With the gradual development of LED lighting, more and more customers will habitually take mobile phones to shoot LED lamps, and determine whether the lamps are harmful to human eyes by observing the presence or absence of stroboscopic. This practice is not entirely correct.
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.
Does the camera shoot with flickering to determine that it will hurt the human eye? In fact, "the sensitive frequency of the human eye to light fluctuations is in the low frequency range." The frequency of warning lights such as ambulances is about 8-10 Hz, which is most likely to cause discomfort and cause 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.
Some engineers pointed out that the current LED power supply can only be achieved by meeting the requirements of no stroboscopic. Approximate: 1. Increase the output electrolytic capacitor. 2. Adopt the valley filling passive PFC scheme. 3 uses a two-stage scheme (ACtoDC, DCtoDC).
First, we discuss the first scheme, "Increase 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, unless the electrolytic capacitor does not cost more, it can not be fundamentally eliminated.
The second way is to use the valley-filled passive PFC scheme, which is also the most mainstream treatment. Non-isolated schemes are commonly used for crystal source or duty cycle schemes. 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.
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