Analysis of LED power supply elimination: general technical route to reduce flicker

 The industry is currently discussing "de-powering", which includes not using a power supply at all or removing some functional modules of the power supply. De-powering can also be done at different levels, including: at the chip level, at the electronic component level, and at the lamp level.

　　Current situation: Multiple patented technology routes using completely different principles have been proposed for de-electricalization at different levels.

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

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

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

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

　　To simplify the drawing and analysis, assume that:

　　(1) The waveform of the AC current after phase shifting 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 only needs to adjust the phase difference between the different input AC currents to achieve the desired result;

　　(2) Within the operating current range, the luminance is basically proportional to the current (Lumileds). Therefore, although the following analysis is only about the current, the conclusions apply to the luminance.

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

　　The general basic principle is: input AC currents of different phases, rectify them separately and add them together to form a total current, and use the total current to drive the LED lamps. The result is:

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

　　(2) The difference between the maximum and minimum values ​​of the total current decreases, so the percentage flicker of the brightness decreases;

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

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

　　Here are a few examples to illustrate how the general principle of reducing flicker is used to reduce flicker.

　　An input sinusoidal AC current: The normalized waveform after rectification is as follows (Figure 16):

　　For two input sinusoidal AC currents with a phase difference of 90°: after rectification separately but without superposition, the normalized graphs of the two pulsating DC currents are shown in Figure 17:

　　The pulsating DC currents obtained by rectifying the two input sinusoidal AC currents with a phase difference of 90° shown in Figure 17 are superimposed to obtain the total current. The waveform of the normalized total current is shown in Figure 18 (the maximum value of the total current after rectifying and superimposing the two input sinusoidal AC currents with no phase difference is 1).

　　In order to demonstrate the function of the general principle of reducing flicker, a comparison is made: in Figure 18, the diamond () represents the normalized total current after two sinusoidal alternating currents with no phase difference are rectified and superimposed, and the square () represents the normalized total current after two sinusoidal alternating currents with a phase difference of 90° are rectified and superimposed (the maximum value of the total current after the two sinusoidal alternating currents with no phase difference are rectified and superimposed is 1).

　　For three input sinusoidal alternating currents with phases of 0°, 60°, and 120° respectively: the waveform of the normalized total current after rectification and superposition is shown in Figure 19 (the maximum value of the total current after rectification and superposition of three input sinusoidal alternating currents with no phase difference is 1):

　　In order to demonstrate the function of the general principle of reducing flicker, a comparison is made: in Figure 19, the diamond represents the normalized total current after three sinusoidal alternating currents with no phase difference are rectified and superimposed, and the square represents the normalized total current after three sinusoidal alternating currents with a phase difference of 60° are rectified and superimposed.

　　Figures 18 and 19 clearly show:

　　(1) The difference between the maximum and minimum values ​​of the total current after the input AC currents of different phases are rectified and superimposed is reduced, so the percentage flicker of the brightness generated by the total current is reduced;

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

　　(3) There is no moment when the total current and the brightness of the light generated are zero.

　　Therefore, the influence of the general strobe on the human eye is reduced.

　　If the number of input AC currents is continued to increase, for example, AC currents with 4 phases of 0°, 45°, 90°, and 135°, or even AC currents with 8 phases of 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5° are input, and then rectified and superimposed, the difference between the maximum and minimum values ​​of the total current is further reduced, and therefore, the percentage flicker of the brightness generated is further reduced; the oscillation frequency of the total current and the maximum value of the brightness generated is further increased.

　　18 and 19 , four alternating currents with a phase difference of 45°, and eight alternating currents with a phase difference of 22.5° are analyzed, and the results shown in FIG. 20 are obtained.

　　In FIG20 , the diamonds represent the percentage of the minimum value to the maximum value of the total current superimposed after rectification (the right vertical axis), and the squares represent the oscillation frequency of the maximum value of the total current superimposed after rectification (the left vertical axis).

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

　　Figure 20 shows:

　　1) For two input AC currents with a phase difference of 90°, the minimum value of the total current after rectification is 70% of the maximum value, and the oscillation frequency of the maximum value is greater than 200Hz. Therefore, the percentage flicker of the light brightness generated by the total current = 18%. This is better than the percentage flicker of 28% of fluorescent lamps using magnetic ballasts (Figure 13b).

　　2) For the three input AC currents with phase angles of 0°, 60°, and 120°, the minimum value of the total current after rectification is more than 85% of the maximum value, and the oscillation frequency of the maximum value is close to 400Hz. Therefore, the percentage flicker of the brightness generated by the total current = 8%. It reaches the level of 8% of the percentage flicker of an incandescent lamp (Figure 14b).

　　3) For the input AC currents with four phase angles of 0°, 45°, 90°, and 135°, the minimum value of the total current after rectification is more than 90% of the maximum value, and the oscillation frequency of the maximum value is close to 500Hz. Therefore, the percentage flicker of the light brightness generated by the total current = 5%. This reaches the level of 5% (Figure 15b) of energy-saving lamps using electronic ballasts.

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

　　The percentage flicker is calculated according to the formula of Energy Star (Figure 21).