Many vendors of industrial bus transceivers showcase their device robustness with a high immunity to electrostatic discharge (ESD). In strong contrast to that, industrial system designers rank ESD as their lowest failure priority and rather focus on the protection against electrical fast transients (EFT) and surge transients. Of course, there are key differences between the various transient immunity tests, and designers can take advantage of several design and layout suggestions for a robust bus node circuit design.
The costly production losses incurred by network downtime due to electrical over-stress make it necessary to protect network nodes against electrical transients caused by electrostatic discharge, inductive switching, and lightning strikes. The International Electrotechnical Commission (IEC) has therefore defined the following three transient immunity test standards to ensure proper circuit operation during and after test:
• Electrostatic Discharge (ESD) Immunity test (IEC61000-4-2)
• Electrical Fast Transient (EFT) Immunity, or Burst test (IEC61000-4-4)
• Surge immunity test (IEC61000-4-5)
The ESD test, simulating the electrostatic discharge of human operators onto electronic equipment, possesses test pulses of less than 100-ns duration with rise times as fast as 1 ns. Although of low energy content, these pulses create sufficiently high currents that can damage the integrated protection cells of bus circuits. A series of 20 consecutive discharges, 10 of positive and 10 of negative polarity with a 1-s interval between pulses, constitutes the minimum ESD test sequence.
The Burst test simulates everyday switching transients primarily caused by inductive switching, relay contact bounce, and other factors. Unlike the single pulses used in ESD testing, an EFT generator produces a train of test pulses, called a burst, of 15-ms duration followed by a pause interval of 285 ms. Thus one burst contains approximately 15,000 transients.
An entire test sequence consists of six 10-s test intervals, with each interval providing around 33 bursts. A test interval is followed by a 10-s pause. The standard requires a minimum test time of 1 minute during which some 3 million pulses are generated. Although short in duration and low in energy, the virtual endless bombardment of test pulses upon the equipment under test (EUT) presents a heavy burden to its internal protection circuitry.
The Surge test is the most severe of all transient immunity tests as it simulates switching transients caused by lightning and switching heavy inductive loads. While rise times are in the lower micro-seconds, the time duration of its test pulses are 1000 times longer than those of ESD and EFT, unleashing very high energy content onto the EUT. Because of the high energy impact, a test sequence is commonly limited to five positive and five negative pulses, while allowing for a 1-minute recovery interval in between pulses.
The importance ranking of the various transient immunities is based on their likelihood of occurrence and their potential damage they can cause to equipment and systems.
In industrial networks, ESD events are of little importance as the human interaction with network components is limited to the installation and maintenance phase, where ESD protective gear must be worn to minimize the risk of static discharge.
On the other hand, burst immunity has the highest priority as it reflects the EUT robustness against everyday switching transients. Right behind follows surge immunity. While surge transients are less likely to occur, their impact is far more severe.
The major components of a well protected bus node comprise transient-absorbing cable, transient-suppressing diodes, and EMC-compliant (electromagnetic capability) layout (see the figure).
Using industrial, shielded bus cable instead of low-cost CAT5 or flat-band cable eliminates the lion’s share of transient energy induced into the bus lines. Cables with braided shields significantly reduce noise coupling into the signal conductors. And with less noise in the signal wires, the transient impact on the bus node electronics is reduced.
Modern Transient Voltage Suppressors (TVS)
Because industrial networks must operate over a wide common-mode voltage range (typically from +12 V to –7 V), the use of bipolar transient suppressor diodes is recommended. Bipolar TVS diodes behave like two Zener diodes of opposite polarity switched in series. The right diagram in the figure shows a typical switching characteristic of a bipolar TVS.
During normal operation, the TVS is high impedance and only a few micro-amps of leakage current flow through the device. Once a transient exceeds the TVS breakdown voltage, the diode conducts and diverts the transient current via a low-impedance path towards ground.
Modern TVS diodes provide the best of both worlds. They have fast response times in the range of picoseconds while possessing high-energy absorption capabilities at the same time.
It was only recently that advances in process technology enabled the manufacturing of fast, low-capacitance TVS diodes. Legacy designs of TVS diodes possess large capacitance in excess of up to 1000 pF, yielding response times of several nanoseconds, which proved too slow for the fast rise times of ESD and burst transients. In addition, their large capacitive loading on the bus often required dropping the data rate to very low levels.
In strong contrast, the modern, more sophisticated suppressor designs possess capacitances between 10 pF and 100 pF, allowing the protection of many bus nodes without a drastic reduction in data rate.
TVS diodes are powerful protectors, but they cannot do the job on their own. Bus circuits, such as transceivers, must provide sufficient internal transient protection to survive any residual transient energy during the clamping process. That’s because a TVS dynamic impedance can cause clamping voltages that might exceed the specified maximum bus voltage of a transceiver.
While transceivers with high ESD ratings can handle this short-term over-stress, weaker components will benefit from surge-rated resistors (RS) switched in series to the transceiver bus terminals. These resistors with typical values of 5 Ω to 10 Ω reduce the current toward the transceiver during the clamping action, minimizing the impact on its ESD cells.
Transient suppressors come in various power ratings. While for data transmission lines, 400-W to 600-W devices are commonly applied, low-impedance power-supply lines typically require larger devices with 1500-W ratings and more.
Another important parameter is the TVS breakdown voltage, VBR. It’s important to ensure that a TVS does not conduct during normal system operation, so any expected fluctuations in the supply voltage or variations of the bus signal levels due to common-mode noise must be taken into account. In this case, the breakdown voltage must be higher than the specified maximum variation in voltage levels.
Good Board Design And Layout
When designing the bus node circuit, it must be understood that real-world transients present an enormous amount of wideband noise in the range of 3 MHz to 3 GHz. Therefore, high-frequency layout practices must be applied to accomplish a circuit board design that is robust against electromagnetic interference (EMI).
Because high-frequency components follow the path of least inductance rather than least impedance, most of the following recommendations aim for the diversion of high-frequency noise through low-inductance paths.
Use a four-layer printed circuit board (PCB) with the stacking order: bus signal layer, ground plane, power plane, and control signal layer. Placing the ground plane next to the bus signal layer establishes controlled impedance traces and provides a low-inductance path for return currents.
TVS diodes should be positioned as close as possible to the bus connector to prevent transients from infiltrating the board circuitry.
Bypass capacitors (typically 100 nF) must be placed as close as possible to every integrated circuit on the board, as they provide the fast switching currents during normal operation. Also, they must bypass noise transients around the IC effectively.
To provide low-inductance ground connections for transient suppressors and bypass capacitors, multiple vias (at least two per terminal) connecting the component terminal to the ground plane are advised.
As transient energy can infiltrate the board through radiating, EMI filtering using simple R-C low-pass filters should be applied to the single-ended data and control lines between the bus transceiver and UART.
Modern transient voltage suppressors allow for the efficient protection of individual bus nodes. While effective transient protection might appear costly at design start, it prevents higher costs in the future due to failures in the field and the network downtime and possible product recalls associated with them.
1. Further information on bus transceivers (LVDS, RS485, CAN, HDMI) is available at www.TI.com/interface.
For information on industrial bus cables, visit www.Beldencable.com.