LED Technology Challenges
Controlling the thermal stability of the LED die is critical to the performance and stability of LED illumination and reliability. The LED architecture inherently produces light from all sides and surfaces of the PN structure in a lambertian distribution (uniform distribution into a 180 degree hemisphere). While this might seem efficient, most of this light is actually absorbed into adjacent die, the mounting substrate, or other surfaces of the LED assembly. This absorption results in an increased thermal loading of the entire LED assembly. This heat must be addressed to obtain maximum light output and reliability. Additionally, for applications that require imaging of the light energy to a small display device (e.g. DLP HDTV), any light that is emitted outside of the system etendue is not useable and only adds to the heat and overall power loading. Controlling this absorption, shaping the light to match the system etendue, and maximizing the thermal efficiency to extract heat from the die are all critical to increasing the light output and usability of the LEDs.
For traditional applications, LEDs are commonly driven in CW (continuous wave – 100% duty cycle) mode. For high brightness applications, however, this is not as desirable. Since the average temperature of the PN junction determines both the light output and lifetime of the LED, it is often more efficient to drive the LEDs with a smaller duty cycle. With a smaller duty cycle, the LEDs can potentially be driven to higher current loads to increase the overall light output while maintaining a lower average temperature of the PN junction. The challenge with this, however, is that the driver circuitry must be able to generate fast switching waveforms, switching large currents in as short a time as only a few microseconds. This certainly presents some challenges for the design of the LED power driver. But, solutions have already been developed with performance that easily meets these requirements.
Another challenge that results from higher thermal loading is that of color shift. As the PN junction changes temperature, the output wavelength of the light can shift by as much as 10nm or more. This color shift obviously impacts the color point for that color, but also impacts the white point for the system since each of the colors are mixed to create white. Fundamentally, to stabilize this color shift, the LEDs must either be run at a lower power or maintain extreme thermal stability. However, with the implementation of some form of system feedback and proper power control algorithms, the stability of the white could be preserved while maintaining high brightness efficiency.
DLP TV with LED Illumination
TI has developed a DLP HDTV system to take advantage of LED illumination with brightness performance that is nearly equivalent to lamp based systems. By utilizing the latest generation of high brightness LEDs and implementing a unique feedback system, it is now possible for DLP HDTV designs to enjoy the benefits of LED illumination. Figure 5 illustrates the basic optical configuration of this system.
Figure 5 – DLP HDTV LED Optical Architecture
Utilizing a unique feedback algorithm, TI has demonstrated that any color shift variations that affect the white point can be controlled to a tolerance beyond what the eye can detect.
The current DLP products implementation with LED technology utilizes a TI DSP component to process system information in real time, offering superior stability over a wide range of operating temperatures while maximizing brightness and reliability.
DLP Products Performance Advantages
The rapid switching capabilities of LED technology match perfectly with the fast switching properties of DLP technology. By taking advantage of the high speed capabilities of the DMD and LEDs, it is now possible to utilize color refresh rates that are much higher than what exists with today’s designs. It is also possible to randomize the color order. Ultimately, images can be created with higher bit depth, better motion fidelity, and higher brightness. By increasing the switching frequency of the LEDs, it is possible to drive them with increased power while minimizing the thermal loading of the PN junction. These fast switching capabilities of DLP technology take advantage of the new LED colors that are becoming available, providing much more flexibility for multiple color configurations using a single DMD device. With a DLP system, the LEDs do not require polarization, reflecting the light precisely off of the DMD mirror surface. The light is used efficiently, only when it is needed. This maximizes brightness and system efficiency while reducing heat. The net result is a lower system cost with higher brightness and larger color gamuts that far exceed those possible by traditional systems utilizing other common illumination sources.
Conclusion
As LED technology developments continue to improve brightness and reliability, LED illumination may become more of a mainstream light source for many future applications. Future developments will be able to take further advantage of the fast LED switching time to improve video performance, enhance contrast without opto-mechanical components, and create adjustable color gamuts that far exceed the possibilities of traditional illumination sources. New products will soon benefit from these fundamental capabilities providing new, unique designs that offer instant on, better colors, and overall better picture using the speed of DLP micromirror arrays. With the advantages of LED and DLP' technologies working together, it is expected that DLP HDTVs will provide even better performance with better reliability far exceeding any existing DLP HDTV product.
References
- Web Article, “A brief history of the
Light Emitting Diode (LED)”, http://www.wavicle.biz/led_history.html, Wavicle LED Lighting Technology, 2002.
- “LEDs Are Still Popular (and Improving) after All These Years”, http://www.maxim-ic.com/appnotes.cfm/appnote_number/1883, Dallas Semiconductor / MAXIM-IC, Application Note 1883, February 2003.
- 3 LEDs 2005, October 2005, San Diego, California, USA
.
- 4 LUMILEDS, Nanoscience and Solid State Lighting, Department of Energy Nanosummit, M.G. Craford, June 2004, Washington, D.C., USA.
About the author
DJ Segler is an engineer in the DLP Products group of Texas Instruments Incorporated. For additional information about DLP technology at TI, please go to ti_dlp@SUTH.COM.
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