IEEE Power Electronics Magazine Compendium - March 2018 - 54

fig 1 When a person's eyes move quickly from side to side,
multiple images are observed in some LED car taillights.

The eye is continually in motion, moving the retinal
image in a succession of drifts and jerks (saccades), even
when we gaze directly at something [10]. These movements
are necessary for vision for, without them, the image fades.
When the gaze is redirected from one point of regard to
another, the eyes make a large saccade, and the flight of the
eye during such a saccade can be extremely rapid, reaching more than 400°/s for the largest saccades. Although the
eyes are open and the retinal cells are functional, we are
usually unaware of what we see during a saccade. This lack
of awareness, called saccadic suppression, arises partly
because what we see before and after the rapid eye movements are relatively stable images, which mask the moving
image that occurs during the saccade. When the image is
illuminated intermittently by flickering light, the image
during the flight of the eye is not a moving image but a succession of discrete images, and these can get confused with
the images before and after the flight of the eye. You become
aware of flickering LED taillights because each saccade is
accompanied by a disconcerting trail of images [11]-[13]. At
night, there is nothing to see before and after the saccade
to mask the intrasaccadic image. The frequency of flicker
at which such intrasaccadic images are visible can be as
high as 2 kHz [14]. During a saccade, the image on the eye is
moved as the eye moves. On the other hand, when the eye
is relatively stable and the objects themselves move rapidly,
we may again notice a similar succession of images. This is
known as the stroboscopic effect, and it occurs at frequencies that are similarly high [15].

Driving LED Lamps May Lead to Flicker
Figure 2 shows the light output of typical commercially available LED Edison socket bulbs with A19 size, all of which are
marketed as 60-W incandescent replacement bulbs and have
about 800 lumens of light output. Most LED bulbs will demonstrate modulating light output that is periodic at twice the
line frequency, which, in this case, is 120 Hz. However, as can
be seen from one bulb, it is possible to design a driver that
exhibits virtually no flicker. High-frequency measurement
noise may be ignored because it is due to the low-cost Texas
Instruments OPT101 photodiode and transimpedance amplifier. To eliminate the measurement noise, the more elaborate
experimental measurement setup described in [2] and [16]
may be utilized. The flickering levels of the LED bulbs depend on the type of driving method that is utilized to convert
the ac input to the required dc through the LED.

IEEE PowEr ElEctronIcs MagazInE

z September 2014

Incandescent bulbs do not require these power electronic drivers since it is possible to send the positive and
negative cycle current directly through their tungsten filaments. The thermal time response for the filament is so slow
that the flicker is naturally filtered to a lower level. On the
other hand, LEDs are unidirectional devices and require
ac-dc conversion. The response time for LEDs is almost
instantaneous, and it is commonly assumed that the luminous intensity of an LED is approximately linearly proportional to the current through the LED. As a result, LED current ripple instantaneously becomes light flicker ripple. This
has caused flicker to reemerge as an important issue in the
design process, even giving rise to new standards groups
such as IEEE PAR1789 that study safe levels [17].
From the experimental plots of luminous intensity in
Figure 2, it is possible to define [2], [18]
Percent Flicker or Mod% = 100 (Max - Min)/(Max+Min),
where Max/Min represent the maximum/minimum measured light intensity from the lamp. It is interesting to note
that there is no dependence on flicker frequency in this definition, although it is known to be an important parameter
in human safety for lighting. From the definition, it can be
seen that Mod% will always be a value between 0% and
100%. A summary of the Mod% of the tested commercially
available A19 LED Edison socket bulbs is presented in
Table 1. For reference, compact fluorescent lamps with
high-frequency ballasts normally have a Mod% of less than
of 10%, while that of fluorescent lamps with magnetic ballasts is 25-50% [2], [16]. Outdoor lighting with high- or lowpressure sodium lamps may have up to 100% modulation.
In the following sections, we present a few of the basic
methods to create the required dc whereby unwanted
flicker may be introduced in LED lighting [2], [19]. There
are many variations of the presented methods.

Full-wave rectifier connected
to lED string
In this approach, shown in Figure 3(a), the ac input source is
sent into a full-wave rectifier, causing the (approximate)
absolute value of the input voltage to be sent to the load. In
- Table 1. Philips and Maxlite are within

the measurement error of the flicker.

lED Bulbs

Mod%

60-W incandescent
Cree
Philips
Utilitech
Ecosmart
GE
MaxLite
Sylvania

7.62%
21.27%
~1%
48.39%
16.96%
19.63%
~1%
29.41%

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018