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10 kV High Voltage Generator with LLC Resonant
Circuit for Sterilizing Microbe Applications
S.-Y. Tseng
H.-C. Lin
#
Y.-D. Chang
##
S.-T. Peng
S.-Y. Fan
GreenPower Evolution Applied Research Lab. #Department of Electrical Engineering ##Linear Motor Research Laboratory
(G-PEARL)
National Chung Cheng University
Department of Electrical Engineering
Department of Electrical Engineering
Wufeng Institute of Technology
Chang Gung University
Ming-Hsiung, Chia-Yi, Taiwan
Kwei-Shan, Tao-Yuan, Taiwan, R.O.C
E-mail: [email protected]
E-mail: [email protected]
TEL: +886-5-2267125
Tel: 886-3-2118800
FAX: +886-3-2118026
Fax:886-3-2118026
Abstract-This paper proposes a high step-up voltage ratio of
full-bridge converter associated with LLC resonant circuit for
sterilizing microbe applications. The proposed circuit structure
adopts full-bridge converter to generate a high DC voltage,
while the LLC resonant circuit is used to recover the energy
trapped in leakage inductance of transformer and to reach
zero-voltage switching feature increasing conversion efficiency.
Moreover, the LLC one can also avoid generating a large
resonant current which occurs in between the large leakage
inductance and the large equivalent capacitor of primary
winding in transformer due to transformer with a high turns
ratio. As compared with a conventional full-bridge converter
with hard-switching circuit or phase-shift control method, the
proposed converter can reduce voltage and current stresses of
switches and increase conversion efficiency with soft-switching
features. Finally, a prototype of high voltage generator under
output voltage of 10 kV and maximum output power of 1 kW
has been implemented to prove the feasibility of the proposed
high voltage generator.
I. INTRODUCTION
Due to the advanced development of semiconductor
technology, design and implementation of high output voltage
generator become more facility. It will expand applications of
pulsed electric field (PEF) technology, such as food
sterilization, waste treatment, pollution control, and medical
diagnosis and treatment [1]-[10]. In particular, when PEFs are
used to sterilize microbes of liquid food, they only cause a
little increase in temperature. As compared with conventional
thermal processing, the PEF method can provide consumers
with safe and nutritious food, and with fresh quality. It can
replace or complement conventional thermal processing
methods.
For food sterilization using PEF processing, researchers
have proposed various processing waveforms, which can be
divided into two groups: wide pulse and narrow pulse. Since
effectiveness of food sterilization using wide pulses is higher
than that using narrow pulses, wide pulses are usually used in
a great deal of liquid food sterilization. In recent years, it is
widely used in fruit juice and milk sterilization. When a wide
pulse with high enough electric intensity is applied to the
suspension, as illustrated in Fig. 1, and creates a voltage
higher then 1 V across the cell membrane, it will induce
irreversible pore changes and kill microbes with a mechanism
of rupturing cell membrane [9]-[10]. This process is called
electroporation and is illustrated in Fig. 2, which was digested
from [10]. Before PEFs are applied to cell, they will create
permeable pores over the membrane and water molecules will
978-1-422-2812-0/09/$25.00 ©2009 IEEE
flow into the cell through the pores. If PEFs are continuously
across the cell membrane, they will cause water influx. When
the repetitive rate and pulse width are large enough, the cell
membrane will be ruptured and cytoplasm will flow outside
the cell. Finally, the cell is destructed.
Outer Membrane
R S1
R S1
E field
E field
Cm
Suspension
Cell
R C2
C S RS2
R C1
Cn
Cn
Rn
C S RS2
R C1
Cm
R S1
R S1
Intracelluar Membrane
Nucleus
Fig. 1. Equivalent model of a cell insidesuspension.
Initial State
EF Excitation
Legend:
Water Influx
Electric Field
Membrane Rupture Cell Destruction
Water
Cytoplasm
Fig. 2. Illustration of electroporation processing of a cell.
In PEF application with wide pulses, electric intensity
varies from ten to tens of kV/cm, pulse width ranges from
several to hundreds of μs and repetitive rate changes from 0.1
Hz to 100 Hz. To realize the above specifications, many PEF
generators (PEFGs) adopt a high voltage generator (HVG)
combined with switch sets with high voltage ratings (SSHVR)
to generate pulse voltage waveforms, as shown in Fig. 3.
