Simultaneous with the rapid advancement of led street light fixture technology over the last several years, a second revolution has also been taking place in solid-state lighting (SSL) luminaire and system design. In the early days of SSL design, we would frequently see obvious engineering mistakes and Frankenstein-looking retrofit lamps and luminaires come into our Durham, Santa Barbara, Hong Kong, Shenzhen, and Shanghai Applications Tech Centers with their creators looking for engineering design help and advice. We still see some of those on occasion, but most of the design errors we have seen in recent history are of a more subtle nature While these mistakes may be less obvious, they can be just as limiting to the market potential of the individual designs -. and detrimental to the reputation of the offering company and the proliferation of SSL as a whole.
Since it was launched in the fall of 2011, Cree Services has tested more than 500 luminaires and retrofit lamps in our Thermal, Electrical, Mechanical, Photometric, Optical (TEMPO) test program. The TEMPO testing program has provided a unique window to observe the rapid acceleration of SSL luminaire design capability and creativity in the industry. That said, there are still a few common mistakes we see even the most experienced SSL designers make. This article discusses the top four design mistakes we see most frequently, and makes some helpful suggestions to avoid them and bring quality SSL luminaires to the market.
Electrical overstress
Our first area of focus is the electrical environment. Electrical overstress (EOS), exposure of a component to current or voltage beyond its maximum specifications, is the leading cause of LED failures, regardless of the manufacturer. LED EOS failures result from excessive localized heat generated by the high current or voltage transient that causes damage to the materials in the LED. Like all semiconductor devices, LEDs have a limited ability to survive overstress.
Cree recently conducted a rigorous regime of testing of three general categories of LEDs: mid-power single-and multiple-chip LEDs, high-power single-chip LEDs, and high-power chip-on-board (COB) LEDs from multiple LED manufacturers. The test consisted of applying square-wave pulses of power levels up to 1700W, with time durations ranging from 0.1 to 70 msec (milliseconds), to each LED to simulate real-world EOS conditions. The failure criteria were based on junction leakage current measurements performed after each pulse. If the leakage current exceeded the device specifications, the result was considered a failure. Fig. 1 shows an example EOS failure of a high-power LED.
FIG. 1. Electrical overstress conditions can cause LED failure.
FIG. 1. Electrical overstress conditions can cause LED failure.
In general, single-chip high-power LEDs with multiple contact vias are most robust against EOS events. Low-power plastic packages are most susceptible to damage from EOS events. COB LEDs with more parallel strings of chips are more robust than COB LEDs with fewer parallel strings. Larger LEDs are more robust than smaller LEDs. For all the tested parts, as the power-to-failure increases, the time-to-failure decreases.
Events ranging from lightning strikes to driver in-rush current at power on can subject led street light fixture to a variety of EOS events. However, failure-inducing EOS events can be avoided by applying proper protection techniques, including metal-oxide varistors (MOV) for over -voltage protection and positive temperature coefficient (PTC) resistors for over-current protection. The general guideline is to design a protection circuit capable of preventing anything greater than a 0.1-msec spike of 2-3 times the maximum rated current of the LED from reaching the LED. A more detailed discussion of this topic can be accessed at www.cree.com / xlamp_app_notes.
Thermal management
Next up, thermal issues can lead to premature LED failure. LEDs are designed to perform over a range of operating temperatures. With all LEDs, the expected lifetime depends on the operating temperature. One of the most critical design parameters for an LED illumination system is the system's ability to draw heat away from the LED junction.
High operating temperatures at the LED junction adversely affect the performance of LEDs, resulting in decreased light output and lifetime. Properly managing this heat is the result of attention to thermal management in the design, assembly, and operation of LEDs in lighting applications. Insufficient thermal management can result in damage very similar in appearance to that caused by chemical incompatibility - a topic we will address shortly; however, unlike chemical incompatibility damage, thermal damage is irreversible.
LEDs are commonly soldered to an FR-4 or metal-core printed-circuit board (MCPCB) that is attached to a heat sink. FR-4 boards are less expensive than MC-PCBs but have higher thermal resistance. COB LEDs are commonly attached to a heat sink using a connector that attaches to the heat sink with screws. A good thermal connection between the COB LED and the heat sink is critical for successful designs. A thermal interface material (TIM) is required for optimal performance. Air is a thermal insulator, so a TIM is needed to fill any voids between the COB LED and the heat sink. Without a TIM, there are a limited number of spots for heat transfer from the LED to the heat sink to occur. With the voids filled by a TIM, heat flows much more freely from an LED to the heat sink In any case, careful consideration must be taken to ensure a sufficient thermal path to ambient is provided Fig 2 shows an example of significant temperature variation -... 22 ° C - in a luminaire due to inconsistent application of the TIM to the heat sink.
FIG. 2. Inconsistent application of thermal interface material can lead to a large temperature gradient.
FIG. 2. Inconsistent application of thermal interface material can lead to a large temperature gradient.
Chemical compatibility
Now let's discuss chemical compatibility. The presence of incompatible volatile organic compounds (VOCs) in LED-based system designs can impair the performance and reduce the lifetime of these illumination systems. Glues, conformal coatings, O-rings, gaskets, and potting compounds are materials frequently used in the construction of LED-based luminaires or lamps, and often contain VOCs. The presence of chemically incompatible VOCs on or near LEDs can degrade the light output levels or cause changes in the chromaticity point of the light. The photograph in Fig . 3 is an example of this VOC degradation on the surface of an LED.
FIG. 3. LEDs can discolor as a result of exposure to incompatible chemicals.
FIG. 3. LEDs can discolor as a result of exposure to incompatible chemicals.
This sensitivity to VOCs is not unique to one LED manufacturer but is a known problem for all types of blue, royal blue, and white-light LEDs. We typically do not see these issues with green, amber, or red LEDs due to the longer , lower energy level of these wavelengths.
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