What is the general technical route for reducing stroboscopic flicker during the de-powering process of LEDs?

Usually, when discussing "de-powering", "de-powering" includes not using a power supply at all or removing some functional modules of the power supply. De-powering can also be carried out at different levels, including: at the chip level, at the electronic component level, and at the lamp level.

The current situation: De-powering is proposed at different levels and using a variety of patented technology routes using completely different principles.

Question: Is there a universal flicker reduction principle that can be applied to all levels and a patented technology route based on this principle, without the need to use different principles to develop different technology routes for different levels?

This article introduces a general principle of reducing flicker and a technical route based on this principle.

The characteristic of this technical route is that its principles can be applied to the chip level, electronic component level, and lamp/circuit level.

The following is a brief introduction to the general principle of reducing flicker.

To simplify drawing and analysis, assume:

(1) The waveform of the AC current after phase shift remains unchanged and is still a sine wave. Although the waveform changes after phase shifting, it does not affect the general principle of reducing flicker. It is only necessary to adjust the phase difference between different input AC currents in order to achieve the required results;

(2) Within the operating current range, the luminous brightness is basically proportional to the current (Lumileds). Therefore, although the analysis below is only for current, the conclusion applies to luminous brightness.

As we all know, the impact of stroboscopic effects on the human eye mainly depends on the difference between the maximum and minimum values ​​of light brightness (percent stroboscopic) and the oscillation frequency of the maximum value.

The general basic principle is: input AC currents of different phases, rectify them separately and then superimpose them to form a total current. The total current is used to drive LED lamps. The result is:

(1) The oscillation frequency of the maximum value of the total current increases, therefore, the oscillation frequency of the maximum value of the brightness increases;

(2) The difference between the maximum value and the minimum value of the total current decreases, therefore, the percentage of brightness flicker decreases;

(3) There is no moment when the total voltage after superposition is equal to 0 (or less than 2.8 volts). Therefore, there is no moment when the lamp does not emit light.

Therefore, the brightness flicker of LED lamps driven by the total current is reduced to the same or even better level than other lamps, that is, this principle can meet the requirements for the flicker of lamps.

Here are a few examples to illustrate how the general flicker reduction principle can reduce flicker.

An input sinusoidal AC current: the rectified normalized waveform is as follows (Figure 16):

For two input sinusoidal AC currents with a phase difference of 90°: after rectification respectively, but without superposition of each other, the normalized graph of the two pulsating DC currents is as shown in Figure 17:

The pulsating DC current obtained by rectifying the two input sinusoidal AC currents with a phase difference of 90° is superimposed to obtain the total current. The waveform of the normalized total current (the maximum value of the total current after rectifying and superimposing two input sinusoidal AC currents without phase difference is 1).

In order to demonstrate the function of the universal flicker reduction principle, a comparison is made: in the middle, the diamond ( ) represents the normalized total current after rectification and superposition of two input sinusoidal AC currents without phase difference, and the square ( ) represents 2 The normalized total current after rectifying and superimposing two input sinusoidal AC currents with a phase difference of 90° respectively (the maximum value of the total current after rectifying and superimposing two input sinusoidal AC currents without phase difference is 1) .

For three input sinusoidal AC currents with phases of 0°, 60°, and 120°: the waveform of the normalized total current after rectification and superposition (respectively rectified with three input sinusoidal AC currents without phase difference) , the maximum value of the total current after superposition is 1):

In order to demonstrate the function of the universal flicker reduction principle, a comparison is made: the diamond represents the normalized total current after three input sinusoidal AC currents without phase difference are rectified and superimposed separately, and the square represents the three phase differences of 60°. The input sinusoidal AC currents are respectively rectified and superimposed to the normalized total current.

Very obvious display:

(1) The difference between the maximum value and the minimum value of the superimposed total current after the input AC currents of different phases are respectively rectified is reduced. Therefore, the percentage of light brightness generated by the total current is reduced;

(2) The total current and the oscillation frequency of the maximum value of the generated light intensity increase;

(3) There is no moment when the total current and the generated brightness become 0.

Therefore, the use of universal stroboscopic effects on the human eye is reduced.

If you continue to increase the number of input AC currents, for example, input 4 AC currents with phases: 0°, 45°, 90°, 135°, or even input 8 phases: 0°, 22.5°, 45° , 67.5°, 90°, 112.5°, 135°, 157.5° AC currents are respectively rectified and superimposed. The difference between the maximum and minimum values ​​of the total current is further reduced. Therefore, the percentage of the generated light brightness is strobe Further decrease; the total current and the oscillation frequency of the maximum value of the generated light intensity further increase.

The diamond represents the percentage of the minimum value and the maximum value of the total current superimposed after respectively rectification (right ordinate), and the square represents the oscillation frequency of the maximum value of the total current superimposed after respectively rectification (left ordinate).

Taking 50Hz alternating current as an example, after rectification, the oscillation frequency of the maximum pulsating current is 100Hz.

1) For two input AC currents with a phase difference of 90°, the minimum value of the total current superimposed after rectification is 70% of the maximum value, and the oscillation frequency of the maximum value is greater than 200Hz. Therefore, the total current produced as a percentage of light intensity strobe = 18%. The percentage flicker is 28% better than that of fluorescent lamps using magnetic ballasts.

2) For three input AC currents with phase angles of 0°, 60°, and 120° respectively, the minimum value of the total current superimposed after rectification is more than 85% of the maximum value, and the oscillation frequency of the maximum value is close to 400Hz. Therefore, the total current produced as a percentage of light intensity strobe = 8%. Reaching the level of 8% of the percentage strobe of incandescent lamps.

3) For four input AC currents with phase angles of 0°, 45°, 90°, and 135° respectively, the minimum value of the total current superimposed after rectification is more than 90% of the maximum value, and the oscillation frequency of the maximum value is close to 500Hz. Therefore, the total current produced as a percentage of light intensity strobe = 5%. Reach the level of 5% flicker of energy-saving lamps using electronic ballasts.

4) The more input AC currents, the smaller the corresponding phase difference angle, the closer the ratio between the minimum value and the maximum value of the superimposed total current after rectification is to 1, and the greater the oscillation frequency of the maximum value. Therefore, the smaller the stroboscopic percentage of light brightness generated by the total current, the greater the oscillation frequency of the maximum light brightness, and the smaller the impact of stroboscopic light on the human eye.

Among them, the percentage strobe is calculated according to the Energy Star formula.