Note: Descriptions are shown in the official language in which they were submitted.
CA 02325691 2000-09-25
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A HIGH-VOLTAGE PULSE GENERATOR
Field of the Invention
The present invention relates to methods and systems for generating high-
voltage
pulses. More particularly, the present invention relates to the design of a
PEF treatment
system for generating high-voltage bipolar and/ or unipolar pulses used to
induce stress
and mortality in biological cells.
Background of the Invention
PEF technology has found a wide range of applications in different areas such
as,
1 o for example, bio-fouling control, non-thermal food processing, odor
control, and NOx
removal. Extensive research has been conducted to study the efficacy of using
PEF as a
non-thermal food pasteurization/sterilization method. However, the application
of PEF
treatment is not straightforward due to a number of factors. Factors affecting
PEF
treatment include, for example, electric field strength, treatment
temperature, stage of
microbial growth, and total treatment time.
In typical PEF treatment systems, high voltage pulses are induced in food
products by specially designed PEF treatment chambers. Fluid food products are
primarily conductive due to the existence of charge carrying particles such
as, for
example, proteins, vitamins and minerals. Therefore, application of a high-
voltage across
a treatment chamber results in a large flux of current flowing through the
food product.
This same current must also flow through the high-voltage pulse generator that
is
generating the high-voltage pulse(s).
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In this regard, a PEF treatment chamber generally has two electrodes that
convert
high-voltage pulses to pulsed electric fields. Food product residing in
between the
electrodes is exposed to this field. For effective treatment, the design of
the treatment
chamber should guarantee a uniform field distribution inside the treatment
zone.
However, due to the large contact area between the two electrodes, the
resistance
therebetween is typically small and often in the range of half an Ohm to a
couple of
Ohms. Consequently, it is difficult for conventional high-voltage pulse
generators to
drive a PEF treatment chamber that has such a small resistance.
Accordingly, conventional high-voltage pulse generators are disadvantageous
for
1 o a number of reasons. One disadvantage is that few high-voltage pulse
generators can
maintain the extremely high currents required due to the low resistances of
treatment
chambers. Additionally, conventional high-voltage pulse generators only
provide for
unipolar pulses that cause the deposition of protein and other charge carrying
particles on
electrodes. Therefore, methods and apparatuses for providing high-voltage
pulses that do
not suffer from these and other disadvantages are desirable.
Summary of the Invention
According to the present invention, methods and systems for generating high-
voltage unipolar or bipolar pulses for inducing changes in biological cells is
provided.
The methods and systems are particularly suited for the pasteurization and/or
sterilization
of, but not limited to, pumpable food products. In accordance with present
invention, a
power source charges an energy storage component, either a capacitor or a
pulse forming
network (PFN). The particular composition of the energy storage component also
influences the shape of high-voltage pulse that is applied (i.e.,
substantially square pulse,
exponential decay pulse, etc.) Multiple switches that are in circuit
communication with
the energy storage component are closed and then opened periodically to
discharge the
energy storage component. The switches are preferably controlled by a trigger
control
system. The periodical discharges result in application of high-voltage pulses
to the load,
where specially designed treatment chambers are connected, exposing biological
cells
inside treatment chambers to intense electric field(s). A high-voltage pulse
transformer is
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preferably connected between the switches/energy storage component and the
treatment
chambers, to allow the switches to operate at different voltage and current
levels or to
operate with other desired features. Repetitive application of short duration,
intense
electric fields induces stress or mortality in biological cells which results
in the
inactivation of food-borne spoilage and pathogenic micro-organisms.
According to one embodiment of the present invention, a system for generating
high-voltage bipolar pulses to induce stress and mortality in biological cells
is provided.
The system includes, for example, a power source, an energy storage component
in
circuit communication with the power source and for storing energy from the
power
1o source, a plurality of switches for opening and closing periodically to
discharge the
energy storage component, and a load comprising at least one Pulse Electric
Field (PEF)
treatment chamber in which biological cells are subjected to PEF treatment.
The system
may additionally include, for example, a pulse transformer in circuit
communication with
the energy storage component and the load and for allowing a plurality of
voltage and
current levels to be generated at the load. The energy storage component
include, for
example, at least one capacitor such that the energy storage component has a
discharge
time constant that is larger than the time interval between the closing and
opening of the
switches.
