Electronic Design

Open LED Shunt Protection Devices Prevent Failure in Multiple LED Strings

LEDs have become quite pervasive and are found in everything from backlighting in televisions, computer monitors, and other displays to transportation uses for instrument and dashboard backlighting, avionics signage, ambient lighting, instrumentation and more.  Low and zero maintenance lighting environments such as street lighting and traffic lights benefit from the longevity and reliability of LED lighting.  Intrinsically safe, LEDs are the lighting option of choice for mining, oil exploration, and emergency lighting applications. 

As LEDs continue to penetrate numerous market sectors and become the preferred method of lighting, the requirement for circuit protection should be considered.  Typical LED lighting designs include solid-state luminaries in which multiple LEDs are connected in a string to achieve the desired level of illumination.  In this configuration, a single open LED can cause the entire string to go dark, resulting in reduced performance, possible maintenance calls and costly warranty returns. 

Adding an open LED shunt protection device that has been designed specifically for LED lighting applications allows the unaffected lights in the string to remain illuminated by shunting current around the inoperable LED.  This article will provide an overview of LED lighting and examine common reasons for LED failures.  It will also discuss the configuration of LED lighting designs and examine the use of next-generation shunt protection devices as an effective circuit protection solution in lighting applications that use multiple LED strings. 

LED Lighting and Causes of Failure

LED designs are easily scalable due to the inherent modularity of the compact LEDs.  For example, multiple strings of 1 Watt LEDs can be configured to create higher power designs.  In common LED street lighting, six strings of 15 LEDs for a total of 90 LEDs are driven by an efficient, high voltage, constant current power supply. 

Accurate current control is essential to govern the power, temperature, and maximum light output of the LEDs.  For ESD protection, most LEDs have an integrated Zener voltage regulator.  While low power LED string designs may utilize Zener diodes in this role, high power LEDs would require very large voltage regulators, and an integrated Zener may not be able to handle the full current of the LED string.

Typical lighting LEDs have a forward voltage of approximately 3 V and forward current of approximately 150 mA.  The maximum light output is governed by the maximum current capability and the temperature.  Accurate control of current is essential to control power and temperature. 

There are three typical failure modes for LEDs, each shown in Figure 1.  As an LED ages, the amount of light it emits decreases until it reaches a percentage associated with “End of Life.”  Second, the LED die can be shorted due to overvoltage, over-current, or over-temperature conditions.  Finally, the wire bond connecting the LED chip to the lead frame may break or burn open.  This often is caused by over dissipation of the LED and results in the optical transparent layer melting.  The forces of the associated cooling and heating can damage the chip’s wire bond, disconnect the integrated Zener voltage regulator, and cause an open LED failure.  A shunt protection device designed specifically for LED lighting applications can protect LEDs from these open circuit failure conditions (see fig. 1).

New Open LED Shunt Protection Devices Safeguard Sensitive LED Designs

Circuit protection beyond a Zener voltage regulator is necessary to allow the remaining LEDs to continue working in the presence of a single open circuit condition.  An open LED shunt protection device can isolate the open circuit and allow current to flow uninterrupted through the string of LEDs.  The next-generation device employs thyristor technology and during normal operation of the circuit there is no current flowing through the device.  Only in the event of an open circuit does the device become active. 

The break-over voltage, VBO, is proportional to the number of intended LEDs in a substring.  It must be high enough not to clip the forward voltage and also lower than the supply voltage.  Whenever the voltage across the device exceeds the rated break-over voltage, the shunt protection device switches to its “on” state.  In this state, the voltage across the device falls to about 1 V and minimizes power dissipation.  The maximum threshold voltage is 1.2 V, which is less than the forward voltage of the LED.  The required current to operate a next-gen shunt protector is less than the current consumed by an operational LED.  Therefore, the luminaire power dissipation is reduced if an LED fails.  A low, standby current when it is in the “off” state provides minimal system power loss.  To ensure that the shunt protection device switches on reliably, the break-over current, IBO, is specified below typical LED operating current and high enough to prevent false triggering.

Next-generation open LED shunt protection devices can 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.  Figure 2 shows examples of possible configurations of LEDs that can be protected with various next-generation shunt protection devices in four voltage options.

Open LED Shunt Protection Devices

Next-generation LED shunt protection devices are designed with a lower trigger current to ensure they operate when necessary and a lower standby current to ensure power efficiency, making them an excellent solution for prolonged operation.  Warranty returns or maintenance calls as a result of an LED failure can be costly in terms of money and time, which is especially unfortunate when a single open LED impairs a large portion of the application.  The latest shunt protection devices provide a circuit protection solution that leads to greater reliability and increased Mean Time Between Failure (MTBF).  When the shunt protection device is connected in a string of LEDs, a single LED failure does not interfere with the operation of the unaffected LEDs and allows the lighting string to continue operating instead of shutting down the entire string of LEDs.  These next-generation devices are also compatible with LED controllers, including those utilizing fast switching for dimming functions and can be operated in the “on” state for the lifetime of the light fixture.

Some specific design considerations must be taken to ensure the optimal use of open LED shunt protection devices in the application:

1. LEDs must be connected in series strings,
2. If strings are put in parallel, a separate current source is needed for each string,
3. The LED current must not exceed the rated max current of the shunt protection device,
4. The current source must have headroom voltage as defined in table 1 below:

Maximum Number of LEDs

Maximum LED Drop (V)

Power Supply Voltage Headroom Required (V)

1

3

13

2

6

12

3

9

17

4

12

21

Table 1: LED Current Source Headroom Voltage Requirement

LED lighting applications are soaring in volume and adequate circuit protection is essential for the longevity and to maintain the affordability of LEDs.  Specialty designed open LED shunt protection devices can help increase the useful life of an LED, minimizing the cost of repairs and replacement with its ability to protect lighting applications that use multiple LED strings.  Bourns has developed a line of new open LED shunt protection devices specifically designed to address the failure rates of early LED designs and to keep LED strings reliably illuminated.  To learn more about open LED shunt protection devices, visit http://www.bourns.com/ProductLine.aspx?name=led_shunt_protectors

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