Nikkei Electronics Asia -- July 2009
Insights
Power Saving on LED Drivers for Traffic, Message Signs

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Jul 30, 2009 00:00 Tiger Yen, RD Engineer, Macroblock Inc.

The worldwide energy shortage has raised public awareness on energy saving issues, and revolutionary LED light sources have therefore been widely adopted for their low power consumption, long life, and fast turn-on times. As LED brightness is adjusted by current, a constant current LED driver is necessary to keep uniform LED brightness.

Furthermore, an LED driver with power saving functions becomes popular in order to meet energy efficiency requirements.

Concerns over Power Saving

In LED traffic sign and message sign applications, only partial LEDs are turned on in a fixed area during a specific time period. The LEDs of the speed limit sign are turned off in the dark area; the other LEDs are turned on in the red and white areas. However, in the dark area, even if the LEDs are turned off, the LED drivers still keep working, which wastes power over a long period of time and shortens the lifecycle of LED drivers.

To maximize the energy efficiency of LED traffic signs and message signs, LED drivers are now designed with power saving functions. For example, when a car passes by, the LED speed limit sign is turned on automatically to display the speed. When no car passes by, the LED speed limit sign is turned off again. Under such conditions, the LEDs do not consume any power, and the LED drivers with power saving functions, such as the MBI5037 from Marcoblock, automatically enter sleep mode, which only consumes less than 0.1mW, dramatically reducing the total power consumption of the whole display panel. In addition, the reduction of power dissipation on LED drivers helps improve the energy efficiency, and the methods may include reducing supply current (IDD), supply voltage (VDD), LED supply voltage (VLED), and sustaining voltage (VDS). The following equations help calculate the power dissipation of LED drivers:

PSD = PDN + PDM,
where PSD is total power dissipation of LED drivers;
PDN is power dissipation of LED drivers in use;
PDN = (IDD x VDD) + (IOUT x Duty x VDS x 16) x N, in which N is the number of LED drivers in use.
PDM is power dissipation of LED drivers that are not in use;
PDM = IDD x VDD x M, in which M is the number of LED drivers that are not in use.

In addition, the following equations help calculate the power loss of an LED driver:

PDRVLOSS = PALLDRV - PUSEDRV ,
where PDRVLOSS is power loss of an LED driver;
PALLDRV is power consumption of all LED drivers; and
PUSEDRV is power consumption of LED drivers which are driving the turned-on LEDs.

Efficiency Improvements

There are different ways to improve the power efficiency of LED display systems, but there are also tradeoffs in power efficiency, system cost and display quality. The following introduces some of the common methods to improve power efficiency, and two of these methods, sleep mode and 0-power mode, have already been applied in some of the newly developed LED drivers to improve power efficiency without increasing the system cost or sacrificing the display quality.

Conventional methods to improve power efficiency of LED drivers for LED traffic signs and message signs involve low operation voltage. Users set the minimum supply voltage for LED drivers, i.e. 3V, to lower the power consumption. From the equation P = I x V, the lower supply voltage helps reduce the power consumption. However, when the VDD is connected to VLED, blue or green LEDs require higher supply voltage; therefore, this method is not always applicable for all application conditions.