What is PWM dimming？

What is PWM dimming？

LED is a diode, which can realize fast switching. Its switching speed can be as high as meager profit. It is unmatched by any light-emitting device. Therefore, as long as the power supply is changed to a pulse constant current source, the brightness can be changed by changing the pulse width. This method is called pulse width modulation (PWM) dimming method.

Figure 7 shows the waveform of this pulse width modulation. If the pulse period is tpwm and the pulse width is ton, then its duty ratio D (or porosity ratio) is ton/tpwm. The brightness of the LED can be changed by changing the duty ratio of the constant current source pulse.

How to realize PWM dimming

The specific method to realize PWM dimming is to connect a MOS switch in the LED load, and the anode of this string of LEDs is powered by a constant current source.

Figure 8 Use PWM signal to quickly turn on and off LED string circuit schematic

Then a PWM signal is applied to the gate of the MOS tube to quickly switch the string of LEDs. So as to achieve dimming. There are also many constant current chips with a PWM interface. It can directly receive the PWM signal, and then output the control MOS switch tube.

So what are the advantages and disadvantages of this PWM dimming method?

The advantages of pulse width modulation dimming

1. No chromatographic shift will occur. Because the LED always works in the division of full amplitude current and zero.

2, can have extremely high dimming accuracy. Because the pulse waveform can be controlled to a very high precision, it is easy to achieve a precision of one ten thousandth.

3. It can be combined with digital control technology for control. Because any number can be easily converted into a PWM signal.

4. Even if the light is dimmed in a wide range, there will be no flicker. Because it will not change the working conditions of the constant current source (boost ratio or step-down ratio), it is less likely that problems such as overheating will occur.

Pulse width dimming problems should be paid attention to

1. The choice of pulse frequency Because the LED is in a fast switching state, if the working 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 operating frequency should be higher than 100Hz, preferably 200HZ.

2. Eliminate the howling of the dimming bow; although the human eye can't detect it above 200HZ, it is within the range of human hearing until 20kHz. At this time, there may be a slight voice. There are two ways to solve this problem. One is to increase the switching frequency above 20kHz, out of the range of human hearing. But too high frequency will 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. The other method is to find out the sound-producing device and deal with it. In fact, the main sound-producing device is the ceramic capacitor at the output end, 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. Yes, tantalum capacitors with high withstand voltage are difficult to obtain, and they are very expensive, which will increase some costs.