Source: Li Jinhua, Vice President, Sales, Zetex Asia Pacific
Undoubtedly, the durability and longevity characteristics of LEDs can come in handy in different applications, with obvious benefits. However, when high-power LEDs are used for high-brightness operation, heat dissipation becomes a critical issue. In fact, less than 20% of the input LED power is converted to light energy, and the remaining 80% is converted to heat. This is really a problem that lighting system designers need to overcome.
If the lighting system is not implemented well, even the most complete thermal design, it is difficult to develop its strengths and play its true purpose. Responsibility for maintaining a safe LED operating environment and reducing the effects of thermal dissipation on LED life falls on the electronic components used to drive the LEDs.
In fact, as long as we look at the component specifications provided by high-brightness LED manufacturers, it is not difficult to determine the main design parameters that need attention, and the negative effects of operating at high temperatures on these devices.
The actual lifetime of an LED is often inversely proportional to the power dissipation and the temperature of the LED junction. The manufacturer can display a mean time between failure (MTBF) of approximately 100 million hours of operation at a temperature of Tj 80 °C. In a practical system, the failure of the LED does not necessarily cause a big problem, but in a system where the heat dissipation is insufficient and the Tj rises to 120 ° C or above, the life of the LED is greatly shortened. In some extreme cases, LEDs will fail in real time.
Thermal design introduces overcompensation to counter the worst environments. However, in some cases, this is impossible. Taking the downlight as an example, it is generally installed in an insulated ceiling space. This space not only hinders heat dissipation, but also does not have enough space to install additional cooling.
The relative luminosity is also inversely proportional to the junction temperature. As the data mutates, the manufacturer estimates that the light output at the maximum junction temperature is reduced by 30%.
Similarly, the lumen maintenance effect is inversely proportional to the junction temperature. At 70 ° C junction temperature, after an LED operation for more than 50,000 hours, it will generally lose 30% of the luminosity output; when the temperature is higher, the loss will be greater, but the official information has not been disclosed.
Therefore, the designer's most important goal is to dissipate the heat of the LED as quickly as possible to keep the junction temperature below the maximum rating and avoid premature failure.
In general, the electronic device used to generate the required LED current can easily introduce a method of detecting excessive temperature, effectively reducing the driving current of the LED and maintaining a stable operating temperature. Although the light output will be slightly weakened, the "life" of the LED is very strong and can operate for a long time.
Taking an LED driver circuit design as an example, the buck converter is equipped with a temperature control function, driving current up to 1 amp, and supply voltage ranging from 4 volts to 6 volts. The circuit uses a 150kΩ NTC thermal tube for temperature sensing. The unit is placed in close thermal contact with the LED. The current flowing through the superheat control tube is multiplied and then added to the peak exchange current to regulate the LED current.
Protect your valuable high-brightness LEDs with the addition of simple, low-cost electronic components. This technique can be applied to many different control systems and is suitable for buck and boost modes of operation using any of the ZXSC family of LED driver ICs. This thermal protection design helps designers of lighting systems achieve smaller, lower cost solutions.
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