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Optimization of Thermal Parameters of LED Luminaires

Determining operating conditions and thermal behaviour of LEDs is the starting point in designing a luminaire and calculating its thermal behaviour. The designer sets the main parameters for removal of heat from its sources (LEDs). The luminaire consists of the LED, a PCB and a heat sink.

There are different ways to perform this calculation: it is possible to use formulae for approximate heat sink calculations or special software, e.g. CFdesign or QLED. These software packags enable a detailed thermal analysis of LED luminaires. It is noteworthy that testing is mandatory for all thermal calculations. The designer must ensure that the heat sink provides the necessary cooling. Ideally, for these purposes the temperatures of p-n junction of the LEDs should be measured. However such measurements require special equipment which is not available in every laboratory.

In some cases, an experienced designer can start making a layout for a luminaire almost without preliminary calculations. The results of any thermal calculations are required to be verified by measurement of temperatures on an actual fixture. By measuring, it becomes clear whether the cooling of the LED chip junction achieves the required temperature or is excessive, which is important from an economic point of view.

There are several approaches to measurement of the thermal behaviour of an LED combined with thermal parameters of LED/heat sink structure, e.g. measurement of the voltage drop on a p-n junction depending on its temperature, or replacement of a diode with an equivalent depending on resistor heating rate and further measurement of a thermal pattern using a thermal imager. Also, there is a method for measuring thermal behaviour of a luminaire within the LED/bond pad/heat sink section.

The Chief Designer of LLC AtomSvet, Dmitry Romanov says, "This method allows you to check the results of calculations quite easily and quickly, select various types of applicable heat sink design solutions and make a more accurate evaluation of the thermal conductivity of materials used in designing a luminaire in all possible modes of operation of an LED."

Solution concept: A temperature sensor is placed directly on a heat-removing bond pad (thermal pad) of a LED, thereby providing a more precise measurement of the temperature at its contact point. The AD7814ARM microchip made by AnalogDevices is used as a sensor. The SPI digital interface allows you to avoid additional errors introduced into the measurement system circuit. Digital measured temperature values are transferred to the module. A built-in ZigBee radio-channel module additionally enables measurement via a wireless interface. The receiver collects, processes and presents the temperature data in a convenient form. The use of wireless network technology enables simultaneous collection of data both from a single sensor and the entire group. An example of practical use of this method is shown in Figure 1.

As we can see, a PCB (made of a tested material) with LEDs is mounted on a heat sink. A temperature sensor is located in the immediate vicinity of the LED crystal, thus allowing you to measure its temperature with the minimum error. By using different materials for PCBs, the thermal interface between the PCB and heat sink, as well as by using various designs of heat sinks, the best combination of these parameters for a particular solution can be selected.

An example of a comparison between different materials for PCBs under various conditions of operation of LEDs is shown in Figure 2. The proposed solution clearly demonstrates an ability to measure the thermal behaviour of an LED efficiently, which significantly shortens the development stages.

AtomSvet’s designers emphasize that with this approach you can effectively create smart luminaires, introducing additional features, such as dimming, into the measuring circuit and adding optional maintenance features for operational control of the lighting, taking into account the locations of luminaires during operation, as well as maintenance because of the gradual contamination of a luminaire or unexpected screening of a heat disperser, etc.

This method of thermal behaviour evaluation has been applied when designing the AtomSvet® Meccano LED luminaire intended to be used specifically in severe conditions with a high concentration of dust and moisture.

The AtomSvet® Meccano luminaire is distinguished by its modular structure, which enables it to be set up for the required value of power and luminous flux.

Using a lens as an external glass enables the AtomSvet® Meccano luminaire to reduce luminous flux losses. An external power source can be replaced if necessary without dismantling the luminaire. Special-purpose optical and electrical features of AtomSvet® Meccano LED luminaires allow them to be used in a wide range of temperatures (-60 to +60°C), in unstable electrical networks, and aggressive environments or constant vibrations.

Figure 1. An example of the use of wireless network technology

Figure 2. An example of comparison between different materials of PCBs in various conditions of operation of LEDs

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