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Issues to be noted in pulse width dimming

Oct 14, 2020

Issues to be noted in pulse width dimming

1. Selection of pulse frequency: Because the LED is in a fast switching state, if the operating frequency is very low, the human eye will feel flicker. In order to make full use of the visual residual phenomenon of the human eye, its working frequency should be higher than 100Hz, preferably 200Hz.

2. Eliminate howling caused by dimming: Although the human eye cannot detect it above 200Hz, it is the range of human ear hearing up to 20kHz. At this time, you may hear a slight voice. There are two ways to solve this problem. One is to increase the switching frequency to more than 20kHz and jump out of the range of human hearing. But too high frequency will also cause some problems, because of the influence of various parasitic parameters, the pulse waveform (front and back edges) will be distorted. This reduces the accuracy of dimming. Another method is to find the sounding device and deal with it. In fact, the main sound-generating device is the ceramic capacitor at the output, because ceramic capacitors are usually made of high dielectric constant ceramics, which have piezoelectric characteristics. Under the action of 200Hz pulse, it will produce mechanical vibration and sound. The solution is to use tantalum capacitors instead. However, high-voltage tantalum capacitors are difficult to obtain, and the price is very expensive, which will increase some costs.

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