There are in the LED lighting power supply design, some design problems: electrolytic capacitor life and the LED does not match, a common cause of LED lights flashing and treatment method, PWM dimming have any impact on the life of the LED, the use of TRIAC dimming control LED brightness of potential problems. The causes and solutions of these problems are discussed in this paper.
The life of electrolytic capacitor is not matched with the LED
An important consideration for LED lighting is that LED drive circuits should be comparable to the working life of the LED itself. Although there are many factors that influence the reliability of driving circuit, the electrolytic capacitors have 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 appropriate electrolytic capacitance.
In fact, the effective working life of aluminum electrolytic capacitors is greatly influenced by ambient temperature and internal temperature rise caused by ripple current acting on internal impedance. The rated life of electrolytic capacitors is based on the maximum rated temperature environment and the maximum rated ripple current. Typical capacitance at 105 ° C rated life could be 5000 hours, rated capacitance than actually suffered from job stress level is lower, the effective working life is longer. Therefore, on the one hand, it is of course possible to extend the working life by choosing electrolytic capacitors with long working life and high rated working temperature. On the other hand, according to the actual stress and working temperature, the low rated working temperature and the capacitance of the rated life can still be selected, thus providing a lower cost solution. In other words, considering the proper stress and working temperature in the design, it can effectively prolong the working life of the electrolytic capacitor and make it more compatible with LED life.
Ansion beauty, for example, semiconductor solid-state lighting energy star standards from the linear LED drive GreenPoint ® reference design chose panasonic em102 ECA - 1 aluminum electrolytic capacitor, the rating of 1000 mu, 25 V, 850 ma F, 2000 hours and 85 ° C. On the assumption that 50 ° C temperature conditions, the capacitance of the usable life of more than 120000 hours. Therefore, it is possible to realize the matching problem of LED drive circuit and LED working life by trying to make the LED drive circuit work in a suitable temperature condition and properly handle heat dissipation.
In general, if the LED driver circuit must be used in electrolytic capacitor, it must strive to control capacitance and working temperature in the application of force, so as to prolong the working life of the capacitor to the greatest extent, in order to match its LED life; On the other hand, designers should avoid electrolytic capacitors as much as possible.
Common causes and treatment of LED lights flashing
Usually the human eye can perceive that the light that is up to 70Hz is flashing, which is not perceived by this frequency. Therefore, in the application of LED lighting, if the frequency of the pulse signal is lower than the low-frequency component of 70Hz, the human eye will feel the flicker. Of course, in specific applications, there are a number of factors that can lead to LED lights flashing. For example, in the offline low-power LED lighting application, a common power topological structure is the isolation type anti-excitation topological structure. With various semiconductor solid-state lighting energy star standards from line 8 w LED drive GreenPoint ® reference design as an example, because of the flyback voltage stabilizer sine square wave power conversion did not provide primary offset constant energy, dynamic self-supply (DSS) circuit may activate and flashing light. In order to avoid this problem, the primary bias must be allowed to discharge at every half cycle, and correspondingly, the value of the capacitance and resistance of the offset circuit needs to be properly selected.
In addition, electromagnetic interference (EMI) filters are also required even in LED applications that provide excellent power factor correction and support TRIAC dimming. The transient current caused by the TRIAC step (step) can excite the natural resonance of the inductance and capacitance in EMI filters. If this resonant characteristic causes the input current to drop to the TRIAC maintenance current, the TRIAC will be closed. After a short delay, the TRIAC usually leads to the same resonance. In the one and a half period of the input power waveform, this series of events can be repeated many times to form the visible LED flicker. In order to cope with this problem, one of the key requirements of TRIAC dimming is that EMI filters have very low input capacitors, and this capacitance should be decoupled through TRIAC and coil impedance. According to the formula, the capacitance reduction in the dimming module can increase the resistance of the resonant circuit, which can suppress the oscillations and restore the desired circuit work.
PWM dimming has an impact on the life span of leds
LED itself has a long life span, and PWM dimming does not damage the expected life of the LED. Even as PWM dimming helps reduce the LED heat, it actually helps extend the life of the LED.
