WO2001084712A1

WO2001084712A1 - Application of voltage pulses to certain types of electrical loads

Info

Publication number: WO2001084712A1
Application number: PCT/US2001/010683
Authority: WO
Inventors: Helmut I. Milde; Sanborn F. Philp
Original assignee: Ion Physics Corporation
Priority date: 2000-05-01
Filing date: 2001-04-03
Publication date: 2001-11-08

Abstract

Two electrodes (Electrode 1,2) are used in the load. Between these electrodes, a pulse voltage is applied via switches (1-4).

Description

APPLICATION OF VOLTAGE PULSES TO CERTAIN TYPES OF

ELECTRICAL LOADS

BACKGROUND OF THE INVENTION

This invention is directed to the application of high voltage pulses of alternating polarity to an electrical load; and is particularly advantageous wherein: (1) The load impedance has a low value - say, 1,000 ohms or less - and (2) the load is electrically "floating;" for example in cases wherein it is inconvenient or impossible to maintain one terminal of the load at ground potential. If the load impedance has a low value, it may be difficult to achieve a rapid risetime for the applied pulse because this risetime is proportional to the ratio of circuit inductance to load impedance. Furthermore, a low impedance application implies a high power, high current application in which the circuit inductance increases as the load impedance decreases; thus compounding the problem of achieving a fast risetime. An example of an advantageous application of this invention would be in the pulsed electric field (PEF) processing of a fluid product, or a fluidizable product. In such an application it is essential that the pulse be very fast - having for example a risetime of the order of one microsecond and duration of approximately 5 microseconds. For higher throughputs in PEF processing, the load impedance is lower and the circuit inductance higher, which makes it progressively more difficult to meet the requirement of a short risetime.

In such low impedance, high power applications it is important to render the inductance as small as possible. This is best accomplished by eliminating the conventional pulse transformer altogether and using only a high voltage pulse forming capacitor and high voltage switches, as is shown in Figure 1. This generator produces square pulses by first charging the two pulse forming capacitors to the voltages to be delivered to the load. Once this is accomplished switch 1 connects capacitor Cj to the load and thus applies a positive pulse to the load. After a time determined by the desired width of the pulse, switch 1 opens and the pulse is terminated. A short period later, switch 2 connects capacitor C₂ to the load for a similar period of time to apply a negative pulse. The disadvantage of this setup is that the individual switches have to withstand twice the pulse voltage during the short period when the other switch is closed. High voltage solid state switches are typically constructed of many series connected switches, which must share equally in the total voltage and must individually be triggered. This requirement means that the cost of high voltage solid state switches increases rapidly as the voltage requirement is increased. Consequently it would be of great benefit, if the voltage per switch could be reduced. One possible circuit that accomplishes this is shown in Figure 2. In this arrangement the two switches in Figure 1 are replaced by 4 switches which each must withstand only a voltage equal to the pulse voltage V_p. A positive pulse is applied when switches 2 and 4 are activated and a negative pulse when switches 1 and 3 are activated. The pulse width is again determined by the duration the switches are in their conducting mode.

The instant invention makes it possible to reduce the voltage of individual switches even further, so that each switch sees only half of the pulse voltage. This requires a modification of not only the pulse generator circuit but also the load configuration. The necessary changes in the load will be illustrated by reference to an application of the instant invention to a PEF processing chamber.

Conventionally, the load is configured in such a way that the high voltage pulse signal is applied to a single high voltage electrode as shown in Figure 3. The general shape of the treatment cell is that of a pipe (usually, but not necessarily of circular cross section). The product fluid enters through a pipe at ground potential and passes out into another pipe at ground potential. The processing chamber - that is, the region or regions where high potential gradients are applied to the fluid - is located primarily in the section surrounded by the insulating pipe, i.e. between the ends of the grounded pipes and the ends of the pipe section at high voltage. The load impedance is primarily determined by the electrical resistance between the high voltage electrode and the grounded pipes.

SUMMARY OF THE INVENTION

The present invention is directed to the application of high voltage pulses of alternating polarity to an electrical load. The innovation in the load is to employ two high voltage electrodes. This is illustrated schematically, in the case of the example of a pulsed electric field processing chamber, in the lower part of Figure 4. Between these two electrodes a pulse voltage of magnitude V_p of either positive or negative polarity, can be applied. If one wished to apply bipolar pulses, the positive pulse would alternate with the negative pulse. The present invention allows for substantial reduction of the voltage of individual switches

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a circuit diagram of a high voltage pulse generator in accordance with the prior art;

Figure 2 is another circuit diagram in accordance with the prior art;

Figure 3(a) is a schematic diagram of a processing chamber arrangement in accordance with the prior art;

Figure 3(b) is another schematic diagram of a processing chamber arrangement in accordance with the prior art; and

Figure 4 is a schematic diagram of a pulse generating circuit and a processing chamber in accordance with the present invention.

DETABLED DESCRIPTION OF THE INVENTION

The upper part of Figure 4 shows an example of an advantageous pulse generating circuit in which each of the high voltage switches requires a voltage rating which is only half the peak voltage of the applied pulse. This electrical circuit is similar to the one shown in Figure 2 with the exception that the electrical ground is located at the center point of the high voltage capacitor(s). With switches 1 and 3 open and 2 and 4 conducting, electrode 1 will receive a negative pulse of ^lA V_p and electrode 2 a positive pulse of amplitude ^λh V_p. Conversely, when switches 2 and 4 are open and switches 1 and 3 conducting, electrode 1 receives a positive pulse and electrode 2 a negative pulse. In either case the voltage between the two high voltage electrodes is V_p. In other words the full voltage is generated while anyone of the individual switches sees only one half of that full voltage. A similar result can be achieved by operating this circuit without center- tapping the capacitor to ground. Such an arrangement would generate the same voltage between the two high voltage electrodes, but the voltage amplitudes between the high voltage electrodes to ground would differ as their impedances to ground differ.

