ESD protection diode selection

Overview of ESD Devices

The function of ESD protection components is to transfer ESD stress from sensitive components, allowing current to flow through the protection components rather than the sensitive components, while maintaining low voltage on the sensitive components; ESD protection components should also have low leakage and low capacitance characteristics, without reducing circuit functionality; It will not cause damage to high-speed signals, and the function of protecting components under multiple stresses will not decrease.

Transient voltage suppressors (TVS), varistors, and polymers are several specialized ESD protection components developed in recent years. The first two components are protected by voltage clamping, while the polymer with conductive particles is protected by a crowbar protection strategy. Varistors and polymers support bidirectional protection, but TVS can support unidirectional or bidirectional protection. Although traditional varistors have certain cost advantages, their biggest problem is that they are too large in size to meet the packaging requirements of handheld devices. In fact, compared with varistors, TVS based on silicon materials and ESD based on polymer materials have better clamping performance, lower leakage, and longer service life.

Comparison of Several ESD Devices

1. Ordinary diodes can only act as voltage clamps and cannot respond to ESD pulses with frequencies up to several hundred megahertz.

2. Varistors/Thermistors/PTC. Varistors will change their characteristics after withstanding one ESD pulse, while ESD protection devices will not change their characteristics even after withstanding tens of thousands of times. 3. Varistors can withstand larger surge currents, and the larger their volume, the greater the surge current they can withstand, up to tens of kA to hundreds of kA; However, the nonlinear characteristics of varistors are poor, with higher limiting voltage at high currents and larger leakage current at low voltages.

4. The nonlinear characteristics of TVS diodes are similar to those of voltage regulators. Before breakdown, the leakage current is very small, and after breakdown, it is a standard voltage regulator. Compared with varistors, TVS diodes have a smaller deviation from the maximum clamping voltage from the breakdown voltage, which is better than varistors, but their current carrying capacity is smaller.

5. In terms of reaction speed, TVS diodes have a fast reaction speed of ps level, while varistors have a slower reaction speed of ns level. In addition, both have large capacitance (TVS diodes also have low capacitance products).

6. TVS diodes have high reliability, are not easily degraded, and have a long service life. However, varistors have poor reliability, are prone to aging, and have a short service life

Firstly, you need to provide the normal operating voltage of the components you want to protect or the allowable voltage range of the pins and wires you want to protect. Otherwise, improper selection of ESD protection diode models will affect normal operation.

Selection of ESD static diodes:

Firstly, Array style ESD protection components should be used to reduce the space and parasitic impedance of the PCB with the minimum number of components;

Secondly, the line capacitance of ESD protection components should be low enough. For example, USB 2.0 requires less than 3pF, USB 3.0 requires less than 0.3pF, and 10/100M LAN requires ESD protection components with less than 3pF;

Thirdly, the clamping voltage of ESD protection components must be low enough to enable the system to operate without interference during ESD. As for how low the clamping voltage is, it depends on the system's noise immunity.