If you are a designer working on field transmitters, you are probably thinking about the physical environment where the system will be installed. Industrial sensor applications require a robust protection scheme since they are likely to encounter damaging surges created by lightning, ground loops, electrostatic discharge (ESD) and electrical fast transient (EFT) bursts. These high surge events could lead to induced voltages onto cables causing large voltage spikes to appear on circuits that were never designed to handle them.
In this post, I will discuss the major challenges when selecting transient voltage suppression (TVS) diodes for ESD and surge protection for field transmitters.
In factory automation and process control, a field transmitter measures critical parameters such as temperature, pressure and flow from the input signal of a sensor. It then converts the measurement into an accurate electrical representation that is transmittable through a robust interface/field bus to the programmable logic controller (PLC) or central unit. Some of the most common communication protocols for field transmitters are IO-Link in factory automation and 4-20mA/Highway Addressable Remote Transducer (HART) loop interfaces in process automation. Figure 1 shows a high-level block diagram of a temperature transmitter, including the signal input/output (I/O) protection.
Figure 1: Temperature transmitter block diagram
As with all systems with an externally exposed interface, your system must have International Electrotechnical Commission (IEC) 61000-4-2 ESD and IEC 61000-4-5 surge protection. The IEC 61000-4-5 surge standard is the most severe transient immunity test in terms of higher current and longer duration, and its application is often limited to long signal and power lines.
In field transmitter applications, there are several downstream components that need to be protected, including multiplexers, analog-to-digital converters (ADCs), 4-20mA transceivers and low drop out (LDO) regulators. Unfortunately, integrated circuit (IC) data sheets generally do not provide a transient voltage immunity rating, which makes it harder to select the right solution to robustly protect your system.
The clamping voltage is the lowest voltage level that your system needs to survive when the TVS diode is providing protection. In other words, it measures how well your protection solution can clamp a transient voltage. The lower the clamping voltage, the better the protection, and the more protection margin you will have for your downstream components. Typical TVS diodes clamp at voltages too high to protect your system, requiring the selection of downstream system components with higher voltage-tolerance ratings, increasing system cost and board area. Therefore, it is recommended to choose a TVS solution with low and flat clamping voltage technology to robustly protect your system.
A typical requirement for industrial field transmitters is to be tested for (and to withstand) 25A (8/20µs) at 1kV, with a 42Ω coupling network during the IEC 61000-4-5 surge immunity test. With such a high power rating, the TVS diode must be able to dissipate and divert high-voltage transients to ground; therefore, you would need to adopt a big solution size that could handle the high power dissipation, resulting in increased board space and design complexity.
Take for example the IO-Link Sensor Transmitter Reference Design (Figure 2), where a big portion of the board space is occupied by traditional TVS diodes used for signal I/O protection, which take up 12.5mm2 of board space for SMA industry standard packages and up to 19.1mm2 for SMB packages. Adopting a small form factor TVS solution saves board space and allows for a much closer placement to the connector in order to keep EMI outside the board area.
Figure 2: Sensor transmitter reference design board
In addition to clamping voltage and package size, leakage current poses yet another challenge when considering TVS diodes for field transmitter applications. At the working voltage, when the diode is not operating in its breakdown region, some current will flow through the diode and can affect system accuracy. Leakage current on the data line negatively impacts signal integrity; therefore, lower leakage enables higher-accuracy 4-20mA current-loop measurements and is necessary in order to prevent offset on 4-20mA loop interfaces.
TI’s new precision surge protection clamp can help solve all three of the surge-protection challenges I’ve described in this post. The TVS3300 can provide up to 30% lower and flatter clamping voltage, a 94% smaller footprint and 58% lower leakage current than traditional SMA and SMB TVS diodes in the market.
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