By Ian Doyle – Marketing Director, ProTek Devices
Electrostatic discharge (ESD) occurs when a charge is exchanged between two objects of unequal charge potential. ESD most often occurs between people and metal objects. It is a result of an electron build-up behaving as a capacitor when dielectric isolation is available. Manmade materials, such as shoe soles, achieve dielectric isolation while someone walks on carpet creating kinetic energy. The potential threat of an ESD transient poses a challenge to system designers to incorporate overvoltage protection. Often, the included protection must have capacitance while not degrading the transmission rate. Traditionally, the use of a high capacitor was used to provide protection. However, due to high speed requirements, the capacitance per line must be significantly reduced. It must be done while still providing overvoltage protection against potential threats. Threats can include ESD, electrical fast transients (EFT), or surges. This paper discusses standard requirements for ESD. It highlights overvoltage protection solutions for the most common high speed interfaces. These include USB, HDMI and Ethernet ports, as shown in Table 1.
The IEC 61000-4-2 ESD Standard
The International Electro-technical Commission (IEC) has defined a Human Body Model (HBM) ESD event. The intent is to guide designers in implementing adequate protections in their electrostatic applications. The IEC defined the HBM ESD discharge impulse, with four levels, in standard 61000-4-2. It has a rise time of less than 1ns and decay time of 60ns. This is highlighted in Figure 1.
As a system requirement, the four levels of IEC 61000-4-2 are shown in Table 2. They are up to ±8 kV contact / ±15 kV air discharges for ESD transients in systems. In some applications, a higher level is specified for enhanced contact and air discharge specifications. This is because human body models can reach ±25 kV in static-rich environments.
To select appropriate overvoltage / ESD protection levels, designers must consider what external transients will be applied to equipment in installation or operational mode. Considerations include ESD, EFT, surge, lightning or mis-wiring. Manufacturers of Integrated Circuits (IC) with electrostatic sensitive devices design a level of protection into the IC. This is to increase robustness during potential board manufacturing overvoltage incidents. However, added protection circuits increases costs in a device by consuming expensive silicon real estate. To minimize such costs, IC manufacturers often design only to the level 1 standard. The IEC 61000-4-2 Level 1 protects against 1-2 kV contact voltage. However, HBM ESD voltages are nature determined – they are real-world and not just for during manufacturing. These real HBM ESD voltages and can be ±15 kV or more, which will damage the IC. So, proper IC protection calls for a Level 1 specification as a secondary protection scheme. Read more.