Electrostatic Discharge(ESD) is the transfer of electrical charge between any two objects. A common example of this is the static shock that a person experiences after walking across a carpet and then touching a metallic object such as a door knob or file cabinet. The “shock” is felt at the point of contact and is typically accompanied by a small spark and a slight snapping sound.
The process of creating an ESD event begins with the generation of static electricity. As one material (in this case the soles of a person’s shoes) comes into contact with another material (carpeted floor) which are then separated, an imbalance of electrical charge is created. The interaction of the soles of the shoes and the carpeted floor causes a build-up of electrons on the surface of the person, while the flooring becomes more positively charged as it gives up electrons. The static electricity continues to increase until a maximum level is reached or the person contacts another surface (in this case, a metal file cabinet) that is at a different electrical potential. The static electricity is then transferred from the person in an electrostatic discharge event.
ESD is different from other, common overvoltage events (switching and surge transients)in that the time it takes ESD to transition from zero to maximum current and voltage is very short. The rise time of an ESD event is less than 1 nanosecond, while the other transients take longer than1 microsecond to reach their peaks.
The International Electrotechnical Commission (IEC) has developed a model of the human-generated ESD event.This model is used in the IEC’s test specification for determining if systems (computers, networks, cell phones, set top boxes, etc.) are susceptible to ESD events. The test specification quantifies the methodology for introducing ESD into the system as well as the various voltage and current levels that define the ESD event. Note again how fast the rise time is for the ESD event.