IEEE Power Electronics Magazine - September 2014 - 23

the two stages. To reduce the value of the dc-link capacitance, some designs sacrifice the performance. On the
other hand, some vendors may utilize only a single-stage
ac-dc approach and only keep the PFC portion of Figure 5
[28]-[31]. In this case, it is likely that the percent flicker
may significantly increase, but there is a benefit of lower
cost as a result of the lower parts count. A compromise
between the two- and single-stage approaches is the single switch converters that integrate two topologies [32].
Finally, it should be mentioned that linear regulators may
be inserted in series with the LEDs to regulate constant
dc so that no flicker appears [33]. This comes at the disadvantage of additional power loss.

Developing Levels of Flicker That Have
Minimal Human Biological Effects
This section presents flicker criteria that can mitigate the
human biological effects. The focus of the results concerns
flicker at frequencies above 90 Hz since this is the dominant
location of flicker for commercial LED driving methods.
However, it is important to mention the following known
criterion to prevent photosensitive epileptic seizures: to
reduce the risk of seizures to individuals susceptible to photosensitive epilepsy, all flicker at frequencies below 90 Hz
must satisfy Mod% # 5%.
There is a significant amount of research from medical and vision science to support the necessity for keeping
flicker below 5% modulation for flicker frequencies below
90 Hz [4]-[6]. It seems reasonable to avoid designing drivers
that cause flicker below 90 Hz. This would, in effect, forbid
the half-bridge rectified ac power line from being directly
used to drive LED strings, but it would also mean that the
bulb should not flicker in this low frequency even when
connected to a dimmer switch. It has already been well

documented that some LED bulbs fail on residential dimmer switches, and these flicker at low frequencies that may
induce photosensitive seizures in some individuals [34].
Note that the 5% modulation criterion does not prevent
discomfort from flicker but only implies that it would
not induce photosensitive seizures. In fact, it is well
understood that in the 1-35-Hz range, a 5% modulation
may be detected by observers since the eye is sensitive
to flicker in the very low frequency range [35]-[39]. IEEE
PAR1789 Standards Working Group [17], as well as other
groups [27], [35], [36], may shortly provide recommended
practices to limit the detectability of flicker below the
CFF in these lower frequency ranges below 90 Hz. However, these flicker frequencies would occur because of
poor power line harmonics rather than the LED driving method. Instead, this research considers the more
common flicker frequencies observed in LED lighting
that occur at or above twice the ac line frequency. For
flicker frequencies, fFlicker, above 90 Hz, Mod% should satisfy the following:
)
■■low-risk level: Mod% # 0.08 fFlicker will mitigate any distractions or negative biological effects caused from flicker
)
■■no-observable-effect level: Mod% # 0.0333 fFlicker .
Note that the Mod% has a maximum of 100%.

Example Calculations
Normally, in lighting, the flicker frequency will have a fundamental component at twice the ac line frequency, i.e.,
fFlicker = 2 ) fac and that fFlicker > CFF.
■■Example 1: In the United States, fac = 60 Hz
* The low-risk level leads to Mod% < 0.08 ) 120 Hz =
10% (rounded to the nearest percent).
* The no-effect level leads to Mod% < 0.0333 ) 120 Hz =
4% (rounded to the nearest percent).

PFC
VLine
85-265 Vac

dc/dc

EMI
Filter

RS1

RS2

RS3

VSense ISense
PFC Controller

dc-dc Controller

fig 5 The typical active PFC circuitry for an ac line input will normally have some flicker frequency at twice the ac line frequency.

September 2014

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2014