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Discussion on the Core Problems of LED Lighting Power Supply Design

2017-08-17

In the design of LED lighting power supply, there are several design problems: the life of electrolytic capacitor does not match with LED, the common causes and treatment methods of LED flicker, the influence of PWM dimming on the life of LED, and the potential problems of using TRIAC dimming to regulate the brightness of LED. In this paper, the causes and solutions of these problems are discussed. An important consideration for LED lighting is that the working life of the LED driver circuit and the LED itself should be comparable. Although there are many factors that affect the reliability of the drive circuit, among them

Discussion on the Core Problems of LED Lighting Power Supply Design

Electrolytic capacitor life and LED does not match the problem

An important consideration for LED lighting is that the working life of the LED driver circuit and 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 impact on the overall reliability. In order to prolong the working life of the system, it is necessary to analyze the capacitors in the application and select the appropriate electrolytic capacitors.

In fact, the effective operating life of aluminum electrolytic capacitors is greatly affected by the ambient temperature and the internal temperature rise caused by the ripple current acting on the internal impedance. The rated life of electrolytic capacitors provided by electrolytic capacitor manufacturers is based on exposure to the highest rated temperature environment and the maximum rated ripple current conditions. A typical capacitor rated life may be 5,000 hours at 105°C, and the lower the operating stress actually experienced by the capacitor compared to the rated level, the longer the effective operating life. Therefore, on the one hand, the selection of electrolytic capacitors with a long rated operating life and the ability to withstand high rated operating temperatures can of course extend the operating life. On the other hand, according to the actual stress and operating temperature, a capacitor with a lower rated operating temperature and rated life can still be selected, thereby providing a lower cost solution; in other words, considering maintaining appropriate stress and operating temperature in the design, The working life of the electrolytic capacitor can be effectively extended, so that it can better match the life of the LED.

For example, ON Semiconductor's Off-Line LED Driver GreenPoint for Energy Star-Compliant Solid State Lighting®The reference design selected Panasonic's ECA-1EM102 aluminum electrolytic capacitor with a rating of 1000 µF, 25V, 850mA, 2,000 hours, and 85°C. Under the assumed 50°C ambient temperature conditions, the useful life of this capacitor exceeds 120000 hours. Therefore, try to make the LED drive circuit work in the appropriate temperature conditions and properly handle the heat dissipation problem, can achieve the LED drive circuit and LED working life matching problem.

In general, if the LED drive circuit must use electrolytic capacitors, it must strive to control the application force and working temperature of the capacitor, so as to maximize the working life of the capacitor, in order to match the life of the LED; on the other hand, designers should also avoid the use of electrolytic capacitors as much as possible.

Common Causes and Treatment Methods of LED Flicker

Usually the human eye can perceive the frequency of 70Hz light flicker, higher than this frequency will not be perceived. Therefore, in LED lighting applications, if the pulse signal appears at a low frequency component with a frequency lower than 70Hz, the human eye will feel the flicker. Of course, in specific applications, there are many factors that may cause LED lights to flicker. For example, in off-line low-power LED lighting applications, a common power supply topology is an isolated flyback topology. GreenPoint of ON Semiconductor's 8W Off-Line LED Driver for Energy Star Solid State Lighting®As an example of the reference design, because the sinusoidal square wave power conversion of the flyback regulator does not provide constant energy to the primary bias, the dynamic self-powered (DSS) circuit may activate and cause flicker. In order to avoid this problem, it is necessary to enable the primary bias to be partially discharged during each half cycle, and accordingly, the magnitudes of the capacitors and resistors that make up the bias circuit need to be properly selected.

In addition, electromagnetic interference (EMI) filters are required even in LED driver applications that provide excellent power factor correction and support TRIAC dimming. The transient current caused by the TRIAC step excites 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, the TRIAC will turn off. After a short delay, the TRIAC will normally turn on again, exciting the same resonance. During one half cycle 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 for TRIAC dimming is that the input capacitance of the EMI filter is extremely low, and this capacitance must be decoupled by the TRIAC and the winding impedance. According to the formula, if the capacitance in the dimming module is reduced, the resistance of the resonant circuit can be increased, and in principle, the oscillation is suppressed and the desired circuit operation is restored.

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 life expectancy of the LED; even because PWM dimming helps reduce the heat generation of the LED, it can actually help extend the life expectancy of the LED. Of course, in system design

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