The advantages and disadvantages of PWM dimming, easy to understand the small 'secret' led dimming
- May 25, 2017 -

PWM dimming is a kind of dimming technology used in LED dimming products. In the analog signal circuit, the digital output control lamp brightness change. Compared with the traditional analog signal dimming has a lot of advantages, of course, in some respects there are some defects, what are the advantages and disadvantages?

We first look at the basic principle of PWM dimming, in fact, in the actual application of products, can be understood as in the LED load into a MOS switch tube, this series of LED anode with a constant current source power supply. A PWM signal is then added to the gate of the MoS tube to quickly switch the led to achieve dimming. The advantages of PWM dimming are obvious, with the following:

First, the PWM dimming, adjust the light precision.

Dimming precision is a common feature of digital signal dimming, because the PWM dimming utilizes pulse waveform signal and high precision.

Second, the PWM dimming, no color bias.

In the whole dimming range, because the LED current is either in the maximum value, or is shut off, by adjusting the pulse duty cycle to change the average current of the LED, so this scheme can avoid the color bias in the current change process.

Third, the PWM dimming, adjustable range is big.

PWM dimming frequency is generally 200Hz (low frequency dimming) more than one 20kHz (high frequency dimming).

Four, PWM dimming, no stroboscopic.

As long as the PWM dimming frequency is higher than $number Hz, the flicker phenomenon of LED is not observed. Will not change the working conditions of constant current source (boost ratio or step-down ratio), also can not occur overheating problems. However, PWM pulse width dimming also has to pay attention to the problem. The first is the choice of pulse frequency: Because the LED is in a fast switch state, if the work frequency is very low, the human eye will be flashing. In order to make full use of the visual residue of human eyes, its operating frequency should be higher than 100Hz, preferably 200Hz.

What are the drawbacks of PWM dimming? Dimming caused by the noise is one, although the eyes of more than 200Hz people can not detect, but all the way to 20kHz is the range of human ear hearing. It is possible to hear the voice of Cecil at this time. There are two ways to solve this problem, one is to increase the switching frequency to more than 20kHz, out of the range of human ear hearing. But the frequency is too high can cause some problems, because the influence of various parasitic parameters, will cause the pulse waveform (front and rear) to produce distortion. This reduces the precision of the dimming. Another way is to find out which devices are audible and process them. In fact, the main audible devices are the output of ceramic capacitors, because ceramic capacitors are usually made of high dielectric constant ceramics, such ceramics are piezoelectric characteristics. The vibration of the 200Hz can produce mechanical vibrations and sound. The solution is to use tantalum capacitors instead. However, high pressure tantalum capacitors are difficult to obtain, and the price is very expensive, will increase some costs.

Summing up, the advantage of PWM dimming is: The application of simple, high efficiency, high precision, dimming effect is good. The disadvantage is because the general led drivers are based on switching power supply, if the frequency of the PWM dimming between 200, led dimming power supply around the inductance and output capacitance is easy to produce human ear audible noise. In addition, when the PWM dimming, the frequency of the adjustment signal and the LED driver chip to the gate control signal frequency closer, the linear effect is worse.