents than the TISP described in the previous protection scheme. A GDT is a gas discharge plasma device that provides a low impedance path to ground to protect against over voltage transients. The selected GDT is the Bourns 2038-15-SM-RPLF.
This third protection scheme offers up to 8 kV contact and 15 kV air discharge ESD, 2 kV EFT, and 6 kV surge protection.
The ADM3485E has a 120 Ω termination attached to the bus pins.
According to IEC 61000-4-2, ESD testing implies using two coupling methods, contact discharge and air gap discharge. Contact discharge implies the discharge gun is placed in direct connection with the port being tested. With air discharge, the charged electrode of the discharge gun is moved toward the port under test until a discharge occurs developing an arc across the air gap. Apply discharges to the screw terminal connector of each bus line.
For IEC 61000-4-4 EFT testing, a capacitive coupling clamp is used to couple the EFT bursts onto the cable connected to the bus lines. The coupling capacitance of the clamp depends on the cable diameter, material of the cables, and cable shielding.
IEC 61000-4-5 surge testing implies the use of a coupling/ decoupling network (CDN) to couple the surge transient into the bus pins. According to the specification, this must be done using two 80 Ω resistors for a two port test. Figure 4 shows the test setup for surge testing. Connect the CDN to the A and B terminals, and the common of the surge generator to the ground connection of the four terminal screw connector.
All protection circuits described were characterized in the lab with the ADM3485E and verified by an external independent EMC compliance test house.