Single-switch three-level boost converter for PWM dimming LED lighting

Abstract

In this paper, a novel single-switch three-level (TL) boost converter for PWM dimming LED lighting is presented. Taking the system efficiency and output ripple into consideration, traditional two-level boost LED driver always employs additional active switch in order to deal with the dimming issue. However, the proposed TL boost converter has overcome the dimming control problem with only one switch by utilizing the "synchronous" PWM dimming approah, hence lower the system cost. Moreover, compared with the standard TL boost converter, the proposed topology not only maintains the advantage of using a lower voltage rating power switch that is rated half the output voltage, but also eliminates one active switch and its associated driving circuit. The operating principle and dimming scheme of the proposed TL boost converter is described in detail. A prototype of 11-W dimmable LED driving converter has been implemented to verify the design and analysis. The experimental result shows that single-switch TL boost converter can regulate the LED current from zero to the rated 350mA with fast slew rate. A peak efficiency of 95% was achieved with 400kHz switching frequency.

Full text

Page 1

Single-Switch Three-Level Boost Converter for
PWM Dimming LED Lighting

Cong Zheng1, Student Member, IEEE
Wensong Yu1, Member, IEEE

1 Future Energy Electronics Center, Bradley Department of Electrical and Computer Engineering,
Virginia Polytechnic Institute and State University, Blacksburg, VA, USA
2School of Information Science & Technology, Southwest Jiaotong University, Chengdu, Sichuan, China
czheng@vt.edu, laijs@vt.edu, wensong@vt.edu, mahongbo81@gmail.com

Abstract—In this paper, a novel single-switch three-level (TL)
boost converter for PWM dimming LED lighting is presented.
Taking the system efficiency and output ripple into
consideration, traditional two-level boost LED driver always
employs additional active switch in order to deal with the
dimming issue. However, the proposed TL boost converter has
overcome the dimming control problem with only one switch
by utilizing the “synchronous” PWM dimming approah, hence
lower the system cost. Moreover, compared with the standard
TL boost converter, the proposed topology not only maintains
the advantage of using a lower voltage rating power switch that
is rated half the output voltage, but also eliminates one active
switch and its associated driving circuit. The operating
principle and dimming scheme of the proposed TL boost
converter is described in detail. A prototype of 11-W dimmable
LED driving converter has been implemented to verify the
design and analysis. The experimental result shows that single-
switch TL boost converter can regulate the LED current from
zero to the rated 350mA with fast slew rate. A peak efficiency
of 95% was achieved with 400kHz switching frequency.
Jih-Sheng(Jason) Lai1, Fellow, IEEE
Hongbo Ma1, 2, Student Member, IEEE
I.
INTRODUCTION
Multilevel power converters have attracted great interest
in all types of power conversion such as ac-dc, dc-ac, dc-dc
and ac-dc-ac since the voltage ratings of the semiconductor
switches can be lower than those of two-level converters [1]–
[4]. The voltage stress of all devices in three-level (TL) boost
converters is typically half of the output voltage, hence the
conduction loss is reduced with a lower on-state resistance,
and a higher efficiency can be achieved [5]–[7]. The standard
TL boost converter is represented in Fig. 1. Each of these
converters consists of two converters that are stacked in a
way that splits the output voltage between two sections. The
output voltage will be split evenly if the switches are
operated with the same duty cycle.
Many TL dc-dc converters adopt soft-switching
technique to obtain higher efficiency [8]–[14]. Nevertheless,
these topologies require additional power switches as well as
their driving circuits, which make them unsuitable for low-
power and low-cost applications, such as LED driver.
Furthermore, additional voltage balance control is needed to
deal with the voltage imbalance issue. Most of today’s
dimmable LED driving circuits are based on two-level
converters [15]–[19]. Reference [15]–[16] introduced a
method that two-level PWM regulator provides dynamic bus
for multistring dimmable LED driver. High dimming ratio
LED driving circuits with fast transient two-level buck and
boost converter are proposed in [17] and [18]. In order to
achieve high efficiency and low output ripple, two-level
boost converter always employs additional switch in order to
deal with the dimming issue, hence the overall cost increases
[18]. Moreover, the voltage stress on each device is higher
than that of TL converters, therefore a smaller voltage range
of device can be selected and the overall efficiency will be
lower.
In this paper, a single-switch TL boost converter for LED
lighting is proposed to further improve the efficiency of
PWM regulator for dynamic bus, such as automotive
This work was sponsored by the National Semiconductor Corporation
in
V
b L
LEDs
1
D
2
D
1S
2 S
1
C
o
C
Fig. 1. Standard two-switch three-level boost
converter.
978-1-4577-0541-0/11/$26.00 ©2011 IEEE2589

