The market for LED-based emergency lighting systems is arguably one of the most mature. Although LED devices have been suitable for simple exit signs for about 20 years now, they have only become feasible for use in emergency lighting systems in the last 6 years. Improvements in the efficacy and color consistency of white LEDs, as well as price reductions, have been the catalysts for change.
Technology continues to evolve and prices erode, making LEDs steadily more applicable in a wider range of emergency lighting systems. Furthermore, innovative design combined with the advantages of LED technology can provide an important competitive edge for lighting designers. Advances in battery/energy storage systems for back-up operation and circuit protection devices, as well as the LEDs themselves, are creating new opportunities.
This article will review the state-of-the-art technology in LED-based emergency lighting systems, with emphasis on new component developments. For example, improved efficiency, cooler running, lower power, and more rugged LEDs are improving reliability and cutting maintenance costs. Typical devices include Osram’s Golden Dragon Plus and Duris P5, Philips Lumileds LUXEON® Z, and the STW8 range from Seoul Semiconductor.
Circuit protection solutions add peace of mind for emergency lighting in safety-critical applications and especially where there is a risk of fire or explosion. Devices such as LED shunt protectors from Bourns and the Polyswitch circuit protectors from TE Connectivity are popular options here.
Finally, battery technology has evolved rapidly in recent years, with the trend towards lithium ion offering smaller, higher-power battery back-up solutions. However, in certain safety-critical applications these are considered a fire risk unless properly protected. An alternative, and sometimes complementary, solution is the ultracapacitor. Eaton’s PowerStor range has already found favor in a range of applications, including emergency lighting.
Wherever people gather, whether in or around buildings, shopping centers, underground car parks, or in aircraft and mass transit transport systems, emergency lighting is an essential factor. Effective lighting is crucial to ensure fast, panic-free evacuation, especially in the case of fire. Well-lit, well-spaced emergency exit signs, indicating the shortest escape route, are important as people quickly become disorientated. Typically, they will try to return by the route which they entered the building, which may not be the most efficient.
Fluorescent lamps are expected to remain in use in older installations, but are increasingly being replaced with LED lamps. LED modules that replace traditional T8 fluorescent tubes are becoming popular retrofit units. Generally, LEDs are demonstrating a number of performance and cost advantages, which are encouraging their more widespread use in many emergency lighting applications. Wherever major lighting systems are being upgraded or replaced with LED technology, it is highly likely that ancillary emergency lighting installations will also be replaced with LEDs.
The growing number of benefits of LED technology is making a compelling case. The longevity and reliability of LED lamps is well suited to emergency systems, reducing service intervals and maintenance costs. Using no lead or mercury, LEDs are considered more environmentally friendly. They are considered to be intrinsically safe, making them the lighting option of choice in mining, oil exploration, and other potentially explosive environments. With no glass content, LEDs are more robust, more resistant to shock and vibration, and less vulnerable to damage from vandals when installed in public locations.
The generally-lower power operation of LEDs compared to traditional lighting technologies is ideal for running lighting systems from back-up batteries or emergency generators that kick-in when mains power has failed.
The nature of LED lamps, as point light sources, makes it easier to control the focus of light to where it is needed; which combined with high optical efficiency, further helps to reduce energy usage. In its simplest form, according to the UK’s ICEL (Industry Committee for Emergency Lighting)1, two 1 W LEDs with a typical 60o beam plus driver, inverter, and a 3.6 V battery, installed at 3 meters above the floor at recommended spacing, should have no problem achieving the required brightness at floor level, according to international standards.
An additional benefit of LEDs, particularly in large-scale lighting scenarios, is the choice of form-factor and the possibility of combining emergency lighting with architectural lighting. Luminaires can be constructed into virtually any size or shape, and then easily incorporated into structures and materials within existing building features. Lighting strips are particularly useful for stairs and hallways, and other modules can be flush-mounted in floors and walls or may be retrofitted into existing structural features. The ability to better conceal emergency lighting, or incorporate it aesthetically into a building, gives far greater flexibility for lighting system designers and their customers.
A case study presented by Osram Semiconductor2 illustrates what can be achieved in this area. The entrance to the Karlplatz Stachus underground (U-bahn/S-Bahn) station in Munich, Germany features a staircase that, due to construction reasons, could only be illuminated by the handrails, totaling 700 meters in length. This lighting doubly serves as the emergency lighting, powered by a battery in the event of a power cut. The installation of Osram LEDs, with their low power consumption, requires just 100 W per 18 meters of handrail illumination.
