LED drive circuit surge protection application

　　LED lights are highly efficient, reliable, and low energy-consuming, and have a wide range of uses, and are often used for lighting , display, signal lights, etc. However, due to the complexity of the LED use environment, especially when LEDs are used outdoors, their drive circuits are very susceptible to overvoltage and overcurrent shocks, which can cause failures or damage, resulting in unnecessary property losses or even casualties. Therefore, when designing LED drive circuits, full consideration must be given and protective measures must be taken to improve circuit reliability and reduce the occurrence of failures. The following is a brief discussion on the protection of LED drive circuits.

　　LED drive circuits are generally composed of several parts, including AC input, rectification, DC/DC conversion, and other modules. According to the different voltages and currents of each module and the surge conditions it may encounter, targeted protection needs to be provided separately.

　　1. Application of surge protection in LED drive circuit

　　In the AC power supply AC input end surge protection scheme, a varistor (MOV) or a gas discharge tube (GDT/SPG) combination can be used for design. In the case of grounding, the concept of differential and common mode simultaneous protection can be adopted as shown in Figure 1. A varistor (MOV) can be connected in parallel between LN to effectively suppress the surge overvoltage generated by the differential mode and protect the subsequent circuit. The circuit connection method of MOV or MOV+GDT/SPG to ground between L/N-PE can effectively discharge the common mode surge energy to the ground to prevent the surge from being introduced into the subsequent circuit and causing damage; if there is no grounding wire in the power supply, as shown in Figure 2, a varistor can be directly connected in parallel between the LN lines for differential mode protection. In order to avoid the possibility of short circuit failure and fire after the MOV protection element fails, TMOV or PMOV can be used for protection. The MOV AC withstand voltage should be at least 1.2~1.4 times higher than the maximum AC working voltage of the line to avoid false operation. When the discharge tube GDT/SPG is used at the same time, the lower limit of the discharge tube breakdown voltage must be at least higher than the maximum peak voltage of the circuit. The withstand current must be selected according to the surge level requirements to meet the requirements of the surge test standard.

　　2. Schematic diagram of AC/DC rear protection circuit

　　After the AC is rectified, the chip in the back-end DC circuit is very sensitive to overvoltage and overcurrent, and the chip is easily damaged. As shown in Figure 3, after rectification, a transient suppression diode TVS is connected in parallel. When overvoltage occurs, the TVS will react at a speed of picoseconds to clamp the excessive voltage within a safe range, thereby protecting the back-end chip from the impact of overvoltage. Abnormal current can be protected by designing a self-recovery fuse PPTC in the circuit. When overcurrent occurs, the impedance of PPTC can increase rapidly, thereby effectively blocking abnormal current until the fault is eliminated. PPTC can continue to restore the low resistance state, so that the circuit can continue to return to normal working state. When TVS is selected, the cut-off voltage is generally 1.2~1.4 times the peak value of the normal working voltage. The power size of TVS should be selected according to the energy of overvoltage. The selection of PPTC should be combined with the circuit working current and voltage for reference. The ambient temperature is also an important key indicator affecting the selection of PPTC. The holding current of PPTC will decrease with the increase of the application environment temperature. The position of PPTC in the circuit is generally connected in series at the front end of TVS. In this way, PPTC can not only effectively protect the circuit chip but also play a certain protective role on the TVS tube, which can greatly improve the service life of the TVS tube.

　　3. Schematic diagram of LED direct drive circuit protection

　　The brightness of LED light is controlled by the current passing through the LED. Unstable current can easily burn out the LED. As shown in Figure 4, a constant current diode can be connected in series in the circuit after the DC/DC module to obtain a stable current. This not only allows the LED to obtain a stable brightness, but also prevents the LED from being burned out due to unstable current. The operating current of low-power LED lamps is generally 10mA to 30mA, and the operating current of high-power LED lamps ranges from 200mA to 1400mA. You can choose a suitable constant current diode according to the required operating current. Since LED lamps are also easily damaged by electrostatic discharge overvoltage interference, the LED lamps at the back end of the DC/DC circuit also need to have certain effective overvoltage protection, generally using TVS tubes.

　　4. Schematic diagram of LED light string protection

　　When multiple LED lamps are connected in series, as shown in FIG5 , once an LED lamp fails to open circuit, the entire LED lamp will affect the normal operation of other LED lamps due to this fault. To solve this problem, an open circuit protection device Tx can be connected in parallel to each LED lamp, so that the use efficiency of each LED can be fully improved. When a single LED fails to open circuit, the LED open circuit protection device Tx connected in parallel will be turned on immediately, so that it can be continuously maintained in the on state, thereby ensuring that other series-connected LEDs in the circuit are not extinguished due to the open circuit fault of a single LED, but the cost of this protection measure is relatively high.

　　In summary, LED drive circuits are generally composed of AC input, rectification, DC/DC conversion, and other modules. Therefore, the overall protection scheme of an LED drive circuit can be referred to as shown in Figure 6:

Figure 6  Schematic diagram of overall protection of LED drive power supply

　　In practical applications, the selection of surge protection component models is closely related to many factors, such as the circuit's operating voltage and current, the circuit's lightning surge test level standard, the working environment, and chip parameters. Therefore, when considering and designing LED driver circuit protection, comprehensive considerations and analysis must be conducted to design a more reasonable protection solution in a targeted manner.