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in
V
b L
LEDs
1
D
2
D
1S
1
C
o
C
3
D

in
V
b L
LEDs
1
D
2
D
1S
1
C
o
C
3
D

(a) (b)
LEDs
in
V
b L
1
D
2
D
1S
1
C
o
C
3
D
2
C

in
V
b L
1
D
2
D
1S
1
C
o
C
3
D
2
C
LEDs
rL

(c) (d)
Fig. 2. Derivation of single-switch TL boost converter.
application with 12V battery as the input. The proposed TL
boost LED driving circuit has solved the dimming issue
encountered in traditional two-level boost converter with
only one switch, hence lower the system cost. In addition,
compared with the standard TL boost converter, this circuit
not only maintains the merit that the voltage stress of the
power switch is half the output voltage, but also eliminates
one active switch and its associated driving circuit. The
operating principle of dimming scheme of the proposed
converter will be discussed, and the related qualitative
analysis will also be described. A hardware prototype has
been built and tested. Experimental results are shown to
verify all designs and analyses.
II.
SINGLE-SWITCH TL BOOST CONVERTER TOPOLOGY
A. Derivation of Single-Switch TL Boost Converter
The derivation of single-switch TL boost converter is
illustrated in Fig. 2.
Step 1: In Fig. 2(a), compared with standard TL boost
converter, a diode D3 is employed to replace the active
switch S2, and hence its driving circuit can be eliminated;
Step 2: Since D3 is used to replace the active switch, it
will block the charging loop of the boost inductor. Therefore,
the position of the input inductor is moved so that this circuit
reveals the boost converter characteristic (Fig. 2(b));
Step 3: In order to form TL characteristic in this circuit, a
flying capacitor C2 is introduced as shown in Fig. 2(c). The
voltage stress of C1 and C2 are naturally balanced, hence the
voltage stress on S1 is half of the output voltage;
Step 4: A small inductor is series connected with D2 to
form a resonant circuit in order to eliminate the inrush
current flowing through the active switch S1 (Fig. 2(d)).
The topology is similar to those which are based on
hybrid switched-capacitor or voltage multiplier cell
technique [20]–[21]. However, in the voltage multiplier cells
applied to non-isolated dc-dc converters, Lr is usually series
connected with C1. When the active switch S1 is off, Lr will
affect the energy transfer from the input toward the output.
While utilizing the proposed configuration, this issue would
be solved so that the energy transfer efficiency will be
increased.
B. Basic Operating Principle of Proposed Single-Switch TL
Boost Converter
Fig. 2(d) shows the proposed circuit that contains a set of
flying capacitors, C1 and C2, an active switch S1, and three
diodes, D1, D2, and D3. C1 and C2 help to keep the voltage
stress on S1 to be half of the output voltage. Lr is a small
inductor to limit the inrush current flowing into S1. As shown
in Fig. 3, the proposed single-switch TL boost converter has
three operational stages.
Stage 1 (0<t<DTs, during dimming active interval): Here
D is the duty cycle, and Ts is the switching period. As shown
in Fig. 3(a), when S1 is on, D2 is also on, and D1 and D3 are
both off. The voltage across the boost inductor is
The current flowing through the inductor rises linearly. At
the same time, C1, C2 and Lr form the resonant circuit to
eliminate the inrush current. The resonant current can be
calculated using (1), where ?V1 represents the voltage
difference between C1 and C2.
Lbin
VV
=
.
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rL
in
V
b L
1
D
2
D
1S
1
C
o
C
3
D
2
C
LEDs

