release time：2020-05-16 source：Shenzhen TOFOLED Lighting Technology Co., Ltd.
1. Circuit composition
When more LED products need to be used, 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 strict usage of LEDs, but also increases the load current of parallel LEDs, and the cost of the driver will increase. The solution is to use a hybrid connection. The number of series and parallel LEDs is evenly distributed. In this way, the voltage distributed on one LED series branch is the same, the current on each LED in the same series branch is also basically the same, the brightness is the same, and the current through each series branch Also similar.
2. Circuit performance analysis
When one LED in a series branch is of poor quality and short-circuited, the current through the series circuit will increase regardless of whether the regulated driving mode or the constant current driving mode is used, and it is easy to damage the LEDs in the series branch. After a large current passes through the damaged string of LEDs, because of the large current passing through, it mostly appears as an open circuit. After disconnecting an LED series branch, if a regulated driving method is used, the output current of the driver will be reduced without affecting the normal operation of all remaining LEDs.
If a constant current LED driving method is adopted, since the output current of the driver remains unchanged, the current distributed among the remaining LEDs will increase, easily damaging all LEDs. The solution is to connect as many LEDs in parallel as possible, so that when a certain LED is turned off, the current distributed in the remaining LEDs is not large, so as not to affect the normal operation of the remaining LEDs.
The advantages of this serial-to-parallel circuit are simple circuit, stable brightness, high reliability, and low device consistency requirements. No special device selection is required, even if a single LED tube fails, it affects the entire light-emitting assembly. Also smaller. In the case of large changes in working environment factors, the effect of using this type of connection light-emitting assembly is ideal.
The problem of the parallel-to-serial mixed connection light-emitting assembly is mainly in a single group of parallel LEDs. Due to the difference in devices and usage conditions, individual LED chips in a single group may lose the PN junction characteristics and appear short-circuited. The short circuit of an individual device will cause the non-failed LED to lose the operating current IF', which causes the entire group of LEDs to go out. The total current passes through the short-circuit device, and the longer short-circuit current will burn the bonding wire or other parts inside the device. , An open circuit appears. At this time, the unfailed LED regains current and resumes normal light emission, but the operating current IF is a little larger than the original. This is the reason why the light-emitting components of this connection form first appeared when several LEDs in a group were turned off together. After a period of time, except one of the LEDs was not lit, the other LEDs returned to normal.
There is another connection method for the hybrid connection, that is, the LEDs are evenly grouped and connected in parallel, and then each group is connected in series. When one LED is of poor quality and short-circuited, no matter whether it is a constant-voltage driving method or a constant-current driving method, the LEDs connected in parallel in this branch will not be lit. If a constant current LED driving method is used, the output current of the driver remains unchanged, except for the parallel branch connected in parallel to the short-circuit LED, the remaining LEDs are working normally. Assuming that the number of LEDs in parallel is large and the drive current of the driver is large, the current through this short-circuited LED will increase. After a large current passes through this short-circuited LED, it will easily become an open circuit. Due to the large number of LEDs connected in parallel, the average distribution current does not change much after one LED is disconnected, and the remaining LEDs can still work normally, so that only one LED is not bright in the entire LED light.
3. The choice of driver
It can be seen from the above analysis that the combination of the driver and the load LED series/parallel mode is very important. When driving a power LED in a constant current mode, it is not suitable to use parallel loads: Similarly, the regulated LED driver is not suitable for series loads.
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