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LED PWM dimming

May 16, 2019

LED PWM dimming


PWM dimming is a mainstream dimming technology applied in LED dimming power products. In the circuit of the analog signal, the brightness of the control luminaire is digitally outputted. This dimming method has many advantages compared with the traditional analog signal dimming. Of course, there are certain defects in some aspects. What are the advantages and disadvantages?

 

Let`s first look at the basic principle of pwm dimming. In fact, in the practical application of the product, it can be understood that a MOS switch tube is connected in the load of the LED. The anode of the string is powered by a constant current source. A PWM signal is then applied to the gate of the MOS transistor to quickly switch the string of LEDs for dimming.

 

The advantages of pwm dimming:

 

First, pwm dimming is precise dimming.

 

Dimming accuracy is a remarkable feature of digital signal dimming common, because pwm dimming uses pulse waveform signals with high precision.

 

Second, pwm dimming, no color difference.

 

In the entire dimming range, since the LED current is either at the maximum value or is turned off, the average current of the LED is changed by adjusting the pulse duty ratio, so the scheme can avoid color difference during the current change.

 

Third, pwm dimming, adjustable range.

 

The PWM dimming frequency is generally 200 Hz (low frequency dimming) to 20 kHz or more (high frequency dimming).

 

Fourth, pwm dimming, no strobe.

 

As long as the PWM dimming frequency is higher than 100 Hz, no flickering of the LED is observed. It does not change the operating conditions of the constant current source (boost ratio or step-down ratio), and it is impossible to overheat. However, PWM pulse width dimming also has problems to be aware of. The first is the choice 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 operating frequency should be higher than 100 Hz, preferably 200 Hz.


What are the disadvantages of pwm dimming?

The noise caused by dimming is one. Although it is not detectable by the human eye above 200 Hz, it is the range of human hearing until 20 kHz. At this time, it is possible to hear the sound of the silk. There are two ways to solve this problem. One is to increase the switching frequency to above 20 kHz and jump out of the human ear. However, too high a frequency can cause some problems, because the influence of various parasitic parameters will cause the pulse waveform (front and rear edges) to be distorted. This reduces the accuracy of dimming. Another method is to find out the sounding device and handle it. In fact, the main sounding device is the ceramic capacitor at the output, because ceramic capacitors are usually made of high dielectric constant ceramics, which have piezoelectric properties. Mechanical vibration occurs under the action of a 200 Hz pulse. The solution is to use a tantalum capacitor instead. However, high-voltage tantalum capacitors are difficult to obtain, and the price is very expensive, which will increase some costs.


To sum up, the advantages of pwm dimming are: simple application, high efficiency, high precision, and good dimming effect. The disadvantage is that since the general LED driver is based on the principle of switching power supply, if the PWM dimming frequency is between 200 and 20 kHz, the inductance and output capacitance around the LED dimming power supply are prone to noise that is audible to the human ear. In addition, when performing PWM dimming, the closer the frequency of the adjustment signal is to the frequency of the LED driver chip to the gate control signal, the worse the linear effect is.