Since HVG must generate an extremely high output voltage
(≈10 kV), it needs a converter with a high step-up voltage
ratio. Moreover, the one with high powering capability is also
required to achieve effectiveness of liquid food sterilization.
As mentioned above, a full-bridge converter associated with a
transformer with high turns ratio is adopted, [11]-[13], as
shown in Fig. 4(a). Due to transformer with high turns ratio, a
1641
large leakage inductance and a large equivalent capacitance,
which is reflected from secondary winding to primary
winding of transformer, will be induced in the primary
winding. As a result, a large resonant current is generated
when switches are within the turn-on transition interval. To
solve this problem, a resonant circuit is introduced into the
full-bride converter [14]-[18], as shown in Fig. 4(b).
II. ANALYSIS OF THE PROPOSED HVG
In order to analysis of LLC resonant circuit, a single
full-bridge converter with one is shown in Fig. 4(c). Its
equivalent circuit can be depicted in Fig. 5. where Rac is the
equivalent load resistance of primary side in the transformer
Tr and Vab is applied to the LLC one. According to equivalent
circuit of LLC resonant circuit, the equivalent resistance Re(ac)
can be determined as
Re ( ac ) =
Fig. 3. Block diagram of a wide pulse PEF generator for liquid food
sterilization.
Although full-bridge converter with the resonant circuit
can avoid generating a large resonant current, it requires a
output inductor to generate dc voltage output, as shown in Fig.
4(b). When the output inductor is placed on output side, it
will suffer a high voltage stress, resulting in a higher cost for
attaining high enough isolation and a larger volume. To
resolve this problem, full-bridge converter with LLC resonant
circuit is adopted. Moreover, the one can help switches to
operate at zero-voltage switching (ZVS) and aid secondary
rectifier diodes to communicate with zero-current switching
(ZCS), as shown in Fig. 4(c). With this circuit structure, the
proposed HVG which adopts LLC resonant circuit can avoid
a large resonant current, reduce weight, size and volume, and
increase conversion efficiency significantly.
8R L ,
(1)
(nπ )2
where RL is load resistance and n is turns ratio of transformer
Tr. To analyze the LLC resonant topology, the resonant
frequency fr1 of Cr and Lr in series is derived by
1
.
(2)
f =
r1
2π Lr C r
Similarly, the resonant frequency fr2 of Cr and (Lr + Lm) in
series is expressed as follows:
1
.
(3)
f =
r2
(Lr + Lm )Cr
2π
Additionally, according to the Fourier theorem, the square
wave, which is applied to LLC resonant circuit, can be
approximated by the fundamental component Vab, and its
maximum value Vab(max) can be derived by
Vab(max) =
4
π
Vi ,
(4)
where Vi is input voltage of full-bridge converter. Therefore,
rms value of Vab can be determined as
Vab ( rms ) =
2 2
π
Vi .
(5)
According to analysis of LLC resonant circuit shown in Fig.
4(c), input to output transfer ratio M of full-bridge converter
is derived by
.
(6)
(sLm // Re(ac ) )
VDC
M=
(a)
nVi sL + 1 + (sL // R )
r
m
e ( ac )
sCr
From (6), it can be seen that when the proposed converter is
operated at different loads, its transfer ratio M will be varied.
In order to attain a constant transfer ratio M for generating
constant voltage output, the proposed one must regulate
switching frequency. Thus, transfer ratio M can be rewritten
as
,
(7)
M=
(b)
M1
M2
Cr
Vi
D3
Lr
IDC
CO
Lm
RL
VDC
1:N
M3
D5
M4
D6
n2
(c)
Q=
Fig. 4. Schematic diagram of full-bridge converter (a) with hard-switching
circuit (b) with resonant circuit, and (c) with LLC resonant circuit for
generating high output voltage.
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1
1
2
2
Lr Lr C r ⎛ ω s ⎞ ⎡ π 2 ⎛ ω s ω r 1 ⎞ ⎤
⎟ + ⎢ Q⎜
⎜
⎟
⎥
−
Lm ω s 2 ⎜⎝ ω r22 ⎟⎠ ⎢ 8 ⎜⎝ ω r1 ω s ⎟⎠ ⎥
⎣
⎦
where ωs (=2πfs) is the angular frequency of switching
frequency fs, ωr1 (=2πfr1) is that of frequency fr1, ωr2 (=2πfr2)
is that of frequency fr2 and Q is quality factor. In (7), quality
factor Q can be expressed by
D4
Tr
VDC
nVi
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Lr
Cr .
