Protection is key to making the most of an LED’s lifetime potential

by Nathan Orton, Design and Quality Manager
Marl International Limited

Service lifetimes of 70,000 – 100,000 hours, and power savings of up to 85% are leading to growing adoption of LEDs in place of other forms of indication and illumination. Incandescent bulbs, halogen lamps and even fluorescent tubes are being replaced in control panels and ambient lighting applications alike.

Since the rated life of the LED can be longer than the service life of the system in which it sits, the component is potentially maintenance free. Not only does this eliminate the direct cost of having a fitter replace the component at intervals, but it also reduces or eliminates the cost of a supporting stock of spares, as well as the need for preventative maintenance inspections.

However, the surrounding environment contains many hazards that can bring about the premature demise of an LED component. Given that an LED has twenty times the operational life of an incandescent bulb, it is also twenty times more likely to encounter abuse or accident. Environmental factors – shock, vibration, moisture ingress - can be to blame. An electrical malfunction or error during installation can expose it to a spike or a reverse current. A silent ‘killer’ is heat. Junction over-temperature can slowly but surely eat away at the life of the component – and impair its efficiency in the process.

A well designed LED fixture provides appropriate protection against such issues, and makes the difference between a component that is essentially maintenance-free, and one that still requires inspection and replacement at intervals.

The environment
In the industrial or transportation environment, specifying an IP67 sealed component that is fully potted for maximum resistance to vibration and shock is a no-brainer. In truth, there is no application where there is zero risk of the component being bathed in coffee or coke, or falling to the floor from an overburdened pair of arms. Marl, for this reason, fully pots almost every LED lamp providing outstanding protection against fluid ingress, shock and vibration. The cost overhead of doing so really isn’t that great, and is infinitely smaller than the cost to the customer than replacing a damaged device in the field.

Marl pots all its LED components to provide protection against damage by fluids, shock and vibration.

To ensure the fixture survives should the equipment be dropped, or worse physically attacked, the front needs to be toughened as well. Precautions that can be taken include the use of shatterproof glass and polycarbonate lenses and metal housings. Where there is serious risk of vandalism, tamperproof fixings can be provided.

Electrical malfunction
In electrical environments that are ‘noisy’ no engineer would dream of fitting an LED without providing appropriate over-voltage and reverse voltage protection. But in the end over-voltages and reversed polarities can come the way of an LED in any circuit. One can never entirely discount the risk of the power supply being connected backwards or adjusted incorrectly.

Fitting a diode or bridge rectifier is straightforward, and need not occupy much space. These simple precautions give reverse voltage protection or allow AC operation, making life easier for the designer, the production engineer and the service manager alike.

This bulb replacement LED incorporates a bridge rectifier to simplify electrical connection – but still fits into the same physical form factor as the bulb it replaces.

Similarly, some LEDs will survive higher levels of overvoltage than others. A 24V LED should not be exposed to more than 24V, but a quality device may survive exposure to as much as 32V. A lesser component will blow at 28V or less. If this sounds a fine distinction, the issue should be considered in the context of our expectations of the component. For an incandescent bulb with a service life of 5,000 hours, the expectation is that it will require frequent replacement. For an LED with a rated life of 100,000 hours, the expectation is that it will not, and it is much more noticeable when it does.

Thermal factors
LEDs are moving from indication to illumination – from the control panel to building and vehicle lighting applications. Although an LED generates minimal heat by comparison with other lighting technologies, it does still generate some. At 1W or above this thermal output can be enough to create issues, especially during prolonged use. Management of the heat output is critical to the longevity for LED components.

The key factor here is the junction temperature of the diode. Keeping the junction cool improves not only the service life but also the efficiency of the LED. The Lamina light engine used by Marl can be used at a junction temperature of 125°C – but is much more efficient if its junction temperature is kept to 55-60°C.

Marl uses a simulation driven design process in order to achieve a thermal design for its products that balances good heat removal with maintaining the minimum size and weight of heat sink. Thermal analysis using SolidWorks COSMOS 2007 software gives the temperature distribution, temperature gradient and heat flowing in the product – as well as the heat exchanged between the product and its environment. The movement of heat through the component and heat sink into the environment can be pictured by plotting heat flux vectors. The investment in software is amply repaid by the results it achieves. All prototypes are subjected to tests in a temperature chamber, and the measured temperatures are normally within 3-4° of the simulation result. Using this process, Marl has never had to redesign a product following the building of the prototype.

A heat distribution image from Marl’s thermal modelling software.

Fit and forget
LED panel indicators and lighting are revolutionising the industry by offering fit and forget illumination that can be all but eliminated from the maintenance schedule. A well designed fixture around the diode itself is needed to allow the LED component to achieve its full potential, in the face of the environmental, electrical and thermal issues it is likely to encounter during its operational life.