Advantage & Disadvantage of PWM Dimming

                                                                         Advantage & Disadvantage of PWM Dimming

pwm, pulse width modulation technology, has applications in many ways. In order to improve everyone's understanding and understanding of pwm, this article will introduce the application of pwm in LED dimming. The main content of this article is to introduce how to realize pwm dimming, the advantages of pwm dimming, and the matters needing attention for pwm dimming. If you are interested in pwm, you may wish to continue reading.

1. PWM dimming

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

Pulse width modulation is an analog control method, which modulates the bias of the transistor base or the gate of the MOS tube according to the change of the corresponding load, so as to realize the change of the conduction time of the transistor or MOS tube, so as to realize the change of the output of the switching regulated power supply. . In this way, the output voltage of the power supply can be kept constant when the working conditions change, and it is a very effective technology to control the analog circuit by using the digital signal of the microprocessor. Pulse width modulation is a very effective technique for controlling analog circuits using the digital output of a microprocessor and is widely used in many fields from measurement and communications to power control and conversion.

The basic principle of pulse width modulation (PWM): the control method is to control the on-off of the switching device of the inverter circuit, so that a series of pulses of equal amplitude are obtained at the output end, and these pulses are used to replace the sine wave or the required waveform. That is, multiple pulses are generated in the half cycle of the output waveform, so that the equivalent voltage of each pulse is a sine waveform, and the obtained output is smooth and has few low-order harmonics. The width of each pulse is modulated according to certain rules, which can not only change the size of the output voltage of the inverter circuit, but also change the output frequency.

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

2. How to realize PWM dimming

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

Figure Schematic diagram of fast switching LED string circuit with PWM signal

Then add a PWM signal to the gate of the MOS tube to quickly switch the string of LEDs. To achieve dimming. There are also many constant current chips with a PWM interface. It can directly accept the PWM signal, and then output the control MOS switch. So what are the advantages and disadvantages of this PWM dimming method?

Third, the advantages of pulse width modulation dimming

1. No chromatographic shift will occur. Because the LED always works at full current and 0.

2. It 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.

3. It can be controlled by combining 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.

4. Issues that should be paid attention to in pulse width 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 human eyes, its working frequency should be higher than 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 buzz Sisisisi. There are two ways to solve this problem. One is to increase the switching frequency above 20kHz, out of the range of human hearing. However, too high frequency will also cause some problems, because the influence of various parasitic parameters will distort the pulse waveform (Trailing edge & rear phase-cuting).

This reduces the accuracy of dimming. Another way is to find out the sound-producing device and deal with it. In fact, the main sound-generating device is the ceramic capacitor at the output, because ceramic capacitors are usually made of high dielectric constant ceramics, which 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. Yes, tantalum capacitors with high withstand voltage are difficult to obtain, and they are very expensive, which will increase some costs.