This article is a basic introduction to the LED circuit provided by the electronic enthusiast website. This paper is divided into three parts: the composition of the LED circuit, the performance analysis of the LED circuit and the choice of the LED circuit driver.
1. The circuit composition of the LED circuit
When multiple LED products are required, if all the LEDs are connected in series, the LED driver needs to input a higher voltage; if all the LEDs are connected in parallel, the LED driver needs to output a large current. If all the LEDs are connected in series or in parallel, not only the strict use of the LEDs is limited, but also the parallel LED load current is large, and the cost of the driver is also increased. The solution is to use a hybrid mode. The number of LEDs in series and parallel is evenly distributed, so that the voltages distributed on one LED series branch are the same, the currents on each LED in the same series branch are basically the same, the brightness is consistent, and the current through each series branch Also similar.
2. Circuit performance analysis of LED circuit
When one of the series branches has a poor quality and short circuit, the current through the series circuit will increase regardless of whether the voltage is regulated or the constant current mode is used, and the LEDs in the series branch are easily damaged. After a large current passes through the damaged LED, it is often broken due to the large current passing through. After disconnecting an LED series branch, if the regulated drive mode is used, the output current of the driver will be reduced without affecting the normal operation of all remaining LEDs.
If the constant current LED driving method is adopted, since the output current of the driver remains unchanged, the current distributed in the remaining LEDs will increase, and it is easy to damage all the LEDs. The solution is to connect the LEDs as much as possible, so that when one LED is disconnected, the current distributed in the remaining LEDs is not large enough to affect the normal operation of the remaining LEDs.
The advantages of the series connection and the parallel connection are that the circuit is simple, the brightness is stable, the reliability is high, and the consistency requirement for the device is low, and no special selection of devices is required, even if the individual LED single tube fails, the influence on the entire illumination assembly is affected. Also small. In the case where the working environment factors vary greatly, the use of such a connection form of the light-emitting element is more effective.
There is another connection method for the hybrid mode, that is, the LEDs are evenly distributed and grouped in parallel, and then each group is connected in series. When there is a poor quality and short circuit of the LED, whether it is a regulated drive or a constant current drive, the LEDs connected in parallel will not light up. If the constant current LED driving method is adopted, since the output current of the driver remains unchanged, except for the parallel branch connected in parallel on the short-circuit LED, the remaining LEDs work normally. Assuming that the number of LEDs connected in parallel is large, the drive current of the driver is large, and the current through the short-circuited LED will increase. After passing a large current through this shorted LED, it is easy to become an open circuit. Since there are more LEDs in parallel, after disconnecting one LED, the average distribution current does not change much, and the remaining LEDs can still work normally, then only one LED in the whole LED lamp does not light.
The problem of the light-emitting elements formed by the first and second series of mixed connections is mainly in a single set of parallel LEDs. Due to the difference in devices and operating conditions, individual LED chips in a single group may lose PN junction characteristics and short circuits may occur. A short circuit on an individual device causes the un-failed LED to lose its operating current IF'', causing the entire set of LEDs to go out, the total current is all passed through the shorted device, and the longer short-circuit current causes the bonding wire or other part inside the device. Burned, there is an open road. At this time, the LED that has not failed regains current and returns to normal illumination, except that the operating current IF is larger than the original. This is the reason why the light-emitting elements of this connection form are first turned off together with a few LEDs in a group, and after a period of time, except for one of the LEDs, the other LEDs return to normal.
3. LED circuit driver selection
According to the analysis, it is very important to select the combination of the driver and the load LED in series/parallel mode. When driving the power LED in constant current mode, it is not suitable to use the parallel load: Similarly, the regulated LED driver is not suitable for the series load.
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