Figure 1: The entrance area of the S-Bahn and U-Bahn underground transport network in Munich’s Stachus, showing the LED-illuminated handrail that doubles as emergency lighting for the staircase if the power fails.
The type and specification of LEDs required for emergency lighting systems will depend largely on individual applications. However, there are some common features that are most desirable, such as low power, cool running, and high luminous efficiency. Cool white LEDs with a color temperature between 3000 and 6500 K, a CRI of 80 or better, and an efficiency of around 75 to 100 lm/W are typical requirements.
Other features to consider, depending on application, include small size, durable and robust housing, ease and flexibility of module design to create strings or other shapes, and the ability to operate from battery back-up or other temporary power sources.
Devices such as Osram Opto Semiconductor’s Duris P5 range of LEDs are clearly suitable for emergency lighting applications. A neutral white (4000 K) device, the GW DASPA1.EC-GUHQ-5L7N-1, provides the high-efficacy, mid-power, and long lifetime performance required for emergency marker lights, such as those used on steps and exit routes. With a typical luminous flux of 29 lm at 100 mA, optical efficiency of 96 lm/W and CRI of 80, the device is supplied in a leadless SMD-2 package. Typical forward voltage is 3.02 V, maximum forward current is 250 mA, and viewing angle is 130o. Within the Duris P5 range are warmer (3000 K) and cooler (5000 K) light output devices with similar specifications to the 4000 K version.
For applications requiring higher luminous efficiency, with albeit higher power consumption, Osram offers the LUW W5AM series in the Golden Dragon Plus line. Typical luminous flux is 116 lm at 350 mA and up to 273 lm at 1 A. Again, designed specifically for marker lights in emergency lighting systems, the devices offer a highly-efficient light source in a very-low profile package. Optical efficiency is quoted at 146 lm/W at 100 mA, maximum forward current is 1 A, typical forward voltage is 3.2 V, and thermal resistance is 6.5 K/W.
Figure 2: Osram’s high-efficiency Golden Dragon Plus LED for marker lighting applications.
Similarly performing devices are available in the Philips Lumileds range. The LUXEON Z 4070 provides a color temperature up to 4000 K and a CRI of 70. Typical luminous flux is 138 lm at 500 mA. Typical efficiency is 95 lm/W. A useful additional parameter quoted on the datasheet is device size versus light output achievable, stated as 63 lm/mm² for the mid-range LED. The devices can be supplied unencapsulated, ideal for incorporating into strips or modules.
Figure 3: Phlips LumiLEDs LUXEON Z LED in a 2.2 mm² micro footprint.
Within its extensive Acrich range of single LEDs and multi-LED modules, Seoul Semiconductor can provide a number of parts at the cooler white end of the spectrum. The 6500 K STW8Q14BE is just one example in the range of single LEDs, aimed at general, interior, and architectural lighting applications. Incorporating ESD protection, the LED itself is encapsulated in silicone and assembled into a heat-resistant thermoplastic body. Electrical specifications include a forward voltage of 3.2 V (typical), forward current of 160 mA (maximum), luminous intensity of 35 lm (3700 to 7000 K), CRI of 80 (minimum), and a thermal resistance of 18°C/W.
While some pre-assembled LED-based emergency lighting modules are supplied with the relevant circuitry to switch automatically from mains to back-up and back again, additional LED shunt protection devices can be installed to help increase the robustness and reliability. Extra protection can help minimize the cost of repairs and replacement of LED lamps, especially when used in long strings of multiple-series LEDs which may be driven from a relatively high voltage compliant, constant-current power supply.
There are a number of benefits to configuring strings of multiple LEDs. A simpler power supply design is a primary advantage, while a high-voltage, low-current power supply improves the efficiency of the power supply itself. Scalability, modularity, and flexibility of form-factor are important factors influencing the choice of strings of LEDs over other lighting technologies. However, the downside is that an open LED on an LED string will switch off all the devices on that string. LED failures can be caused by high temperatures, electrostatic discharge (ESD), or overvoltage and overcurrent events.
Low-power LED strings can be easily protected by Zener diodes. For high-power applications (operating at higher than 75 mA), LED shunt protection devices are recommended, and in active mode, will dissipate less power than the open LED would have done. These thyristor-based devices operate by shunting the current around the inoperable LED, creating a bridge, to ensure the healthy diodes in the string remain illuminated.
