Research on strobe problem of white LED illumination based on different driving conditions

The main features of white LEDs are: low voltage, small size, high efficiency, environmental protection and long life. At present, the research and development of illumination sources with white LEDs as illuminating sources is strong, and new products emerge in an endless stream, and have been actively promoted and applied within a certain range. Compared with conventional mature illumination sources typified by incandescent lamps and high and low pressure gas discharge lamps, although the industry's research on white LED illumination sources is constantly deepening, since white LEDs have fast response with ms level, they can work in DC. And the current-type device [1] under the condition of unidirectional high-frequency pulse current, when used as an illumination source, there is little "inert" illumination of the conventional light source, which may cause more serious problem of light output stroboscopic effect and affect normal use [2] ], [3], [4], it is necessary to pay sufficient attention to the stroboscopic problem of white LED illumination source under different driving conditions, in order to better meet the requirements of various lighting scenes and obtain a wider range of Practical application.

1 White LED driving and stroboscopic characteristics

At present, the preparation principle of commercialized and widely used white LEDs is to use blue LEDs to excite yellow fluorescent powder YAG:Ce to obtain white light output. The efficiency of white light output is directly related to the light-emitting efficiency of blue LEDs, the efficiency of phosphors, and the matching between them. Since the white LED is a current-type device with a fast response of ms level and can work under DC and unidirectional high-frequency pulse current conditions [1], it may cause a serious problem of light output strobe as an illumination light-emitting device. For the sake of analysis, a test circuit as shown in Fig. 1 was built to test the current and light output characteristics of the LED under DC linear and AC mains driving conditions.

1.1 DC linear drive

Linear regulator (steady current) power supply is one of the earliest forms of drive (Figure 2a, b). Since the LED is a current device, the light output characteristic directly responds to the characteristics of the driving current. Therefore, the light output of the LED driven by the linear regulated (steady current) power source has no stroboscopic phenomenon, which is a smooth straight line, as shown in FIG. 2c. (The upper line in Figure 2 is the light output, the lower line is the drive current, the same below).

1.2 power frequency exchange mains drive

Due to the universality and economy of AC mains, directly using the power frequency 50Hz AC mains to drive LED is a more practical driving method [5], [6]. Since the characteristics of the LED must work under the condition of unidirectional current (including pulsating current), the circuit configuration of the power frequency AC mains drive is to make the forward current flowing through the LED unidirectional and have a constant current. Features, mainly in the following forms:

1) Power frequency AC half-wave drive

The characteristic of power frequency AC half-wave drive is that a single LED flows through a one-way pulsating sinusoidal half-wave current. The specific circuit components are mainly:

A diode guide:

The characteristics of diode guiding are: using a certain size of rectifier diode D to reverse current blocking, so that the current flowing through the LED is a positive pulsating sinusoidal half-wave current, the specific circuit is shown in Figure 3a.

B diode bypass:

Diode bypass is characterized by a certain specification of rectifier diode D for bypassing the reverse current to ensure that the current flowing through the LED is a positively pulsating sinusoidal half-wave current. The specific circuit is shown in Figure 3b.

C double LED in parallel:

The parallel connection of double LEDs is: using the same type of LEDs in reverse parallel, so that they work in the forward and reverse directions of the current respectively, that is, the current is automatically guided, so that the current flowing through the single LED is a positive pulsating sinusoidal half-wave current, which is generally improved. The efficiency of current utilization has doubled the overall light output. The specific circuit is shown in Figure 3c.

The LED current and light output driven by the power frequency AC half-wave are shown in Figure 3d.

2) Power frequency AC full wave drive

Power frequency AC full-wave drive can improve the light output efficiency of a single LED, mainly using a full-wave rectifier to convert the power frequency AC current into a one-way pulsed DC, so that the LED flows through a one-way pulsed sinusoidal full-wave current. The specific circuit, current, and light output characteristics are shown in a and b of Figure 4, respectively (the deadband current present in Figure 4b is due to the minimum turn-on voltage that the LED has).

According to the formula of the stroboscopic wave depth of the light source: H = [(Φmax - Φmin) / Φmax] 100% (where: H - wave depth, Φmax - light output peak, Φmin - light output valley)

To analyze, whether it is sinusoidal half-wave or sinusoidal full-wave AC current to drive the LED, the fluctuation of its light output is very large, and the undulation depth of the stroboscopic wave is as high as 100%, which will lead to poor quality of the illumination source. It is not suitable for the occasions where high-quality lighting is required, so it cannot be used as a mainstream lighting driver. It is just a simple and transitional way.

