The majority of LED fixtures must be constructed of more than a single LED if the desired brightness is to be achieved. This means that a decision must be made on how to wire the LEDs together in a single fixture. There are three different configurations usually used to do this, (Series, Parallel and Matrix), each with its own advantages and disadvantages.
Series This is the most straightforward way to connect your LEDs is in a series configuration in which the cathode of each driver is connected to the anode of the next (see figure below).
1.The main advantage of this implementation is the fact that all possible LED current imbalances are avoided through using a single string (the same current must flow though each LED).
2. Since no external circuitry is required to control the current level through each LED, it is also the most efficient configuration that can be used.
3.Any failure which blows an LED short will have no effect upon the remainder of the LEDs.
1. In order to generate more light in a single fixture, more LEDs must be added to the string. Since the string voltage is proportional to the number of LEDs in the string, long strings can generate very high voltages which can translate into a safety issue (for example, a string of 48, 3.5 V LEDs would have a string voltage of 168 V).
2. Any failure which blows an LED open will break the circuit of the string, and will cause the lamp to stop functioning.
Parallel To decrease the potential high string voltages of the series configuration, another option is to use multiple parallel strings of LEDs. These parallel strings will be made up of less LEDs in series, and so the string voltage would be reduced by a factor equal to the number of parallel strings (for example, 48 LEDs can be wired in 8 parallel strings of 6 LEDs each, which would have a string voltage of 21 V)
1.The output voltage of the driver used can be kept relatively low (important for Class 2 requirements).
1.Small differences in the forward voltages of various LEDs can cause significant imbalances in current due to the non-linear relationship between voltage and current due to the non-linear relationship between voltage and current in LEDs. This difference in forward voltage can be caused by inherent differences in the LEDs physical makeup, as well as voltage shifts due to temperature differences.
2. A resistor placed in each string makes the voltage – current relationship of the string more linear thus balancing the current. This however decreases the efficiency of the system.
3. Any failure of an LED (open or short) can cause stress in the remaining LEDs.
Matrix In order to improve upon the Parallel String configuration, it is possible to make additional connections to form a Matrix. In a Matrix configuration, multiple LEDs are connected in parallel in ‘bands’ and each of these ‘bands’ is connected in series to form a Matrix.
Using the example used above in the Parallel String configuration, (48 LEDs wired in 8 parallel strings of 6 LEDs each), the Matrix configuration would be 6 ‘bands’ of 8 parallel LEDs connected in series. In this configuration, the string voltage is still 21 V, so the same voltage is applied across each LED as the parallel configuration, and the same drive current flows through each LED.
1.The output voltage of the driver used can be kept low (important for Class 2 requirements).
2.A single LED failing short would take out the band of LEDs, but the remaining LEDs will operate without too much additional stress.
1. Balancing (or load sharing) of the LEDs is still an issue.
2.It is more complex than the other configurations.
Independent Strings The most robust solution is to use a driver with multiple outputs, each one regulated with independent constant current control. This would give all the load balancing advantages of series connected LEDs, while limiting the forward voltage of the strings of LEDs (the advantage of the Parallel configuration)
This type of driver will be significantly more expensive due to the requirement of multiple independently regulated outputs.