Several LED protection circuit design solutions

White light LED is a voltage-sensitive device. In actual operation, the current is capped at 20mA. However, the current often increases due to various reasons during use. If no protective measures are taken, the LED will be damaged after the increased current exceeds a certain time and amplitude . The main reasons for LED damage are:

①Sudden increase in power supply voltage. ②A short circuit of a certain component or printed line or other wire
in the circuit forms a local short circuit in the LED power supply path, which increases the voltage in this place. ③A certain LED is damaged due to its own quality reasons and forms a short circuit, and its original voltage drop is transferred to other LEDs. ④The temperature inside the lamp is too high, which deteriorates the characteristics of the LED. ⑤Water has entered the interior of the lamp, and water is conductive. ⑥During assembly, anti-static work was not done well, so that the interior of the LED has been damaged by static electricity. Even if normal voltage and current values ​​are applied, it is very easy to cause damage to the LED.

So, how can I protect the LED circuit ? Here are some tips:

1. Use fuses (tubes) in the protection circuit.

Since fuses are disposable, slow to react, poor in effect, and inconvenient to use, fuses are not suitable for LED lamps . Because LED lamps are now mainly used in urban lighting projects and lighting projects. It requires the LED protection circuit to be very demanding: when the normal current is exceeded, the protection can be immediately activated, so that 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, without affecting the operation of the LED. The circuit cannot be too complicated, the volume cannot be too large, and the cost must be low. Therefore, it is difficult to implement it using a fuse.

2. Use transient voltage suppression diodes (TVS for short)

Transient voltage suppression diodes are a high-efficiency protection device in the form of a diode. When its two poles are subjected to a reverse transient high-energy impact, it can immediately reduce the high resistance between its two poles to a low resistance at a very short speed of 10 to the negative 12th power of seconds, absorb surge power up to several kilowatts , and clamp the voltage between the two poles at a predetermined voltage value, effectively protecting the precision components in the electronic circuit. Transient voltage suppression diodes have the advantages of fast response time, high 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 is not easy to find TVS devices that meet the required voltage value. The damage of LED lamps is mainly caused by overheating of the chip due to excessive current. TVS can only detect overvoltage but not overcurrent. It is difficult to choose the appropriate voltage protection point, so this device cannot be produced and is difficult to use in practice.

3. Choose a resettable fuse

The resettable fuse is also called a polymer positive temperature thermistor PTC, which is composed of polymers and conductive particles. After special processing, the conductive particles form a chain-like conductive path in the polymer. When the normal working current passes (or the component is at normal ambient temperature), the PTC resettable fuse is in a low resistance state; when an abnormal overcurrent passes through the circuit (or the ambient temperature rises), the heat generated by the large current (or the ambient temperature rises) causes the polymer to expand rapidly, which cuts off the conductive path formed by the conductive particles, and the PTC resettable fuse is in a high resistance state; when the overcurrent (overtemperature state) in the circuit disappears, the polymer cools and the volume returns to normal, in which the conductive particles re-constitute the conductive path, and the PTC resettable fuse is in the initial low resistance state. In the normal working state, the resettable fuse generates very little heat, and in the abnormal working state, it generates very high heat and has a large resistance, which limits the current passing through it, thereby playing a protective role. In the specific circuit, you can choose:

① Shunt protection. Generally, LED lamps are divided into many series branches. We can add a PTC component in front of each branch for protection. The advantage of this method is high accuracy and good protection reliability.

② Overall protection. Add a PTC component in front of all light beads to protect the entire lamp. The advantage of this method is simplicity and does not take up volume. For civilian products, the results of this protection in actual use are satisfactory.