Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

2.1 How to implement PWM dimming
Concrete realization of PWM dimming bunch into a MOS switch(Figure 8) in the LED load  , this string of LED's anode power supply with a constant current source .

Then add a PWM signal to grid of MOS tube to quickly switch to LED string. In order to achieve dimming. There are many chip itself with a constant current PWM interface, can directly accept the PWM signal, then output control MOS switch. Then what advantages and disadvantages of this PWM dimming method has?
2.2 The advantage of PWM dimming
1. Not produce any chromatographic shift. Because the LED is always working at full rate between current and 0.

2. Dimming can be very high accuracy. Because the pulse shape can control to a very high precision, so it is easy to achieve the precision of one ten thousandth.
3. And digital control technology can be combined to control. Because any number can easily transform into a PWM signal.
4. Even in a wide range dimming, flicker will not occur. Because it would not change the working conditions of constant current source (step-up ratio or step-down ratio), not to overheating and other issues.
2.3 The attentions should be paid about PWM dimming

1. The choice of pulse frequency : Because the LED is in fast switching, if the operating frequency is low, the human eye will feel flashes. To take full advantage of the phenomenon of human visual residues, and its frequency should be higher than 100Hz, preferably 200Hz.
2. Eliminate howling that the dimming cause: Although the human eye can not detect more than 200Hz, but they are all human auditory range till 20kHz. At this time it is possible to hear the slightest sound. There are two ways to solve this problem, first, the switching frequency up to 20kHz or more, out of the range of human hearing. But the frequency is too high can cause some problems, due to various parasitic effects, will make pulse waveform (before and after the edge) produces distortion, which reduces the accuracy of dimming. Another method is to find the sound device and to be addressed. In fact, the main sound output device is a ceramic capacitor, as ceramic capacitors are usually made of high dielectric constant of the ceramic, such ceramics have piezoelectric properties. Under the pulse of 200Hz will produce mechanical vibration and sound. The solution is to replace the tantalum capacitors. However, a high voltage tantalum capacitors is difficult to obtain, and the price is very expensive, will increase some costs.