ESD protection by selecting the right components

As signal speeds increase and supply voltage rails reduce, the ICs become more sensitive to ESD transients. And the more devices are connected, the more important it becomes to protect each device in a wider system against ESD. What must be considered when selecting the right component?

As the Internet of Things (IoT) keeps expanding, analysts predict that tens of billions of connected electronic devices will come online in the next few years. Most likely, the total number of electronic devices for general use is even higher. Many of these devices will perform important roles in our lives, such as consumer devices for home automation. Others might be embedded deeply and hidden from view, such as inside vehicles and smart buildings. All of these are susceptible to electrostatic discharge (ESD). As the IoT connects more things to make networks of networks, any part of these billions of devices could become critical. Consequently, it is even more important that the ESD protection of these devices is carefully designed.

Even after a PCB has been assembled and installed in a device, ESD remains one of the main causes of failure. Modern integrated devices are designed to be robust so that malfunctions can barely occur during operation. Therefore, the operating conditions pose the greatest challenge. While engineers can take into account extreme environmental conditions such as humidity, temperature, and vibration, ESD remains largely unpredictable. The best approach is to adequately protect the devices from electrostatic discharge.

Standards such as IEC 61000-4-2 define protection levels that depend on the strength of the discharge voltage used for the test. For example, IEC 61000-4-2 defines a discharge voltage of ±2 kV for a contact as well as for air discharge, while Level 4 specifies a contact discharge of ±8 kV and an air discharge of ±15 kV. To ensure reproducible results, the discharge method and the test transients' shape are also precisely specified. The human-body model is widely used for testing purposes and is defined by standards such as MIL-STD-883 and JEDEC JS-001.

How ESD affects electronic components

The yellow and black warning symbols used by manufacturers to identify electrostatic-sensitive devices are commonplace in the industry. ICs based on a metal oxide process to form transistors with insulated gates are well known for their sensitivity. It must also be considered that as supply voltages fall – a widely used technique for reducing power consumption –, the low inherent ESD protection of the components will further decrease.

Almost all general-purpose ICs are now based on CMOS processes, and many have supply voltage rails that are 1 V or lower. This makes them more susceptible to the transient effects of ESD. If exposed to a discharge with high voltages and currents, parts of an IC can be immediately destroyed or slightly damaged so that its service life is effectively shortened. Due to these different degrees of severity, it is becoming increasingly difficult to assess the state of health of an integrated circuit.

Transients can fade quickly without causing any damage, or they can irreparably destroy interconnects. Due to the nature of the substrate, the lead frame, and the packaging, it is difficult to integrate adequate ESD protection into the ICs themselves. Therefore, this must be attached externally and placed in such a way that it intercepts the discharge immediately. Usually, this means that the protection must be placed as close as possible to the weak point.

As design practices change, it is important to understand how these new features may affect the type of protection selected. Often many options are available, but some design features, such as high-speed serial buses, limit the effectiveness or applicability of some ESD components. Parameters such as insertion loss and breakdown voltage must be evaluated concerning the characteristics of the signals to be protected to stay within the functional margin.

Understanding ESD protection components

ESD protection components are essentially based either on semiconductors in the form of transient voltage suppressors (TVS) – basically diodes (Figure 1) – or on ceramic materials in the form of varistors. Modern varistors are usually multi-layer components (MLVs; Figure 2). These have a certain capacitance, which can be useful but can also degrade the switching behavior of high-speed buses. If there is no such limitation in insertion capacitance, multi-layer ceramic capacitors (MLCCs) are also suitable for ESD protection.

Figure 1:

Transient voltage suppressors (TVS) are basically diodes that are used for ESD protection.

Figure 2:

Ceramic-based multi-layer varistors (MLVs) are also used for ESD protection.

Insertion loss due to capacitance is a key factor in deciding whether to use semiconductor-based TVS or ceramic-based MLV components. In many cases, both solutions should be largely interchangeable, but the insertion loss of the individual components is different. For example, for a 1 Mb/s signal, the protection device should have a low insertion loss at a frequency of 0.5 MHz. This is explained in more detail in the datasheet.

