How to choose ESD protection components

This article will analyze some of the types of protection products that system designers can use and compare their characteristics. To ensure the robustness of the system when subjected to ESD events, these products must be tested according to standards such as IEC 61000-4-2. System designers use a variety of methods to ensure that products meet mainstream ESD standards, including addressing enclosure design, circuit board design, component selection, and even software fixes. One important method is to use protection components at critical circuit nodes such as input and output (I/O) connectors. ESD protection components are often called transient voltage suppressors (TVS).

ESD protection principle of TVS

Many integrated circuits (ICs) have potentially sensitive inputs, which makes them susceptible to damage when input voltages are much higher than normal (e.g., under ESD stress). The area between the normal operating voltage and the voltage at which the device begins to be damaged is the safe overvoltage region. There is a slight overlap between the safe overvoltage region and the device damage region because if a large overvoltage only lasts for a very short time, the device may be able to withstand it even if it is not in the safe overvoltage region. The job of the TVS is to maintain the input voltage within the safe overvoltage range when an ESD event occurs, while not affecting system performance during normal operation. TVS devices are placed near where ESD events may enter the system, with the goal of limiting the voltage at sensitive nodes and diverting current to less sensitive nodes, such as ground level. To achieve this function, the TVS must have high impedance within the normal operating voltage range and low impedance outside the normal operating voltage range, so that the current can be diverted directly away from sensitive nodes and transient voltages can be limited.

The basic requirements for TVS depend on the specific application, but in general they are as follows: (1) be able to operate properly under the expected ESD stress; (2) have high impedance (low leakage) within the normal voltage range; (3) have low impedance outside the normal voltage range; (4) have a turn-on voltage suitable for the application; (5) be able to quickly switch from high impedance to low impedance during stress; and (6) have a capacitance that is not too high for the target application. Before comparing specific TVS device types, it is important to understand the two classifications.

Unidirectional vs. Bidirectional Protection: Both unidirectional and bidirectional TVS devices suppress both positive and negative stresses. The difference between these two terms can be best understood in terms of the voltage range over which the TVS maintains a high impedance, low leakage state. This voltage range determines the types of circuit nodes that the TVS device can protect. Bidirectional TVS devices have symmetrical characteristics with respect to zero volts (Figure 1). Bidirectional devices are best suited for protecting circuit nodes where the voltage is symmetrical or bidirectional around zero volts. Unidirectional TVS products have characteristics that are asymmetrical with respect to zero volts. Unidirectional TVS devices are ideal for protecting circuit nodes where the voltage polarity is always the same, such as unipolar voltages such as 0 to 5 volts.

电压箝位与消弧：电压箝位器件的工作原理是从低电压时的高阻抗转换至高于导通电压时的低阻抗，且没有负阻抗区域(图2)。这器件通过提供接地的低阻抗通道，对高于导通电压的电压进行箝位。消弧(crowbar)器件在低电压时也呈高阻抗，但当电压较高时触发新的导通机制，使电流增大，并伴随着电压下降。因此，消弧器件拥有负阻抗区域。某些消弧器件的触发电压可能非常高。如果消弧TVS触发器的速度足够快，就可以经常提供保护，即便电压已经达到可能导致器件损坏的电平。消弧器件有时也称作“骤回”器件，因为电压骤然下降。

There are three main technologies used in ESD TVS devices: varistors, polymers, and silicon diodes, each with its own unique characteristics.

1. Metal Oxide Varistor (MOV)

压敏电阻在小电流和低电压下具有高阻抗，但在高电压和大电流下，它们的阻抗大幅下降，因此它们属于电压箝位器件。

Varistors are bidirectional protection devices with a wide range of current and voltage protection capabilities, suitable for applications ranging from high-voltage transmission lines and lightning protection to small ESD surface mount devices. However, their large capacitance relative to their conductivity means that their use in high-speed signal line protection is limited. Varistors will also degrade after multiple stresses, even at levels far below those caused by a single stress.

2. Polymer surge suppressor

Polymer surge suppressors are arc-extinguishing devices and are always bidirectional protection devices. They have very low capacitance, making them attractive for high-speed applications. However, their disadvantages are high on-state voltage, relatively poor on-state impedance, and a tendency to degrade when subjected to multiple stresses.

3. TVS diode

如今大多数的二极管都是采用硅制造的固态器件。它们为双端器件，很容易让一个方向上的电流流过，但在相反方向上，它们呈现高阻抗，直到两端电压达到击穿电压。二极管本质上为单向器件，保护方式为电压箝位。

The characteristics of a diode depend on the doping levels of the N and P regions, which are located at different distances from the junction. Adjusting the doping levels can create diodes with reverse bias breakdown voltages ranging from hundreds of volts to just a few volts. Diodes designed with a well-defined reverse bias breakdown voltage are usually called Zener diodes.

Diode-based TVS products offer versatility not found in other ESD protection products—they offer both unidirectional and bidirectional protection options. The basic diode is a unidirectional product and the only unidirectional protection component. Bidirectional protection can be easily achieved by combining two diodes in series. Bidirectional protection can be achieved with a common cathode or common anode configuration. Bidirectional protection performance can be achieved with a pair of unidirectional TVS devices. There are a variety of bidirectional diode-based TVS devices on the market, where both diodes are located in the same package, and are often even integrated on a single silicon substrate.

In the past, silicon TVS devices had a disadvantage in protecting low-voltage, high-speed signal lines due to their high capacitance. However, recent technological advances have eliminated this disadvantage. ON Semiconductor's new product, the ESD9L5.0, combines the advantages of silicon device protection with the low capacitance required by high-speed applications. The product behaves like a simple Zener diode. In fact, the ESD9L5.0 contains a Zener diode with a low breakdown voltage and a pair of standard diodes with a high breakdown voltage (and therefore low capacitance).

Comparison of protection components

Table 1 summarizes the basic characteristics of the three types of TVS devices mentioned above. Selecting the appropriate protection device should take into account many factors, among which the key determining factor is the characteristics of the protected circuit.

Circuit nodes with asymmetrical sensitivity to stress may require unidirectional protection that only TVS diode products can provide. High-speed applications require very low capacitance, which makes polymer devices attractive. Polymer devices can meet the requirements for low capacitance and protection capabilities. In order for polymer TVS products to be used in high-speed applications, high-speed nodes need to operate under transient high voltage to conduct the polymer TVS and provide medium impedance in the on-mode.

由于成本低、不要求高压导通，压敏电阻常常具有吸引力。如果它们被制造得足够大以提供具有足够低的导通阻抗，从而提供充足的保护，那么它们的电容通常对高速应用而言就太大了。TVS二极管产品具有很好的箝位能力，如今市场上也有超低电容的TVS产品，甚至适合最高速的应用。二极管也颇具吸引力，因为它们能够用作单向保护器件，匹配当今许多高速数字信号的电压范围。