Key points for the application of overcurrent protection devices for LED lighting

LED has the distinct characteristics of low power consumption, long life, and environmental protection. After penetrating from small-size backlight applications to large-size panel backlights, it is now further expanding into general lighting applications. It has been rapidly developing in landscape, architecture, commerce, automobile, mobile lighting and other fields.

In the face of many application fields of LED lighting, there are various driving solutions because the market has not formed a unified standard. However, from the perspective of overcurrent protection, it can be roughly divided into two basic types according to the input current of the lamp body: DC input and grid AC input. The main difference between the two types is whether the driving power supply has an AC to DC conversion module. Of course, there are also many low-cost solutions such as resistors and RC step-down on the market. There are no EMI rectification and constant current drive modules, but since the selection requirements of the primary-side fuse are consistent with Figure 2, it is classified as the second type here.

There are three main purposes for overcurrent protection of LED lighting:

First, to protect personal and property safety, protective devices are often added for this purpose in applications where the power input voltage is relatively high.

Second, isolate the subsystem from the external system. If the external system becomes unstable, it will affect the safety of the subsystem. On the other hand, if the components inside the subsystem are damaged, such as the short-circuit failure mode of the parallel components, or the originally safe insulation level inside is destroyed due to external factors, causing the internal circuit to short-circuit to the ground, if there is no fuse to isolate the system from the outside and no effective measures are taken in time, the fuse system of the external system will be activated, which will cause the entire power supply system to be interrupted, which will indirectly cause unexpected losses in important places such as hospitals, and may even cause casualties, so sometimes it is necessary to add 2 fuses at the power input end.

Third, protect important and valuable electronic components or devices. For LED lighting fixtures, the cost of LED lamp beads accounts for a large part of the total cost of the fixture. In non-isolated power drive or non-constant current drive circuits, the output end of the drive needs to be protected from overcurrent.

Due to the different architectures of power input, there are significant differences in the focus of selecting overcurrent protection devices:

1. When selecting DCin fuses of DC input type, special attention should be paid to the temperature reduction coefficient parameters of the fuses. Since high-power LEDs generate a lot of heat, the temperature inside the LED lamp cup is relatively high. If a fuse with a larger temperature reduction is selected, a larger current specification will be selected. Under the same working current, the protection capability of the fuse with a large current specification will be relatively lower. In addition, at the position of DCin, since capacitor filtering is used at the back end, a relatively large startup pulse current will be generated. Therefore, when selecting a fuse at this position, it is necessary to pay attention to strictly consider the pulse conditions. Otherwise, the wrong option will easily cause the fuse to be broken by the startup pulse, and it will be difficult to pass many power-on and power-off tests and surge current tests. It is recommended to use products with strong pulse resistance.

2. For the selection of fuses at the output end of the driver, in addition to paying attention to the fuse temperature reduction coefficient, it is also necessary to consider the fuse's fusing speed index. Since the current fluctuation here is not large, in the event of circuit abnormalities or component failure, the circuit needs to be quickly cut off to protect the rear-end LED light string. It is recommended to select a fast-acting type and a fuse with a small temperature reduction at this location.

Generally, there are many SMD low-voltage fuses available on the market for the above two occasions, such as AEM Technology's SolidMatrix® technology fuses, with sizes ranging from 0402 to 1206 and current specifications ranging from 0.5 to 30A. There are many different series, specifications, and characteristics of fast-break, fast-fast-break, high-pulse resistance, slow-break, etc. for engineers to choose from.

3. For the ACin position of AC input LED lighting, especially LED spherical bulbs, it is necessary to consider both the size of the fuse and the withstand voltage of the fuse. However, the fuses on the market that can withstand a wide AC voltage range are generally large in size, and there are few small-volume SMD fuses with a withstand voltage of 250VAC. To address this application dilemma, AEM Technology has launched the AirMatrixTMAF2 series of SMD fuses, which are only 6.1mm*2.5mm*2.2mm in size and can withstand a voltage of 250VAC. At the same time, they also have the advantages of high consistency, low internal resistance, and high pulse resistance, which will bring a new application experience to all engineers.

Some engineer friends believe that fuses can be cut off in case of overcurrent regardless of whether they are used on the positive or negative poles or the neutral or live wires, and can play a protective role. Here I would like to share my experience with all engineer friends. Fuses need to be placed on lines with a potential difference with the ground. Fuses placed on the ground wire can also cut off the circuit, but after the cut-off, there is still a potential difference between the entire lighting appliance and the ground. Once an internal device short-circuits, it will cause circuit danger. Therefore, in countries where some sockets can be plugged in reverse, such as Germany or China, from a safety perspective, it is necessary to consider adding fuses to the LN of some LED lighting fixtures that can be plugged in reverse.