White LEDs are voltage-sensitive devices. In actual work, the current is 20mA as the upper limit, but the current will often increase due to various reasons in use. If the LED circuit protection measures are not taken, this increase After a large current exceeds a certain time and amplitude, the LED will be damaged. This article mainly introduces the causes of LED damage and circuit protection.
White LEDs are voltage-sensitive devices. In actual work, the current is 20mA as the upper limit, but the current will often increase due to various reasons in use. If the LED circuit protection measures are not taken, this increase After a large current exceeds a certain time and amplitude, the LED will be damaged.
The main causes of LED damage are:
1. Sudden increase in power supply voltage. There are many reasons for the sudden rise of the power supply voltage, such as overcurrent caused by overvoltage events such as transient lightning strikes, transient switching noise of switching power supplies, power grid fluctuations, power quality problems, or improper use by users It may cause the power supply voltage to suddenly increase.
2. A short circuit of a component or printed line or other wire in the circuit forms a partial short circuit of the LED power supply path, which increases the voltage in this place.
3. An LED is damaged due to its own quality and thus forms a short circuit, and its original voltage drop is passed on to other LEDs.
4. The temperature in the lamp is too high, which deteriorates the characteristics of the LED. When the junction temperature in the LED chip is increased by 10 ° C, the luminous flux will be attenuated by 1%, and the LED life will be reduced by 50%.
5. Water has entered the lamp, and the water is conductive.
6. The anti-static work was not done during assembly, which caused the interior of the LED to be damaged by static electricity. Although the normal voltage and current values ​​are applied, it is also easy to cause damage to the LED.
These reasons will cause the LED current to rise significantly, and soon the LED chip will be burned due to overheating. According to our experience, most LEDs are short-circuited after being burned, and a few are open circuits. Each LED has a voltage drop of about 3.2V. If it burns out, the LED string will be broken.
How to protect the circuit?
1. A fuse (tube) is used in the protection circuit
Because the fuse is disposable, and the reaction speed is slow, the effect is poor, and the use is troublesome, the fuse is not suitable for use in the finished LED lamp, because the LED lamp is now mainly in the city's glorious project and lighting project. It requires the LED protection circuit to be very harsh: protection can be started immediately when the normal use current is exceeded, the power supply path of the LED is disconnected, so that the LED and the power supply can be protected, and the power supply can be automatically restored after the entire lamp is normal. Does not affect the LED work. The circuit should not be too complicated, the volume should not be too large, and the cost should be low. Therefore, it is very difficult to realize by fuse.
2. Use transient voltage suppression diode (referred to as TVS)
Transient voltage suppression diode is a high-efficiency protection device in the form of a diode. When its two poles are impacted by reverse transient high energy, it can reduce the high resistance between its two poles to low resistance at a very short time of 10 negative 12th power seconds, absorbing surge power of up to several kilowatts. The voltage between the two poles is clamped to a predetermined voltage value, which effectively protects the precision components in the electronic circuit. Transient voltage suppression diodes have the advantages of fast response time, large transient power, low leakage current, good consistency of breakdown voltage deviation, easy control of clamping voltage, no damage limit, and small size.
However, in actual use, it will be difficult to find TVS devices that meet the required voltage values. 52soluTIon authoritative data description: The damage of the LED light bead is mainly caused by the overheating inside the chip of the current ambassador. TVS can only detect overvoltage but not overcurrent. It is difficult to choose the appropriate voltage protection point, this device cannot be produced and is difficult to use in practice.
3. Select self-recovery fuse
Self-recovery fuse, also known as polymer polymer positive temperature thermistor PTC, is composed of polymer and conductive particles. After special processing, the conductive particles form a chain-shaped 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-impedance state; when an abnormal overcurrent passes through the circuit (or the ambient temperature increases), the large current (or the ambient temperature increases) The generated heat causes the polymer to expand rapidly, which cuts off the conductive path formed by conductive particles. The PTC self-recovery fuse is in a high-resistance state; when the overcurrent (overtemperature state) in the circuit disappears, the polymer cools and the volume recovers Normally, the conductive particles reconstruct the conductive path, and the PTC resettable fuse assumes the initial low resistance state. In normal working state, the self-recovery fuse has very little heat. In abnormal working state, its heat is very high and its resistance value is very large, which also limits the current passing through it and thus plays a protective role. Of course, these reliable fuses can be purchased at large professional exhibitions such as Shenzhen Aidzz.
Speaking back to the specific circuit, we can choose:
â‘ Shunt protection. Generally, LED lights are divided into many serial branches. We can add a PTC component in front of each branch to protect separately. The advantages of this method are high accuracy and good reliability of protection.
â‘¡Overall protection. Add a PTC component in front of all the light beads to protect the whole lamp. The advantage of this method is that it is simple and does not occupy volume. For civilian products, the results of this protection in actual use are satisfactory.
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