A New Energy Recovery Circuit for the Capacitive
Load using the Current-Balance Transformer
J. B. Baek, J. H. Park, B.H. Cho
Department of Electrical Engineering, Seoul National University
San 56-1, Silim-dong, Seoul, 151-742, Korea
E-mail: bjbsla@pesl.snu.ac.kr
Abstract- This paper proposes a new recovery circuit for the
capacitive load. Connecting the one side of windings to the load
together, the resonant current is shared in both windings, hence
the conduction loss is reduced. Simple circuit structure with
transformer achieves the reduced the number of devices.
Furthermore, the current-balance transformer enables ZVS
operation of the switching device. To confirm the validity of the
proposed circuit, prototype hardware with 12-inch mercury-free
flat fluorescent lamp is experimented. The results are compared
with a conventional energy-recovery circuit in perspective of
luminance performances.
I.
VS
SX1
SY1
DX1
DY1
CP
DX2
L
L
SX2
SX4
SY4
DY1
SY2
Fig. 1 Conventional Webber’s ERC
INTRODUCTION
Cells of the PDP are electrically modeled as an equivalent
panel capacitance (CP) because the dielectric and MgO layers
cover the electrodes in the unit cell [1]. Recently, research
interest toward developing a high luminance and high
luminous efficiency mercury-free flat light sources has gained
momentum [2]-[7]. It is also modeled as an equivalent panel
capacitance (CP). When the sustain voltage (VS), which is the
AC square wave with a high peak-to-peak value, is applied to
the sustaining and scanning electrodes, energy as much as
CPVS2 is lost in each transient. To minimize this energy loss
and improve the power efficiency of the plasma discharge
system, an energy recovery circuit (ERC) is necessary. [8][18]. Webber’s type energy recovery circuit (ERC) which
employs series resonance is one of the most popular schemes
in the industry [19]. Fig. 1 shows the basic scheme of the
Webber’s ERC. This ERC uses auxiliary capacitor to recover
and inject the energy in the capacitance Cp. Through this
series LC resonance, the panel voltage changes with three
states such as –Vs, 0 and Vs. The auxiliary capacitor has
approximately ten times larger capacitance than a panel
equivalent capacitance and it operates as constant voltage
source. The voltage is maintained as Vs/2 because recovery
and injection has same amount in a period.
However, it reveals potential areas of improvements such as
the energy recovery efficiency and the complexity of the
circuit. It has the high conduction loss due to series-connected
bidirectional switches and the possibility of in-rush current by
failure of ZVS due to the voltage drop from the MOSFET’s
parasitic resistance. Also, it needs extra unidirectional diodes
to block the bidirectional voltage. In this paper, a new ERC
using the current-balance transformer is proposed to improve
the energy recovery efficiency and to reduce the part count of
the circuit. Each operational mode of the proposed ERC is
explained in the following sections. A prototype hardware
using 12-inch mercury-free flat fluorescent lamp (MFFL) is
implemented to verify the circuit operation. Its luminance
performance is compared with the conventional Webber’s
ERC. Experimental result indicates that under low luminance,
luminance efficiency was increased by 15%.
II. ENERGY RECOVERY CIRCUIT WITH CURRENT-BALANCE
TRANSFORMER
The proposed ERC has the similar operating sequence with
other conventional ERC: capacitive load charging, sustain,
energy recovery, and erased-holding. Fig. 2 shows the circuit
diagram of the proposed ERC. It has symmetric structure that
the load is located between twin bridges for bipolar operation.
