Driving high brightness LEDs

May 01, 2020 //By Mark Patrick
Driving high brightness LEDs
High brightness light emitting diodes (HBLEDs) are bringing about a revolution in lighting - with applications ranging from the humble flashlight to car front lighting. There are still trade-offs, however, in relation to cost and benefit, with there being a need to have a long lifespan to fully justify their use.

Apart from manufacturing quality, operating temperature is the main factor when it comes to longevity. This means that careful heatsink design, along with a driver that keeps the current at an optimal level while contending with abnormal conditions (such as cooling loss - which could otherwise result in early failure and expensive replacement costs) will be mandated. To increase the benefit side of the equation, the driver can also provide features such as dimming, fault protection and the ability to control multiple strings of LEDs. In this article, we look at the background, the cost/benefit factors and how LED drivers are selected for different example applications.

 

Dip don’t dazzle!

Back in the 1970s, in the UK, there were regular public service announcements on the TV. They ranged from advice on sheltering under kitchen tables in case of nuclear attack to how to cross roads safely (explained by David Prowse, who would later be cast as Darth Vader in the Star Wars movies) and how to drive safely. One of the campaigns was ‘Dip Don’t Dazzle!’ - reminding that ‘full beam’ could blind the drivers of oncoming traffic. It was hardly a big problem at the time - with the yellowish light, cloudy lenses and tarnishing reflectors …but today, the piercing HBLEDs of cars (and even pedal bikes) are a distinct hazard. HBLED technology has made it possible to replace incandescent bulbs, fluorescent tubes and sodium/mercury vapour lamps with far higher performance products. The market agrees, with a projected $22B global revenues by 2023 at 4.9% CAGR (according to Global Market Insights).

HBLED basics

LEDs exploit the characteristic of a semiconductor p-n diode junction - where photons are emitted by electroluminescence when electrons recombine with electron ‘holes’ crossing the semiconductor band gap as the junction is forward biased. The amount of doping of the semiconductor affects the band gap size which in turn affects the energy and hence frequency or perceived colour of the photons emitted. Modern HBLEDs are typically a high-power blue emitter behind a transparent melding that has been impregnated with a cerium-doped yttrium aluminium garnet phosphor (Ce3+:YAG), which emits yellow light. The combination of blue and yellow then gives white with good colour rendition. Semiconductor material and packaging advances for better thermal performance have now resulted in single HBLEDs that can produce over 100lumens/W. Other LED phosphor combinations can give even better colour characteristics (but with lower efficiency), and white can also be derived from three separate red, green and blue LEDs (though with poorer colour stability in relation to temperature and time). These RGB systems are useful, however, where dynamic colour change is required - in applications such as mood lighting or stage illumination.


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