Blog

Why are some fine-pitch displays operating like toasters?

Manufacturers are continuing to develop finer pitch displays, but as the size gets smaller, the heat goes up to almost 165 degrees Fahrenheit. So what is happening that the surface of the display is a perfect temperature to cook an over-easy fried egg, and how can we fix it?

November 3, 2016 by Gary Feather — CTO, NanoLumens

I was recently wandering around an international digital signage conference, and I became more excited about the capabilities of this industry to innovate in fine-pitch.  We are, quite frankly, exceeding everyone's expectations. The display systems continue to get thinner and lighter. The interfaces between the cabinets are transitioning to cable-free. The weight in kg/m2 continues to decline and the ability to build finer pixel pitch displays is exploding. Everywhere I look there is 2.5-millimeters, 1.8-mm, 1.6-mm and all the way down to 0.9-mm pixel pitch. Now we all know that tighter pitch displays are tougher to assemble while still maintaining the uniformity of a seamless display. We also know they can be easily damaged and that many manufacturers are working to make them more robust. And, we also know that we have to use smaller and smaller packages; from 2121 (2.1-mm on a side) LED packages all the way down to 0707 LED packages. 

With this trend in tighter resolution I also noticed another trend; the tighter pitch displays are getting dimmer in specified nits, and the display surfaces are getting hotter with the smaller the pitch. I had a chance to measure a few stellar 1.2-mm displays thermally at full brightness, and measured temperature on the surface at over 73 degrees Celsius on both displays. Even though the LEDs are packed tighter, it would seem with the lower level of nits (cd/m2), the displays would be cooler. So what is happening that the surface of the display is a perfect temperature to cook an over easy fried egg (165 degrees Fahrenheit)?

Who's turning up the heat?

More analysis reveals the problem. The answer is contained in the fact that in addition to being closer and for adding more heat in a smaller area, for the finest pitch, the LED die are also getting smaller and therefore, efficacy (mcd/mA) and performance are dropping. Also, pixel pitch densities with the desired brightness without disproportionate increases in power dissipation in a smaller area translates to higher operating temperatures of display electronics such as components, power supplies and LEDs.

This translates to lower MTBFs (mean time between failure) and rising ppm failure rates of the individual LEDs to a level making support (reliability and life) of the display below the desired requirements, such as  cCurrent 6mm pixel pitch LED digital Ssgnage with D65 (6500K) white point average around ~8cd/W at 2000 nits. Or, high contrast 4-mm displays at 2000 nits that can be ~4cd/W, dissipating almost twice the power for the same per unit area brightness (nits). Each of these products maintains operating temperatures below 50 degrees (25 degree rise over ambient).  The industry challenge should be to keep the efficacy above 2 cd/W as pixel pitches drop from 2.5 mm down to 1.25 mm and 0.9-mm pixel pitch. But at this time, cd/W are dropping, nits are dropping and the surface temperatures are rising. 

There is also a need for a lower cost per RGB SMD LED pixel in order to increase the market and to drive the LED harder and for more time to create the desired brightness. The result is that the two fine pitch 1.25-mm pixel pitch LED displays tested at a system level, were below 1 cd/W. Proactively the industry must respond with actions to stop the fall in cd/W. 

How do we cool the heat?

First, LED die sizes must not be allowed to shrink below a reasonable square mils level. While smaller die for the R and G and B means less semiconductor material, and also a higher yield and compound lower cost, the system impact on energy can be significant. Second, the drive currents must be adjusted and set at a level to maintain years of safe and reliable operation. Increasing drive currents above this level will result in earlier failures. Third, drive circuits and drive timing must be designed to minimize the power dissipation, which may require a lower level of drive multiplexing (1/8 verses 1/32). Finally, regardless of the power generated, the thermal design of the fine -pitch display must be improved to assure the customer that the surface temperature of the display will stay at a "safe –to-the-touch" level. Expectations are usually in the 50 degree range.

Good digital signage display designs always requires a system level approach. After understanding all the requirements of the customers, the end display system must meet the firm requirements. Designs must avoid excessive operating temperatures and remove design elements that lower the cd/W to an unacceptable level. Expecting more from the display designers will yield better displays with longer life and greater reliability.

Image via iStock 

About Gary Feather

---

None

Connect with Gary: