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AB20-5 Datasheet(PDF) 3 Page - Lumileds Lighting Company |
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AB20-5 Datasheet(HTML) 3 Page - Lumileds Lighting Company |
3 / 16 page 3 Signal Lamp Design Process The design of an LED signal lamp consists of four inde pendent but interrelated designs: optical design, mechanical design, thermal design, and electrical design. The optical design is needed in order to design the secondary optics elements, such as reflectors or lenses, which are mounted in front of the LED emitters. In addition, the outer pillow lens needs to be designed in order to generate the desired output beam pattern. The optical design of an LED signal lamp is not unlike that of an incandescent signal lamp, except that the LED emitters have a much smaller geometry and a different optical radiation pattern. A mechanical design is needed in order to generate the desired mechanical drawings for the outer case, outer lens, and possibly internal secondary optics. The mechanical design would also include the selection of materials used for the signal lamp assembly. The mechanical design is not unlike the mechanical design of an incandescent signal lamp. The purpose of the thermal design is to evaluate the heat flow from the LED emitters to the ambient air and to reduce the thermal resistance as much as possible. For best results, the LED signal lamp should be designed to minimize self heating of the LED emitters. SuperFlux LEDs are limited to a maximum junction temperature of 125°C. In addition, all LEDs experience a reduction in light output at elevated temperatures. This phenomena is fully reversible, such that the light output returns to its original value when the in the temperature returns to its initial value. However, selfheating causes an undesirable reduction in the luminous flux output of the LEDs. The thermal design of an LED signal lamp differs from that of an incandescent design. For an incandes cent design, the design focus is to choose plastic materials that can withstand the heat generated by the bulb. For the LED lamp design, the focus is to protect the LEDs from high temperatures and to optimize the optical performance. The purpose of the electrical design is to choose the appro priate forward current through the LED emitters and ensure that this current stays within an acceptable range during worstcase operation at the extremes of ignition voltage and temperature. Also, the electrical circuit configuration deter mines the luminous intensity matching between the emitters within the LED signal lamp. In addition, the electrical design can also protect against EMC transients, and highvoltage and lowvoltage transient conditions. In many cases, an elec trical design is not needed for an incandescent signal lamp since the bulb can be driven directly from the ignition voltage. These four design processes are interrelated. For example, the mechanical drawings used to construct the signal lamp cannot be completed until the optical, thermal, and electrical designs are finished. Since these different design processes are interrelated, it is not uncommon to design the LED signal lamp using estimates for these different factors and to iterate the optical, mechanical, thermal, and electrical designs based on bench testing of prototype signal lamps. A flow chart of the basic design process for an LED signal lamp is shown in Figure 1.1 and consists of the following steps: 1. Define external operating parameters for the signal lamp. These parameters are usually specified by the car manu facturer or defined in various automotive specifications. These parameters include: • Operating and storage temperature requirements for the signal lamp. • Photometric test conditions of the signal lamp (i.e., whether testing is done at initial turnon at room temperature, after a 30 minute warmup at room temperature, or over some operating temperature range). • Design voltage (the voltage at which the photometrics will be tested). • Operating voltage range (i.e., 9 V to 16 V). • Transient operating voltage range (i.e., 24 V for 1 minute). • EMC transients applied to the signal lamp (i.e., SAE J1113 pulses 1 through 7 and theamplitude and dura tion of each pulse). • Whether any additional photometric guard band is required above the minimum photometric require ments defined by the SAE or ECE standards. Please refer to AB206 for a summary of environmental strife tests that have been used to validate Super Flux LEDs as well as suggested assembly validation tests for automotive applications. SuperFlux LEDs in Automotive Application Brief AB201 (5/04) |
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