IEEE Power Electronics Magazine Compendium - March 2018 - 55

this case, the current through the LEDs has a
waveform shape similar to a scaled absolute
value of a sine wave. That is, the rectified
sine wave may be of the form V p sin (~t) ,
where V p is the amplitude of the sine wave,
~ is the angular frequency in radians
~ = 2 ) r ) fac, and fac is the ac mains power
line frequency. Similarly, it is possible to use
the so-called ac-LED approach, as shown in
Figure 3(b). Two strings of LEDs are powered in parallel, with the anode of one paralleled string connected to the cathode of the
other parallel string. When the ac line voltage
is positive, energy drives one of the LED
strings. When the ac line voltage is negative,
the other paralleled LED string is driven. At
most, one of the LED strings has a current
through it. The combined LED current
would, therefore, in both cases have a shape
as in Figure 3(c), which has Mod% = 100%
with flicker frequency fFlicker equal to twice
the ac mains frequency, i.e., fFlicker =120 Hz
(North America) or fFlicker =100 Hz (Europe).
The benefits of the ac-LED approach are
the minimal part count and the elimination
of high-frequency power electronic drivers,
but this comes at a cost of extremely high
ripple-much higher than in line-frequency
magnetic ballast for fluorescent lamps.
Alterations on the ac-LED approach may
include ac stacking, which varies the number of conducting LEDs depending on the
voltage level of the input source [19]-[21].
Then, it may be possible to add holdup
capacitors of reasonable size to subsequently provide energy to the LEDs, even
when the input voltage becomes low. Such
approaches can partially reduce the flicker
modulation percentage [22].

Tek Stop
Maximum
Ch2 Mean
4.47 V
Minimum

Ch2 Maximum
6.44 V

2
Mod% = 48.4%

Ch2 2 V

BwM 10 ms A Ch2
50%
(a)

4.68 V 8 May 2014
18:21:39

Tek Stop

Ch2 Mean
4.48 V
Ch2 Maximum
4.60 V

2

Ch2 Minimum
4.32 V

Mod% = ~ 1%

Ch2 2 V

BwM 10 ms A Ch2
(b)

4.48 V

50%

8 May 2014
18:42:52

Tek Stop

Ch2 Mean
4.47 V

simple Dimming PwM circuits
It is common to dim LEDs by pulsing the
current through them, as shown in Figure 4.
The average luminous intensity of the LED
can be adjusted by varying the length of
time that the LED current is high and low.
Thus, PWM dimming circuits may be
designed to operate at any frequency. This is
true whether the input source is ac or dc
power, although Figure 4 assumes a dc input
source, perhaps coming from a prior ac-dc
conversion. In Figure 4(a), the switch is in
series so that when it is on, the current will
flow through the LED. When the switch is
off, no current flows through the LEDs. In
Figure 4(b), the current flows through the

Ch2 Minimum
2.24 V

Ch2 Maximum
5.12 V

2

Ch2 Minimum
3.44 V
Mod% = 19.6%

Ch2 2 V

BwM 10 ms A Ch2
50%
(c)

4.68 V

8 May 2014
18:29:13

fig 2 The typical plots of luminance light flicker in commercial LED A19 bulbs
(60-W incandescent replacement bulbs). Mod% = (Max luminance - Min
luminance)/(Max luminance + Min luminance). Mod% for the three bulbs are
(a) Utilitech, 48.4%; (b) Philips, <1%; and (c) GE, 19.6%.

September 2014

z	IEEE PowEr ElEctronIcs MagazInE

55
Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018
