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Power, Control and Management of Lighting Applications

Tue, 09/15/2009 - 6:28am
by Richard Zarr, National Semiconductor

Richard_Zarr texturedWhen lighting applications started using solid-state sources, engineers began to understand the issues in the migration away from incandescent bulbs. It is fairly well known that LED sources lack the IR spectrum of their filament based counterparts requiring thermal management via conduction rather than emission. Driving and managing these solid-state light sources can be challenging, especially for those customers unfamiliar with the electronics, thermal management and optical properties of LEDs.

New technologies such as photonic lattice LEDs have incredible luminous intensity and can easily replace specialty bulbs in areas such as LCD projectors and light engines.  Additionally, these devices are finding their way into general lighting applications as the market expands.  An interesting phenomenon with high brightness LEDs is the relationship between operating temperature and life span.  LEDs degrade with time and the affect is accelerated with temperature.  A specification called L50 is the elapsed time from when the light source is new until it degrades to 50% of the original intensity, thus the L (life) and 50 (50%).  It can range from several thousand hours to well over 200,000 hours however, it drops dramatically as the junction temperature increases.

Once extremes are exceeded, damage is permanent and cannot be reversed so it is important to keep the LED’s junction temperature below the maximum number specified by the manufacturer to guarantee the L50 specification.  To accomplish this, manufacturers are adding thermal sensors to light engines and modules.  These sensors are typically resistive (RTDs) and interface easily with control systems that fold-back the current to limit junction temperature.  One such device that has this feature is the National Semiconductor LM3424 LED driver with thermal fold-back. The function can also be added to any standard LED driver by providing an additional control loop that limits the drive current while monitoring the LED’s temperature.

Another problem with the adoption of LEDs in replacing incandescent bulbs is the retrofit market.  Many people may opt to add standard dimmers to their lighting system.  LEDs are not powered like standard filament bulbs, but require a constant current.  If a string of LEDs is powered from an off-line supply, and a dimmer which truncates components of the AC cycle is placed in series, the off-line supply will simply regulate the current until the energy in each cycle falls below the shut-down point.  This translates into on and off – even with the dimmer in the circuit. 

To get around this, a solid-state lighting system must monitor the signature of the dimmer and modulate the current to the LED string.  In this manner, the LEDs will see a lower or higher average current based on the dimming signature and appear to dim or brighten as the dimmer is adjusted. National’s LM3445 TRIAC dimmable LED driver has this function built in, but it could be done discretely as well. 

Most high brightness LEDs are actually a blue with a phosphor placed in the optical path. The phosphor absorbs some of the light and re-emits yellow light through a process known as Stokes Shift.  If you simply reduce the current of the LED, the optical output power falls which causes the phosphor to reduce emission resulting in a blue shift.  This can be avoided using pulse width modulation to dim the LED which provides a constant current when on, but when averaged over time appears to increase or decrease.

There are a few issues with migrating to high brightness LEDs for general lighting, but with a little mechanical and electronic help, these new light sources will provide end customers with a very long service life – maybe never requiring replacement.
 

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