LED lighting power supply design of the core challenges

          LED lighting power supply design, there are several design challenges: electrolytic capacitor life and the LED does not match, a common cause of LED lights flashing and approach, PWM dimming LED impact on the life of the regulation of the use of TRIAC dimming LED brightness potential problems. Expand the paper discusses the causes and solutions for these problems.

 

Electrolytic capacitor life and the LED does not match the problem

 

An LED lighting important consideration is the LED drive circuit and LED operating life itself should be able to be compared. While the impact of the driving circuit reliability There are many factors, but electrolytic capacitors have a crucial impact on overall reliability. In order to prolong the working life of the system, the need for targeted analysis of capacitance, and select the appropriate electrolytic capacitors.

 

In fact, the effective working life of aluminum electrolytic capacitors by ambient temperature and internal temperature influence by the action on the internal impedance of the ripple current results in large part. Rated life electrolytic capacitors electrolytic capacitors manufacturer is based on exposure to the maximum rated temperature environment and the application of the maximum rated ripple current testing conditions. At 105 ° C typical capacitance may be 5,000 hours rated life, work stress capacitance actually incurred lower compared to the nominal level, the effective working life is longer. Thus, on the one hand, selection of nominal long working life and can withstand high temperature rating of electrolytic capacitors to extend the working life course. On the other hand, according to the actual temperature stress and work, you can still select a lower rated operating temperature and the rated lifetime of the capacitor, thereby providing a more cost-effective solution; another point of view, considering the stress and work to maintain proper temperature in the design can effectively extend the working life of the electrolytic capacitor, making it more and LED lifetime match.

 

For example, the Semiconductor meet the "Energy Star" solid-state lighting standards GreenPoint® Offline LED Driver Reference Design chose Panasonic ECA-1EM102 electrolytic capacitors, rated at 1000μF, 25 V, 850mA, 2,000 hour and 85 ° C. Assuming 50 ° C ambient temperature conditions, the useful life of this capacitor more than 120,000 hours. Therefore, try to make the LED drive circuit at a suitable temperature condition and properly handle the heat problem, it can achieve matching LED drive circuit and LED operating life.

 

Overall, if the LED driver circuit must be used in electrolytic capacitors, it must strive to control capacity and work force suffered application temperature, to the greatest extent prolong the working life of the capacitor, to match with the LED life; on the other hand, designers also should be avoided as much as possible the use of electrolytic capacitors.

 

LED lights flashing common causes and treatment approaches

 

Typically the human eye can perceive frequencies up to 70Hz light flicker, above this frequency is not perceived. Therefore, in the LED lighting applications, if the pulse frequency is lower than the low-frequency component signal appears 70Hz, the human eye will feel flashes. Of course, in specific applications, there are many factors that could cause LED flashes. For example, in the off-line low-power LED lighting applications, a common power supply topology is isolated flyback topology. Semiconductor accord with "Energy Star" solid-state lighting standards 8W Offline LED Driver Reference Design GreenPoint® example, because the flyback regulator sine square wave power converter does not provide a constant bias to the primary energy, dynamic self Power Supply (DSS) circuit may activate and trigger light flashes. To avoid this problem, the primary bias must be able to discharge part of each half cycle, and accordingly, the need to select the appropriate capacitance and resistance values ​​which constitute bias circuits.

 

In addition, even when using offers excellent power factor correction, support TRIAC dimming LED driver applications also require electromagnetic interference (EMI) filter. Transient current by the TRIAC step (step) caused fires EMI filter inductor and capacitor natural resonance. If this resonance characteristics cause the input current falls below the TRIAC to maintain current, TRIAC will turn off. After a short delay, TRIAC conduction usually will inspire the same resonance. In the input power of a half cycle of the waveform, this series of events may be repeated several times, so as to form a visible LED flashes. To address this issue, a key TRIAC dimming input capacitance EMI filter requirements is very low, and this impedance decoupling capacitors to be able to pass TRIAC and winding. According to the formula, the dimming module capacitance decreases, then it is possible to increase the resistance of the resonant circuit, on the principle of oscillation is suppressed, recovery circuit you want.

 

PWM dimming impact on the life of LED

 

LED itself life is very long, PWM dimming and will not hurt the life expectancy of the LED; even as PWM dimming LED helps reduce the heat, in fact, can also help prolong LED life expectancy. Of course, in the system design, the need to effectively detect and control LED temperature to ensure reliable operation of LED, reflects its long life and low maintenance cost advantages.

 

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