Static Electricity Generation Mechanism

Usually, static electricity is generated by friction or induction.

Tribostatic electricity is generated by contact friction or the movement of charge generated in the process of separation of two objects. Static electricity left over from friction between conductors is usually relatively weak due to the strong conductivity of the conductors. The ions formed as a result of friction will move together rapidly and will be neutralized during and at the end of the friction process.

After friction, the insulator can generate a high electrostatic voltage, but the amount of charge is very small. This is due to the physical structure of the insulator itself. In the molecular structure of an insulator, it is difficult for electrons to move freely without being bound to the nucleus. Therefore, friction results in only a small amount of molecular or atomic ionization.

Induced static electricity is an electric field created by the movement of electrons in an object under the influence of an electromagnetic field. Induced static electricity can only be produced in conductors. The effect of the space electromagnetic field on the insulator is negligible.



Electrostatic Discharge Mechanism

220V mains power can kill people, but thousands of volts cannot. What is the reason? The voltage at both ends of the capacitor meets the formula: u = q / C. According to this formula, when the capacitance is too small, a small amount of charge will produce a high voltage.

Usually the capacitance of our bodies and the objects around us is very small. When a charge is created, a small amount of charge will also produce a high voltage. Due to the small amount of charge, the current generated during discharge is very small, the time is very short, the voltage cannot be maintained and drops in a very short time. Because the human body is not an insulator, static charges accumulated all over the body will accumulate when there is a discharge path, so people feel that the current is larger, and people feel like electric shock. After static electricity is generated in conductors such as the human body and metal objects, the discharge current will be relatively large.

For materials with good insulation performance, the amount of load produced is very small; on the other hand, the charge produced is difficult to flow. Although the voltage is high, when there is a discharge path somewhere, only a very small gap at the contact point and nearby can flow and discharge, while the charge at the non-contact point cannot be discharged. Therefore, even with a voltage of tens of thousands of volts, the discharge energy is very small.

Therefore, although the static voltage of the plastic turnover box, packaging foam and chemical fiber carpet is very high, the discharge energy is very small.



Damages of Static Electricity to Electronic Components

Static electricity is harmful to LEDs and is not a unique "patent" of LEDs. Even widely used diodes and triodes made of silicon materials are under threat. Even buildings, trees and animals can be damaged by static electricity (lightning is a type of static electricity, which we will not consider here).

So how does static electricity damage electronic components? I don't want to go too far, let's just talk about semiconductor devices and it's limited to diodes, triodes, ICs and LEDs.

Causes of Damage

The damage done by electricity to semiconductor components is ultimately the introduction of current. Under the influence of current, the device is damaged by heat.

If there is current, there must be voltage. However, semiconductor diodes have a P-N junction, whether forward or reverse, the P-N junction will have a voltage range that blocks current. The front barrier is low and the back barrier is much higher.

In a circuit where the resistance is large, the voltage intensifies. But looking at the LED, when forward voltage is applied to the LED, when the external voltage is less than the threshold voltage of the diode (the size corresponds to the forbidden bandwidth of the material), there is no forward current, and all the voltage is applied to the PN junction.

When reverse voltage is applied to the LED, all of the voltage is applied to the P-N junction when the external voltage is less than the reverse breakdown voltage of the LED. Currently, no matter the LED's virtual solder joints, bracket, P area or N area, there is no voltage drop because there is no current.

When the PN junction is broken, the external voltage will be shared by all resistors in the circuit. Where the resistance is larger, which part carries the higher voltage. In the case of the LED, it is natural for the P-N junction to carry most of the voltage. The thermal power produced at the PN junction is the voltage drop across it multiplied by the current value. If the current value is not limited, excessive heat will burn the PN junction and the PN junction will lose its function and pass through.



Why Are ICs More Afraid of Static Electricity?

Because the area of ​​each element in the IC is very small, the parasitic capacitance of each element is also very small (usually the circuit function requires very small parasitic capacitance). Therefore, a small amount of electrostatic charge will produce a high electrostatic voltage, and the power tolerance of each component is usually very small. Therefore, electrostatic discharge can easily damage the IC.

But ordinary discrete components, such as ordinary low-power diodes and low-power transistors, are not very afraid of static electricity, because the chip area is relatively large, and the parasitic capacitance is relatively large, and it is not easy to accumulate. voltage on them in general static determination.

Low-power MOS transistors are easily damaged by static electricity due to the thin gate oxide layer and small parasitic capacitance. Generally, the three electrodes are short-circuited after the packaging is completed, and the short-circuit wire generally needs to be removed after soldering is completed in use.

And because of the high-power MOS tube, large chip area, normal static electricity will not damage them. So you will find that the three electrodes of the power MOS tube are no longer protected by shorting lines (the first manufacturers still shorted them before they left the factory).

The LED actually has a diode and its area is huge relative to each component in the IC. Therefore, the parasitic capacitance of the LED is relatively large. Therefore, static electricity cannot damage the LED in general situations.

In general, static electricity, especially static electricity generated on the insulator, has a very high voltage, but the amount of discharge charge is very small, and the duration of the discharge current is very short.

The voltage of static electricity induced on the conductor may not be very high, but the discharge current can be very large and is usually a continuous current. This is very harmful for electronic components.




Why Doesn't Static Electricity Often Damage LEDs?

Let's first look at an experimental phenomenon. A piece of metal iron plate has 500V static electricity. Put the LED on the metal plate (pay attention to the placement method to avoid the following problems). Are you saying that the LED will be damaged?

Here, the premise for the LED to be damaged should generally be applied with a voltage greater than the breakdown voltage, that is, the two electrodes of the LED must touch the metal plate at the same time and have a greater voltage. breakdown voltage.

Since the iron sheet is a good conductor, the induced voltage is uniform throughout it. The so-called 500V voltage is relative to ground. Therefore, there is no voltage between the two electrodes of the LED, so naturally there will be no damage. If you do not connect one electrode of the LED to the iron plate and the other electrode is not connected by a conductor to ground or another conductor (hand or wire without insulating gloves).


LED Damage Conditions Caused by Static Electricity

The above test case reminds us that when the LED is in an electrostatic field, one electrode must be in contact with the electrostatic body and the other electrode must be in contact with the ground or other conductors to be damaged. In actual production and practice, with small volume LED, such a thing is unlikely to happen, especially at parties. Accidental events are possible. For example, the LED is on an electrostatic body, and one electrode is in contact with the electrostatic body, and the other electrode is immediately suspended. If someone touches the hanging electrode at this time, it may damage the LED.

The above phenomenon tells us that the problem of static electricity cannot be ignored. Electrostatic discharge requires a conductive circuit and will not harm if static electricity is present. When only a small amount of leakage problem occurs, it can be considered a static accidental damage problem. If a large amount occurs, it is more likely to be a chip contamination or stress issue.