LED Circuit Protection

White LED is a voltage-sensitive device. In actual operation, the upper limit of the current is 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. This article mainly introduces the causes of LED damage and circuit protection.

White light LEDs are voltage-sensitive devices. In actual operation, the upper limit of the current is 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:

1. Sudden increase in power supply voltage. There are many reasons for the sudden increase in power supply voltage, such as transient lightning strikes, transient switching noise of switching power supplies, overcurrent caused by overvoltage events such as power grid fluctuations, power supply quality problems, or improper use by users, etc., which may cause the power supply voltage to suddenly increase.

2. A short circuit of a component, printed line or other wire in the circuit forms a local short circuit in the LED power supply path, causing the voltage in this place to increase.

3. If an LED is damaged due to its own quality reasons and forms a short circuit, its original voltage drop will be transferred to other LEDs.

4. The temperature inside the lamp is too high, which deteriorates the characteristics of the LED. When the junction temperature inside the LED chip rises by 10°C, the luminous flux will decrease by 1%, and the life of the LED will be reduced by 50%.

5. Water has entered the lamp, and water is conductive.

6. When assembling, the anti-static work is not done well, so that the internal part of the LED has been damaged by static electricity. Even if the normal voltage and current are applied, it is very easy to cause damage to the LED.

These reasons will cause the LED current to increase significantly, and soon the LED chip will burn out due to overheating. According to our experience, most LEDs burn out due to short circuits between the two poles, and a few are open circuits. Each LED has a voltage drop of about 3.2V. If it burns out and the circuit is open, the LED string will be damaged.

How to protect the circuit?

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. 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 both 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 using fuses.

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

Transient voltage suppression diode is 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 up to several kilowatts of surge power, clamp the voltage between the two poles at a predetermined voltage value, and effectively protect the precision components in the electronic circuit. Transient voltage suppression diode has 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 is not easy to find TVS devices that meet the required voltage value. The authoritative information of 52solution.com states that the damage of LED light beads 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 normal working conditions, the resettable fuse generates very little heat, and in abnormal working conditions, it generates very high heat and has a large resistance value, which limits the current passing through it, thereby playing a protective role. Of course, these reliable quality fuses can be considered to be purchased at large professional exhibitions such as the Shenzhen Electronics Exhibition (aidzz).

Back to the specific circuit, we can choose:

①Branch protection. Generally, LED lights 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 that it is simple and does not take up space. For civilian products, the results of this protection in actual use are satisfactory.