Composition and performance of LED circuit

1. Circuit composition

When more LED products are needed, 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 in parallel not only limits the usage of LEDs, but also increases the load current of parallel LEDs and the cost of the driver. The solution is to use a mixed connection. The number of LEDs in series and parallel is evenly distributed, so that the voltage distributed on an 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 an LED in a series branch has poor quality and short circuits, regardless of whether the voltage-stabilized drive mode or the constant current drive mode is used, the current passing through the series circuit will increase, which can easily damage the LEDs in the series branch. After a large current passes through the damaged LED string, it often appears as a short circuit due to the large current passing through. After disconnecting an LED series branch, if the voltage-stabilized drive mode is used, the output current of the driver will decrease without affecting the normal operation of all the remaining LEDs.

If a constant current LED drive is used, since the output current of the driver remains unchanged, the current distributed in the remaining LEDs will increase, which can easily damage all LEDs. The solution is to connect as many LEDs in parallel as possible, so that when one LED is disconnected, the current distributed in the remaining LEDs is not large, which will not affect the normal operation of the remaining LEDs.

The advantages of this series-first-parallel circuit are simple circuit, stable brightness, high reliability, and low consistency requirements for devices. There is no need to specially select devices. Even if a single LED tube fails, the impact on the entire light-emitting component is small. In the case of large changes in working environment factors, the light-emitting element using this connection form has a more ideal effect.

There is another way to connect the parallel-parallel mode, which is to evenly distribute the LEDs and connect them in parallel, and then connect each group in series. When one LED is of poor quality and short-circuited, no matter whether it is a voltage-stabilized drive mode or a constant current drive mode, all the LEDs connected in parallel in this branch will not light up. If a constant current LED drive mode is used, since the output current of the driver remains unchanged, except for the parallel branch connected in parallel to the short-circuited LED, the rest of the LEDs will work normally. Assuming that there are a large number of LEDs in parallel and the drive current of the driver is large, the current passing through this short-circuited LED will increase. After a large current passes through this short-circuited LED, it is easy to become an open circuit. Since there are many LEDs in parallel, after disconnecting one LED, the average distribution current does not change much, and the rest of the LEDs can still work normally, then only one LED in the entire LED lamp will not light up.

The main problem with the light-emitting element formed by the mixed connection of parallel first and then series 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 short-circuit. The short circuit of individual devices will cause the LEDs that have not failed to lose their working current IF'', causing the entire group of LEDs to go out, and the total current will all pass through the short-circuited device. The short-circuit current for a long time will cause the bonding wires or other parts inside the device to burn out, resulting in an open circuit. At this time, the LEDs that have not failed to fail regain current and resume normal light emission, but the working current IF is slightly larger than before. This is the reason why several LEDs in a group of light-emitting elements with this connection form go out together first, and after a period of time, except for one LED that is not lit, the other LEDs return to normal.

3. Driver selection

From the above analysis, it can be seen that the matching selection of driver and load LED series/parallel mode is very important. When driving power LED in constant current mode, it is not suitable to use parallel load: similarly, voltage-regulated LED driver is not suitable for series load.