LED lighting system protection

In this circuit, the AC fuse provides the basic fire protection to deal with the main system fault that may cause the overcurrent condition, but it must be able to tolerate the surge of 3KA to 6KA without breaking. In the case of downstream component failure, DC fuses can be used for fast over current protection in DC to DC converter or LED drive circuit.

Many of the protective devices can be used for power supply and LED drives, and there are many publications that can provide guidance for selection. The circuit in Figure 1 is an example of switching power supply protection in LED street lighting systems. In this circuit, the AC fuse provides the basic fire protection to deal with the main system fault that may cause the overcurrent condition, but it must be able to tolerate the surge of 3KA to 6KA without breaking. In the case of downstream component failure, DC fuses can be used for fast over current protection in DC to DC converter or LED drive circuit.

Figure 1:LED street lighting circuits, for example, including switching power supply (SMPS) related to the protection device.

On the AC input side of the circuit, overvoltage events and voltage transients are also required. They are usually caused by nearby lightning strikes, but they can also be generated by transient state changes on the power line. Typical protection for overvoltage conditions is a metal oxide varistor (MOV) that can be combined with a transient voltage suppressor (TVS). Circuits for power protection also need to be isolated from the ground to prevent possible shock hazard (these specifications are included in IEC/UL60950-1, UL1449 and IEC/UL6500). Figure 2 shows the solutions to meet these requirements. The design incorporates a metal oxide varistor with a transient voltage suppressor.

Figure 2: for example LED lighting power supply input overvoltage protection scheme.

In addition, the overvoltage protection should also be considered for the LED driver chip. The proper decoupling capacitor, together with the TVS device for the line driver rated supply voltage, will provide a very stable design. Some LED drive manufacturers are aware of the inclusion of open circuit LED strings, but the circuit should not be confused with the circuit of the protection string, or if the LED fails, to keep the LED string in operation.