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Why Does LEDs Flicker And How To Stop It

May 23, 2019

Why Does LEDs Flicker And How To Stop It

LEDs flicker when their light output fluctuates. This fluctuation happens because your dimmable light-emitting diodes are designed to switch on and off at very high speed.


There are two main reasons why visual strobo can be felt:


1. The output ripple frequency is low. Under special conditions (for example, sudden entering the light area from the no-light area), you will also feel the strobe around 100Hz. The old movie frame rate is 24fps, but if the frequency of illumination is The flash frequency is around 60Hz, and everyone can't stand it. Computer monitors and TVs eliminate the old frame rate system and it's easy to understand.


2. Output ripple voltage or current 100Hz output ripple, when the ripple voltage is less than 5%, it does not feel stroboscopic. At this time, the ripple current may be much smaller than 5%, and the implementation is somewhat difficult. The frame rate of a mobile phone or camera is generally around 30, and the high-speed motion camera can reach 400fps. Shooting with strobe light, if the strobe frequency can not exceed 4 times the frame rate set by the shooting device, you will see the light flashing or even shaking on the shooting device, and the result of the shooting is also the same. Therefore, when shooting the display of the old CRT monitor, you will often see the bar moving up. The lower frequency stroboscopic, although we can not immediately feel it, but in such a long-term light, people are very tired, easy to get myopia and other eye diseases.


At present, LED power supply can simply meet the requirements of no stroboscopic:


1. Increase the output electrolytic capacitor


2. Adopting the valley filling passive PFC scheme


3. Adopting two-stage scheme (AC/DC, DC/DC)


The first scheme "increases the output electrolytic capacitor", this scheme can theoretically use the electrolytic capacitor to absorb part of the AC ripple, but the actual experience tells us that when the ripple control is within a certain range (10%), It is difficult to further reduce it, unless the electrolytic capacitor is added to the cost without any cost.


The second way is to use the valley-filled passive PFC scheme, which is also the most mainstream treatment. Isolation schemes can use either core or IWATT (the earliest solution, which has now been largely phased out). Two large capacitors and three diodes are used for power factor correction. Because there is a large electrolytic capacitor behind the rectifier bridge, the AC ripple is absorbed, and the current through the inductor or transformer to the secondary is DC.


The third method is to adopt a two-stage scheme. By adding a DC/DC to the existing isolated power supply of our company, the influence of AC ripple can be completely eliminated. Electrical parameters can also fully meet the certification standards. However, this solution has a certain increase in cost. It requires an additional power management chip and some peripheral circuits, and the total cost will increase.