Basic concepts and principles of static electricity generation and
Need to know diode-based static electricity countermeasures


Static electricity is a phenomenon in which electric charges are accumulated on the ground and insulated objects due to friction, peeling, etc.

The instantaneous current can reach several A, but the actual flow of electricity is very short, so it is rare for people to be injured by static electricity. However, electronic components, semiconductors, and products can easily be damaged by static electricity, and in the case of chemicals, it can even lead to explosions.

Therefore, not only circuit designers but also plant/facility managers must have a broad understanding of static electricity. This is necessary to prevent accidents in advance, prevent damage to products, and prevent disasters caused by explosions.

Through this article, we will look at the basic concepts of static electricity, the principles of its generation, and other pulse voltage sources, and learn about their characteristics and methods of countermeasures against static electricity based on diodes.


Understanding Electric Fields
Frictional electricity was discovered around 600 BC by Thales of Greece, who observed that dust stuck to decorative amber as he cleaned it.

The total amount of (+) and (-) charges in a substance is normally neutral. However, when two substances rub against each other, electrons move from one substance to the other. At this time, the electricity that is not moving inside the material is called static electricity, and there is no essential difference from the electric current that drives the circuit.

Discharge occurs when a charge is generated in an insulator and the charge is transferred to a conductor by direct contact or induction. This movement of charge is called discharge, and at this time, it appears as if current is flowing for a short moment. This is also called electrostatic discharge.


EOS and ESD
EOS is an abbreviation for 'Electrical Overstress', which literally means a state where electricity is under a lot of stress. It means all stresses that exceed the absolute rating set by the product. It also includes ESD, EFT, etc.

ESD is a discharge phenomenon in which a finite amount of charge moves rapidly between two objects with different potentials. In simple terms, it is a phenomenon in which a well-coupled state is rapidly coupled when a finger is touched. This phenomenon is considered to be a current movement because the charge moves, and the image emitted at this time is a spark.


Electrostatic transfer principle and strength
There are four principles and intensities of electrostatic transfer: △Direct Connection △Secondary Arcing △Electric Field △MAgnetic Field.

Direct Connection is the principle that the discharge is directly transmitted to the signal line or power line and is directly conducted to the internal circuit. Secondary Arcing is the principle that the discharge occurs in the external case and the secondary discharge is transferred to the internal circuit.

Electric Field refers to the case where an electric field is formed by discharge and transmitted through electrostatic coupling to the internal circuit. Lastly, the MAgnetic Field is a case where a magnetic field is formed by a discharge and is inductively coupled to the internal circuit and transmitted.

The intensity of the shock ranges from 1 to 12, with 12 being the highest, meaning you'll feel like your entire hand has been hit hard.


Diode Specifications
There are eight diode specifications: VR (reverse voltage), VF (forward voltage), IF (forward current), IFM (peak forward current), IFSM (surge current), P (permissible power), trr (reverse recovery time), and C (capacitance).

ESD measures
VCC is the most commonly applied method, which is to place one of the two diodes on (+) and the other on (-) to configure two in series. This method has an operating structure in which the diodes each take on a role for positive (-) polarity static electricity and positive (-) polarity static electricity. Therefore, it is also configured in the ports of the IC's internal circuit.

TVS (Transient Voltage Suppression) is an equivalent circuit that consists of two Zener diodes facing each other. This device has a diode turn-on characteristic in the forward direction and a breakdown in reverse bias, making use of the characteristic of rapidly turning on in the reverse direction.

A varistor (variable resistor) is a semiconductor element whose resistance value changes depending on the voltage applied across the element, and its composition is a compound semiconductor mainly composed of ZnO (zinc oxide). At the rated voltage, the insulation resistance of the insulator is maintained, but when an instantaneous overvoltage occurs, the resistance value rapidly decreases to a conductor of several mΩ or less.


PCB Countermeasures
ESD countermeasure components should be placed close to external I/O connectors and the length of the trace (PCB pattern) of the ESD component should be minimized. If countermeasures are not taken at the source of noise (surge), there is a risk that the surge pulse will be transmitted to the adjacent PCB.

Therefore, it is not important to use a lot of ESD countermeasure parts because if the ground pattern to absorb noise (surge) is unstable, it will have no effect.

The grounding wire of the ESD device must be connected to the chassis or power ground, and if connected to the signal ground, there is a concern that the signal ground may shake due to a momentary surge voltage. In cases where there is no chassis or power ground or circuit connection is not possible, the PCB pattern must be processed thick and short over a wide signal area.

Capacitance is designed sharply so that ESD can stay and explode. If it explodes, the line will be damaged, so it is designed at 45 degrees. EMI noise that is designed at 90 degrees is radiated a lot.

Also, for ground setup, the design must be done at the corners. At this time, it should be designed in a round shape to minimize ESD impact.


Ethernet device layout
The reason for opening up the Ethernet device is that when ESD comes in, the charge circulates. As it circulates, it generates a magnetic field, and this loop antenna phenomenon can be a problem. So you have to block the path by opening it. Then, it can escape to the ground through the Monting Hole. In addition, there are measures to allow it to escape through the capacitor.


Induced current and induced electromotive force in switching circuits
Induced electromotive force refers to the spark phenomenon caused by switching. When initially closed, energy is generated in the reverse direction. Since this phenomenon does not have energy, it appears as induced electromotive force.

When the switch is opened after a certain period of time, energy is continuously stored and then the current is extracted momentarily. When the current is extracted and there is no path, a large induced voltage is generated when it goes backwards. The greater the inductance, the stronger the effect.

Therefore, this phenomenon must be prevented, and the circuit for this countermeasure is the snubber circuit. This happens a lot in relays that turn on and off high voltages that humans cannot do, and the relay itself has a lot of inductance.

If you model it, the current will continue to flow when closed and energy will be stored. The moment you open the switch, the stored energy will appear and be withdrawn. In this way, voltage is created instantaneously. This problem occurs because the voltage is large. So if this keeps happening over and over again, the system will take damage.

Therefore, if you have a snubber circuit using a diode, you must take measures to ensure that the remaining current can be absorbed within the diode.

At this time, you can use a diode, a resistor, or a capacitor. In addition, when it is expressed as voltage or current according to time change, when it is considered as frequency, a capacitor with frequency pass characteristics is consumed in the resistance value.

The short pulse voltages that occur momentarily can be considered as noise when viewed on a frequency diagram. That is why a snubber circuit is installed.

The countermeasure against static electricity using diodes is to use TVS diodes, Varistors, etc. to counteract or bypass diode properties. It requires a lot of know-how and requires multifaceted research, but it has the disadvantage of creating ground paths and forming many bypass paths.

Static electricity can easily damage electronic components, semiconductors, and products, and can cause chemical explosions at any time, resulting in disaster. Therefore, we need to understand, learn more about, and be careful about static electricity.