1.3 high frequency switching constant current source drive

Since the high frequency of the switching power supply can bring a series of advantages, and the technology is mature, the current mainstream LED lighting driving method adopts a high-frequency power conversion mode, and the output thereof is a stable current containing a periodic pulsating component. . According to different circuit topologies, the magnitude of the current ripple component of the output is also different. Typical architectures for LED high-frequency constant-current driving power supplies include buck converters, boost converters (inductive boost and switched capacitors), and buck-boost converters. According to whether the electrical input and output are directly connected, it is divided into two categories: isolated and non-isolated [1]. Due to the different architectures of the circuits, the output current waveforms are varied and varied. In order to categorize the problem, the test circuit shown in Figure 1 is still used to test the LED current and light output characteristics by simulating the current output of the high-frequency switching constant current source, and the average value of the forward current flowing through the LED during the test. constant. For this reason, the high-frequency driving current and light output characteristics of LEDs under different waveforms, different frequencies and different output current ripples [7], [8] were tested.

1) Light output under different waveform driving conditions

For the test of LED light output characteristics under different waveform driving conditions, the sinusoidal full-wave (multiplier), pulsating DC, triangular wave and square wave currents with a frequency of 30KHz are used to maintain the average value of the driving current. The actual waveform obtained is shown in Figures 5a-d.

It can be seen from the comparison of Figures 5a-d that the overall light output of the LED is well responsive to the waveform of the drive current, with stroboscopic effects of different depths. Analyze and summarize as follows:

The degree of fluctuation of the light output of ALED is directly related to the degree of pulsation of the drive current. The larger the pulsation component, the greater the fluctuation of the light output, and the more stroboscopic;

B. In the driving of four different waveform currents, the square wave current drive (equivalent to LED running in PWM dimming mode) has the largest stroboscopic depth (>95%), and the triangular wave current drive is second (~80%), sinusoidal The wave current is driven again (~65%), and the pulsating DC drive is minimal (<10%);

In practical applications, different drive current forms are selected according to the requirements of different lighting. In order to obtain a light output with good illumination quality, it is necessary to use a driving power source with a small pulsation component.

2) Light output under different frequency driving conditions

For the test of LED light output characteristics under different frequency driving conditions, DC currents with pulsating components with frequency ranges from 30KHz to 60KHz and constant current average values ​​are respectively used to drive the LEDs. The actual waveform is shown in Figure 6a-d.

It can be seen from the comparison of Fig. 6a-d that the stroboscopic degree of the light output of the LED has a good correspondence with the frequency of the driving current, and the analysis and summary are as follows:

A does not change other parameters in the driving of the LED. When the frequency is gradually developed from a lower direction to a higher direction, the fluctuation of the light output will be greatly reduced. In Figure 6, when the frequency is 30KHz, the fluctuation depth of the light output is about 18%, and when the frequency is 60KHz, the fluctuation depth of the light output is about 8%;

The reason that the increase of the same driving current frequency causes the fluctuation of the LED light output to decrease is mainly due to the fact that under the condition of high frequency, the afterglow effect of the LED phosphor gradually appears, so that the fluctuation of the light output is reduced;

C adopting high-frequency current drive is one of the important technical measures to reduce the fluctuation of LED light output and improve the quality of illumination source.

3) Light output driven by different levels of current pulsation

For the test of LED light output characteristics under different current pulsation driving conditions, the pulsating DC current with the frequency of 30KHz and the average value of the current is used to drive the LED respectively. The actual waveform is shown in Figure 7a-c.

It can be seen from the comparison of Figures 7a-c that DC currents with different pulsating components are used to drive the LEDs, the light outputs of which have different stroboscopic effects. The analysis is summarized as follows:

A is driven by high pulsating current (Fig. 7a, pulsation component is 100%), the light output of the LED has strong fluctuations, and the depth of stroboscopic is about 28%; under the condition of medium pulsating current driving (Fig. 7b, The pulsation component is 44%), the stroboscopic depth of the LED light output is about 20%; under the condition of low pulsating current driving (Fig. 7c, the pulsating component is 8.3%), the depth of the LED light output strobe is about 3%. Can be regarded as a source of no-frequency flash, comparable to linear drives;

The size of the BLED drive current pulsation component will directly determine the fluctuation of the LED light output, that is, the stroboscopic depth. Therefore, the technical specifications of the driving power supply must be selected according to actual requirements to meet the requirements of use.

2 Analysis of white LED lighting strobe

Within the allowable range, the light output of the LED is almost directly responsive to the magnitude and waveform of the input drive current, and there is almost no illumination inertia of a conventional conventional light source - the thermal inertia of an incandescent lamp, a halogen lamp, and the phosphor afterglow effect of a fluorescent lamp. Therefore, the current fluctuations driven by it directly affect the fluctuation of the light output, which requires special attention in design and use. For LED light sources with high illumination quality requirements (such as teaching, office, display, display, etc.), special consideration should be given to the current output characteristics of their drivers, reducing current fluctuations and additional disturbances of higher harmonics, and reducing LED light. The stroboscopic effect of the output.

