Discussion on the key technical problems of power supply design in LED lighting

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. A Morimi semiconductor Senior Application Engineering Manager in the text of the text of the causes of these problems and solutions are discussed in detail in the paper.

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

LED lighting is an important consideration is that the LED driver circuit and the working life of the LED itself should be able to compare. Although there are many factors that affect the reliability of the drive circuit, the electrolytic capacitor has a crucial impact on the overall reliability. In order to prolong the working life of the system, it is necessary to analyze the capacitance in the application and select the appropriate electrolytic capacitor.

In fact, the effective working life of the electrolytic capacitor is largely affected by the ambient temperature and the internal temperature rise caused by the ripple current acting on the internal impedance. The rated life of the electrolytic capacitor provided by the electrolytic capacitor manufacturer is based on the conditions of exposure to the highest rated temperature and the maximum rated ripple current. At 105 C, the rated life of a typical capacitor may be up to 5000 hours, and the lower the working stress of the capacitor, the lower the rated level. Therefore, on the one hand, it is possible to extend the operating life of the electrolytic capacitor, which has a long service life and is capable of withstanding a 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, A Morimi semiconductors comply with the "Energy Star" solid state lighting standard offline LED drive Green Point? Reference design chose Panasonic ECA-1EM102 aluminum electrolytic capacitor, the rated value of 1000 F, 25V, 850mA, 2000 hours and 85 degrees C. At a temperature of 50 degrees C, the lifetime of the capacitor is more than 120 thousand hours. Therefore, try to make the LED drive circuit work in the appropriate temperature conditions and properly deal with the problem of heat dissipation, we can achieve the LED driver circuit and the LED life cycle matching.

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 methods of LED lamp flashing

Usually the human eye can perceive the frequency of 70Hz light flashes, above this frequency will not perceive. Therefore, in the LED lighting applications, if the pulse signal frequency is lower than the low-frequency component of the 70Hz, the human eye will feel the flicker. Of course, in specific applications, there are a variety of factors may cause LED lights flashing. For example, in a low power LED lighting application, a common power supply topology is an isolated flyback topology. The A Morimi GreenPoint line with the "Energy Star" solid state lighting standard 8W offline LED driver? Reference design, for example, since the sine wave power conversion of the flyback regulator does not provide constant power to the primary bias, the dynamic power supply (DSS) circuit may be activated and led to flash. In order to avoid this problem, the primary bias must be partially discharged in each half cycle, and accordingly, the amount of capacitance and resistance that is appropriate to form the bias circuit is required.

In addition, even in the use of excellent power factor correction, TRIAC dimming LED driver applications, but also the requirements of electromagnetic interference (EMI) filter. The transient current caused by the TRIAC step (step) will excite the natural resonance of the inductor and capacitor in the EMI filter. If the resonant characteristics cause the input current to fall below the TRIAC sustain current, the TRIAC will turn off. After a short delay, the TRIAC will normally turn on and excite the same resonance. In the 1.5 cycle of the input power waveform, this series of events may be repeated several times to form a visible LED flash. In order to deal with this problem, TRIAC dimming is a key requirement of the EMI filter input capacitance is very low, and this capacitor to be able to TRIAC and winding impedance decoupling. According to the formula, if the capacitance of the dimming module is reduced, the resistance of the resonant circuit can be increased.

The influence of PWM dimming on the life of LED

LED itself has a long life, PWM dimming and does not damage the life expectancy of LED; and even because of PWM dimming to help reduce LED heat, in fact, can also help extend the life expectancy of LED. Of course, in the design of the system, it is necessary to effectively detect and control the temperature of LED, to ensure the reliable work of LED, and to reflect the advantages of long life and low maintenance cost.

The potential problems of using TRIAC dimming to control LED brightness

The current TRIAC dimming is a popular market dimming mode. The TRIAC dimmer was originally designed for incandescent lamps, but leading vendors such as on semiconductor have also introduced a LED driver that supports TRIAC dimming, which is ideal for TRIAC dimming in LED lighting. But this method also has its limitations, such as increasing the complexity of the circuit, the influence of power factor, dimming level is relatively limited. A Morimi semiconductors recently launched LED driver for residential and commercial LED lighting applications NCL30000. The device uses a critical conduction mode (CrM) flyback architecture to provide a power factor above 0.95 with a single-stage topology, thus eliminating DC to DC (DC-DC) conversion segments, simplifying the circuit. This device provides high efficiency even in low power, in line with the requirements of various specifications and the overall system requirements. In addition, this device is compatible with the front TRIAC dimmer and the trailing edge transistor dimmer, depending on the dimmer used, the LED light output can be adjusted to be less than 2%, providing excellent dimming performance.

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