According to a second embodiment of the present invention, a system for
generating high-voltage bipolar pulses to induce stress and mortality in
biological cells is
provided with an H-bride switching configuration. The system includes, for
example, a
power source, an energy storage component in circuit communication with the
power
source and for storing energy from the power source, a plurality of switches
for opening
and closing periodically to discharge the energy storage component which are
colifigured
in an H-bridge configuration, and a load comprising at least one Pulse
Electric Field
(PEF) treatment chamber in which biological cells are subjected to PEF
treatment. The
load is preferably in circuit communication with the switches through a bridge
portion of
the H-bridge configuration.
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It is therefore an advantage of the present invention to provide a high-
voltage
pulse generator system for effective PEF treatment that generates bipolar
and/or unipolar
high-voltage pulses.
It is a further advantage of this invention to provide a high-voltage pulse
generator system for effective PEF treatment that reduces or eliminates
protein build-up
on PEF treatment electrodes.
Brief Description of the Drawings
In the accompanying drawings which are incorporated in and constitute a part
of
the specification, embodiments of the invention are illustrated, which,
together with a
lo general description of the invention given above, and the detailed
description given
below, serve to example the principles of this invention.
Figure 1 is the schematic diagram of a first embodiment of a high voltage
bipolar
pulse generator.
Figure 2 is the schematic diagram of a second embodiment of a high voltage
bipolar pulse generator.
Figure 3 is the schematic diagram of a third embodiment of a high voltage
bipolar
pulse generator.
Figure 4 is the schematic diagram of a fourth embodiment of a high voltage
bipolar pulse generator.
Figure 5 is the schematic diagram of a fifth embodiment of a high voltage
bipolar
pulse generator.
Figure 6 is the schematic diagram of a sixth embodiment of a high voltage
bipolar
pulse generator.
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Figure 7 is the output voltage and current waveforms of the generator shown in
Figure 2. Pulse duration of both positive and negative pulses is 3 s, delay
time in
between the two pulses is 12 s, and load resistance is 40052.
Figure 8 is the output voltage and current waveforms of the generator shown in
Figure 3. Pulse duration of both the positive and negative pulses is 4 s,
delay time in
between the two pulses is 12 s, and the load resistance is 30052.
Figure 9 is the output voltage waveform of the generator shown in Figure 6.
Figure 10 illustrates the inactivation of E. coli 0157:H7 and E. coli 8739 in
apple
juice samples by PEF treatment.
Detailed Description of Illustrated Embodiment
PEF treatment systems typically employ high-voltage pulses to induce stress
and
mortality in biological cells. One such PEF treatment system is disclosed in
U.S. Patent
5,690,978 to Yin et al. which is hereby fully incorporated by reference. The
present
invention provides a bipolar high-voltage pulse characteristic that reduces
and/or
eliminates the migration of charge carrying particles in the food product,
such as proteins,
from migrating in the direction of the electric field and depositing on one
electrode--as
occurs in typical unipolar PEF treatment systems. This migration of charge
carrying
particles has been known to cause local field distortions between the
treatment electrodes
and leads to arcing and non-uniform PEF treatment. Additionally, the present
invention
provides a switching circuit which is capable of maintaining high currents
and/or voltages
through small resistive loads, such as PEF treatment chambers.
Figure 1 shows a circuit diagram 100 of a high-voltage bipolar pulse
generator,
including a power source V, an energy storage component 106 having inductors
L11,
L 12, L 13 and capacitors C 11, C 12, and C 13, a set of switches U 11 and U
12, transformer
T11, and a load 104 having one or more PEF treatment chambers. The power
source V
of circuit 100 is a DC power source, which can be any one of the following: an
AC
system with a rectifier and a regulator, commercial DC power supply, capacitor
charging
power supply, resonant charging system or any system that can provide desired
voltage
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and energy levels. Two charging resistors R11 and R12 and a protective diode D
are
optionally shown for the purpose of isolation and protection.
The energy storage component 106 shown in Fig. 1 is a pulse forming network
(PFN). The number of capacitor-inductor combinations (e.g., L11 and C11, L12
and
C12, and L13 and C13), as well as the values of the capacitors and inductors
used are
dependent on specific design characteristics (i.e., pulse duration, amplitude,
etc.) and are
calculated and selected to suit. During the energy storage component 106
charging
period, the capacitors thereof are charged up to the line voltage of the power
source V.
