Parallel Strings LED Drivers Configuration Guide
In order to reduce the maximum string voltage and/or to introduce a little more fault tolerance, a common configuration is to utilize multiple paralleled strings of LEDs. Taking the example where 24 LEDs were used to construct a lamp, these LEDs could be arranged in 4 paralleled strings of 6 LEDs each. This configuration is illustrated below in Figure 2.
Basic Operation
Total string voltage is reduced from the Series Configuration by a factor equal to the number of paralleled strings. Current is divided between the various strings based on how well each of the strings are matched. Small differences in the Vf of the various LEDs can cause significant imbalances in current. A resistor in each leg is typically employed to help balance the current. While in theory a constant voltage source could be used to drive the system, the total current can change significantly at the boundaries of the voltage regulation band or when the forward voltage drop of the LEDs varies. Therefore it is highly desirable to employ a constant current driver in this configuration.
Advantages
This configuration requires the use of only a single driver and output voltage can be kept rather low. By properly selecting the value of the resistance, reasonable current sharing can be achieved among the paralleled LED strings.
Disadvantages
The improved accuracy of current sharing is achieved at the expense of increased power dissipation in the resistors. Any failure of an LED open or short can cause significant stress on the remaining LEDs.
Failure Modes
If a single LED fails short, the string in which the failed LED resides will take significantly greater than its share of the current. The stress on the LEDs in this string will increase significantly thus increasing the likelihood that a second LED will fail. The LEDs in each of the other strings will dim considerably as the total current is fixed by the rating of the driver. If any LED fails open, then the LEDs in that string will cease to operate but the current in each of the other strings will increase by a factor of 1/(s-1) where s is the total number of strings.
LED Driver Configurations Guide
- Series LED Driver Configuration
- Parallel Strings LED Driver Configuration
- Matrix LED Driver Configuration
- Multi-Channel LED Driver Configuration
- Top 10 Questions and Answers About LED Drivers
- LED Lighting Power Management
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