Efficacy
The holy grail of the light source. Offering a white light of up 150 lm/w (in laboratory tests). This is the area where the most focus is being placed. The efficacy of LEDs is very much affected by temperature. LEDs utilise a Heat Sink, a die cast fitting to dissipate heat away from the PCB. The lower the temperature, the higher the efficacy. Average efficacy would be 50-100 lm/w at present but this figure is increasing constantly.
Life
The other key factor in the focus by lamp manufacturers and luminaire designers on LEDs is that they offer unprecedented life of 50,000 hours plus. Some LEDs even promise 100,000 hours. Because LEDs offer such extreme life, one of the biggest challenges is how to achieve actual performance over life measurements i.e. 'real life' data. Testing an LED for 24 hours a day, 365 days per year, would only provide 8,760 hours of data. So, an LED rated at 100,000 hours would naturally require 11.5 years to produce 'real life' data. With the pace of technological development and the need to commercialize LEDs, this is obviously not workable.
Some manufacturers have agreed to use a 6:1 ratio to ensure data can be gathered and extrapolated. So, 1,000 hours actual would equate to 6,000 hours and 2,000 hours actual would be 12,000 hours.
Lumen Maintenance
The lifespan of an LED depends on its operational and environmental temperature. At room temperature, LEDs (and LED modules) have a very long lifespan of up to 50,000 working hours. In contrast to filament lamps, where a break in the helix (filament) means the end of its life, total failure of an LED is extremely rare. Its light intensity also declines much more slowly: this property is known as degradation.
The period of degradation of the original luminous flux defines the lifespan of LEDs. The degradation of the luminous flux is strongly dependent on the temperature of the light emitting surface in the semiconductor crystal. There must therefore be no build-up of heat in the operation of an LED: the conducting plate or additional heat sink must reliably divert the heat. A too high environmental temperature will equally lead to a decrease in the luminous flux.
Until recently, LED reliability claims were covered under the blanket lumen maintenance statement "70% lumen maintenance at 50K hours," However, this is not ideal for LEDs.
Mortality Curves
A conventional lamps' life is characterized by a mortality curve. This refers to a percentage of the lamps that catastrophically fail. For instance, the most common mortality rating is based on the time at which 50 per cent of lamps will have failed catastrophically, commonly known as a B50 – see graph.
As LEDs experience a gradual reduction in light output during operation and generally do not catastrophically fail, more than 50% of LEDs will still provide a good measure of light at less than the 70% lumen maintenance (known as L70) threshold. The Lighting industry has thus sought to develop alternative methods of defining the mortality of LEDs.
One manufacturer now offers mortality curves based on 2 options. There is the standard 70% lumen maintenance (L70) at 50% survival rate (B50) or an alternative 70% lumen maintenance (L70) at 90% survival rate. The latter measure is known as B10. This is a new and revolutionary way to measure LED lifetime and offers customers the advantage of knowing how long their LEDs will produce optimal light output for.
Control Gear (Drivers)
LED Drivers are current control devices that replace the need for resistors. LED Drivers respond to the changing input voltage while maintaining a constant amount of current (output power) to the LED as its electrical properties change with temperature. The voltage versus current characteristics of an LED is much like any diode. Current is approximately an exponential function of voltage, so a small voltage change results in a large change in current. It is therefore important that the power source gives the right voltage.
If the voltage is below the threshold or on-voltage no current will flow and the result is an unlit LED. If the voltage is too high the current will go above the maximum rating, heating and potentially destroying the LED. As the LED heats, its voltage drop decreases, further increasing current. Consequently, LEDs should only be connected directly to constant-voltage sources if special care is taken.
The majority of LEDs require Direct Current. Depending on the type of operation, there are two different methods of control for LEDs and LED modules: constant voltage and constant current.
Constant Voltage Driver
The voltage regulated control of LEDs is characterised by the fact that the diodes are operated with a 'constant voltage'. In this case standard proprietary "direct current" equipment can therefore be used as the power supply. This method of operation permits easy control of light intensity in LEDs by pulses (switching on and off) of the power supply. With this method it is necessary to limit the current in LEDs, because the forward tension leaks strongly. An incorrectly defined operating current limit can lead to destruction of LEDs and their operational and control equipment.
