However, since indoor SSL fixtures are directly connected to the AC line, like with legacy lighting, there is a risk that 100-Hz or 120-Hz flicker could occur as a result of driving current ripple at the supply's output. Strobe flicker has become one pending issue remaining in LED lighting application.
The Illuminating Engineering Society of North America (IESNA) released the definition of "percent flicker" and "flicker index" in the ninth edition of The IESNA Lighting Handbook shows how the metrics are defined.
Percent flicker is a relative measure of the cyclic variation in output of a light source (i.e., percent modulation). This is also sometimes referred to as the "modulation index."
Flicker index is defined as a “ relative measure of cyclic variation in the output of various sources at a given power frequency.” It takes into account the waveform of light output as well as its amplitude. The flicker index value assumes from 0 to 1.0, with 0 for steady light output. High values indicate an increased possibility of noticeable flicker.
Flicker index has a significant effect on how light makes people feel. Higher flicker index means more sensitivity to human eyes and a poorer comfort level. According to the paper” The evaluation of flicker in LED luminaires” written by Michael Grather. Table presents the flicker percent as well as flicker index of typical light engines showed as bellow:
|Max.||Min.||Avg.||Percent Flicker(%)||Flicker Index|
|T5 HO elec.||10.52||9.960||10.20||2.734||0.0036|
|LED at DC||43.4||41.0||42.2||2.84||0.0037|
|LED with flicker||15.996||0.0555||6.3026||99.309||0.4498|
In accordance with some relevant findings, LED light source is almost linear with LED driver current output. You can see the linearity in graphs that clearly plot forward electrical current relative to luminous flux. Such makes it obvious that electrical current is the critical reason for lighting flicker.
How to reduce strobe flicker of LED light, there remains improvement on LED driver design. When we discuss flicker problem of LED-based light, we are focused on indoor LED lighting application, especially commercial lighting area. Commercial lighting requires PF values greater than 0.9. Given high PF requirements as well as elimination of lighting flickering, here summarized some major driver topologies that are used in indoor LED lighting application.
|Design Complexity||Cost||Efficiency||AC Input Range||Ripple Current|
|Single-stage Active PFC||Low||Low||Medium||Wide||Large|
|Single-stage PFC+Ripple Suppressor||Medium||Medium||2-3%lower||Wide||Small|
Figure above depicts a two-stage design that includes a passive PFC stage along with a switching DC/DC converter second stage. The PF design is often referred to as valley fill as capacitors keep the output from falling to low levels. Thanks to the valley fill circuitry and bulk capacitor, the current ripple of this scheme is small and easy to control. This structure is widely used in low-cost offline adapters and chargers. However, the drawback of the passive scheme is that it is not suitable for higher power over 20W because of its poor EMC issues at higher power levels. In addition, the passive scheme is not suitable to achieve wide universal input voltage range such as 100-240VAC.Single-stage Active PFC:
The single-stage approach with active PFC that is depicted as a widely adopted topology for wide input range LED drivers. The topology delivers excellent power conversion efficiency and PF value with wide load range. While the drawback is the high current ripple that leads to visible or invisible 100–120-Hz flicker. Good designs can reduce the current ripple to a relatively low value; however, the ripple is normally still higher than the previous two-stage scheme. One interesting feature of the single-stage topology is that the ripple is greatly affected by the different voltage and current characteristics that are specific to each LED load. Driver designers are seeking better ways to control the ripple in the single-stage design.
Active PFC Plus Switching DC/DC:
This way is to solve the current ripple problem by means of adding an active second stage behind the active PFC stage. Such driver topology is with addition of a DC/DC converter stage. But the additional DC/DC stage in the driver comes with a cost increase of 15–20%. This circuitry greatly lowers the output current ripple and makes the output almost an ideal DC at the expense of losing 2–3% efficiency. Moreover, this structure can cover most of the power levels required in indoor applications and is widely used.
Single-stage PFC Plus Ripple Suppressor:
Luckily, there is another good solution for lowering the output current ripple with a circuit that's far simpler than a switching DC/DC stage. You can segment a single-stage design with a relatively simple linear ripple-suppressor circuit such as the one depicted in Figure above.
The modified-single-stage topology utilizes a uniquely designed linear regulator, which can greatly reduce the output current ripple from single-stage PFC constant-current output with only 2–3% efficiency loss. The approach offers additional benefits. Adding a switching DC/DC stage to a driver handicaps the EMC performance in most of the cases, while adding the linear regulator does not. The better EMC performance allows the ripple suppressor to be flexibly utilized with existing single-stage LED drivers by SSL manufacturers. The addition of the circuit to the output is far more cost effective than buying another driver or switching DC/DC converter to get much better light output.
The key advantage of the ripple suppressor is that it provides a very simple and flexible way to reduce the flicker of the design we already have at a minimal and very reasonable cost.
There are multiple ways to create excellent LED driver with low ripple and flicker to lighting environment, each of which has its advantages and drawbacks. While selecting LED lighting solutions, try to consider the comprehensive factors to reach your requirement.