Replacement of Conventional Light Sources by LED

The latest LED technology opens up wide areas for new applications, new technical possibilities and reduced costs. The main advantages of the state of the art LED light sources are:

• The life time of an LED reaches or frequently exceeds the life time of the application and there are often no special changes necessary. If one defines the failure criteria as end of life when the light intensity of an LED has reached 50% of the initial light intensity, depending on the model, one can calculate numerical life times above 100,000 hours for LED Chips based on GaAs. In the case of GaN – based chips (blue, true green, white) the life time strongly depends on the packaging technology and could be in the range between 7000 hrs and 100,000 hrs.
• In the case of colored light the power consumption is below the power consumption of white light sources with filter glass (see table below). White LEDs can now reach the level of fluorescent triphosphor tubes and standard white LEDs show a better efficiency than incandescent or halogen lamps.
• The radiant intensity of an LED is controllable in the whole range without any change in the light properties (e.g. dimming of incandescent lamps leads to a shift in color temperature) and any time loss (switching time of LEDs for general lightening < 1 μs). The PWM (pulse width modulation) is the recommended method for the dimming of LEDs. Dimming by reduced DC current may lead to inhomogeneous appearance of LED arrays.
• Compared to the high efficient light sources of fluorescent tubes, compact lamps and metal halide high-pressure lamps the switching time of LEDs is very short. This plays an important role especially for applications like head lamps and brake lights in automotive and traffic applications.
• LEDs provide excellent stability for mechanical and thermal stress. The reliability of an LED is described by the different failure mechanisms, the spontaneous total failure of the LED and the reduced light intensity of the LED. To calculate the reliability of an LED array one should use the data for spontaneous failure. They should be in the range of 10 ppm/h within the first hour of operation including solder processes and below 0,1 ppm after the first hours of usage.
• The light quality of incandescent lamps is excellent (CRI 100%). White LEDs provide color rendering indices between 70% and 85%, better for daylight (6000 K) than for warm white (2900 K) LEDs.

The table below lists the properties of different light sources including costs of ownership:

The calculation is based on a price of 0,1 € / kWh. This LED- data has been taken from different data sheets of top suppliers. Due to the diverse applications the costs of maintenance and ownership need to be calculated against the relevant application. The loss of power supplies has not been considered. One key aspect in the application of LEDs as light sources is the management of thermal loss. This problem is new for LED applications, previous typical slogans are “LEDs are low cost, low heat, and have low failure rates”. The LED producers resolved the problem of the thermal resistance between the LED chip and the connecting pad with different solutions like metal core PCB, specially designed lead frames, to based solutions and ceramic-based packages.

Good solution can provide a thermal resistance between p/n junction and solder pads below 20 K/W. The excess heat can be removed by different passive and active ways. The metal core P.C.B. or flexible P.C.B. usually directly mounted on a heat sink or air fan or in specials cases water cooling is used to keep the p/n junction temperature below the critical limit. In addition one should consider that the radiant intensity has a negative temperature coefficient. As an example red LEDs have an TC in the range of -0,5 % / °C and an increase of the p/n junction temperature from room temperature (20°C) up to 85 °C reduces the radiant intensity by 32.5%.

The picture shows a solution for the thermal management of a warm white LED source for a street lamp. The warm white LED modules for this street lamp were developed by OSA Opto Light GmbH, Germany.

1. 1.In this case an LED with a thermal resistance below 10 K / W has been mounted on a metal core PCB. The circuit also includes intelligent temperature protection. The metal core PCB is directly mounted onto a passive cooling system and onto the metal housing of the lamp itself. In operation this system provides in equilibrium a temperature gradient between the p/n- junction and the metal lamp housing below 50°C.
2. 1.In the case of a direct replacement of an existing light source by an LED light source a few aspects have to be carefully considered, including..
3. The thermal management of an incandescent filament or tungsten halogen lamp is designed on the fact, that the thermal radiation is the way to remove the heat from the lamp. Thus the thermal conductivity of the socket is very low. A replacement process has to ensure that the LED is protected from high temperature.
4. The power supplies of most conventional lamps are constant voltage sources, typical between 6V / 12V and 220V. LEDs require constant current sources and have a forward voltage between 1.5 V (IRED) and 4 V Power supply. A very common way is a current limitation resistor or a constant current source with bipolar transistor. The power consumption at the resistor determines the total power consumption and not the LED itself. In the case of a 24 V System the power consumption of a red LED sign (UF = 2 V, I= 30 mA, P=60 mW) is 720 mW. State of the art power step down converters are a way to solve this (also thermal) problem.
5. Conventional white light sources are used for colored signs and indicators in combination with a filter glass. Often the phantom effect of the filter glass is desired. The replacement should consider the absorption of the LED emitted light carefully.
6. Conventional light sources are compared with high end white, blue and green LED chips insensitive to ESD / short high voltages. The whole unit (often the LED itself) has to provide adequate protection.
7. Especially yellow and 565 - 570 nm green LEDs show a visible shift of the dominant wavelength over the required temperature range. An easy but expensive way to resolve this problem is to combine a white LED with a color filter.

Considering all the aspects shown above, the direct replacement of conventional lamps is an interesting way to increase the reliability of systems and to reduce the real costs of ownership. The state of the art LEDs show clear advantages for all applications, where colored light sources are necessary. In near future we will see white LED light sources with further cost benefits and increased external efficiency, which will again show clear advantages over conventional light sources.