RL
(8)
When ratio α (=Lm / Lr) and frequency gain γ (=fs / fr1) are
defined, transfer ratio M can be further rewritten as
.
(9)
M=
V DC
nVi
1
⎛1
1
⎜⎜ + 1 −
αγ 2
⎝α
2
⎞
⎞
π 4 2⎛ 1
⎟⎟ +
Q ⎜⎜ − γ ⎟⎟
64
⎠
⎝γ
⎠
2
According to (7), the DC characteristic of the LLC
resonant converter can be illustrated in Fig. 6. From Fig. 6, it
can be observed that operational regions of LLC resonant
circuit can be divided into three regions: region I, region II
and region III. According to different operation region, the
proposed converter has different operational features. In the
following, features of each operational region are briefly
described.
output diodes can not be operated with ZCS, resulting in
voltage spike across output diodes.
B. Region II (fr2 < fs < fr1)
When fr2 < fs < fr1, operational region of the proposed one is
on between frequency fr1 and fr2. The converter can be
operated at a higher gain and with ZVS feature. Additionally,
the proposed one can operated in two different resonant
periods. When switches are communicated, resonant inductor
Lr and capacitor Cr will start to resonate. The resonant period
enters the first resonant period. Within this time period,
inductor Lm is clamped to output voltage and is linearly
increased. When resonant current iLr is equal to current iLm,
the second resonant will occur, which is the resonant between
Cr and Lm in series with Lr. This resonant period will last till
the primary switches have been turned on again. During the
second resonant time period, the current of secondary side in
transformer remains zero. As a result, secondary output
diodes are operated with ZCS.
C. Region III (fs < fr2)
When fs < fr2, operational region of the proposed one is on
the left hand side of fr2. The proposed converter is operated
with ZCS. It is not suitable for power MOSFET applications.
As mentioned above, the proposed converter is designed in
region II to increase conversion efficiency.
Fig. 5. Equivalent circuit of LLC resonant circuit.
Fig. 6. Plots of characteristic of the LLC resonant converter.
A. Region I (fs < fr1)
When fs < fr1, operational region of LLC resonant converter
is on the right hand side of frequency fr1, operational principle
of the proposed converter is similar to traditional series
resonant converter. Therefore, it can reach ZVS feature. In
this operational region, resonant components Lr and Cr act as
series resonant, while Lm is clamped by output voltage and
does not participate in the resonant process. As a result,
III. OPERATIONAL PRINCIPLE OF THE PROPOSED PVG
In the liquid food sterilization system, the PEFG shown in
Fig. 7 consists of two circuits: a high voltage generator (HVG)
and a switch set with high voltage ratings (SSHVR). They
can process power from input voltage Vac to treatment
chamber (TC) for sterilizing microbes. The HVG adopts two
full-bridge converters connected in parallel to generate a high
dc-link voltage VDC, and its soft-switching features which
include ZVS in switches and ZVS in output diodes are
achieved with LLC resonant technology. The SSHVR is
composed of four sets of high-voltage switch, in which each
of the high-voltage switches is formed with 15 MOSFETs in
series, to chop a DC voltage to pulse voltage. Fig. 4(c) shows
schematic diagram of single full-bridge converter with LLC
resonant circuit. According to operational principle of the
proposed one, its operational modes can be divided into 6
modes over one switching cycle, as shown in Fig. 8, while its
key waveforms are shown in Fig. 9. In the following, each
operational mode is described briefly.
Fig. 7. Schematic diagram of a complete PEF generator.
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(a)
(b)
Fig. 9. Key waveforms of full-bridge converter with LLC resonant circuit
over one switching cycle.
Mode 1 [Fig. 8(a); t0 ≤ t < t0]: Before t0, switches M1 and M3
are in the on state, while M2 and M4 are in the off state.
Within this time interval, magnetizing inductor Lm, leakage
inductor Lr and capacitor Cr form a resonant network and
they are in the resonant state. In addition, output diodes D1 ~
D4 are in reversely bias, and the energy stored in output
capacitor CO is supplied to load RL. When t = t0, switches M2
and M3 are turned off. Resonant inductor Lr, capacitor Cr and
parasitic capacitors CM1 ~ CM4 connect in series and they will
start to resonate. Within this time interval, since resonant
current iLr is greater than iLm, current of secondary side of
transformer is not equal to zero. Therefore, diodes D1 and D4
are forced to forwardly conduct. As a result, magnetizing
inductor of secondary side is clamped to output voltage VDC.
