In the design of LED lighting power supply, there are several design problems as follows: the electrolytic capacitor life does not match the LED, common causes and treatment methods of LED flicker, the influence of PWM dimming on LED life, and potential problems of controlling LED brightness by TRIAC dimming.
This paper discusses the causes and solutions of these problems.
The problem that the electrolytic capacitor life does not match the LED
An important consideration for LED lighting is that the LED drive circuit should be comparable to the working life of the LED itself.
Although there are many factors that affect the reliability of the drive circuit, the electrolytic capacitance is of great importance to the overall reliability.
In order to prolong the working life of the system, it is necessary to analyze the applied capacitance and select appropriate electrolytic capacitance.
In fact, the effective working life of aluminum electrolytic capacitors is largely influenced by ambient temperature and internal temperature rise caused by ripple current acting on the internal impedance.
The rating life of the electrolytic capacitor provided by the manufacturer is based on exposure to the highest rated temperature and application of 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, selecting electrolytic capacitors with a long rated working life and capable of withstanding a high rated working temperature can certainly extend the working life.
On the other hand, according to the actual stress and working temperature, capacitance with lower rated working temperature and rated life can still be selected to provide a lower cost solution.
On the other hand, considering the proper stress and working temperature in the design can effectively extend the working life of the electrolytic capacitor and make it better match the 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 (including F, 25 V, 850 ma, 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, the matching problem between LED drive circuit and LED working life can be realized by making LED drive circuit work in appropriate temperature conditions and properly dealing with heat dissipation.
In general, if the electrolytic capacitor must be used in the LED drive circuit, efforts must be made to control the applied force and working temperature of the capacitor, so as to extend the working life of the capacitor to the maximum extent, so as to match the LED life.
On the other hand, designers should avoid using electrolytic capacitors as much as possible.
Common cause and treatment of LED lamp flicker
Typically, the human eye can perceive light flashes with a frequency of up to 70Hz, but not at higher frequencies.
Therefore, in the application of LED lighting, if the frequency of the pulse signal is lower than 70Hz, the human eye will feel the flash.
Of course, there are many factors that can cause LED lights to flicker in concrete applications.
For example, in offline low-power LED lighting applications, a common power topology is the isolated flyback topology.
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 able to discharge at each half cycle part, and accordingly, the values of capacitance and resistance that make up this bias circuit need to be properly selected.
In addition, EMI filters are required even in LED drive applications that provide excellent power factor correction and support TRIAC dimming.
The transient current caused by TRIAC step (step) will induce the natural resonance of inductance and capacitance in EMI filter.
If this resonant characteristic causes the input current to drop below the TRIAC maintenance current, the TRIAC will close.
After a short delay, the TRIAC usually leads, triggering the same resonance.
This series of events may be repeated several times over the half-cycle of the input power waveform, resulting in visible LED flashes.
To address this problem, a key requirement for TRIAC dimming is that the input capacitance of the EMI filter is extremely low and should be decoupled through TRIAC and wound 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 can be suppressed and the desired circuit can be restored.
How PWM dimming affects LED life
The life of the LED itself is very long. PWM dimming will not damage the life expectancy of the LED.
Even as PWM dimming helps reduce the LED's heat, it actually helps extend the LED's life expectancy.
Of course, in the system design, it is necessary to effectively detect and control the temperature of LED, ensure the reliable operation of LED, and reflect its advantages of long life and low maintenance cost.