First, why should we protect the LED?
White LEDs are gaining more and more people's daily lives because of their many advantages, and their usage is now very huge. It is a new device that has its own characteristics. White LEDs are voltage sensitive devices. The current should not exceed 20mA when working with each LED. If too many LEDs are used, it will be easily burned. If the LED is used normally, its life is very long. But in actual use, LEDs tend to be easy to break. What is the reason? In fact, it does not take into account the use characteristics of the LED and add protection to it.
LEDs are optoelectronic semiconductor devices that are susceptible to electrostatic shock during assembly. This requires electrostatic protection during the assembly process. We have found that many manufacturers do not have this concept or do not understand it at all. This is not acceptable.
In actual operation, the LED is limited to a current of 20 mA, but the current is often increased due to various reasons in use. If no protective measures are taken, the increased current exceeds a certain time and amplitude after the LED. It will be damaged.
Second, the cause of LED damage
1. Sudden rise in the supply voltage. There are many reasons why the power supply voltage suddenly rises. For example, the quality of the power supply or the improper use of the user may cause the power supply voltage to suddenly rise.
2. A short circuit of a component or printed line or other conductor in the line forms a partial short circuit of the LED power supply path, causing the voltage in this place to increase.
3. An LED is short-circuited due to its own quality, and its original voltage drop is transferred to other LEDs.
4. The temperature inside the lamp is too high, which makes the characteristics of the LED deteriorate.
5. The inside of the lamp is filled with water, and the water is electrically conductive.
6. At the time of assembly, there is no anti-static work, so that the inside of the LED has been damaged by static electricity. Despite the application of normal voltage and current values, it is extremely susceptible to LED damage.
These reasons will cause a significant increase in LED current, and soon the LED chip will be burnt due to overheating. According to our experience, most of the LEDs are short-circuited after being burned, and a small part is broken. Each LED has a voltage drop of about 3.2V. If it is broken, it will not emit light if it is broken. If this voltage is shorted, it will be transferred to other LEDs, causing more current to other LEDs. Other LEDs will be burned faster and even critically. It is extremely easy to cause a major accident with small damage. LEDs are generally installed at high places, which is not easy to install and is more difficult to repair. Therefore, the protection of LED is the actual demand, but it has not been valued by everyone at present, and it is also a problem that many people have no choice but to deal with.
Third, how to protect the LED
The protection of LEDs is the first thing we think of is the use of fuses, but the fuses are one-off, and the reaction speed is too slow, the effect is poor, the actual use is also very troublesome, so the fuse is not suitable for use in the current LED light products, Because LED lights are now mainly in the city's brilliance engineering and lighting projects. In response to this actual demand, we have done a lot of experiments, and according to the requirements of the project, we have summarized the characteristics of the LED protection circuit. It is very demanding: it can start the protection immediately after the normal use current, and let the LED supply path It is disconnected, so that the LED and the power supply can be protected. After the whole lamp is normal, the power can be automatically restored without affecting the LED work. The key is because it is a civilian product, and the added circuit can not be too complicated and the volume cannot be too large. The cost is low. These requirements are contradictory and mutually restrictive, and it is very difficult to achieve.
First, you should determine which protection circuit and protection device to use.
1. We can choose to use transient voltage suppression diode (TVS for short). The transient voltage suppression diode is a high efficiency protection device in the form of a diode. When its two poles are subjected to reverse transient high energy impact, the high resistance between the two poles can be immediately reduced to low resistance at a very short time of 10 minus 12 power seconds, absorbing up to several kilowatts of surge power. The voltage between the two poles is clamped to a predetermined voltage value, which effectively protects the precision components in the electronic circuit. The transient voltage suppression diode has the advantages of fast response time, large transient power, low leakage current, good uniformity of breakdown voltage, easy control of clamping voltage, no damage limit, and small volume.
However, it is not very satisfactory in actual use. The first is to find a TVS device that meets the required voltage value is not easy. TVS devices are mainly used for lightning protection and lightning protection, as well as overvoltage absorption above 220V, and the supply voltage of LED lamps is generally 24V or 12V. The TVS products of this voltage value are few and the test is not good. At the same time, we know that the damage of the LED bead is mainly caused by the overheating of the chip and the internal overheating of the chip. TVS can only detect overvoltages and cannot detect overcurrents. Overvoltage is definitely the cause of overcurrent, but it is difficult to master the proper voltage protection point. It is difficult to use this device and it is difficult to use it in practice.
