1. Circuit composition

When using a large number of LED products, if all LEDs are connected in series, the LED driver will need to output a higher voltage; if all LEDs are connected in parallel, the LED driver will need to output a larger current. Connecting all LEDs in series or parallel not only limits the strict usage of LEDs, but also increases the load current of parallel LEDs and the cost of drivers. The solution is to adopt a hybrid connection method. The number of LEDs in series and parallel is evenly distributed, so that the voltage distributed on one LED series branch is the same, the current on each LED in the same series branch is basically the same, the brightness is consistent, and the current passing through each series branch is also similar.

2. Circuit performance analysis

When there is a poor quality LED short circuited on a series branch, regardless of whether the voltage regulation driving method or the constant current driving method is used, the current passing through the series circuit will increase, which can easily damage the LED in the series branch. After a high current passes through the damaged LED string, it often manifests as a circuit breaker due to the large current passing through. After disconnecting one LED series branch, if a stable driving method is used, the output current of the driver will decrease without affecting the normal operation of all remaining LEDs.

If a constant current LED driving method is used, due to the constant output current of the driver, the current allocated to the remaining LEDs will increase, which can easily damage all LEDs. The solution is to parallel as many LEDs as possible, so that when one LED is disconnected, the current allocated to the remaining LEDs is not large, and it will not affect the normal operation of the remaining LEDs.

The advantage of this series connected and then parallel connected circuit is that it is simple, has stable brightness, high reliability, and has low requirements for device consistency. It does not require special device selection, and even if individual LED single tubes fail, the impact on the entire light-emitting component is relatively small. In the case of significant changes in work environment factors, the use of this type of connection for light-emitting components yields ideal results.

The main problem with the light-emitting component composed of a mixed connection of parallel and serial connections is that in a single group of parallel LEDs, due to differences in devices and usage conditions, individual LED chips in a single group may lose their PN junction characteristics and experience short circuits. Individual device short circuits can cause the remaining LEDs to lose their operating current IF, resulting in the entire LED group being extinguished. The total current passes through the short circuited device, and prolonged short circuit currents can burn out the bonding wires or other parts inside the device, leading to an open circuit. At this point, the LED that has not failed regains current and resumes normal light emission, but the operating current IF is slightly higher than before. This is why this type of connected light-emitting component first turns off several LEDs in a group, and after a period of time, except for one LED that does not light up, the other LEDs return to normal.

There is another way to connect the hybrid system, which is to evenly distribute the LEDs and group them in parallel, and then connect each group in series. When one LED has poor quality and short circuits, regardless of whether it is driven by voltage regulation or constant current, all LEDs connected in parallel in this branch will not light up. If a constant current LED driving method is used, due to the constant output current of the driver, except for the parallel branch connected to the short-circuit LED, all other LEDs work normally. Assuming there are a large number of LEDs in parallel and the driving current of the driver is high, the current through this short circuited LED will increase. After a high current passes through this short circuited LED, it is easy to become an open circuit. Due to the large number of parallel LEDs, after disconnecting one LED, the average distribution current does not change much, and the other LEDs can still work normally. Therefore, only one LED in the entire LED light is not lit.

3. Selection of Drivers

From the above analysis, it can be concluded that the selection of the combination of driver and load LED series/parallel is very important. When driving power type LEDs with constant current, parallel loads are not suitable. Similarly, voltage regulated LED drivers are not suitable for series loads.