advantage & disadvantage PWM

PWM, pulse width modulation technology, is used in many ways. In order to improve everyone's understanding and understanding of pwm, this article will introduce the advantages and disadvantages of pwm's application in LED dimming.

Advantages of PWM Dimming

1. Does not produce any chromatographic shift. Because the LED always operates between full amplitude current and 0. Good color consistency and high brightness levels. Over the entire dimming range, since the LED current is either at its maximum value

, or it is turned off, and the average current of the LED is changed by adjusting the pulse duty cycle, so this scheme can avoid color shift during the current change.

2. Can have extremely high dimming accuracy. Because the pulse waveform can be completely controlled to a high precision, it is easy to achieve a precision of one ten thousandth. The duty cycle adjustment range can reach up to 1% at low frequency dimming

- 100%. No matter what the dimming ratio is, the LEDs always work under constant current conditions.

3. It can be controlled in combination with digital control technology. Because any number can be easily transformed into a PWM signal.

4. Even if the light is dimmed in a wide range, there will be no flickering phenomenon. Because the working conditions (boost ratio or step down ratio) of the constant current source will not be changed, problems such as overheating are less likely to occur.

5. Can provide larger dimming range and better linearity. The PWM dimming frequency is generally 200Hz (low frequency dimming) to above 20kHz (high frequency dimming). As long as the PWM dimming frequency is higher than 100 Hz, observe

No flickering of LEDs. PWM dimming ratio up to 5000:1.

6. The LED driver chips produced by most manufacturers support PWM dimming. This is divided into the following 3 cases: only with PWM dimming input pin, no analog dimming pin; set one for PWM dimming and analog dimming respectively

A dimming input pin is shared with analog dimming.

7. When using PWM dimming, the conversion efficiency of the LED driver is high.

What are the disadvantages of PWM (pulse width modulation) 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 flickering. In order to make full use of the residual visual phenomenon of the human eye, its working frequency should be high

at 100Hz, preferably 200Hz.

2. Eliminate the whistle of the dimming bow; although the human eye cannot detect it above 200HZ, it is within the range of human hearing until 20kHz. At this time, you may hear Sisi's voice. solve this problem

There are two ways to solve the problem. One is to increase the switching frequency to above 20kHz, out of the range of human hearing. However, if the frequency is too high, it will also cause some problems, because the influence of various parasitic parameters will make the pulse wave

The shape (front and rear edges) is distorted.

This reduces the accuracy of dimming. Another way is to find out the sound-producing device and deal with it. In fact, the main sounding device is the ceramic capacitor at the output, because ceramic capacitors are usually made of high dielectric

It is made of ceramics with electrical constants, and these ceramics have piezoelectric properties. Under the action of 200HZ pulse, it will produce mechanical vibration and make sound. The solution is to use tantalum capacitors instead. For high withstand voltage tantalum

It is hard to come by, and it is very expensive, which will add some cost.

• Relatively more expensive

• If the drive is running below 100Hz, the peripheral vision will flicker

• When the driving frequency is low, the stroboscopic effect is obvious in a fast moving environment.

• The rise and fall of current in PWM dimming can cause electromagnetic interference (EMI).

• Performance issues can occur when the driver is mounted away from the light source.