Thursday, January 17, 2013

LED Sign Tech & Spec: Power Supplies

-Deacon Wardlow
Power supplies are the beating heart of an outdoor LED system. I wrote a brief article on this topic previously  covering some scenarios on off-the-shelf product vs custom specified components. This article is specifically taking a look at the heart which pumps power through a system.

Power supplies primarily serve the purpose of AC to DC conversion and power flow control. In a PC, the power supply runs proper levels of power to different components in the system. In an LED system, the power supply takes 120V (or 240V) AC service and converts it to DC for the control components and LEDs to use. As little as 8 years ago, power supplies were built with this primary purpose in mind and had little-to-no safeties built into the system.

Current (quality) power supplies have several control factors and you should know what they are and how they work. If the manufacturer you're working with can't say their power supplies handle ALL of the features mentioned, you should be concerned as these are standard feature sets. Make them prove it...

Here are the standard features and what they mean:

SMPS Universal AC input / Full range
In a SMPS (switched-mode power supply) AC input is directly rectified and filtered to obtain DC voltage. Switched-mode power supplies are regulated to keep the output voltage constant. These employ a controller which monitors current drawn by the load. As power output requirement increases, the duty cycle increases thus ensuring the system has the power it requires.

Built-in active PFC function
Power Factor Correction (PFC). This is a somewhat complex system which could take pages to explain. Simply put active power factor correction consists of a number of capacitors which are switched by means of contactors. These contactors are controlled by a regulator that measures power factor in an electrical network. Depending on the load and power factor of the network, the power factor controller will switch the necessary blocks of capacitors in steps to make sure the power factor stays above a selected value. This keeps everything in the system even and regulates the power going through to the system, alleviating spikes/drops in power which could shorten the lifespan of your display.

Protections against Short circuit / Overload / Over voltage / Over temperature
Quality Power supplies often include safety features such as current limiting which protect the device from harm. In the event an abnormal high-current power draw is detected, the power supply recognizes this as a direct short and will shut itself down before damage is done. The same goes for excessive heat levels (which directly affect performance).

Forced air cooling by built-in DC Fan
Many power supplies are built with passive thermal management (perforated aluminum casing). Unfortunately this only serves low-end power systems. An LED display often requires significant power and can create high levels of heat (BTUs - British Thermal Units) which need to be mitigated within the system. 

Built-in fan speed control
Simply having a forced air cooling system isn't enough. The fans need to be managed with controls set to run at varying required speeds (from ) - 100%) when thermal sensors detect levels above a given threshold.  This ensures the system continues to operate optimally. As heat increases, a power supplies efficiency decreases and its output drops. Keeping the power supplies cool is incredibly important to ensuring the system performs at its best.

Fixed switching frequency at 100KHz
Resulting DC voltage is switched on and off at a high frequency by electronic switching circuitry.This produces an AC current which passes through a transformer or inductor. High frequency switching enables the use of  filter capacitors that are much smaller, lighter, and less expensive than pass-through systems (systems which use control chips to filter/convert power). After the inductor , the high frequency AC is rectified and filtered to produce the DC output voltage. If the SMPS uses an adequately insulated high-frequency transformer, the output will be electrically isolated from the mains; this feature is often essential for safety.

While this is a little drier than the regular articles posted, it's important to understand the heart of LED Display systems. A lot of manufacturers cut corners with the power supplies and they're hoping buyers won't know the difference. These seem like small, unimportant things (a power supply is a power supply, right?) but they have significant importance to the overall  lifetime optimal performance of your display. The more you know, the better you can source the best solution for your company and clients.

*I invite you to comment here and/or email me directly with requests at Vantage LED has white paper resources and more educational material on the website (, please check it out when you have a moment. Note all posts/thoughts/writings are strictly the viewpoint of me and me alone and do not reflect nor speak for Vantage LED’s beliefs, attitudes, thoughts, etc. unless specifically stated.

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