A set of two switches U11 and U12 are closed periodically to discharge the
energy storage component 106. Each time, only one switch is closed and
involved in
discharging the PFN while the other switch remains open. The switches U11 and
U12
are preferably switching devices which are normally open and only close upon
actuation
such as, for example, thyratrons. The PFN controls, through trigger device
102, the
discharge of energy and assists in opening the switches again till energy
stored in PFN is
transferred to a load 104 of PEF treatment chambers and current passing
through the
switch decreases to zero. The switches U l I and U12 are preferably grounded
and,
therefore, the corresponding driving circuits do not need to be floated at
operating
voltage. In all embodiments, the switches U11 and U22 are triggered by trigger
device
102. The trigger device 102 is preferably a signal generator configured to
generate a low-
voltage square wave control signal of particular frequency "f', voltage, and
duty ratio
including time between pulses "dt" and pulse duration "T". However, switches
Ul 1 and
U12 may also be triggered by a computer control system, such as a PEF
treatment master
control system, configured with particular frequency, voltage, and duty
ratios.
Additionally, the waveform characteristics can be manually set or adjusted, or
set or
adjusted in real-time or by computer control based on a feedback system
regulating the
entire PEF treatment process. Additionally, in all embodiments, the load 104
includes at
least one PEF treatment chamber and preferably between 2 to 8 PEF treatment
chambers.
In the embodiment of Figure 1, a high-voltage pulse transformer Ti l and PEF
treatment chambers form the load 104 of the pulse generator. The periodical
discharges
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of the switches U11 and U12 apply pulses to two primary windings of the
transformer
T11. The PEF treatment chambers are connected to the secondary winding of the
high-
voltage pulse transformer T11. The transformer T11 serves as an isolation and
control
device. Therefore, the voltage and current levels experienced by the PEF
treatment
chambers can be different from the levels at the primary side of the
transformer T11. For
example, a low-voltage power source could be used with a step-up transformer
to provide
high-voltage pulses across the PEF treatment chambers, or a high-voltage power
supply
with a step-down transformer can be used to generate a lower-voltage at the
PEF
treatment chambers. Therefore, the current level at the primary side of the
transformer
1 o can be many times smaller than the current passing through the PEF
treatment chambers
connected to the secondary side of the transformer. The terminals of the
transformer Tl 1
are preferably arranged as shown in Fig. 1. When the set of switches Ul 1 and
U12 are
closed alternately, pulses of the same polarity are applied to the primary
windings of the
transformer T l 1 in alternate fashion resulting in bipolar pulses across the
PEF treatment
chambers. When only one of the switches is involved in the discharge, high-
voltage
unipolar pulses, either positive or negative in polarity, are generated across
the PEF
treatment chambers. Different types of waveforms can be generated using the
system
shown in Fig. 1. For example, by changing the energy storage component 106 to
a pure
capacitor bank, exponential decay pulses that are either bipolar and/or
unipolar are
generated and applied to the PEF treatment chambers. However, the preferred
waveform
is a substantially square pulse waveform.
Referring now to Figure 2, a second embodiment 200 of a high-voltage bipolar
pulse generator having a power source V, a resistive element R21, an energy
storage
component 206, a set of two switches U21 and U22, a transformer T21, and a
load 104
having at least one PEF treatment chamber is shown. The power source V is
similar to
the power source used in Fig. 1. A capacitor C 21 is used as the energy
storage
component 206. The power source V charges the energy storage capacitor C21
during a
charging period. The two switches U21 and U22 are periodically closed for a
short
period of time to discharge the energy storage capacitor and then opened. As
described
above, each time only one switch is involved in discharging the capacitor C21
while the
other switch remains open. Switches U21 and U22 are similar to switches U 11
and U 12
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in that they are normally open and are actuated by a control signal. Suitable
switching
devices include, for example, CROSS-TRON, TETRODE, POWER MOSFET, IGBT,
GATE-TURN-OFF SCRs or other switches with switching characteristics that meet
the
above-described requirements.
A high-voltage pulse transformer T21 and PEF treatment chambers form the load
104 of the system. The PEF treatment chambers are connected to the secondary
windings
of transformer T21. The terminals of the transformer's primary windings are
preferably
arranged and connected as shown in Figure 2. When the two switches U21 and U22
alternately discharge capacitor C21, pulses are applied to the two primary
windings of the
1o pulse transformer T21. As described above, high-voltage bipolar pulses are
generated at
the secondary winding of transformer T21 and are applied to the PEF treatment
chambers. Also as described above, when only one of the switches is involved
in the
discharge, high-voltage unipolar pulses, either positive or negative in
polarity, are
generated across the PEF treatment chambers.