2. We can choose to self-recover the fuse. The self-recovery fuse is also called a polymer polymer positive temperature thermistor PTC, and is composed of a polymer and conductive particles. After special processing, the conductive particles form a chain-like conductive path in the polymer. When the normal operating current passes (or the component is at normal ambient temperature), the PTC resettable fuse is in a low-resistance state; when there is an abnormal overcurrent in the circuit (or the ambient temperature rises), the large current (or the ambient temperature rises) The generated heat causes the polymer to expand rapidly, which cuts off the conductive path formed by the conductive particles. The PTC self-recovery fuse is in a high-resistance state; when the overcurrent (over-temperature state) disappears in the circuit, the polymer cools and the volume recovers. Normal, in which the conductive particles reconstitute the conductive path, the PTC self-recovery fuse is in an initial low resistance state. In the normal working state, the self-recovery fuse has little heat, and in the abnormal working state, its heat is very high, which limits the current passing through it, thereby protecting. It is small in size, low in cost, and can be used repeatedly. It realizes automatic exit of protection and automatic exit; it is not easy to be damaged by solid-state package impact resistance; we found in actual test: because it is a heat sensitive device, subject to temperature The impact is great. Since the PTC is packaged inside the luminaire, the bead must be heated to affect the performance of the PTC. For luminaires that have already been identified, the PTC can be selected by experiment. A more reliable method of use is to keep it away from the hot lamp beads.
In a specific circuit, there are two ways to choose:
1, shunt protection. The general LED light is divided into a number of serial branches. For example, 24V voltage, we are using 7 LED beads in series and then add a resistor, the current is generally 17~19mA, we can choose 7 integer multiple beads to combine into a whole lamp. We can add a PTC component in front of each branch for protection. The benefits of this approach are high accuracy and good reliability of protection.
2. Overall protection. A PTC element is added in front of all the beads to protect the entire lamp. The benefit of this approach is simple and does not account for size. We generally choose this method. In the case of household products, the result of such protection in actual use is still satisfactory.
The selection of PTC is very particular. We have explored the more accurate corresponding values ​​through a long time of experimentation.
Fourth, the electrostatic protection of LED
All matter is made up of atoms, which have electrons and protons. When a substance acquires or loses electrons, it becomes negatively charged or positively charged. These charges accumulate on the surface of the material and we call the object electrostatically charged. The accumulation of charge is usually caused by the separation of materials from each other, and can also be caused by friction, which is called triboelectric charging. There are many factors that affect the accumulation of charge, including contact pressure, friction coefficient, and separation speed. The electrostatic charge will accumulate continuously. If there is no bleed passage, this value will eventually reach a high level until the charge generation stops, the charge is vented, or sufficient strength is reached to break through the surrounding medium. After the dielectric is broken down, the electrostatic charge is quickly balanced, and the rapid neutralization of this charge is called electrostatic discharge. Due to the rapid bleeder voltage on a small resistor, the bleeder current can be large, possibly exceeding 20 amps. If this discharge is done by electrostatically sensitive components, such a large current will be designed to have only a turn-on voltage of 3V and LEDs with a current of 20 mA cause serious damage.
1. Why should we raise the awareness of static electricity protection?
Before the 70s of this century, many static problems were caused by people's lack of awareness of static electricity protection. Even now, many people suspect that electrostatic discharge will cause damage to electronic products. This is because most of the electrostatic discharge damage occurs below human perception, because the human body's perceived voltage for electrostatic discharge is about 3KV, and many electronic components are damaged at hundreds of volts or even tens of volts. Usually, electronic devices are electrostatically discharged. There is no obvious boundary after the damage, and the components are mounted on the equipment and then tested. As a result, many problems occur and the analysis is quite difficult. Especially for potential damage, it is difficult to measure significant changes in performance even with precision instruments. However, experiments in recent years have confirmed that the reliability of electronic products has dropped significantly after a certain period of time. The damage caused by static electricity is absolutely true.
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