If there is an open-circuit in the LED string, all LEDs will be extinguished. Therefore, the industry has proposed a new LED open circuit protector solution to ensure that when one of the LED light strings fails, the other LEDs can still work normally, and also prevent damage caused by lightning or electrostatic discharge (ESD).
The LEDs are usually connected in series in the string. If one of the LEDs in the string is broken due to an open circuit, all LEDs in the string will go out. But if an LED is shorted, the rest of the string is essentially unaffected. In view of this, the industry proposes an LED break protector that can break one of the LEDs in the light string, but the other LEDs remain in normal operation and can prevent other electronic faults at the same time.
The LED open circuit protector can be connected in parallel with each LED in the string to shunt the tripped LED current to keep the rest of the string continuing to operate. The protector can also function when an overvoltage occurs in the circuit and causes an overcurrent in the string, or when the electrode is reversed.
Industry design engineers have tried many components such as Zener Diode, Metal Oxide Varistor (MOV) and Silicon Controlled Rectifier (SCR) to achieve LED break protection. However, LED circuit breakers are still the industry's first choice because LED circuit breakers are designed for these applications. As shown in Figure 1, when an LED has an open circuit fault, the voltage across it attempts to rise to the constant current output voltage of the power supply. The LED open circuit protector senses this voltage rise and begins to conduct, limiting its voltage drop to around 1.3 volts (V) to keep the rest of the LED string working properly.
Figure 1 When an LED has an open circuit fault, the LED circuit breaker begins to conduct, limiting its voltage drop to 1.3V and keeping the rest of the string working properly.
LED circuit breakers are available with different characteristics and voltage ratings. Some protectors protect individual LEDs in the string, while others allow design engineers to secure two to five LEDs with a single protector for cost savings. The LED circuit breakers introduced by industry players in previous years have been able to provide open-circuit LEDs and Lightning Surge protection, but they are not ideal for ESD events and polarity reversal protection. A new generation of LED circuit breakers provides both ESD and reverse polarity protection, and designers should select a new generation of components based on possible fault conditions and needs in the application.
ESD and lightning protection
ESD can enter the LED string when assembled or installed in the field. This may come from the machine to the LED string or the human body touches the LED string; the lightning can enter the LED string in two ways - the nearby lightning strike may be in A transient in the power supply, coupling the LED driver into the string, or a nearby lightning strike may be on the LED driver supply rail or may cause electrical transients on the LED string.
Lightning surges have a rise time of 8–10 microseconds (Figure 2), while the rise time of ESD events is only a few nanoseconds (ns) (Figure 3), where the difference depends on the detection method. Early LED circuit breakers were only resistant to lightning strikes, but were not resistant to ESD events. A new generation of LED circuit breakers, with the Littlefuse PLED 5 series as an example, can handle ESD events at ±15 kV air discharge, ±8 kV contact discharge, and ±40 amps (A) 5/50 Nai Second (ns) Electrical Fast Transient (EFT).
Figure 2 Lightning strikes have a rise time of 8 to 10 microseconds.
Figure 3 The ESD event has a rise time of a few nanoseconds. LED circuit breaker protects against ESD at +-15kV air discharge and +-18kV contact discharge
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