In the design of LED lighting power supply, there are the following design problems: the life of electrolytic capacitors does not match the LED, the common causes and treatment of LED lamp flicker, how PWM dimming affects the life of the LED, and the use of TRIAC dimming to adjust the LED Potential problems with brightness. Zheng Zongqian, senior application engineering manager of ON Semiconductor, discussed the causes and solutions of these problems.
The problem that the life of the electrolytic capacitor does not match the LED
An important consideration for LED lighting is that the operating life of the LED drive circuit and the LED itself should be comparable. Although there are many factors that affect the reliability of the drive circuit, electrolytic capacitors have a crucial impact on the overall reliability. In order to extend the working life of the system, it is necessary to analyze the capacitance in the application in a targeted manner and select the appropriate electrolytic capacitor.
In fact, the effective working life of electrolytic capacitors is largely affected by the ambient temperature and the internal temperature rise caused by the ripple current acting on the internal impedance. The electrolytic capacitor rated life provided by the electrolytic capacitor manufacturer is based on exposure to the highest rated temperature environment and the application of the maximum rated ripple current. At 105 ° C, the typical rated life of a capacitor may be 5,000 hours. The lower the working stress actually experienced by the capacitor than the rated level, the longer the effective working life. Therefore, on the one hand, the selection of electrolytic capacitors with a long rated working life and capable of withstanding high rated working temperatures can of course extend the working life. On the other hand, according to the actual stress and operating temperature, capacitors with lower rated operating temperature and rated life can still be selected, thereby providing a lower cost solution; from another perspective, consider maintaining appropriate stress and operating temperature in the design , Can effectively extend the working life of electrolytic capacitors, so that it can better match the life of the LED.
For example, ON Semiconductor's GreenPoint® reference design that meets the "Energy Star" solid-state lighting standard selects Panasonic's ECA-1EM102 aluminum electrolytic capacitor, which is rated at 1,000µF, 25 V, 850 mA, 2,000 hours and 85 ° C. Under the assumption of an ambient temperature of 50 ° C, the usable life of this capacitor exceeds 120,000 hours. Therefore, trying to make the LED drive circuit work at a suitable temperature condition and properly deal with the heat dissipation problem can realize the matching problem between the LED drive circuit and the working life of the LED.
In general, if electrolytic capacitors must be used in the LED drive circuit, it must strive to control the application force and operating temperature of the capacitor, so as to maximize the working life of the capacitor to match the life of the LED; on the other hand, the designer Electrolytic capacitors should also be avoided whenever possible.
Common causes and treatment of LED flashing
Normally the human eye can perceive light flicker at a frequency of 70 Hz, and above this frequency it will not perceive it. Therefore, in LED lighting applications, if the low frequency component of the pulse signal frequency is lower than 70 Hz, the human eye will feel the flicker. Of course, in specific applications, there are many factors that may cause the LED lights to flicker. For example, in offline low-power LED lighting applications, a common power supply topology is an isolated flyback topology. Taking ON Semiconductor's 8W off-line LED driver GreenPoint® reference design that meets the "Energy Star" solid-state lighting standard as an example, because the sine square wave power conversion of the flyback regulator does not provide constant energy to the primary bias, dynamic Self-powered (DSS) circuits may activate and cause light flicker. In order to avoid this problem, it is necessary to enable the primary bias to be partially discharged every half cycle, and accordingly, the magnitudes of the capacitance and resistance constituting the bias circuit need to be appropriately selected.
In addition, even in LED driver applications that provide excellent power factor correction and support TRIAC dimming, electromagnetic interference (EMI) filters are required. The transient current caused by the TRIAC 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 holding current, TRIAC will turn off. After a short delay, TRIAC will usually turn on again, exciting the same resonance. During one and a half cycles of the input power waveform, this series of events may be repeated multiple times, resulting in visible LED flashes. In order to deal with this problem, a key requirement of TRIAC dimming is that the input capacitance of the EMI filter is extremely low, and this capacitance must be able to be decoupled by TRIAC and winding impedance. According to the formula, if the capacitance in the dimming module is reduced, the resistance of the resonant circuit can be increased, in principle, the oscillation is suppressed and the desired circuit operation is resumed.
How does PWM dimming affect the life of LEDs
The life of the LED itself is very long, and PWM dimming does not damage the expected life of the LED; even because PWM dimming helps reduce the amount of heat generated by the LED, it can actually help extend the life expectancy of the LED. Of course, in the system design, it is necessary to effectively detect and control the LED temperature to ensure the reliable operation of the LED, reflecting its advantages of long life and low maintenance cost.
Potential problems of using TRIAC dimming to adjust LED brightness
At present, TRIAC dimming is a popular dimming method in the market. TRIAC dimmers were originally designed for incandescent lamps, but leading suppliers such as ON Semiconductor have also introduced LED drivers that support TRIAC dimming, which is ideal for providing TRIAC dimming in LED lighting. But this method also has its limitations, such as increasing circuit complexity, affecting power factor, and relatively limited dimming level. ON Semiconductor, as the premier high-performance, energy-efficient silicon solution supplier for green electronic products, has recently launched the NCL30000 LED driver for residential and commercial LED lighting applications. This device uses a critical conduction mode (CrM) flyback architecture to provide a power factor higher than 0.95 in a single-segment topology, thus eliminating the DC-DC (DC-DC) conversion stage and simplifying the circuit. This device provides extremely high energy efficiency even at low levels, meeting various specifications and overall system light efficiency requirements. In addition, this device is compatible with leading-edge TRIAC dimmers and trailing-edge transistor dimmers. Depending on the dimmer used, the LED light output can be adjusted to less than 2%, providing excellent dimming performance.
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