LED lighting products strobe causes and solutions

The traditional fluorescent lamp is directly used for 50Hz AC, and its strobe is 100Hz. When shooting with a digital camera, water ripple will occur due to the different sampling frequency. The LED luminaire is powered by DC power, and the light emitted by the light source will also be in the form of DC (small fluctuations or pulsations superimposed on the DC basis). From a physical point of view, the volatility of the luminescence is indeed far lower. The degree of fluctuation in the illumination of a light source operating at an alternating current. However, its input power supply is still in the form of AC, it is difficult to completely avoid AC ripple through the LED light source. Therefore, there will be flashing when the LED lamp is photographed with a digital camera.

Is there any damage to the human eye when the camera is flickering? In fact, "the sensitive frequency of the human eye to light fluctuations is in the low frequency range", such as the frequency of warning lights such as ambulances is about 8-10 Hz, the most easy to let The human eye is uncomfortable and causes everyone to be alert. These frequencies are much lower than the optical frequency range of the source of normal AC power supply operation, so "there is no uncomfortable situation when the normal human eye operates at the current frequency of the civilian power supply." At present, the optical frequency of the light source widely used in the world is 50-60 Hz (including LED display, LED TV), so it is even more inaccurate to say that the ripple current of the LED lamp output 100 Hz will affect the comfort of the human eye.

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

1, the output ripple frequency is low

Someone else will experience a strobe of around 100Hz under special conditions (for example, suddenly entering a light area from a matte area). The old movie frame rate is 24fps, but if the strobe frequency of the illumination is around 60Hz, everyone Can't stand it, computer monitors and TVs eliminate the old frame rate system, it is easy to understand. 2. The output ripple voltage or current output ripple of 100Hz does not feel stroboscopic when the ripple voltage is less than 5%. 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 strobe, 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, the LED power supply can be achieved simply by satisfying the requirement of no stroboscopic. Approximate:

1. Increase the output electrolytic capacitor

2. Adopting the valley filling passive PFC scheme

3 adopts 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. Non-isolated solutions are commonly used in Shanghai Jingfeng Mingyuan or Shanghai duty cycle scheme. The isolation scheme can be cored or IWATT (the earliest solution, which has now been largely phased out). The circuit principle is as follows. 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.

However, there are certain problems in the valley filling scheme. Firstly, the non-isolated valley filling scheme will have uncontrolled output no-load voltage, which will easily cause lamp bead damage. Secondly, it is impossible to make 90-265V full voltage input. The output voltage valley of the valley filling circuit is only half of the valley value of the electrolytic filter circuit. The output voltage of the valley-filled rectification method is much lower than that of the ordinary rectified output voltage. It may be that after filling the valley, the input at low pressure is insufficient. Furthermore, harmonic testing simply cannot be passed regardless of the isolated or non-isolated valley-filling scheme. The power factor cannot be fully achieved above 0.9. Therefore, such a solution cannot fully meet the UL, CE and other certification standards. See attached page for test data.

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, and it is necessary to add one more power management chip and some peripheral circuits, and the total cost should be increased by about 7 yuan. The following are two common stroboscopic power supplies tested:

Figure 1. Filling the valley PFC isolated solution

Figure 2. Non-isolated valley-filled PFC non-isolated solution

At present, a well-known chip design company in the United States is developing a true strobe-free implementation through internal chip adjustment. The technology adopts the method of first boosting and stepping down, and the whole process is completed by one chip, and smooth DC output is realized under the premise of ensuring PFC and THD. The product is currently in the research and development stage and has not yet been launched.