A longer life for LED Power Electronics
One of the greater challenges with LED lighting is the electronic driver’s robustness to normally occurring transients or power surges in the network. Many everyday examples have shown that the active electronics in the light sources find it hard to handle the effects in a use environment, in which incandescent bulbs and lamps with passive electronics have functioned fine for decades. A new project funded by the Innovation Network Smart Energy (INNO-SE) will characterise the electronic environment with regard to the occurrence of these surges.
Big advantages of LEDIt is not difficult to see the advantages of LED lighting. Major energy savings, as well as political pressure for uniform and better lighting, has led to a variety of infrastructure applications such as street lighting and lighting in large buildings, where LED lighting is part of the solution. The switch to LED light sources has been relatively easy to integrate into new as well as existing installations. In terms of investment, the switch has had financial advantageous or at the very least not been detrimental, as the savings gained by the long lifetime and low energy consumption have been able to balance out the costs of investing in a replacement of light sources. However, practical experience has shown that it has not always turned out as expected. There has been discussions in specialised trade magasines about particular vulnerability to voltage transients (short-term high voltage of many kV) in the case of lightning strikes, but also generally poor durability, which does not measure up to the expectations of a 2-digit lifetime.
Vulnerable electronic driversPut simply, an LED light source consists of two parts: the light element itself (the LED chip) and the power electronics (the driver), which supplies the power to the light element. The light elements, the LED diodes, have after several years of development achieved a high level of quality where light output and lifetime are favourable. In principle, the driver is simply a switch mode power supply that can deliver suitable power to the light element. In some designs it is also possible to operate various types of electronic dimming of the luminosity, which makes the lights more user-friendly and offers additional energy savings.
However, even simple drivers consist of far more components than the previous era’s passive connections. This more complex structure means that there is also a greater risk that a vital component will break, and the active electronics are more vulnerable to environmental influences, e.g. temperatures, humidity, and voltage transients (power surges). The stress from the electrical environment can be severe. Figure 1 shows a test where a surge reaches up to many kA in the PCB traces of a power supply.
Fictional lifetime dataTraditionally, the lighting industry has indicated the lifetime for a light source based upon the lifetime of the light element, because this was the crucial factor. Other components in the light source had almost always had a much longer lifetime. This was the case with the incandescent light bulb, the halogen bulb, fluorescent tubes, etc. However, with LED lighting the situation has changed radically, because even if the LED light element itself has an extremely long lifetime, the many other components, particularly in the electronic driver, have a far shorter lifetime. Clearly, the lifetime of the overall system is then the most critical component.
The industry has however retained a tradition for stating the lifetime for the light element itself, perhaps because this offers a very positive claim for the technology. In reality, however, this lifetime indication does not cover the entire device. The typical lifetime indication, e.g. 50,000 hours, is currently referred to as “lumen maintenance life”, i.e. an indication of how much luminosity will drop over time. A typical drop of 70% of the original light output is often used as the criteria. This only covers the LED light element, and then only its change in light. But the LED element rarely fails, and even then not usually catastrophically. This is not the case however for the many electrolytic capacitors, varistors, diodes, transistors and other components that make up the driver, and when the driver fails, the light disappears.
Between two worlds – manufacturers of consumer electronics – infrastructureThe driver circuits are often purchased from sub-suppliers, which manufacture very large numbers of the units. Large-scale production helps to keep prices down, and the drivers are supplied for consumer products all over the world. However, the consumer market is not typically a market where you can expect the electronics to last for 10 or 20 years.
This is contrary to infrastructure installations, where the aim is to ensure flawless street lighting without the need for maintenance throughout at least a decade. The infrastructure’s need for a long lifetime under demanding installation conditions is a strict requirement for electronic designs. The use environment has been well described in regard to temperatures, humidity, and mechanical influences, but the electrical influences are not so familiar. The on/off function, voltage transients, and overtones (harmonic interference) in a power grid with multiple light sources can be a much greater challenge for the electronics than expected.
Measuring and simulation of the environmentThe purpose of the innovation project is to measure and characterise the electrical environment of large-scale installations with LED light sources. Together with a number of industry partners as well as AAU (Aalborg University), DELTA (a part of FORCE Technology) will undertake measurements and calculations of the electrical environment. These activities will lead to a better description of the electrical requirements for the lighting network, and thus lead to better opportunities for specifying and designing durable electronics, thereby allowing the full benefits of LED light technology to be obtained. The owners of lighting installations, e.g. municipalities with street lighting and companies with larger buildings, can then look forward to reduced costs.
The results of the project will be communicated to the industry through professional networks and with the publication of an official report. The project is expected to be completed at the end of 2017.