Furthermore, the capacitance of varistors can be used as part of measures against electromagnetic interference (EMI) at the circuit level. This characteristic has been rarely used to date. However, as manufacturers develop more advanced production techniques, they can better control the tolerance of the capacitance of a varistor. Thus, the capacitance can be used as part of the EMI filter. The capacitance of an MLV can range from 1 to over 100 pF, and even higher for SuperHigh Capacitance Varistors (SHCVs), much higher than a TVS diode.

Comparing ESD protection options

Since the way these two components absorb transient overvoltages, engineers can use TVS and MLV solutions in many applications as an alternative. They are often similar in size and have a compatible footprint on the PCB. Due to their small size, they can be placed close to the point of potential ESD exposure from an external source, such as an exposed interface or charging port.

Figure 3:

A guide to selecting the right ESD technology

As the diagram in Figure 3 shows, MLVs cover a wider application area than TVS diodes and are available in both commercial and automotive grades. The following considerations can help design engineers make more detailed comparisons.

• Operating voltages: At comparable sizes, an MLV and a TVS are likely to have similar response times, in the low nanosecond range, and protect similarly well against overvoltage (Figure 4). With a lower breakdown voltage, a TVS diode is preferable in lower voltage applications as it clamps transient voltages faster.
• Robustness: With increasing operating temperature, the performance of TVS components decreases. However, MLVs are based on ceramics and are therefore more robust and operate better at higher temperatures without losing performance up to +150 °C. In comparison, the performance of the silicon-based TVS diodes starts to degrade at temperatures above +25 °C.
• Maturity and cost: Both technologies are technically mature, but MLVs are more expensive due to the lower production volumes worldwide. However, new and smaller components have made them more attractive for smaller applications such as wearables. Higher production volumes have reduced the average selling price to comparable and competitive cost levels.
• Capacitance and EMI Filtering: As the parasitic capacitance of an MLV can be controlled during manufacture, it can provide not only ESD protection but also EMI suppression, replacing two separate components - TVS diodes and MLCC capacitors – and saving PCB space. It is not possible to control the capacitance of TVS diodes in the same way without additional process costs (see Figure 5).

Figure 4:

Reaction of TVS and MLV to an overvoltage event

Figure 5:

Function of ESD Notch Filters

• Insertion loss: This largely depends on the capacitance of the component and the signal frequencies on the lines being protected. Consequently, this is more relevant at higher frequencies, e.g., >1 GHz, and calls for the solution with the lowest capacitance. At lower voltages, this is likely to be the TVS diode, but at higher voltages, the advantage shifts back toward the MLV.
• Leakage current: This is not a major design consideration when choosing between MLV and TVS diodes. Both technologies exhibit some leakage current and are largely comparable.
• Physical size: As MLV technology has evolved, manufacturing processes have also become more sophisticated. MLVs are now available in sizes similar to multilayer ceramic capacitors (MLCCs) and their size can be further reduced. It must be kept in mind that the level of protection is still proportional to the physical size of the component. Currently, 01005 is the smallest package available for both TVS and MLV technology.
• Interchangeability: Both technologies ensure a similar level of ESD protection in the same footprint for applications with low-frequency and low-voltage signal lines. With increasing frequencies, TVS is preferable. However, with increasing voltage, MLV is the better choice. The design team must carefully examine the application and the available components to determine the most suitable technology.

Conclusion

Electrostatic-sensitive devices must be protected against transient ESD events. As supply voltages decrease and data rates increase, the operating conditions determine how the protective measures must be designed. Insertion loss and parasitic capacitance are becoming ever more important and are increasingly being taken into account.

Improved manufacturing processes provide more control of the tolerances for the capacitance of MLVs. This allows engineers to use them not only as ESD protection but also as EMI filters at the same time, which saves costs and space on the PCB.

Since TDK Electronics offers both TVS diodes and MLVs, designers can select the most suitable solution for their application for all critical points on a PCB from just one supplier. The more devices are connected, the more important it becomes to protect each of them from ESD.