White LEDs are voltage-sensitive devices. In actual operation, the current is limited to 20 mA, but the current is often increased due to various reasons in use. If no protective measures are taken, this increased current The LED will be damaged after a certain amount of time and amplitude.
The main causes of LED damage are:
1 Sudden rise in the supply voltage.
A short circuit of a component or printed wiring 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 A certain LED is damaged due to its own quality and thus forms a short circuit, and its original voltage drop is transferred to other LEDs.
4 The temperature inside the lamp is too high, which deteriorates the characteristics of the LED.
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.
So, how do I protect the LED circuit?
1. Use a fuse (tube) in the protection circuit
Since 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 glory engineering and lighting engineering of the city. It requires the LED protection circuit to be very demanding: the protection can be started immediately when the current is exceeded, the LED power supply path is disconnected, the LED and the power supply can be protected, and the power 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 implement the fuse.
2. 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, in actual use, it has been found that it is not easy to find a TVS device that satisfies the required voltage value. The damage of the LED beads is mainly caused by overheating inside the chip due to excessive current. TVS can only detect overvoltages and cannot detect overcurrents. It is difficult to master the proper voltage protection point, and the device cannot be produced and is difficult to use in practice.
3. Select self-recovery fuse
The self-recovering 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. In the specific circuit, you can choose:
1 branch protection. The general LED light is divided into a number of serial branches. We can add a PTC component in front of each branch to protect them separately. The benefits of this approach are high accuracy and good reliability of protection.
2 overall protection. A PTC component is attached to the front of all the beads to protect the entire lamp. The benefit of this approach is simple and does not account for size. For civilian products, the result of this protection in actual use is still satisfactory.
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