Alfredo Ochoa, Product and Applications Engineering Manager, and Sudesh Nayak, Manager, New Product Development, ProTek Devices
Industrial applications today face a serious challenge from improper circuit protection, costing money and lost time. Also, a negative brand image is often tied to system down time. Companies are better served, in terms of real dollars and perceived brand image, by avoiding product recalls. Proper circuit protection is one way to ensure this.
Any industrial application where humans or the environment directly interface with it is susceptible to current and voltage surges. So, these applications would need circuit protection. The common ones include point of sale (PoS) systems that have card readers, modems and serial ports or residential and commercial energy meters and smart meters, industrial displays, appliances, general control systems, instrumentation, photovoltaic systems, LED lighting, and many more. Any of these industrial applications are at an inherent risk from transient electrical threats that can include electrostatic discharges (ESD), electrical fast transients (EFT), surges, lightning or improper wiring.
Industrial Communications Systems
Communications systems are ubiquitous in industrial applications. For example, there is widespread use of RS-485, USB2.0, ProfiNet or ProfiBus setups. Traditionally a capacitor was used to protect these data buses from spurious fast transients such as ESD and EFT. However, data rates nowadays are steadily rising to a point where the capacitance can cause significant packet losses. A solution to remedy this would be a transient voltage suppressor diode (TVS diode).
A TVS diode is designed to act only at or above a voltage exceeding its minimum breakdown voltage (BVmin). Each TVS is rated for a stand-off or working voltage (VSW) which is well under its minimum breakdown voltage. Once the voltage across the TVS exceeds the breakdown voltage, it offers a low impedance path to the circuit ground. Thus, in an instance of transient overvoltage, the line that is protected by the TVS is clamped at a voltage closer to the working voltage of the application. A TVS is usually connected line-line or line-circuit ground. The advantages that a TVS provides are a noticeable reduction in capacitance and increased repeatability in terms of surge protection.
As mentioned, capacitance can be one of the factors that needs consideration when selecting a TVS for a high-speed data application. While large capacitance can easily mitigate the fast transients, it can also hinder data at higher baud rates. A large capacitance from the data line to ground with the line/trace resistance, forms a low pass filter that can affect rise and fall times of clock/data signals and even cause loss of signal in power to ground.
Protecting Industrial Power Supplies
An industrial power supply is an obvious requirement in any electrical application. Power supplies, owing to their inputs and internal circuit components, can be susceptible and even be a source of spurious voltage spikes. However, power supplies are less immune to ESD events. This is due to the use of passive components such as capacitors and inductors that are inherently robust. Ferrite beads are usually added in power supplies to add inductance that limits the impulse amplitudes under fast transients.
While these traditional solutions often provide adequate protection, these passive components can deteriorate over time and usage. As a result, transient energy would be fed forward into the system power rail, causing electrical damage in other areas of the system downstream. For this reason, overvoltage protection solutions with a fast response time that divert or shunt the transient energy to ground must also be considered for power supplies and power rails.
Another susceptible region of an Industrial design that is prone to transient stress are sensors. In industrial applications with sensor use, it’s ideal to use TVS solutions that can each handle ESD levels per standard IEC 61000-4-2 and EFT levels per IEC 61000-6-4.
Surge protection per IEC 61000-4-5 is also critical for the design engineer to understand what installation classification is required for a system. They include:
· Class 0: well-protected electrical environment, often within a special room
· Class 1: partly protected environment
· Class 2: electrical environment where the cables are well separated, even at short runs
· Class 3: electrical environment where cables run in parallel
· Class 4: electrical environment where the interconnections are running as outdoor cables along with power cables and cables used for both electronic and electric circuits
· Class 5: electrical environment for electronic equipment connected to telecommunication cables and overhead power lines in a non-densely populated area
Once the classification is known, the level of protection required can be determined using the IEC 61000-4-5 class levels.
Click image to enlarge
Figure 2. Peak current values required per voltage class and the equivalent limiting current resistors used, per IEC61000-4-5
To protect design investments, industrial application providers must consider comprehensive overvoltage and overcurrent protection devices throughout their product ranges. The use of proper circuit protection solutions should seamlessly integrate within the various electronics design requirements for today’s modern industrial electronics systems and sub-systems. Proper circuit protection also helps meet all relevant standards, which may be requirements in many industrial applications. Properly executed, they can also cost-effectively mitigate expensive redesigns or help avoid product recall scenarios.