LED lights strobe how to break? The analysis of the actual solution selection method for power supply topology flashing

LED solid-state lighting (SSL) more efficient, more durable, replace the traditional incandescent and fluorescent lamps is an undeniable trend of lighting industry. However, due to solid state lighting device directly connected to the AC line, like the traditional lighting, because the current ripple driving power output, there is a risk of 100 Hz or 120 Hz flashing. Flicker can make people feel uncomfortable, even the eye may not detect flicker, also can cause headaches and other diseases. LED driver design rigorous can minimize flicker, and ensure that the solid-state lighting energy efficiency.

In fact, LED and lighting manufacturers are eager to solve the flicker problem, most manufacturers want to drive solutions, because the drive determines the degree of flicker. This paper will explain the reason and characteristics of the scintillation, described in LED lighting can occur, and explain how to weigh the driver engineers of different topologies to find the best cost / benefit combination. We will also describe the realization of a cost-effective and flexible circuit with ripple suppression without blinking LED lighting.

Market expectations

Because LED is a new generation of light source, not only is the solid-state lighting market expectations of higher power, but also has better lighting environment.

However, the same with the traditional lighting technology, most of the LED lamp is connected directly to the 50 or 60Hz AC power supply. Even if the rectification to 100Hz or 120Hz frequency, because the frequency involved is relatively low, any associated with the line flicker may be detected by the naked eye. In fact, the rectification circuit may lead to flicker.

The use of single stage power factor correction structure at the same time (PFC) and output drive current LED driver is particularly vulnerable to the effects of scintillation. Among other reasons, effects of flashing will be LED ripple current. However, there are many ways to solve this problem, including the ripple suppression circuit. Solid state lighting product development personnel need assessment methods, choose both cost and can meet the application requirements of the driver. In different applications, different degrees of flicker is acceptable. For example, due to flicker but not in system design for indoor applications can play a role in the streets or in outdoor area lighting.

The effect of scintillation

According to the American Association of electrical and Electronic Engineers (IEEE) report published in 2010 pointed out that the lights flicker frequency 3 - 70Hz range is very easy to perceive, let a person feel unwell. Even repeated repeated geometric patterns and static flash may also lead to the sudden onset of illness, the incidence rate is about 0.025%. This type of flickering through the drive can be easily solved. If you see this frequency in the output ripple current waveform, we usually think of the driver is not stable.

But now, people began to pay more attention to long-term effects of exposure in the range of 70-160Hz under high frequency flicker. This flicker may cause discomfort, headache and visual impairment. Some researchers even claim that the retina can feel up to 200 Hz flicker, but tests showed that more than 160 Hz, flashing on the health effects can be neglected. As mentioned in the rectifier circuit 100 and a frequency of 120 Hz, which will focus on reducing the range of flash. In fact, effects of flashing 100Hz or 120Hz on human health is not only the frequency, including physical and physiological factors.

Definition of flashing

We need to understand the characteristics of flash. Illuminating Engineering Society of North America (IESNA) defined in the "IESNA lighting Handbook ninth edition issued a" flicker percentage "and" scintillation index ".

Flashing percentage refers to a relative measure of light output cycle changes (i.e. modulation percentage), also sometimes referred to as "modulation index". Refer to figure 1, according to the maximum (A) and minimum (B) to calculate the percentage of flashing light output level is calculated by dividing the sum and difference of A B A and B, the percentage is the percentage of flashing.

Figure 1.IES flicker index

The definition of the scintillation index in the IESNA manual is in a given power frequency, a reliable relative measure of various source output cycle change. It takes into account the waveform and amplitude of light output." The scintillation index assumed value of 0 to 1, stable light output is 0. The higher the value indicates the possibility of flashing lights and stroboscopic effect increased significantly. Again with reference to figure 1, by Area 1 and Area 2 Area 1 is calculated by dividing the sum of the scintillation index.

As mentioned earlier, in addition to the frequency, the scintillation index has significant effect on people's sensitivity. The scintillation index is high, means that the eye is more sensitive, more comfortable degree of difference. A study in Table 1 from the Michael Grather paper, display the scintillation index in different light under the engine.

Solid state lighting output

Having defined the flash, consider the LED light source is how to work. LED light output and drive current had a linear relationship. Look at any of the big power LED data table, you can see the arrangement of linear forward current and flux in the graph. This is very clear: the driving current is the key source of LED lights flashing, constant current power supply is the main responsibility of the LED driver.

When discussing 100-120 Hz flashing, people most often is concerned with indoor lighting applications. There are a lot of LED indoor lighting scheme can provide constant current drive. For example, in the AC rectifier, a simple current limiting resistor, linear regulator and semiconductor switching pulse width modulation (PWM) control is possible. But these programs are not in the scope of discussion, because they do not provide the required indoor commercial application