In the design of LED lighting power supply, there are several design problems: electrolytic capacitor life and LED does not match, LED light flashes in the common causes and treatment measures, PWM dimming of LED life, what is the effect of using TRIAC dimming control LED brightness potential problems. In this paper, the causes and solutions of these problems are discussed.

The problem that the life of electrolytic capacitor does not match LED

An important consideration of LED lighting is that the LED drive circuit and the life of the LED itself should be comparable. Although there are many factors that affect the reliability of the drive circuit, the electrolytic capacitor has a crucial influence on the overall reliability. In order to prolong the operating life of the system, it is necessary to analyze the capacitance in the application and select the proper electrolytic capacitor.

In fact, the effective operating life of the aluminum electrolytic capacitor is greatly affected by the ambient temperature and the internal temperature rise caused by ripple currents acting on the internal impedance. The lifetime of electrolytic capacitors supplied by electrolytic capacitor manufacturers is based on exposure to the highest rated ambient temperature and maximum rated ripple current applied. At 105 degrees C, the typical capacitor life rating may be 5000 hours, and the actual work stress of the capacitor is lower than the rated level, and the effective working life is longer. Therefore, on the one hand, it is possible to extend the operating life of the electrolytic capacitor by selecting the rated operating life and the high rated operating temperature. On the other hand, according to the actual stress and working temperature, still can choose a relatively low capacitance rated temperature and rated life, so as to provide a lower cost solution; from another perspective, consider maintaining proper stress and temperature in the design, can effectively prolong the working life of the electrolytic capacitor. It can match with the LED life.

For example, offline LED driver GreenPoint reference design in accordance with the "A Morimi semiconductor solid-state lighting standard energy star chose ECA-1EM102 aluminum electrolytic capacitor Panasonic, its rated value is 1000 F, 25 V, 850mA, 2000 hours and 85 degrees C. The capacitor has a lifetime of more than 120 thousand hours, assuming 50 degrees C ambient temperature. Therefore, in order to make the LED drive circuit work at the proper temperature condition and properly deal with the heat dissipation problem, the matching problem between the LED drive circuit and the operating life of the LED can be achieved.

In general, if the LED driver must use the electrolytic capacitors in the circuit, the applied force and working temperature that it is necessary to control the capacitance of the capacitor, so as to prolong working life, in order to match and LED life; on the other hand, the designer should as far as possible to avoid the use of electrolytic capacitor.

Common causes and treatment of LED light flashing

Generally, the human eye can detect light with frequencies of 70Hz, which is not sensed at this frequency. Therefore, in LED lighting applications, if the pulse signal appears below the low-frequency component of 70Hz, the human eye will feel blinking. Of course, in specific applications, there are a variety of factors that may cause LED lights to flicker. For example, in offline low power LED lighting applications, a common power topology is an isolated flyback topology. 8W offline LED driver GreenPoint from reference design by A Morimi in accordance with the "semiconductor solid-state lighting standard energy star" as an example, the sine wave regulator side flyback power conversion does not provide constant energy to the primary bias, dynamic self supply (DSS) circuit can be activated and flashing. To avoid this problem, it is necessary to allow primary bias to be discharged at each half cycle, and accordingly, the values of the capacitance and resistance that constitute the bias circuit need to be appropriately chosen.

In addition, electromagnetic interference (EMI) filters are also required, even in the LED drive applications that provide excellent power factor correction and support TRIAC dimming. The transient current caused by the TRIAC step (step) will excite the natural resonance of the inductance and capacitance in the EMI filter. If this resonant characteristic causes the input current to fall below the TRIAC sustaining current, the TRIAC will turn off. After a short delay, the TRIAC usually switches on and fires the same resonance. In the 1.5 cycle of the input power waveform, this series of events may be repeated several times to form visible LED flashes. To cope with this problem, a key requirement of TRIAC dimming is that the input capacitance of the EMI filter is very low and that the capacitor is decoupled through the TRIAC and the winding impedance. According to the formula, if the capacitance of the dimming module is reduced, the resistance of the resonant circuit can be increased, and the oscillation is suppressed in principle, and the desired circuit work is resumed.

How does the dimming of PWM affect the life of LED?

LED itself has a long life, and PWM dimming does not damage LED's life expectancy. Even PWM dimming helps reduce LED heating, and actually helps to extend LED life expectancy. Of course, in