A current-balance transformer is connected between switches
and the load. In this case, the turn ratio of the transformer is
1:1 to balance the current. Using this current-balance
transformer, the current is split into each winding in every
operational mode. Therefore the current always flow two
switches, and conduction loss decreases in half. It reduces the
possibility of ZVS failure, because the resonant energy loss
decreases due to the on-resistance. In-rush current caused by
ZVS failure at the beginning of sustain discharge is reduced,
which is critical to the EMI and device life-span. Furthermore,
in case of the Webber’s ERC, it needs diodes which are
necessary for bidirectional voltage blocking. In case of the
proposed ERC, it needs no diodes and current-balance
transformer has a filtering function thus it provides a
protection from EMI noise. Fig. 3 shows the key waveforms
of the proposed ERC. As shown in Fig. 3, there are 8 different
operational modes in a cycle, and the first 4 modes are
symmetric to the following 4 modes.
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SY3
SX3
1676
I Cp =
VS
sin ω0 ( t − t0 )
2Z 0
(1)
VCp =
VS
(1 − cos ω0 ( t − t0 ) )
2
(2)
where, ω0 =
1
LC p
Z0 =
L
Cp
VS
SX3
SX1
Fig. 2 Proposed ERC with Auto-Transformer
SY3
SY1
CP
TX
A. Operational Mode Analysis
SX4
SX2
x Mode 1 [t0 < t ≤ t1]:
TY
SY4
X-board
SY2
Y-board
(a) Mode 1
Before t0, all the bottom switches (SX2, SX4, SY2, SY4) are
turned on, and the panel voltage (Vcp) is zero. The load
current (Icp) is also zero. At the beginning of Mode 1, Sx1 is
turned on, the current is shared in Sx1 and Sx4. The current
resonates through the leakage inductance of transformer (L)
and the panel capacitance (Cp) as equation (1). Due to the
current-balance transformer, the current is distributed to the
two switchds with reduced equivalent resistance (Rds_on),
and the conduction loss decreases. The transformer-tap
voltage reaches Vs and SX3 can be turned on under ZVS
condition in Mode 2. Both SY2 and SY4 are kept on, which hold
transformer-tap voltage to the ground. SX4 can be turned off
under ZCS condition.
The panel voltage (Vcp) increases from zero to Vs in
resonant fashion. Fig. 4(a) shows the path of conducting
current in Mode 1.
VS
SX1
SX3
SY3
SY1
CP
TX
SX2
TY
SX4
SY4
SY2
SY3
SY1
(b) Mode 2
VS
SX1
SX3
CP
TX
VX1
SX2
TY
SX4
SY4
SY2
SY3
SY1
VX3
VX2
(c) Mode 3
VX4
VS
VY1
VY3
SX1
VY2
SX3
CP
VY4
ICp
t0 t1
t2 t3
mode 1
t4 t5
mode 3
VCP
mode 2
t6 t7
TX
mode 7
mode 5
mode 4
mode 6
SX2
SX4
SY4
SY2
mode 8
(d) Mode 4
Fig. 4 Operational mode diagrams of the proposed ERC
Fig. 3 Key waveforms
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TY
1677
x Mode 2 [t1 < t ≤ t2]:
TABLE I
COMPARISON OF THE NUMBER OF DEVICES
In the previous mode, SX4 is turned off under ZCS condition
during the anti-parallel diode on. Mode 2 starts when Vcp
reaches Vs. At that moment, the anti-parallel diode of SX3
starts to conduct and SX3 is turned on under ZVS condition.
The switches of Y board are still grounded and those of X
board maintain Vs. Through the current sharing, the
conduction loss also decreases significantly. Fig. 4(b) shows
the current sharing path in this mode.
Magnetics
Storage
Capacitor
Webber’s
8
4
2 Inductor
4
Proposed
8
0
2 transformer*
0**
III. EXPERIMENTAL RESULTS
To verify the performance of the proposed ERC, a
prototype hardware with a capacitor load has been built. The
capacitance is 88[nF] to simulated the MFFL load and the
switching frequency is 20kHz. Fig. 5 shows the experimental
waveforms of the current (Icp) and the voltage (Vcp) of the
load. The figure shows the panel current (Icp) and a winding
current (ISX3 – ISX4), and the switches share the load current
equally through the transformer winding. In case of Webber’s
ERC, the panel current is same as the switching current.