The current iLm of primary side linearly increases.
Additionally, resonant current iLr increases with the resonant
mode. Since resonant current iLr increases, voltages across
body diodes CM1 and CM4 are released from Vi to 0. Those
across CM2 and CM3 are charged from 0 to Vi.
(c)
DM1
M1
CM1
M2
IDS2
ID
DM2
CM2
Cr ir
D1
Lr
D2
Tr
im
Lm
Vi
M3
IDS3
DM3
CM3
M4
1:N
DM4
CM4
D3
IC
IDC
CO
RL
VDC
D4
(d)
Mode 2 [Fig. 8(b); t1 ≤ t < t2]: At t1, since energies stored in
CM1 and CM4 are discharged to 0, body diodes DM1 and DM4
are in forwardly bias. At the moment, switches M1 and M4 are
turned on. They are operated with ZVS at turn-on transition.
Moreover, resonant inductor Lr and capacitor Cr form a
resonant network and they will start to resonate. Therefore,
resonant current iLr increases with the resonant manner.
Within this time interval, diodes D1 and D4 still keep in the
forwardly bias state. Thus, current iLm also increases with the
linear manner. Additionally, energies are transferred from
input voltage Vi to load through transformer and diodes D1
and D4.
(e)
(f)
Fig. 8. Equivalent circuit of full-bridge converter with LLC resonant circuit
operated in region II for each operational mode over one switching
cycle.
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Mode 3 [Fig. 8(c); t2 ≤ t < t3]: When t = t2, resonant current
iLr is equal to current iLm. At the moment, current of secondary
side in transformer decreases to 0. As a result, diodes D1 and
D4 are reversely biased. Resonant inductor Lr, magnetizing
inductor Lm and capacitor Cr form a resonant network and
they will begin to resonate. During this time interval, currents
iLr and iLm decrease with the resonant manner. Additionally,
energy stored in output capacitor CO supplies power to load.
1644
Mode 4 [Fig. 8(d); t3 ≤ t < t4]: When t = t3, switches M1 and
M4 are turned off. Resonant inductor Lr, capacitor Cr and
parasitic capacitors CM1 ~ CM4 form a resonant network and
they will start to resonate. Since resonant current iLr is greater
than iLm, current of secondary side is not equal to 0. As a
result, diodes D2 and D3 are in forwardly bias and secondary
winding is clamped to output voltage VDC. Therefore, current
iLm linearly decreases. Moreover, resonant current iLr increases
with the resonant mode. Since resonant current iLr increases,
voltages across CM2 and CM3 are discharged from Vi to 0,
while those across CM1 and CM4 are charged from 0 to Vi.
Mode 5 [Fig. 8(e); t4 ≤ t < t5]: When t = t4, voltages across
CM2 and CM3 are clamped to 0 and body diodes DM2 and DM3
are in forwardly bias. At the same time, switches M2 and M3
are turned on. Thus, switches M2 and M3 are operated with
ZVS at turn-on transition. In addition, resonant inductor Lr
and capacitor Cr form a resonant network and they will start
to resonate. Therefore, resonant current iLr increases with the
resonant manner. During this time interval, diodes D2 and D3
are still in forwardly bias. Thus, current iLm linearly increases.
Moreover, energies are transferred from input voltage Vi to
load through transformer and diodes D2 and D3.
Mode 6 [Fig. 7(f); t5 ≤ t < t6]: At t5, resonant current iLr is
equal to current iLm. As a result, current of secondary side
decreases to 0 and diodes D2 and D3 are reversely biased.
Therefore, D2 and D3 can reach ZCS feature. Within this time
interval, resonant inductor Lr, magnetizing inductor Lm and
capacitor Cr form a resonant network and they will keep in
the resonant state. Therefore, currents iLr and iLm decrease
with the resonant manner. Additionally, energy stored in
output capacitor Co supplies power to load. When t = t6,
switches M2 and M3 are turned off. Operational modes of one
switching cycle are complete.
IV. EXPERIMENTAL RESULTS
To verify the analysis and feasibility, a prototype of the
proposed HVG was implemented with the following
specifications.
