As a new green lighting technology, high-power white LED has many advantages, such as high light conversion efficiency, low energy consumption, long life and no need to maintain. It has been applied to various lighting fields. LED long life is the safe working environment based on LED for lighting, there are two main threats affect their life: one is the electric shock, the current is applied on the LED exceeds the maximum rated current of the LED technology in the data sheet, including electric shock caused by overvoltage; another is overheating damage. These damage can be shown as an immediate failure of the device, and may also occur for a long time after an overcurrent shock and shorten the working life of the LED.

The cause of damage to LED lamps and lanterns

The failure of LED lamps is due to failure of power and drive, and two is from the failure of the LED device itself. Usually the damage to the LED drive power and drive comes from the overpower impact (EOS) of the input power supply and the breakdowns at the load end. The overpower impact of the input power often causes the damage of the driving chip in the driving circuit, and the breakdown of the passive components such as the capacitance. The short-circuit fault at the load side may cause overdrive of the drive circuit, and the driving circuit may cause overheating damage due to short circuit damage or short circuit fault. The failure of LED device itself is mainly in the following cases.

1. electrostatic discharge event

Electrostatic discharge (ESD) damage is a transient overvoltage harm most common is a highly integrated semiconductor device manufacturing, transportation and application, and the LED lighting system shall meet the IEC61000-4-2 standard of "the human body electrostatic discharge mode 8kV contact discharge, to prevent electric shock failure in the system may be caused by electrostatic discharge.

The performance of the LED PN junction array will be reduced or damaged, as shown in Figure 2. The internal failure of the LED chip caused by the ESD event discharge path, which may be only partial function damage, can also cause permanent damage to the LED.

2. transient overcurrent events

Transient overcurrent event refers to the current through the LED exceeds the maximum rated current of the LED technology in the data sheet, this may be due to the large current generated directly may also be indirectly produced by high voltage, such as lightning and transient switching power supply transient switching noise, power fluctuations caused by overcurrent overvoltage events. These events are transient and have very short duration. We usually call them peak peaks such as "current peak" and "voltage spike". The transient overcurrent events also include the LED power supply, or the transient overcurrent at the time of charged plug - in.

For the LED lighting in a car, the transient surge load of ISO7637-2 is an important threat to its normal work.

The failure mode of LED after electric shock is not fixed, but it usually causes damage to the welding line, as shown in Figure 1. This damage is usually caused by a great transient overcurrent. In addition to causing a welding line to burn, other parts of the welding line may be damaged, such as sealing material.

Common cause and protection scheme of LED lamps and lanterns

Heat management and fault overheating protection are a challenge for LED lighting design which is nearly 80% energy into heat. Both theory and practice have shown that the performance and life span of LED is closely related to the PN junction working temperature of LED. When the temperature of the LED chip is increased by 10 degrees C, the luminous flux will decrease by 1%, and the life of LED will be reduced by 50%.

Protection of power and drive circuit

Because the LED power drive circuit is easily damaged by overcurrent and short circuit fault, we should take full account of the protection measures of various fault states in the drive circuit design, so as to improve the reliability of the circuit and reduce the repair rate. The LVR series PTC of Tyco Electronics can achieve overcurrent and short circuit protection for AC power supply, and ROV is used for overpower shock and surge protection. The POLYZEN series is an over voltage and over current integrated protection element used for DC power input port, and the surface mount Series in LED lamp string is used to protect overload or open circuit fault.

Common cause and protection scheme of LED lamps and lanterns

Overcurrent and overheating protection of PPTC in LED lighting

Although many high-end LED drivers are integrated with over current and overheating protection functions, in many cases, especially in terms of overheat protection, discrete component schemes are still needed as a cooperative protection scheme, so that lamps and lanterns are of higher reliability and longer life.