However, in proposed ERC, the panel current is splitted up by
two winding of the current-balance transformer. It means that
conduction loss decreases a half.
Fig. 6 shows the ZVS operation in the voltage waveforms
of gate-source and drain-source. In Mode 1 and Mode 3, the
proposed ERC achieves ZVS turn-on.
Fig. 7 shows luminance efficiency comparison at different
luminance. In most of operating region, the proposed ERC has
higher efficiency than a Webber’s ERC. It is note worthy that
the luminance efficiency was increased more at lower
luminance region.
In this mode, the load voltage starts to decrease and the
stored energy in the load capacitance is recovered. The
operation is quite similar to Mode 1 except turning on SX for
load capacitance discharge. Y board is tied on zero voltage.
When SX2 is turned on, the stored charges in the load start
moving through SX2 and SX3 with the current-balance
transformer. Then the energy starts recovering into the power
source by the LC resonance. This mode also includes currentsharing operation and obtains reduction of conduction loss.
The current path is shown in Fig 4(c). When the panel voltage
drops from Vs to zero under resonance as equation (4), ZCS
condition of SX3 is achieved and SX4 also satisfies ZVS
condition. The equation of the panel voltage and current is as
follows:
V
(3)
I Cp = − S sin ω0 ( t − t2 )
2Z0
VS
cos ω0 ( t − t2 )
2
Diode
* use the leakage inductance of the transformer
** be replaced by input voltage source
x Mode 3 [t2 < t ≤ t3]:
VCp =
Switch
(4)
x Mode 4 [t3 < t ≤ t4]:
After the resonant current becomes zero in Mode 3, the ZCS
condition of SX3 is achieved and Vcp reach zero voltage.
When the load voltage reaches zero voltage, the diode of SX4
is conducted and that the switch is turned on in ZVS condition.
Then, the load voltage maintains zero voltage until new
operating cycle begins. Fig 4(d) shows the current path in
Mode 4. It can be oscillated when the energy is remained.
Sx2 gate
waveform
Vcp
Icp
B. Device Count Comparison
Using the current-balance transformer, the proposed ERC
reduces the number of devices. Table 1 shows the comparison
of the number of device count with Webber’s ERC. Diodes
used to block the bidirectional voltage are removed in the
proposed one. The resonant inductor is replaced by the
leakage inductance of the transformer. Because the input
voltage source acts as the buck storage device, storage
capacitors are eliminated.
ISX3 - IISX4
SX3 - ISX4
Fig. 5 Experimental Result using Capacitor load
(2μs/div, 100V/div, 10A/div)
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REFERENCES
Vgs of S x4
[1]
ZVS of Sx4
[2]
Vds of S x4
[3]
[4]
[5]
[6]
Vgs of S x3
ZVS of Sx3
[7]
Fig. 6 switch voltage waveforms (0.5[㎲/div]
ch.1: [20V/div] ch. 2: [100V/div], ch. 3: [20V/div] )
[8]
[lm/W]
[9]
32
28
[10]
24
[11]
20
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[12]
Webber’s ERC
Proposed ERC
4000
5000
Luminance [cd/m 2]
6000
[13]
Fig. 7 Luminance efficiency comparison
[14]
IV. CONCLUSION
This paper proposes a new energy-recovery circuit for the
AC driving with capacitive load using the current-balance
transformer. It improves the luminance efficiency through the
current sharing which results in the reduction of the
conduction loss. It also helps the ZVS condition which not
only improves the efficiency but also overcomes the EMI
problem caused by ZVS failure. Furthermore, the proposed
ERC reduces the device counts, and thus cost. A prototype
was built to verify the performance of the proposed scheme,
and compared with a Webber’s ERC.
[15]
[16]
[17]
[18]
[19]
ACKNOWLEDGMENT
This study is partly supported by Samsung SDI, and
Research Center for Energy Conversion and Storage.
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