rL
in
V
b L
1
D
2
D
1S
1
C
o
C
3
D
2
C
LEDs

(a) (b)
rL
in
V
bL
1
D
2
D
1S
1
C
o
C
3
D
2
C
LEDs

rL
in
V
bL
1
D
2
D
1S
1
C
o
C
3
D
2
C
LEDs

(c) (d)
Fig. 3. Operation modes of single-switch TL boost converter.
1cos
o
Lr
i
o
V
t
Z
ω
Δ
=
(1)
where
12
112
T
()
o
o
rrr
L CL CC
π
ω =
==
?
(2)
12
rr
o
r
L
C
L
?
Z
CC
==
(3)
Stage 2 (DTs<t<Ts, during dimming active interval): As
shown in Fig. 3(b), when S1 is off, D2 is also off, and D1 and
D3 conduct together. Since the voltage stress on C1 and C2
are naturally balanced,
1
C
V
=
voltage across the boost inductor Lb is
current flowing through the inductor falls linearly. Based on
the voltage second balance on Lb during one cycle, the input
and output voltage relationship can be derived by
V DTVV
=−
2
1
D
−
The current ripple of the boost inductor Lb can be expressed
as
VV D
I DT
L
2
0.5
V
Co
VV
=
. Therefore, the
0.5
o
VV
=−
Lbin
. The
(0.5)(1)
insoins
D T
−
(4)
oin
VV
=
(5)
inin
Lbs
bbs
L f
Δ==
(6)
Stage 3 (Dimming inactive interval): This stage contains
three sub operation modes. Fig. 3(c) shows that, when the
dimming signal is inactive, only D1 is still working while
other devices are turned off. In this case, the boost inductor
Lb begins to discharge. When the input current falls to zero,
D1 turns off. In the meanwhile, the output capacitor Co starts
discharging to the load as shown in Fig. 3(d). Co should be
selected carefully to meet both the output ripple requirement
and the dimming speed requirement. Since this schematic
has relatively high output-input voltage ratio, the LED output
voltage will easily decrease to the threshold voltage, and
there will be no output current flowing through the LED,
which implies that this topology is suitable for dimmable
LED lighting, despite only one active switch is required.
III. DIMMING SCHEME AND CLOSED-LOOP CONTROL
A. Dimming Schemes for Boost LED Drivers
For traditional two-level boost LED driver, when the
output-input voltage ratio is not big enough, the input voltage
will be always larger than the threshold voltage of the LED
string. Therefore, an active switch is always series connected
with the LEDs, serving as a dimming control switch. In this
way, the cost of the LED driver will be increased.
For the proposed single-switch TL boost LED driver,
there are three potential dimming schemes which will be
demonstrated in the following paragraphs.
The first dimming approach is shown in Fig. 4(a). The
principle is applying PWM dimming signal directly to the
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main switch, but leaving the reference signal uncontrolled.
From the simulation results of this dimming method
illustrated in Fig. 4(b), there is a huge spike on the LED
current when the dimming signal is on. The reason is that
during dimming off interval, the feedback signal is far away
from the reference, hence the integrator capacitor will be
saturated, resulting severe current spike during LED turn on
interval.
The second dimming methodology is directly applying
PWM signal to the reference of the LED current, while the
gate signal applied into the main switch is still from the
driver IC without any modification (Fig. 5). In this case, the
LED current rising transition and falling transition are
relatively slow, due to the integration effect of the
compensator. Especially for the dimming inactive interval,
the reference is zero, and the error signal will be extremely
small. Therefore, theoretically the compensator will not be
able to make the LED current down to zero due to the
integration effect.
The proposed dimming approach is called “synchronous”
PWM dimming. As shown in Fig. 6, the PWM dimming
signal is both applied to the main switch and the reference of
the LED current at the same time. Based on the proposed
synchronous dimming scheme, the integrator saturation issue
appeared in the dimming approach I will be avoided and
hence the LED current spike during turn on interval will be
reduced. Furthermore, the dynamic response of LED current
will be improved compared with using dimming strategy II.
B. Closed-Loop Compensator Design
Since the proposed single-switch TL boost LED driver is
suitable for automotive application with 12V battery as the
input, and, the input voltage may varies between 9V to 16V,
depending on the state of charge (SOC) of the battery. A
class of bode plots for different input voltage conditions are
simulated and presented in Fig. 7.
In order to calculate the resonant frequency, the output
capacitor Co and flying capacitor C1 are both converted to be

