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Engineering Edge

The Latest Philips TVs make LEDs Dance

“Dancing” above refers to dimming and boosting of LEDs in time with the picture

By Wendy Luiten, Philips

In recent years there has been an explosion in LED-LCD displays, and a proliferation of LED TVs in people's homes. They offer a great picture, great styling with a thin form factor, increased functionality like 3D and internet access, and great value for money, but cooling the LEDs, while steadily increasing power densities in ever thinner product enclosures poses a big challenge for designers.

In an LED, die (junction) temperature affects both performance and lifetime as LED lumen efficacy is lowered and the color temperature shifted. Heat degrades the epoxy lens, and both the absolute temperature and the temperature distribution over the LCD screen can lead to performance and lifetime issues.

Figure 4. Simulation Results with Valve Closure Time of 20 Seconds at the end of the Cycle

Non-uniform LED temperatures lead to unwanted spatial non-uniform lighting in terms of color uniformity and brightness. Effective heat spreading is therefore a key goal of the thermal design. The temperature of the LCD has to be just right. Too hot (typically 60 - 70°C ) and optical materials age, too cold and the switching is sluggish, causing fast changing pictures to display blurred – very performance degrading in modern high resolution 3D TVs! At a system level, the temperature the user experiences when touching the TV is a key safety issue.

While it is entirely feasible to do electrical and optical tests in a standalone setting and directly translate the results to the system situation, for thermal this is not the case. The presence of the set back cover, which causes an additional thermal resistance from the display back to the room temperature, and second, the direct thermal interaction between the display and boards illustrates the difficulties encountered in co-design.

Philips have been pioneers in the TV industry since the 1940s, so the chances are the first TV you ever saw was a Philips. In fact, it was probably the first TV your grandparents ever saw. Philips has used FloTHERM extensively in the design of TVs since the software was first released in the early 1990s.

“FloTHERM has long been an important tool to our LED-LCD TV thermal design, and is routinely used at module (display) level and at system (TV) level as the temperature distribution in the LED-LCD display is a system-level issue due to the strong thermal interactions. FloTHERM helps us select and optimize the thermal solution so we have confidence from a very early design stage.”

G.A. Luiten, Philips Research, Eindhoven, Netherlands

Today TP Vision, the maker of today's Philips TVs, uses FloTHERM to look at all aspects of the thermal design of LED LCD TVs from the LED package, module, up to the full TV in different environments, starting from conceptual design through to the final product, to optimize all aspects of the thermal design. Factors considered include evaluating different design architectures, such as direct lit display vs. edge lit displays, and optimizing the cost benefit of different cooling solutions.

While LEDs are used to light the display, there is a world of difference between using LEDs for display backlighting and using LEDs in a lighting product. In lighting applications LEDs are typically used in a steady state manner, with timescales in the order of hours or longer. In a TV two different timescales prevail. Heating and steady state behavior of the set as a whole is governed by the average power consumption, and this has a long timescale similar to domestic lighting applications. However, the momentto-moment changing of the video content happens at a much higher frequency, and this creates an additional much shorter timescale.

A further complicating factor is the thin film transistor (TFT) panel that covers the LCD-LED display. Light emitted from the TV is a combination of the light emitted bythe backlight and the state of the pixels in the TFT panel. If a TFT pixel is open, light goes through to reach the viewer. If a TFT pixel is closed, light is blocked. In display with a static light emitting backlight, pixels are predominantly open in a bright image, and pixels are predominantly in the 'closed' state if the image is dark. However, from a picture quality and energy consumption point of view, a static light output from a backlight is not optimal.

To improve the picture quality (deeper black) and reduce energy consumption it is common to dim the LEDs in dark scenes. A refinement is to use 2D (or local) dimming, where the LEDs are dimmed not only in time, but also depending on location, providing a further improvement both in picture quality (higher contrast over the screen area) and in energy consumption. However, it is possible to go further still. The timescale over which the area of a TV lit by a single LED changes is one to two orders of magnitude smaller than typical thermal time constants for display LED packages.

As well as dimming LEDs during dark scenes it is also possible to boost their light output for short periods of time. LED dimming and boosting scenarios, while not primarily intended as thermal control measures, are very beneficial to the thermal management of LED-LCD TV sets as the associated LED temperature is more highly correlated to the average LED power, which is much lower than the peak. The result is exceptional picture quality.

Philips TV takes a similar approach for the ambilight feature: the LED temperature is determined by the average LED power dissipation, and large instantaneous LED peak powers can be allowed to increase the immersive experience. Tight thermal management algorithms are deployed to prevent LED boosting from adversely affecting lifetime and reliability of the display.

FloTHERM® simulations were performed on a stand-alone direct lit display, cooled by a heat transfer coefficient typical of natural convection including radiation on the front and on the back. Figure 4 illustrates an important trade-off between the number of LEDs and thermal issues.

The calculated temperature field compares well to the measured temperatures on the front of a direct lit TV set, in Figure 6 In the direct lit TV, the effect of hot air rising is visible, as higher temperatures at the top of the display. Also, the positions of the three boards are visible as locally higher screen temperatures. The infrared picture confirms that screen temperatures are well below the aging limit of approximately 60 °C (in 35 °C ambient) and that the temperature difference over the screen is around 8 °C Figure 17 shows the infrared images of the front of a side lit TV, equipped with internal graphite heat spreaders. Comparison of the infrared images with the simulation results shows good agreement. The measurement confirms that the screen is critical with respect to the aging criterion in the zone directly adjacent to the LED bars, and that there is roughly 20 °C temperature difference between the high temperatures at the side and the temperatures in the center.

Acknowledgement:

With thanks to TP Vision Innovation Lab, Eindhoven and TP Vision TV Development, Bruges, formerly known as Philips TV

 
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