2.1 stroboscopic analysis of traditional illumination sources

Traditional lighting sources have a certain frequency of flash memory, but due to different working principles, the stroboscopic size of different light sources is not the same [9], [10], respectively, as follows:

1) Incandescent and halogen lamps

Incandescent and halogen lamps use the thermal effects of current to make the filament glow incandescently. Due to the thermal inertia of the filament in the incandescent state, it is less sensitive to the response of current fluctuations and has a certain inertia, so it has a certain effect on reducing the fluctuation of the light output.

2) Low pressure fluorescent lamp

The luminescence of low-voltage fluorescent lamps (energy-saving lamps) is operated by the combination of filaments and phosphors. The driving is divided into power frequency (inductive) and high-frequency (electronic). Due to the thermal inertia of the filament and the advantage of the afterglow of the phosphor, the combined effect of the two has a certain effect on reducing the fluctuation of the light output. Among them, the working current frequency of the power frequency fluorescent lamp is low, the stroboscopic frequency is serious, and the working current frequency of the high frequency fluorescent lamp is high, which is more advantageous due to the thermal inertia of the filament and the advantage of the afterglow of the phosphor, so the fluctuation of the light intensity output is small. The strobe is low, which is an ideal illumination source. The specially designed high-frequency fluorescent lamp can be used as a stroboscopic illumination source.

3) HID illumination source

Commonly used HID lighting sources mainly include: HPS lamps, metal halide lamps. However, due to its high power and high heat generation, most of it works under power frequency driving conditions. Therefore, although there is also a thermal inertia effect of the filament, and it has a certain effect on reducing the fluctuation of the light output, the fluctuation of the light output is still relatively high. Large, stroboscopic is also more serious, mainly used in lighting occasions that do not require high stroboscopic performance, such as: urban roads, open-air billboards, squares, architectural decorative lighting, etc.

Table 1 gives the strobe of the traditionally tested traditional illumination source and an improved technical solution for reference.

Table 1 Test results and improvement measures of strobe of traditional illumination source

2.2 Strobe analysis of LED illumination source

Through the above actual test and analysis and induction, it can be seen that there are many reasons for the strobe of the LED light output, which is determined by the characteristics of the device itself and the different drive modes. To this end, it is necessary to study the stroboscopic characteristics of the various drivers described above, and propose targeted solutions and improvements.

In summary, since the light output of the LED has very good current-frequency response characteristics, this characteristic is necessary for application in dynamic display conditions, and is an advantage; and for lighting applications, it may be a "disadvantage", centralized analysis as follows:

First: in the low-frequency and LED PWM dimming mode [11] (non-100%) operating conditions, the curve of its light output is almost consistent with the current curve. At this time, since the driving current is unidirectionally pulsating, the light output also has a one-way pulsation characteristic, and the fluctuation of the light output is large. According to the definition of stroboscopic depth, the stroboscopic depth of the light output is 100%;

Secondly, when applied under high frequency conditions, the fluctuation of the light output is reduced due to the effect of the afterglow of the phosphor, which has a good tendency;

Third: the current output characteristics of the LED lighting driver will directly affect the stroboscopic output of its optical output, and the current output characteristics of the driver have direct and circuit topology, design, device selection, overall cost and The service life is related. To this end, it is necessary to carefully identify the objects, occasions, and technical requirements of the LED illumination source. It is not just enough to illuminate the LED light source, but it is necessary to design a high-quality illumination source that meets the requirements.

Table 2 shows the stroboscopic effect, influencing factors and improvement measures of LED light output under different driving conditions, as a summary of the above analysis of LED stroboscopic problems.

3 Conclusion

Although LEDs have very good lighting characteristics. However, it is still in the stage of promotion and application, and there are many problems that require constant research. Since the light output of the LED has very good current response characteristics, the stroboscopic problem may be more prominent than the conventional light source, and further consideration is needed in view of the following aspects.

1) The possibility of LED power frequency drive, and the severe stroboscopic problem of light output under PWM dimming mode operation conditions;

2) When driving under high frequency conditions, the afterglow effect of the LED phosphor is enhanced, the fluctuation of the light output is reduced, but the improvement is limited, and the phosphor with long afterglow characteristics can be studied to improve;

3) The stroboscopic size of the LED illumination light output directly affects the quality of the illumination. For lighting, it is not only satisfied that the LED is lit, but a high-quality illumination source that meets the actual use requirements is the right way.