●input voltage: AC 110 V (with voltage double);
●switching frequency fS: 30 kHz ~ 50 kHz,
●output voltage VDC: 10 kV,
●output current IDC: 0.1 A, and
●output power PO: 1 kW.
To consider high step-up ratios and isolation requirement,
the proposed HVG adopts two sets of full-bridge converter
and two sets of transformer to generate output voltage of 10
kV, as shown in Fig. 5.That is, each transformer will boost up
a 5 kV dc-link voltage. Additionally, the components of the
proposed HVG are determined as follows:
● switches M1 ~ M4: IRFP 460,
● turns ratio of transformer Tr1, Tr2: 10,
● magnetizing inductance Lm1 or Lm2: 3 mH,
● resonant inductance (include leakage inductance) Lr1 or Lr2:
1 mH,
● resonant capacitor Cr1 or Cr2: 15 nF, and
● diodeD1~ D4: UF 1010 (7 diodes connected in series).
high equivalent capacitance appearing at the primary side of
the transformers, and there is no ZVS feature. Figs. 11, 12
show the measured voltage VDS and current IDS of switches M1
and M2 when the proposed HVG is respectively operated at
10 % and 50 % of full load. From Figs. 11, 12, it can be
observed that switches M1 and M2 can be operated with ZVS
at turn-on transition. Measured voltage VD and current ID
waveforms of output diode is illustrated in Fig. 13, from
which it can be seen that output diode can reach ZCS feature.
Fig. 14 shows efficiency measurements of the proposed HVG,
from which it can be seen that the maximum efficiency can
reach as high as 93 % at 90 % of full load, and around 91 %
under full load. The output voltage VDC and current IDC is
shown in Fig. 15, illustrating that output voltage ripple is very
low within 1 %. As mentioned above, the proposed HVG is
relatively feasible in high output voltage applications, which
has been verified by the experimental results.
(VDS : 100 V/div, IDS: 5A/div, 5 μs/div)
Fig. 10. Measured voltage VDS and current IDS waveforms of switch in
full-bridge converter with phase-shift control method.
(VDS : 200 V/div, IDS: 1A/div, 5 μs/div)
(a)
(VDS : 200 V/div, IDS: 1A/div, 5 μs/div)
(b)
Fig. 11. Measured voltage VDS and current IDS waveforms of switches (a) M1
and (b) M2 in the proposed HVG under 10% of full load.
The measured waveforms of voltage VDS and IDS of switch
M1 show in Fig. 10 when full-bridge converter with the
conventional phase-shift control method is adopted. From Fig.
10, it can be seen that high spike current occurs due to the
978-1-422-2812-0/09/$25.00 ©2009 IEEE
1645
V. CONCLUSIONS
This paper has proposed a full-bridge converter with LLC
resonant circuit to form a high voltage generator. The
proposed HVG can use resonant technology and transformer
with high turns ratio to reach a high step-up voltage ratio. By
adopting the LLC resonant circuit, energy trapped in the
leakage inductor can be recovered, ZVS features can be
achieved and current spike can be suppressed effectively. In
the paper, analysis of the generator has been presented in
detail, from which design equations and circuit parameters
are derived. Experimental results have verified that the
proposed HVG can achieve high efficiency over a wide load
range. It is relatively suitable for PEFG applications.
VDS1
IDS1
(VDS : 200 V/div, IDS: 1A/div, 5 μs/div)
(a)
VDS2
[1]
IDS2
[2]
[3]
(VDS : 200 V/div, IDS: 1A/div, 5 μs/div)
(b)
Fig. 12. Measured voltage VDS and current IDS waveforms of switches (a) M1
and (b) M2 in the proposed HVG under 50% of full load.
[4]
[5]
[6]
[7]
[8]
(VDS : 2 k V/div, IDS: 100 mA/div, 10 μs/div)
Fig. 13. Measured voltage VD and current ID waveforms of output diode.
[9]
Efficiency(% )
100
90
80
70
60
50
40
30
20
10
0
[10]
[11]
[12]
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
LLC resonant circuit
Hand-switching circuit
[13]
Load (% )
Fig. 14. Comparison of converter efficiency between full-bridge converter
with hard-switching circuit and with the proposed LLC resonant
circuit.
[14]
[15]
[16]
[17]
(Vo: 5 kV/div, Io: 100 mA/div, 5μs/div)
Fig. 15. Measured output voltage VDC and output current IDC under full load
condition.
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[18]
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