In the past, light-emitting diodes ( LEDs ) were only used as indicator lights. It has low power consumption, low current, and the heat generated is not a problem. However, the times have changed. Now the current of LEDs has been upgraded from a few milliamps to amps; in some cases, the power of LEDs has jumped from a few milliwatts to more than 10W; and the heat generated by LEDs - well, to be sure, now It has become a problem that cannot be ignored.
In the past, the power of LEDs was so low, so the waste heat was very low, and it was basically not burned out, Rick Zarr, a National Semiconductor technology expert, said: "However, the new LEDs generate a lot of heat, so if you Without proper control, it will have a big impact on their service life. An LED bulb can be used for 10, 20 or 30 years, but if you do not control it properly, it may be scrapped in two years.
That's why project engineers now need to focus on LED drivers . At its simplest, the driver controls the input current and input voltage and then reconfigures them for use by the LEDs. In this respect, the drive is much like a ballast that has been used on fluorescent lamps for decades.
Although, on the other hand, drives have become more complex, they have played a more important role in the new era of high-power, high-current, high-heat LEDs. The features of the current drive were unimaginable ten years ago. For example, by inducing current, they can estimate the brightness level of the LED and make corresponding changes. They can compensate for changes such as heat or aging, and even use LEDs for lighting applications. In addition, by using the "heat return" scheme, it is possible to avoid the heat of the LED reaching a limit that impairs its service life.
Thanks to this type of innovation, new LEDs are finding a home in many unforeseen applications. The car uses the LED as a brake light, "the rear view mirror footlights (Note: the rear view mirror footlights are mounted under the mirrors on both sides of the car to illuminate the lights around the body, when entering a darker parking garage) Make the car look very beautiful, but also illuminate the muddy road in the wasteland)" even the headlights. Televisions use LEDs as backlights; municipalities use LEDs as street lights; consumer electronics manufacturers are using them for netbooks, tablets and GPS systems.
"The development of LEDs is better year after year," said Peter Di Maso, marketing manager for lighting power products at Texas Instruments. “These improvements make LEDs available for general lighting, which is why there is such an urgent need for better drives.â€
Growing demand
Of course, not all forms of lighting require a drive. For example, incandescent bulbs have long been optimized for our popular 110V power supply, so no additional circuitry is needed to adapt the input current to the output.
However, LEDs are more complicated. “When you use LEDs, you first need to understand the input power and its changes,†says Steve Bowling, application manager for Microchip's 8-bit microcontroller products division. “At the output, engineers need to know their optical requirements and want to produce The lumen value. Finally, the ultimate goal is to have the drive provide a constant current source to the LED."
Thanks to what is known as Haitz's Law, although driving LEDs has become more complicated and more necessary. Haitz's Law, a law named after Roland Haitz, a retired scientist at Agilent Technologies, points out that the cost of producing 1 lumen of LEDs is 10 times lower every 10 years, and the brightness produced by each package increases. 20 times. For LED users, Haitz's law means increased demand, which in turn means that the task of developing new technologies flows downstream.
"People want to ignore the LED lights when they are in place," National Semiconductor's Zarr said. “They didn’t want to bother thinking too much, so the burden was shifted to the manufacturer of the LED lights, and then the LED manufacturers moved the pressure to their downstream drive manufacturers.â€
Avoid fever
For drive manufacturers, the challenge is to add the necessary features in a small package. The driver chip is very small and typically measures 10 x 10 mm or less. And, as LED manufacturers incorporate LEDs into bulb designs to replace incandescent bulbs, future driver chips need to be made smaller.
As they turn to such applications, drivers need to have the ability to avoid overheating of the LEDs. To do this, many vendors tend to help dissipate heat by selling radiators and fans to their customers. Although more and more LED users want to minimize the heat of the LED before it needs to be dissipated, it is necessary to start with the driver.
"In an ideal world, LEDs can last for thousands of hours," Bowling said. “But to achieve this, the LED must be operated within its normal operating temperature range. If you put the LED in a small space, then you need to solve the problem of how to dissipate heat.â€
Drive manufacturers are helping engineers deal with this type of problem by developing newer and better features. For example, National Semiconductor has integrated a feature called "thermal foldback" into several devices -- the LM3464 and LM3424. By using a thermal return mechanism, the new device regulates the current so that the LED is always operating within its maximum operating range.
“Thermal return is not exactly designed to dissipate heat for the purpose of protecting the bulb,†says Zarr. “If you are in a situation where the LED is approaching its critical operating temperature, the drive will fold back the current and dim the bulb.â€
The driver will also give LED users other reasons to limit the current flowing through the device. For example, Analog Devices can use its ADP1650 driver to control the current in high-brightness LEDs that are often used in camera flashes.
"From a brightness point of view, LEDs can be easily controlled by current," said Jose Rodriguez, technical director of power management at Analog Devices. “So we set the current inside our drive and tightly control it.â€
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