Bourns is a leading supplier of LED shunt protection devices. Their LSP0600BJR-S operates at 6 V. To maximize the level of protection available, designers should use one device per LED. Alternatively, to minimize costs, other parts in the range offer protection to 9, 13 and 18 V, can be used to isolate groups of two, three or four LEDs respectively.
Bourns has produced a useful white paper3 that explains the power supply issues concerning the compliance voltage of a constant current power source. Example circuits and designs are provided, highlighting the demands made on the power supply in specific failure situations incorporating the Bourns LSP series devices. Guidance on appropriate power supply architectures for LED strings is also included.
Protecting LED lighting fixtures, as well as emergency lighting batteries from overcurrent and heat damage, is the primary function of the Raychem circuit protection products from TE Connectivity. The company points out that, although considerably longer lived than the lighting technologies it replaces, LED lifespan depends on a number of factors: junction temperature, operating voltage, and operating current.
Resettable polymeric positive temperature coefficient (PPTC) devices, such as those in TE’s enduring PolySwitch and PolyZen ranges, have demonstrated their effectiveness in protecting LED circuitry. Typical applications include power input protection from power line coupled transients and surges, LED driver circuit output protection, and LED ESD protection. They are particularly effective in high-reliability emergency lighting installations because they reset themselves after the fault is cleared and the power is cycled.
Figure 4: Selection from the broad range of PolySwitch resettable devices, which can be used to protect LED emergency lighting circuitry.
According to the manufacturer, most LED applications use power conversion and control devices to interface with the power source to control power dissipation from the LED driver. Protecting these interfaces from overcurrent and overtemperature damage is the role of resettable PPTC devices such as those in the PolySwitch range.
In typical operation, the fuse has a low resistance value. In the event of an overcurrent condition, the device trips into a high resistance state. This helps protect the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low, steady-state level. The device remains in the latched position until the fault is cleared. Once power to the circuit is cycled, the device resets, the current flow resumes and the circuit is restored to normal operation. Further detailed information and circuit diagrams can be found in an earlier Digi-Key TechZone article.4
Many LED driver ICs require a protected DC input to provide stable current to the diode. A PolySwitch device, in series with the LED driver IC together with a parallel voltage limiting device, such as a Zener or transient suppression diode, help provide effective protection. The PolySwitch components are available in a range of sizes, with tiny 1608-packaged devices for the most compact applications, saving board space and cost. Designers can select current ratings from 50 mA to 3 A and voltage ratings from 6 to 60 V.
A typical device for protecting ICs is the 6 V nanoSMDC150F, featuring a current-hold of 1.5 A, a trip rating at 3 A, and maximum current of 100 A. Time to trip is 0.3 s.
Supercapacitors are increasingly finding favor as a back-up power source in certain applications, and especially in safety-critical systems, where lithium-ion batteries may pose a fire hazard. Compared to batteries, supercapacitors are capable of delivering more than ten times the power and more than a thousand times the cycle life. They offer energy density three orders of magnitude higher than traditional electrolytic capacitors.
They also work well in tandem with conventional battery back-up technologies, to enhance the performance and lifetime of such systems. A feature of interest in emergency lighting applications is their use in hold-up and bridging power systems, ensuring constant power even when mains power fails. Supercapacitors are considered more environmentally friendly than batteries, and can help designers downsize their battery needs as well as extend battery life.
A leading player in the supercapacitor market is Eaton. Their XB and XV series cylindrical devices deliver 300 to 400 F capacitance, with operating voltages of 2.5 V and 2.7 V respectively. Featuring ultra-low ESR for high power density, these devices ensure minimal voltage drop during peak current demand. They are considered particularly suitable for UPS and back-up power systems, as well as emergency lighting installations.
A typical device in the XB range is the PowerStor XB3550, delivering 300 F capacitance at 2.5 V. Top of the XV range is the PowerStor XB3560 supercapacitor, delivering 400 F capacitance and rated at 2.7 V.
Figure 5: Eaton’s PowerStor supercapacitors for back-up and power-bridging functions in a range of safety and industrial applications.
Although a less-than-glamorous application, emergency lighting systems are benefiting from technology advances, not only in LEDs themselves, but also in ancillary subsystems for the all-important circuit protection and battery back-up functions. Emergency lighting designers have the opportunity to revamp and differentiate their product offerings in this sector, both for new installations and retrofit units.