The two outer insulators in Figure 4 can be made approximately half the length of the middle insulator. In this case, all three zones inside the insulators act as processing zones. The disadvantage of this is that the load impedance is thereby lowered to approximately half the load impedance of the arrangement shown in Figure 3(a). This lower impedance of the load makes it more difficult to obtain a fast risetime and also requires a higher current carrying capability of the high voltage switches. The effect on the risetime is partially offset by the lower inductance of a set of switches rated at V_p compared to a set of switches rated at V_p. If the impedance is still too low and one does not wish to select a cell configuration with dielectric inserts, as shown in Figure 3(b), one can make the lengths of the outer insulators much longer (approximately 10 times longer) than the length of the insulator in the middle. In this way the impedance is primarily determined by the impedance between the two high voltage electrodes. In other words, the load presented by the processing chamber - as well as the degree of processing - can be controlled by the outer insulators. This can be done without changing the load presented by treatment zone between electrodes 1 and 2 alone; and without affecting the degree of processing, which occurs, between electrodes 1 and 2.

For most applications, the amplitude of the positive and negative pulse should be of equal amplitude. On the other hand, should one desire to generate pulses of unequal amplitude, this could be achieved by charging the two sides of the high voltage capacitors unequally. By this means the pulse of the positive polarity could be made larger than the pulse of the negative polarity or vice versa. This increase or decrease would show itself only across the outer insulators. The voltage between the two high voltage electrodes - electrodes 1 and 2 - would be unchanged. If a situation existed where only unequal pulses should be used in processing, one could eliminate the effectiveness of the middle zone by making its insulator much longer than the outer insulators so that the ohmic losses in this central zone would be much smaller (say, by a factor 10) than the ohmic losses in the two remaining sections. During processing, the fluid is heated by ohmic losses in the fluid. Consequently, the resistivity of the fluid decreases as it passes through the three processing regions and the current is higher in the third treatment zone compared to the first or second. This imbalance in current could partially be offset by increasing the length of the third treatment zone. This remedy can be used only up the point where the applied voltage becomes inadequate to achieve effective processing.

In actual practice, the processing chambers illustrated in Figures 3 and 4 may be one of a plurality of such chambers connected sequentially, or in parallel, with means of cooling the product fluid after passage of one or more treatment zones.

The application of this invention to the PEF processing of fluids has been discussed as an example of an advantageous application. It will be seen, however, that other applications of the subject invention can be made; and it is not limited to the example that has been presented.

Claims

What is claimed is:

1. Apparatus for the application of positive and negative pulses to a load, said apparatus having a first electrode and a second electrode spaced from said first electrode, comprising: a power supply of positive and negative polarity; pulse forming capacitors comprising a first leg in electrical communication with said positive polarity of said power supply and a second leg in electrical communication with said negative polarity of said power supply, each capacitor being grounded; a first switch in electrical commumcation with said positive polarity and with said first electrode; a second switch in electrical commumcation with said positive polarity and said second electrode; a third switch in electrical communication with said negative polarity and said first electrode; a fourth switch in electrical commumcation with said negative polarity and said second electrode; and pulse generating means for generating said positive and negative pulse.

2. The apparatus of claim 1, wherein said first and second legs of said capacitors are equally charged to generate positive and negative pulses of equal amplitude.

3. The apparatus of claim 1 , wherein said first and second legs of said capacitors are unequally charged to generate positive and negative pulses of unequal amplitude.

4. The apparatus of claim 1, wherein said first and second electrodes are spaced by an insulator.

5. The apparatus of claim 4, wherein said first electrode is insulated by a second insulator opposite said insulator between said first and second electrodes, and wherein said second insulator has about one-half the length of said insulator between said first and second electrodes.

6. The apparatus of claim 4, wherein said first electrode is insulated by a second insulator opposite said insulator between said first and second electrodes, and wherein said second insulator has about ten times the length of said insulator between said first and second electrodes.

7. The apparatus of claim 4, wherein said first electrode is insulated by a second insulator opposite said insulator between said first and second electrodes, said second electrode is insulated by a third insulator opposite said insulator between said first and second electrodes, and said second and third insulators have different lengths.

8. The apparatus of claim 4, wherein said first electrode is insulated by a second insulator opposite said insulator between said first and second electrodes, said second electrode is insulated by a third insulator opposite said insulator between said first and second electrodes, and said second and third insulators have the same lengths.

9. The apparatus of claim 7, wherein said second and third insulators are shorter than said insulator between said first and second electrodes.

10. The apparatus of claim 8, wherein said second and third insulators are shorter than said insulator between said first and second electrodes.

11. The apparatus of claim 9, wherein said second and third insulators are about 10 times shorter than said insulator between said first and second electrodes.

12. The apparatus of claim 1, wherein said load comprises fluid between ground and said first electrode, fluid between said first electrode and said second electrode, and fluid between said second electrode and ground.

13. The apparatus of claim 1, wherein said first, second, third and fourth switches are connected in a bridge circuit.

14. The apparatus of claim 1, wherein each of said capacitors is connected in series and said capacitors are centrally grounded.