To generate square wave pulses, the capacitance of the energy storage
capacitor
C21 is selected so that a discharge time constant of the circuit shown in
Figure. 2 is
comparably larger than the time interval between the closing and the opening
of the
switches U21 and U22. When the switch closing and opening time intervals are
larger
than five times the discharge time constant, exponential decay pulses are
generated and
2o applied to the PEF treatment chambers.
Referring now to Figure 3, a third embodiment 300 of a high-voltage bipolar
pulse generator system is shown. The system has a power source V, resistive
element
R3 1, an energy storage component 306, a set of four switches U3 1, U32, U33,
and U 34,
a transformer T31, and a load 104 having one or more PEF treatment chambers.
The
power source V and energy storage capacitor C31 are similar to the power
source and
energy storage capacitor C21 of Figure 2. The power source V charges the
energy
storage capacitor R31 during the charging periods. The set of four switches
U31, U32,
U33, and U34 are arranged in an H-bridge switch configuration with transformer
T31.
The load 104 of PEF treatment chambers is in circuit communication with the
switches
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through a bridge portion of the H-bridge configuration. The switches U3 1,
U32, U33,
and U34 comprise the same types of switching devices as those already
described in
connection with the embodiments of Figures 1-3.
The four switches operate in pairs, U31 and U34 are a pair and U32 and U33 are
another pair. When a pair of switches is opened or closed, the two switches
that comprise
the pair are opened and closed at the same time. The two pairs of switches are
closed and
open periodically to discharge the energy storage capacitor C3 1. However,
each time
only one pair of switches is closed while the other pair remains open. In this
manner, the
pair of switches in each leg of the H-bridge (i.e., U31-U32 and U33-U34) could
never be
closed at the same time.
A high voltage pulse transformer T31 and PEF treatment chambers form the load
104 of the system. The PEF treatment chambers are connected to the secondary
windings
of the transformer T31. When the pairs of switches (e.g., U31-U34 and U32-U34)
discharge capacitor C31 alternatively, pulses were applied to the primary
winding of the
pulse transformer T3 1. As a result thereof, high-voltage bipolar pulses are
generated at
the secondary winding of the transformer T31 and are applied to the PEF
chambers.
To generate square wave pulses, the capacitance of the energy storage
capacitor
C 1 should be selected such that a discharge time constant of the circuit
shown in Fig. 3 is
comparably larger than the time intervals between the closing and the opening
of the
pairs of switches. When they time intervals are larger than five times the
discharge time
constant, exponential decay pulses are generated and applied to the PEF
treatment
chambers. If only one pair of switches are closed and opened all the time,
high voltage
unipolar pulses, positive or negative in polarity, are generated. As described
above, when
only one pair of switches is involved in the discharging procedure, high-
voltage unipolar
pulses, either positive or negative in polarity, are generated across the PEF
treatment
chambers.
Illustrated in Figure 4 is a fourth embodiment 400 of a high-voltage bipolar
pulse
generator system of the present invention. The system has a power source V,
resistor
R41, an energy storage component 406 having capacitor C41, switches U41, U42,
U43,
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and U44, and a load 104 having at least one PEF treatment chamber. The power
source
V, energy storage capacitor C41 and switches U41-U44 are identical to those
used in the
third embodiment 300 and operate in the same manner. The load 104 and PEF
treatment
chambers are connected between two legs (i.e., U41-U42 and U43-U44) of an H-
bridge
switch configuration, as shown in Figure 4. The load 104 of PEF treatment
chambers is
in circuit communication with the switches through a bridge portion of the H-
bridge
configuration. As described in the embodiment of Figure 3, periodic closing
and opening
of the pairs of switches (e.g., U41-U44 and U42-U43), as triggered by trigger
device 102,
results in the application of bipolar (or unipolar) pulses across the PEF
chambers. In the
lo embodiment of Figure 4, a pulse transformer (shown in embodiment of Figure
3) is not
required if the power source V can provide the desired voltage levels and
switches U41-
U44 can handle the necessary voltage levels and corresponding switching
current levels.
