High power LED is more and more used in traffic lights, luminous billboards, street lights, airport runways, railway crossings, medical / surgical lighting and traffic signals. They have long service life (high-power white LEDs usually have a service life of 35000 to 50000 hours) and low power consumption. However, the LED will also be damaged due to overvoltage, overheating and even reverse power connection caused by lightning stroke and electrostatic discharge (ESD).
Because LEDs are usually connected in series in the lamp string, if one LED in the lamp string is open due to fault, all LEDs in the string will go out. Littelfuse provides led open circuit protector. The protector can be connected in parallel with each LED in the lamp string. It can bypass the LED with open circuit fault to keep the rest of the lamp string working. When overvoltage occurs in the circuit and leads to overcurrent in the lamp string, or when the electrodes are reversed, the protector will also work.
When an LED has an open circuit fault, the LED open circuit protector will start to turn on, limit its voltage to 1.3V, and keep the rest of the series to continue to work.
In order to achieve this, designers have tried many devices, such as zener diode, metal oxide varistor and thyristor rectifier, but they are not as good as LED open circuit protector, because led open circuit protector is designed for this application. As shown in Figure 1, when an LED has an open circuit fault, the LED open circuit protector senses the voltage rise and begins to turn on, limiting its voltage drop to about 1.3V to keep the rest of the lamp string working normally.
Led open circuit protector has different characteristics and rated voltage. Some protectors are designed to protect a single LED in the lamp string; Others allow designers to protect two, three, four or five LEDs with a single device to save costs. Littelfuse is a new generation of LED open circuit protector, which can provide ESD protection at the same time.
ESD and lightning protection
ESD can enter the LED light string during assembly or field installation. Lightning can enter the LED light string in two ways: nearby lightning may cause transient in the power supply, which can be coupled to the LED driver and enter the LED light string; Or nearby lightning strikes may cause electrical transients on the power supply rail of the LED driver or on the line connected to the LED lamp string.
The new generation of LED open circuit protector, such as littlefuse pled 5 series, can cope with ESD events of ± 15kV air discharge, ± 8Kv contact discharge, and EFT events of ± 40A 5 / 50 ns (fast pulse group).
Polarity reverse protection
When an LED light string equipped with pled5 is reversely connected to the power supply, all protectors will begin to turn on, shunting the current on these protected LEDs to protect them from injury. After the reverse electrode is corrected, the protector will close and the LED will work normally.
Responsible editor; zl