Constant Current Driver
The current regulated control of LEDs has advantages for constant operation and in the efficacy (lumen/watt). IN this instance, it is important to use a predetermined current. The appropriate wiring, in most cases includes a governor, ensures constant operation. Strongly fluctuating forward tensions play only a small role in this method of operation as the voltage to the LEDs adjusts in proportion to their operational current so that they are not overloaded.
RGB Colour Controllers
Dimming
Dimming of an LED can be done by either reducing the current level through the diode ( DC-dimming, analogue dimming) or by applying PWM-dimming (short for Pulse Width Modulation) to the LED.
DC-Dimming
DC-dimming is a straightforward solution to reduce the thermal load (and brightness) of an LED. For example, reducing the LED's current from 350 mA down to 250 mA, will reduce the thermal load on the LED accordingly. Varying the current of LED may however have side effects on the light output of the LED. LED can have a noticeable dependency of the output colour on the current that is applied; this is also referred to as a colour-shift of the LED. For white LED reducing (or increasing) the LED current may lead to a change of the white-point. It is important to check whether any colour-shift occurs with DC dimming and whether it is acceptable to the particular application. If the colour-shift is too strong, PWM-dimming can help reduce this effect. In particular for RGB applications it is advisable to use devices with PWM dimming.
PWM-Dimming
PWM-dimming utilizes a different method for reducing the average current through the LED: the current applied to the LED is turned on and off at a high frequency (e.g. 300 Hz) while keeping the current level fixed (e.g. at 350 mA). The average value of the current flowing through the LED is then determined by the length of the on-period as compared to the off-period (the duty-cycle).
The above charts show dimming at 25%, 50% and 100% and the resulting, average current flow through the diode. Since the current through the LED remains unchanged at different dimming levels, there is also no colour-shift introduced due to a change in current. This ensures best performance of the LED in both RGB and white light applications.
Low Power LEDs & High Power LEDs
Low Power LEDs are mostly single-die LEDs used as indicators, and they come in various-sizes from 2 mm to 8 mm, through-hole and surface mount packages. They are usually simple in design, not requiring any separate cooling body. Typical current ratings range from around 1 mA to above 20 Ma, with a luminous flux of approx 1 lm. The small scale set a natural upper boundary on power consumption due to heat caused by the high current density and need for heat sinking.
High power LEDs (HPLED) can be driven at hundreds of mA (vs. tens of mA for other LEDs), some with more than one ampere of current, and give out large amounts of light i.e. in excess of 120 lm. Since overheating is destructive, the HPLEDs must be highly efficient to minimize excess heat; furthermore, they are often mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will burn out in seconds.
Junction Temperature
LED Junction Temperature is the temperature at the light emission point at the heart of an LED device (called the 'p-n junction').
This is a critically important parameter for high reliability LED applications because both LED lifetime and LED light output are directly proportional to junction temperature. By controlling LED junction temperature through a variety of thermal management techniques, optimally efficient LED lighting designs with very long service lives can be realised.
Heat Sink
The key to a successful design starts with the transfer of LED heat. Each custom LED lighting design involves the concept of efficiently transferring as much heat as possible away from LED PN junction. The process begins within the LED lamp, where thermal energy released into an integrated slug can potentially exit the light emitting diode. Modern surface mount LED lamps depend on the thermal efficiency of this slug.
In addition, a thermally stressed LED lights will lose efficiency and light output will diminish. If the LED thermal management continues to race out of control, the LED junction may break down causing a state of complete thermal runaway. The result is typically catastrophic failure. Other affects of overstressed LEDs may include broken wire bonds, delaminating, internal solder joint detachment, damage to die-bond epoxy, and lens yellowing.
Power Factor
LED drivers are designed to convert mains voltage to DC with power factor pre-regulated for LED lighting. Thereby avoiding the problems associated with the higher wattage fluorescent circuits where the phase shift between the supplied voltage and current becomes significant.
