The simplest of the various configurations is the series configuration. The LEDs are all simply connected in series. The cathode of the first LED connected to the anode of the second and so on. This configuration is illustrated in Figure 1.
The string voltage is a function of the number of LEDs in the string and the typical forward voltage (Vf) of each LED. For example, 24 LEDs with a Vf of 3.5V would operate at a string voltage of 84V. A single current source provides power to each of the LEDs which, by design, all receive exactly the same current.
The implementation is very simple with only a single circuit. There are no current imbalance issues as all LEDs receive identical current. System efficiencies tend to be high as it is easier to optimize efficiency with high voltage and low current. No ballast resistors are required which tend to reduce efficiency. The most common LED failure mode has negligible impact on the system.
Output voltage can become very high which may translate into a safety issue. An open anywhere in the string whether it is a mechanical failure or a failed LED can cause the entire lamp to cease operation.
In this configuration if a single LED fails short there is virtually no impact on the other LEDs. Each continues to be powered with the same current as it was in normal operation. Overall less power is consumed and the lamp decreases in brightness by 1/n where n is the number of LEDs in the lamp. If single LED fails open then the current path is broken, all LEDs are turned off and the lamp ceases to operate.
For an application where system reliability is of primary concern, an OVP device can be connected in parallel with a certain number of adjacent LEDs (failure group). The OVP device normally consists of an SCR and some voltage detection circuitry. The SCR is open normally and activates short if any of the LEDs in the failure group fails open. In this case, all other failure groups remain functional. Brightness of the lamp is then reduced by 1/f where f is the number of failure groups.
A typical LED lamp consists of many 1 to 2W LEDs operating together. There are a variety of configurations for powering these LEDs. This application note discusses a variety of these configurations. For each configuration a diagram is presented along with a description of basic operation and a discussion of the various advantages and disadvantages. In addition, there is a brief description of what happens if a single LED fails either open or short. For reference, the most common failure mode for LEDs is to fail short.