(a)

(b)
Fig. 4. Circuit diagram and simulation results for dimming
scheme I.
0
0.2
0.4
0.6
0.8
1
Gate
0
20
40
60
80
100
Vds
0.090.0920.0940.0960.0980.1
Time (s)
0
2
4
I(LED)

Fig. 6. Circuit diagram for “synchronous” PWM dimming
strategy.

Fig. 5. Circuit diagram for dimming scheme II.
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Page 5

paralleled with the clamp capacitor C2. Base
on each capacitor, the equivalent capacitance
12
2
12
12
()
C
4
eqCCoo
o
CC
C
V
C
VCC
=
=
⋅
++ ⋅
??
The equivalent inductance is similar
traditional two-level boost converter
2
(1)
b
D
eq
L
L
=
−

Therefore, the double pole frequenc
represented as
0
1
L C
2
eq eq
f
π
=

Fig. 8 shows the open-loop bode
( ) ( )
iPWM c
sH G G s
⋅⋅⋅
designed based on the worst operation condi
id
G
, where the PID

Fig. 7. Bode plots under different input volt
Gain /
-40
-30
-20
-10
0
10
20
freq / Hertz
100
200 4001k2k 4k10k
Phase / degrees
-200
-150
-100
-50
0
9V
12V
14V
16V
Fig. 8. Open-loop bode plot with PID co
10
2
10
3
10
4
135
180
225
270
315
360
P.M.: 29.1 deg
Freq: 1.26e+004 Hz
Frequency (Hz)
Phase (deg)
-20
0
20
40
G.M.: 10.6 dB
Freq: 8.12e+004 Hz
Stable loop
Closed-Loop Bode Plot
Magnitude (dB)
ed on the voltage
e is derived as by
2
2
()
o
CC
VV
⋅
(7)
to that of the
(8)
cy f0 could be
(9)
plot including
compensator is
ition: Vin = 9V.
IV. EXPERIMENTAL V
A laboratory prototype of the
LED driver is built to verify the op
of the proposed topology and
parameters of the prototype conve
input voltage Vin is set from 9V
frequency fs is 400kHz, the dimmin
400Hz, the boost inductor Lb is
capacitors C1 and C2 and output c
ceramic X7R capacitors, and the sm
50nH. The load consists of a str
connected, which yields a total thr
The type of diodes D1, D2 and D3 is
MosFET S1 is Si4840DY. The contr
Fig. 9.
ILED an
Fig. 10.
ILb and
Fig. 11.
Vo and


tage conditions.
20k40k 100k

ompensator.
10
5
VERIFICATIONS
single-switch TL boost
peration and performance
dimming strategy. The
erter are as follows. The
V to 16V, the switching
ng signal frequency fdim is
35?H, the two flying
capacitor Co are all 1?F
mall resonant inductor Lr is
ring of 11 LEDs series
reshold voltage of 36 V.
PDS540, and the type of
roller IC is UCC35705.

d Vds.

d Vds.

d Vds.
2593