In the LED industry, it seems that static electricity is the number one enemy of damaged LEDs. But I don't think so. 1. Mechanism of static electricity generation Usually, the generation of static electricity is caused by friction or induction. Frictional static electricity is generated by the contact of two objects in contact with friction or the generation of charge during separation. The static electricity left by the friction between the conductors is usually weak, because the conductors have strong electrical conductivity, and the ions generated by the friction will move together and neutralize during the friction process and at the end of the friction. After the insulator is rubbed, a higher electrostatic voltage may be generated, but the amount of charge is small. This is due to the physical structure of the insulator itself. In the molecular structure of the insulator, it is difficult for electrons to move freely from the bond of the nucleus, so the friction can only produce a small amount of molecules or atomic ionization. Inductive static electricity is an object in an electric field. Under the action of an electromagnetic field, electrons in an object move to form an electric field. Inductive static is generally only produced on conductors. The effect of the space electromagnetic field on the insulator is negligible. 2. Electrostatic discharge mechanism The 220V mains can kill people, but the voltage of thousands of volts on people can not kill people. What is the reason? The voltage across the capacitor satisfies the following equation: U = Q / C. According to this formula, when the capacitance is small, a small amount of charge generates a high voltage. Usually our body, the objects around us, the capacitance is very small, when the charge is generated, a small amount of charge will also produce a very high voltage. Since the amount of charge is small, the current formed during discharge is very small, the time is very short, the voltage cannot be maintained, and the time is reduced in a very short time. Since the human body is not an insulator, the static charge accumulated throughout the body will be collected in the case of a discharge path, so the current is felt to be larger and there is a feeling of electric shock. When a conductor such as a human body or a metal object generates static electricity, the discharge current will be relatively large. For materials with good insulating properties, one produces a very small amount of charge, and on the other hand, the generated charge is difficult to flow. Although the voltage is high, when there is a discharge path at a certain point, only the contact point and the charge in a very small range can flow and discharge, and the charge at the non-contact point cannot be discharged (who calls it an insulator). Therefore, there is a voltage of tens of thousands of volts, and the discharge energy is also negligible. As shown in Figure 8. Therefore, although the electrostatic voltage of the plastic turnover box, the packaging foam, the chemical fiber carpet, etc. is very high, the discharge energy is very small. 3, the harm of static electricity to electronic components Static electricity can be harmful to LEDs. It is not a unique "patent" for LEDs. It is also a common diode and triode made of silicon material, which is also threatened. Even buildings, trees, and animals can be damaged by static electricity (thunder electricity is a kind of static electricity, we will not consider it here). So, how does static electricity damage electronic components? I don't want to go too far. I only talk about semiconductor devices, and I am limited to diodes, transistors, ICs, and LEDs. The damage to the semiconductor components by electricity is ultimately accompanied by current. Under the action of current, the device is damaged due to heat. To have current, there is a voltage. However, semiconductor diodes have a PN junction, and either the forward or reverse direction, the PN junction has a voltage range that blocks the current. The positive barrier is low and the reverse barrier is much higher. In a circuit, where the resistance is large, where is the voltage concentrated. But look at the LED, when the voltage is positively applied to the LED, when the external voltage is less than the threshold voltage of the diode (the size corresponds to the material forbidden band width), there is no forward current, and the voltage is all added to the PN junction. When the voltage is reversely applied to the LED, when the external voltage is less than the reverse breakdown voltage of the LED, the voltage is also added to the PN junction. At this time, the virtual solder joint of the LED is also worth mentioning, the bracket is also called, the P zone is also the N zone. There is no voltage drop! Because there is no current. When the PN junction breaks down, the external voltage is shared by all the resistors on the circuit. Which part of the resistance is large, and which part is responsible for the high voltage. As far as LEDs are concerned, it is natural that the PN junction bears most of the voltage. The thermal power generated at the PN junction is the voltage drop across it multiplied by the current value. If the current value is not limited, too high heat will burn out the PN junction, and the PN junction will lose its effect and pass through. Why ICs are more afraid of static electricity, because the area of ​​each component in the IC is very small, and the parasitic capacitance of each component is very small (often the circuit function requires very small parasitic capacitance), so a small amount of electrostatic charge will High electrostatic voltages are generated, and the power tolerance of each component is usually small, so electrostatic discharge can easily damage the IC. However, the usual discrete components, such as ordinary low-power diodes and low-power transistors, are not very afraid of static electricity, because their chip area is relatively large, and the parasitic capacitance is relatively large. Generally, static static voltage does not easily accumulate high voltage on them. Since the low-power MOS transistor is thin and has a small parasitic capacitance, it is easily damaged by static electricity. Usually, after the package is completed, the three electrodes are short-circuited and then shipped out. It is also often required to remove the short-circuit line after the welding is completed. For high-power MOS tubes, due to the large chip area, the general static electricity will not damage them. So you will see that the three electrodes of the power MOS tube are now protected from short-circuit lines (the early manufacturers still shorted them and shipped them out). The LED actually has a diode whose area is very large compared to each component in the IC. Therefore, the parasitic capacitance of the LED is relatively large. Therefore, static electricity in general can not damage the LED. In general, static electricity, especially static electricity generated on insulators, has a high voltage, but the amount of discharge charge is extremely small, and the discharge current duration is short. The static electricity induced on the conductor may not be very high, but the discharge current may be large and often a continuous current. This is very harmful to electronic components. 4. Why do static electricity damage LEDs not happen often? Let's look at a test phenomenon first. A piece of metal iron plate with 500V static electricity, put the LED on the metal plate (pay attention to the method to avoid the following problems), everyone said that the LED will be damaged? Here, the LED is to be damaged, and should generally be applied with a voltage greater than its breakdown voltage, that is, the two electrodes of the LED are to be in contact with the metal plate at the same time and have a voltage greater than the breakdown voltage. Since the iron plate is a good conductor, the induced voltages everywhere are equal. The so-called 500V voltage is relative to the ground. Therefore, there is no voltage between the two electrodes of the LED, and naturally no damage is caused. Unless you touch one electrode of the LED to the iron plate, the other electrode you connect to the ground or other conductor with a conductor (hand or wire without insulated gloves). The above test phenomenon suggests that when an LED is in an electrostatic field, it must be an electrode that contacts the electrostatic body, and the other electrode that is in contact with the ground or other conductor may be damaged. In actual production and application, with such a small size of LED, there is very little chance of such a thing happening, especially in batches. Occasional events are possible. For example, the LED is on the electrostatic body, and one electrode is in contact with the electrostatic body, and the other electrode is just floating. When someone touches the electrode that is suspended, the LED may be damaged. The above phenomenon tells us that the problem of static electricity is not negligible. Electrostatic discharge is to have a conductive loop, and it is not damaged by static electricity. When only a very small amount of leakage problems occur, the problem of accidental damage to static electricity can be considered. If it occurs in a large amount, it is more likely to be a problem of chip contamination or stress. KNM2 Series Moulded Case Circuit Breaker
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