What is ESD?
Electrostatic discharge (ESD) is a phenomenon of a sudden and momentary flow of electric current (i.e., sudden discharge of charges) between two electrically charged objects at different potentials. The build-up of static electricity between two objects is caused by the event called tribocharging or electrostatic induction. A visible (sometimes invisible) spark occurs when two differently charged objects are brought very close to each other causing the dielectric breakdown between them. These visible or sometimes invisible sparks are called ESD.
Static electricity due to tribocharging is one of the causes of ESD. In tribocharging, the separation of electric charges (static electricity) between the two objects is created when these objects are brought into contact and then separated. When one object is positively charged, and another one is negatively charged. Examples of tribocharging include rubbing a balloon against a sweater, walking across a carpet, picking up a polythene bag, walking on a vinyl tiled surface, working on a bench, and so on.
Tribochraging results in static electricity (creation of electric potential between two objects) and can lead to ESD when the objects of opposite polarities are brought very close to each other. For example, a human walking across a carpet has the static potential of 30 kV. If that human touches a sensitive electronic device/component, it results in an ESD event.
Electrostatic induction is another cause of ESD. Electrostatic induction occurs when an object is placed in an electric field. When an object like a piece of metal comes in the contact with an electric field, the electrons of the metal are attracted towards the external source, resulting in the positive charge being distributed at the other end. If the external source is a negative charge then the polarity of the charged ends reverses. For example, charged surfaces of synthetic materials (i.e., plastic, Styrofoam, etc.) or bags can induce potential on nearby ESD sensitive devices via electrostatic induction, and an ESD event may occur when a conductive metal tool touches these objects.
The other causes of ESD events in the electronics field are the handling of printed circuit boards or troubleshooting electronic devices without using an electrostatic wrist strap, and rapid movement of air near electronic equipment.
An ESD event occurs with peak impulse current in the Ampere range having rise time in the range of pico-seconds to the nanoseconds. For example, an ESD event due to a human may have rise times ranging from approximately 200 ps to greater than 10 ns with peak impulse currents from a few amperes to greater than 30A. A person may/may not feel the discharge, but that is sufficient to damage sensitive electronic components. Hence, it is necessary to test the electronics products against ESD.
ESD event occurs due to the potential differences therefore, the major ESD protection approach is to ground all the elements or bring all elements to the same potential. The standards ANSI ESD S20.20 and EN100-015/1-1991 cover the requirements for designing, establishing, implementing, and maintaining ESD protection.
Figure 3: ESD pulse as per IEC 61000 4-2
The IEC 61000-4-4 standard is the international EMC standard to conduct an ESD immunity test on equipment to check the ability of the equipment against ESD. As per this standard, ESD pulses (between 4 kV and 8 kV) are used during the immunity test. Here, ESD pulses are applied directly or indirectly (via air) only to the points and surfaces that can be touched by the user during usual operation/handling (e.g., cleaning or adding consumables when the EUT is powered). Typically, the EUT will be exposed to at least 200 ESD pulses, 100 pulses with negative and 100 pulses with positive polarity, at a minimum of four test points. This standard describes the ESD immunity test of the EUT for Human Body Model (HBM), Machine Model (MM), and Charged-Device Model (CDM).
What is an ESD Generator?
Figure 4: ESD Generator
An ESD generator is used to simulate/generate the ESD pulses as per EMC standards such as IEC 61000 4-2 to test the immunity of the equipment against ESD. An ESD generator generates ESD pulses to simulate the Human Body Model (HBM), Machine Model (MM), and Charged-Device Model (CDM).
How is an ESD Pulse is Generated?
A simplified circuit of an ESD generator is shown below. Let us discuss the HBM model.
Figure 7: Simplified circuit of the ESD Generator
As per IEC standard, the charging resistor Rc has a value of 50 to 100 M ohms, the total value of energy-storage capacitor Cs and distributed capacitance Cd is Cs + Cd = 150 pF± 10%. The capacitor Cs represents the electrostatic capacitance of the human body. The discharge resistor Rd represents the resistance of the human skin (330 Ohms ± 10%). Initially, the capacitor (CS + Cd) is charged to a certain value through the charging resistor Rc by closing the switch (S1). During charging, the discharging switch (S2) is in an open state. After charging, the switch (S1) is opened, and now the switch (S2) is closed, now the capacitor (Cs + Cd) starts to discharge, which results in electrostatic discharge on the EUT occurs.
Key Specifications of an ESD Generator:
Discharge network/pulse network: Represents the discharge network capacitor and resistor values of the ESD generator. For example, 150 pF, 30 Ω.
Rise time: Represents the impulse current rise time of ESD event in ns (nanosecond). It is the time taken by the current from 10 % to reach 90% of the peak value.
Discharge modes: Air discharge (AD) / Contact discharge (CD).
Air discharge voltage: Represents the amplitude of the air discharge voltage in kV that can be provided by the ESD generator.
Contact discharge voltage: Represents the amplitude of the contact discharge voltage in kV that can be provided by the ESD generator.
Discharge Polarity: Represents the Polarity of the ESD voltage from the ESD generator. The ESD generator has the capability to generate burst voltage in Positive/negative/alternating polarity
Repetition Frequency: Represents the number of ESD pulses per second.
Discharge repetition CD: Represents the time gap range provided by the ESD generator for the repetitive contact discharges. It is represented in second.
Trigger mode: Represents the triggering mode of the ESD generator. The ESD generators are available with Auto/Manual mode triggering options.
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