Referring now to Figure 5, a fifth embodiment 500 of the high-voltage bipolar
pulse generator system is illustrated. The system has a power source V, an
energy
storage component having two energy storage capacitors C51 and C52, a set of
switches
U51 and U52, a pair of diodes D51 and D52, a pulse transformer T5 1, and a
load 104
having one or more PEF treatment chambers. The power source V is similar to
the
power source used in Figures 1-4. The power source V charges the energy
storage
capacitors C51 and C52 during alternating charging periods. The two switches
U51 and
U52 are periodically closed for a short period of time and then opened, as
triggered by
trigger device 102, to discharge the energy storage capacitors C51 and C52.
Each time
only one switch (e.g., U51) is involved in discharging its respective energy
storage
capacitor (e.g., C5 1) while the other switch remains open (e.g., U52).
Switches U51 and
U52 are of the type already described in connection with the earlier
embodiments.
As described earlier, to generate square wave pulses the capacitance of the
energy
storage capacitors C51 and C52 should be selected so that a discharge time
constant of
the circuit shown in Figure 5 is comparably larger than the time interval
between the
closing and opening of switches U51 and U52. When the time intervals are
larger than
five times the discharge time constant, exponential decay pulses are generated
and
applied to the PEF treatment chambers. If only one of the two switches is
closed and
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opened all the time, high-voltage unipolar pulses, positive or negative in
polarity, are
generated.
Referring now to Figure 6, a sixth embodiment 600 of the high-voltage bipolar
pulse generator system of the present invention is shown. The system has a
power source
V, isolation transformer T61, diode circuit with four diodes D61, D62, D63,
and D64, a
pulse forming network having a capacitor C61 and two inductors L61 and L62, a
set of
switches U61 and U62, diode D65, pulse transformer T62, resistor R61, and a
load 104
having one or more PEF treatment chambers. The operation of this embodiment is
similar to that of the first embodiment of Figure 1. Briefly, power source V
charges
l o energy storage capacitor C61 through diodes D61-D64 and inductors L61 and
L62.
Trigger device 102 causes switches U61 and/or U62 to periodically discharge
energy
storage capacitor C61 through pulse transformer T62 to the PEF treatment
chambers of
load 104.
Preferably, the present invention is useful in connection with PEF treatment
of
liquid food product and other pumpable substances wherein high voltage bipolar
pulses
are used to induce stress and mortality in microorganisms, biological cells,
spores, or
other particles. However, the present invention can also be used in any
application that
requires a pulse generator which can maintain high current levels through
small resistive
loads. Additionally, the present invention can also be used in any application
that
requires bipolar and/or unipolar high voltage pulses. For example, the high
voltage pulse
generator of the present invention may be particularly useful in welding
applications.
The invention also relates to methods of inducing stress and mortality in
microorganisms,
biological cells, spores, or other particles with PEF's and the novel high
voltage pulse
generator.
In all of the described embodiments, the high-voltage pulse generator system
preferably includes, for example, the following operating characteristics:
1. Operating input voltage: minimum 120 VAC (alternatively, any
standard or nonstandard single or three
phase voltage).
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2. Operating voltage output: 0 to 200 kV and preferably 5 to 150 kV.
3. Peak current: 0 to 50 kA and preferably 10 A to 5 kA.
4. Polarity: preferably bipolar and/or unipolar.
5. Wave shape: preferably substantial square wave.
6. Pulse duration: preferably .5 to 20 microseconds.
7. Output Frequency: preferably 500 Hz to 20 kHz.
Figure 7 is the output voltage and current waveforms of the generator shown in
Figure 2. Pulse duration of both positive and negative pulses is 3 s, delay
time in
between the two pulses is 12 s and the load resistance is 400SZ. Figure 8
illustrates the
1o output voltage and current waveforms of the generator shown in Figure 3.
Pulse duration
of both the positive and negative pulses is 4 s, delay time in between the
two pulses is
12 s, and the load resistance is 30091. Figure 9 is the output voltage and
current
waveforms of the generator shown in Figure 6. Figure 10 illustrates the
inactivation of E.
coli 0157: H7 and E. coli 8739 in apple juice samples by the PEF treatment of
the present
invention.
While the present invention has been illustrated by the description of
embodiments thereof, and while the embodiments have been described in
considerable
detail, it is not the intention of the applicants to restrict or in any way
limit the scope of
invention to such detail. Additional advantages and modifications will readily
appear to
those skilled in the art. For example, the waveforms may sloped or rounded
pulses.
Therefore, the invention, in its broader aspects, is not limited to the
specific details, the
representative apparatus, and illustrative examples shown and described.
Accordingly,
departures may be made from such details without departing from the spirit or
scope of
the applicant's general inventive concept.
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