Jan 24, 2017: Usually, LED streetlights have LED luminaries configured in series of strings of LEDs. This type of configuration has several benefits, for example, a number of LEDs can be arranged in series to make the the power supply design simple. Secondly, high voltage, low current power supply improves efficiency of the power supply, and thirdly, modular design can allow easy scaling, for example, using 1 W LEDs – 8 W, 16 W, 24 W or 32 W designs can easily be created with multiple strings of 8 x 1 W LEDs.
However, in this type of configuration, an open LED can affect the performance of the streetlight and cause complete darkness. This also affects the reliability, performance and maintenance of the streetlight, leading to high cost of maintenance. When an LED string is highly stressed, the LEDs develop an open circuit, which happens due to repeated cooling and heating stresses inside an LED package.
But this problem has a solution. An LED shunt protector (LSP) device can be added to the LED string design, which shunts current around the inoperable LED, and enables the unaffected LEDs in the string to remain illuminated. The electronic shunt current bypass such as an LSP device isolates the open circuits, and enables the current to flow uninterrupted through the LED string. Since the device is inactive, no current flows through it. The device becomes active only when a circuit is open.
What is an LED shunt protector (LSP)?
An LSP product is designed to make an open LED circuit failure mode tolerable. To comply with Energy Star rating, LED lighting fixtures must have a three year warranty, and produce >70 % of new light output for 35k hours (residential: 25k hours).
While low power LED strings can use zener diodes to rectify this problem, high power LEDs require very large and expensive zeners.
Bourns offers LSP devices for all types of LEDs that allow the remaining LEDs in a string to continue to work when an LED fails. LSP devices in active mode dissipate less power than the LED.
The advanced LSP devices feature various voltage options and are designed to protect substrings of one, two, three, or four LEDs. The number of LEDs in a substring is the number of LEDs that will go dark when one of the LEDs in the substring fails.
How the LSP works
LSP devices start working where the breakover voltage is proportional to the number of LEDs in a substring, which is high enough not to cut out the forward voltage but lower than the open circuit LED voltage when the breakover current is drawn.
When the voltage across the device exceeds the rated breakover voltage, the LSP device switches to its ?on? state, lowering the voltage to 1V. It minimises power dissipation and the maximum threshold voltage is 1.2V, which is less than the forward voltage of the LED.
The LSP?s ?off? state features a low standby current ensuring minimal system power loss. The LSP device?s required operating current must be less than the current consumed by an operational LED. Therefore, using a switched mode constant current supply actually reduces the luminaire power dissipation if an LED fails. To reliably switch on the LSP, the breakover current is specified below the minimum LED operating current and high enough to prevent false triggering.
For more info: http://www.bourns.com/productline.aspx?name=led_shunt_protectors