Digital signage, area illumination, and display back-lighting require large numbers of LEDs, especially of the high-brightness types. A simple arrangement is adequate for driving a single or a few LEDs. However, driving large numbers of LEDs presents a different challenge. There are issues like the optimum overall interconnection topology, the options for powering them, and then the control of the array.
A drive current from 20 to 60 mA is adequate for most single LEDs with a voltage drop of 1.8 to 4 VDC. For instance, a red LED has a nominal voltage drop of 1.8 VDC, and a white LED as high as 4.0 VDC, with other colors in between. Therefore, driving LEDs means changing over to constant-current supplies or drivers rather than using the more common constant-voltage supplies. Most designers are often less familiar with constant-current modes of supplies and their implications.
In concept, the power supply and the driver chain for LEDs are fairly simple. However, driving an array of LEDs is both simple and difficult. While driving a single LED is simple as it is a low voltage and low current load, and the driver has to operate at an efficient dc-to-dc conversion frequency of between 100 kHz and 1.5 MHz. The difficult part is the driver has to supply a constant current, which places new and difficult demands on the design of the circuitry.
Typically, the LED driver for a multi-LED array is the final stage in the power-supply chain, beginning with the AC input. There are optional low-voltage dc-to-dc conversion stages between the two, and ultimately the final DC voltage-to-current conversion immediately before the actual LED drive.
For creating an array of multiple LEDs, it is necessary for the designer to first decide the optimum combination of series, parallel, and series-parallel topology they must use. It is possible to wire up arrays of multiple LEDs as series-only, parallel-only, or as a combination of series and parallel. Each combination brings its own trade-offs for deciding the driver, its cost, reliability, physical layout options, and failure/fault handling. They must also consider the unavoidable thermal considerations and heat dissipation.
In a series-only configuration, a single power rail of the power supply supplies all the LEDs. Therefore, the current flowing through each LED is the same, allowing more or less equal brightness from the LEDs. However, an open-mode failure in any LED serves to shut down the operation for the entire chain. To prevent this from happening, most designers add a small value resistor across each LED, thereby adding to cost and space requirements.
The series-only combination has another issue, that of compliance voltage. As the chain grows longer, so must the voltage go up to deliver the current to the chain, as this voltage is the sum of the drops across all the LEDs. Regulating 20 to 60 mA at a high voltage of 150 VDC is difficult.
A parallel combination of LEDs means the driver must supply only a low compliance voltage, equal to the voltage drop across one LED. However, the power supply now has to supply a fairly large amount of current, equal to the sum total of currents of all the LEDs.