By John Hu
In-field power supply failure is a major cause of downtime in LED Displays. This is especially a critical problem in the Digital Outdoor and Live Broadcast applications. This white paper explores the technologies available to achieve 100 percent power supply up time that are now available in the market.
In the world of LED displays, “resiliency and redundancy” are buzzwords for critical applications such as broadcast TV and digital out-of-home — where every minute of up-time is sponsored. The power system, and in particularly the power supply unit (PSU) is an area susceptible to failures and continues to be an area of innovation for designers.
According to Meanwell, an industry leading power supply provider, the MTBF¹ (Mean Time Between Failures) on the widely used SP320-5 is 207,000 hours. (MIL-HDBK-217F (25°) There still remains a probability of outage due to normal wear and tear, operating in non-laboratory environments, outlier events or unpredictable external factors such as a power surge or unpredictable weather related events.
Most displays on the market today (2013) are not equipped with any resilient power system. Thus, if and when a PSU goes out, the LED tiles which it drives will inevitably go dark. Most designs use one PSU to drive 3 to 6 tiles, based on screen pitch, tile size and power consumption of each tile. This means up to six tiles would go out in a chain of dark line across the screen when a failure occurs.
According to industry leader Daktronic’s product page:²
“Fully Sealed, 2-to-1 Power Supplies
… Your message remains readable even if a power supply fails because the failed power supply only affects two modules. Other LED digital billboard distributors may have a single power supply delivering power to large groups of modules. When a power supply fails, it affects a vast section of a digital billboard and the advertisement becomes unreadable.”
What this tells us is that if a PSU goes out on a Daktronics system, two LED tiles will be affected. This is an advantage over others that drive more than two tiles per PSU.
With resiliency implemented, any dark tile on a screen due to power can be eliminated. As soon as one PSU goes down, the back-up source is immediately activated. This switch over is instantaneous and without any visible interruption to the audience. The probability of a black tile, approaches to zero as it’s correlated to the probability of two or three PSUs going out at the same time. Even in the extreme unlikely event that multiple PSUs go out simultaneously, the feedback and intelligence function will reduce the power draw requirement and dim the affected screen area so the area is dimmed, but never goes completely dark.
Every single PSU is paired with a backup unit. This is the most straightforward approach but the weight of panel and increased cost is the tradeoff. Assume there are 4 PSUs per a 3×4′ (1×1.2m) panel and 4 pounds per PSU on average would result in a total increase of 16 pounds per panel. Cost of additional PSU needs to be factored in but the benefits should outweigh the cost incurred from down time on the screen.
This setup is where a group of say four or five power supply is used in tandem with a backup unit (in which case would be four or five, plus one). An intelligent feedback system is activated to monitor the number of active PSUs. If one PSU goes out, the redundant grid isn’t affected as there’s ample overhead. In the extremely unlikely case that more than two to three units goes out at the same time, the intelligence on the Smart system can reduce the screen brightness of the affected area to say, 50 percent, thus reducing the overall load. The cost adders and weight increase of the the n+1 solution is marginal and thus by far it’s the most advanced in the market.
A component that goes hand in hand with “resiliency” is the “feedback” mechanism. As a PSU goes out, feedback is activated, and the service provider is made aware of the outage. The location and exact number of the PSU is also proactively identified so that a scheduled, targeted and swift maintenance event can be made. Feedback is commonly delivered by email or text message.
The mechanism by which the control system of the display works in conjunction with the PSUs to reduce the brightness of the affected area or to activate PSUs on reserve.
This blog was reprinted with permission from John Hu / QST and originally appeared here.