The transition to light-emitting diodes (LEDs) from conventional forms of lighting has not been without issues. However, the high light output vs. input power and long operating lifetimes make LEDs attractive for many applications, such as automotive systems. Engineers have quickly learned that the main challenge in using LEDs has been a thermo-electric issue. The low forward voltage drop of an LED diode junction (typically 0.7V for a single junction) means that additional circuit elements must provide the voltage reduction from a typically higher system voltage.
Depending upon the LED selected, the range of forward voltage drop is 0.7 to 4.5VDC. The current per device is between 10 and 300mA (short peaks can be higher). However, the maximum current in the device follows a strict temperature curve. As the temperature increases, the maximum forward current decreases. The combination of ambient temperature, power dissipated in the LED package, and power dissipated in the LED circuit elements determines the type of thermo-electric system that must be employed.
The traditional design process for an electronic product includes circuit design, schematic capture of the circuit, creation of a bill of materials (BOM), and a printed-circuit-board (PCB) layout. Special material considerations must be given for designs requiring controlled impedances, with circuit density impacting such layout considerations as the use of multilayer circuits and blind, buried, and/or filled viaholes.
More steps are involved in the LED circuit design process, including circuit design, schematic capture, selecting the optical wavelength (usually expressed in °K), determining the intensity (in lumens), selecting the LED device for the circuit and how many are needed, determining the forward voltage drop of the device and the operating current required for the target wavelength, determining the system voltage (such as 12VDC for an automotive application), calculate the power consumed by each LED, calculate the power in the associated circuit elements (such as resistors) in series with each LED, and find the sum of the power consumption for all of the circuit elements (in W). In addition, the LED circuit design process requires determining the surface area required for the various resistors and LEDs (in cm2), calculating the power per area (W/cm2) by dividing the total power (in W) consumed by the surface area of the LED circuit, designing a thermal management system to transfer the amount of heat generated by the circuit (in W/cm2), to a heat sink or enclosure or other thermal mechanism, selecting a PCB support structure (laminate), creating a BOM, and making a layout for the LED PCB.
Controller circuitry for LED systems can be as varied as the applications. But these design concerns are well documented and available. A more complicated issue can be the BOM creation, which is more than just translating a parts list from schematic capture to a list of real manufacturers' parts numbers. Selecting an appropriate LED starts with determining the required wavelength, and then finding the intensity (in Lumens) needed for the application. Often, multiple LEDs are required to provide the desired intensity.
Normally, this would be the end of the electronic design phase and PCB layout would begin. However, with a higher-power LED circuit design, a designer must also account for thermal management. A traditional epoxy-glass PCB with thickness of 0.062-in. (1.54mm) will not work in most LED applications because of its poor thermal conductance. The laminate material for an LED PCB must be capable of conducting heat away from the LEDs, with thermal conductance measured in watts per meter per degree Kelvin (W/m°K). This information is typically provided on a circuit laminate's data sheet.
Thermal resistance, which is inversely proportional (1/X) to thermal conductance, can be calculated by a circuit's area and length. A circuit material with proper thermal conductance should be capable of transferring the heat generated by the LEDs and associated components through the reinforcing material. This can be determined by selecting a maximum temperature for the LED circuitry and subtracting the maximum ambient temperature of the operating environment. This drop in temperature must be achieved across the material of the LED PCB.
Thermal resistance for PCB materials varies with thickness. A designer can choose between a thick material with low thermal resistance and a thinner, lighter material with higher thermal resistance. A typical solution is to use a thermally conductive backing material laminated to the PCB, such as aluminum or copper, to help dissipate heat, which can be a concern for higher-power LED PCBs. Most low-power designs (power density of less than 2W/in.2) can use epoxy-glass laminates.
Single-sided PCBs for higher-power LED designs are typically based on materials with the highest thermal conductance. But a single-sided design can be difficult to fit all of the interconnect circuitry (components and traces) on the PCB. When a design cannot fit on one side, a double-sided (or multilayer) PCB approach must be used.
Double-sided circuit designs require the use of an insulating layer between the bottom-side copper traces/pads and the metal backing material. This dielectric insulating layer can add significant thermal resistance, so the laminate should be chosen for high thermal conductance (and lower thermal resistance), although the material cost tends to increase with increasing thermal conductivity.
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