IEEE Power Electronics Magazine Compendium - March 2018 - 56

I_LED
LED

LED

LED

LED

LED String

ac
50~60 Hz

(a)

LED Current

I_LED
LED LED LED LED
ac
50~60 Hz

LED LED LED LED

I_LED

R2

LED Current (A)

R1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

LED Current Versus Time

0

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
Time (s)
(c)

Or a
Capacitor
(b)
fig 3 The two methods to drive LEDs at twice the line frequency. (a) A full-bridge rectification: rectify ac and send to the LED
string. (b) The opposite connected parallel strings: directly power two LED strings with opposite anode/cathode connections.
(c) The current/luminous output in the LEDs for both approaches: simulation of current through HB LEDs. The luminous intensity
is proportional to the current, causing the lamp to flicker at twice the line frequency (shown periodically every 1/120 s) [2].

LEDs when the switch is off, while there is no current in the
LED when the switch is on. In either case, the LED current
looks similar to Figure 4(c) and has a flicker frequency of
f = 1/T, where T is the period of the PWM. The flicker frequency for PWM dimming in commercial LED lamps typically varies from 120 to 480 Hz [16], but it is also possible to
perform high-frequency dimming [23].
It should be mentioned that there are alternative
approaches to dimming, such as amplitude dimming, in which
the current through the LED is continuous and not pulsing.
By reducing the value of this continuous current (amplitude),
the luminance is dimmed. This approach can avoid flicker.

Power Factor correction circuitry
Even when more sophisticated high-frequency switching
power supplies with power factor correction (PFC) circuits

Vin

Vout

Vin

LED

are used to drive LEDs from ac mains, such as in Figure 5,
there is often flicker at twice the line frequency. Depending
on the design of the circuitry, the harmonic content of this
flicker may vary from being small and unnoticeable to significant in magnitude. The percent flicker depends on the
controller design, the circuitry being used, and filter component values, among other factors.
Figure 5 shows a typical circuitry that may be used
in driving the LEDs in a bulb. A two-stage approach [19],
[24]-[26] is illustrated because this can have a reduced
flicker Mod%. The first stage may be used to achieve
PFC to meet typical requirements, such as a power factor greater than 0.7 given by EnergyStar requirements
[27]. The second stage can be used to reduce the percent flicker. An energy buffer, such as a dc-link reservoir
capacitor, is usually used to absorb the ripple between

Vout

LED

Vin
PWM

PWM

FB

FB

dc/dc Converter

R

(a)

dc/dc Converter
(b)

ILED

Vin

R

T-Period
On

Off

(c)

fig 4 Sending a PWM current through LEDs: (a) series PWM dimming, (b) parallel PWM dimming, and (c) the PWM current
through the LED in either method.

56

IEEE PowEr ElEctronIcs MagazInE

z	September 2014

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