Abstract:
The invention provides a power supply apparatus for supplying electric power to a capacitive load. The apparatus has a transformer, a positive half-period driver and a negative half-period driver supplying positive and negative half-periods of voltage to the first coil. The second coil forms an electric resonance circuit and supplies electric voltage to the load. Zero crossings of the voltage supplied to the first coil are determined from a third coil on the transformer, and alternation between positive and negative half-periods of voltage supplied to the first coil is done at the zero crossings of the voltage supplied to the first coil.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of and claims the benefit and priority to U.S. patent application Ser. No. 12/520,495, filed on Dec. 2, 2009, which is a U.S. National Phase application of PCT International Application Number PCT/EP2007/064053, filed on Dec. 17, 2007, designating the United States of America and published in the English language, which is an International Application of and claims the benefit of priority to U.S. Provisional Application No. 60/876,050, filed on Dec. 20, 2006. The disclosures of the above-referenced applications are hereby expressly incorporated by reference in their entireties 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to power supply apparatuses for supplying electric power to a capacitive load. The invention also relates to a method for operating such power supply apparatuses having a capacitive load such as an ozone generating device coupled to the power supply apparatus. Furthermore the invention relates to a high voltage transformer suitable for use in such power supply apparatuses. 
         [0004]    2. Description of the Related Art 
         [0005]    An example of capacitive loads is an ozone generating device coupled to a power supply apparatus generating an AC voltage to be supplied to the ozone generating device. Such power supply apparatuses have an inductive output impedance, and when the ozone generating device is connected to the output of the power supply apparatus, the inductive output impedance of the power supply apparatus and the capacitive impedance of the ozone generating device form a resonance circuit having a resonance frequency. Such ozone generating devices are driven at frequencies and voltages that are sufficiently high to produce a corona discharge in the ozone generating device. Air containing oxygen (O 2 ) such as atmospheric air or pure oxygen is supplied to the ozone generating device, the corona converts oxygen molecules (O 2 ) in the ozone generating device to ozone (O 3 ), and air with an enhanced content of ozone in comparison to the air supplied to the ozone generating device is supplied from the ozone generating device. The amount of ozone produced by the ozone generating device increases with the voltage supplied to it, and for minimizing losses in the supply apparatus driving the ozone generating device the power supply apparatus should be operated at or near the resonance frequency. In practice, however, for several reasons the resonance frequency may not be constant and may vary over time and as a function of operating parameters including temperature and pressure in the supplied air/oxygen; exchanging the ozone generating device or parts thereof, e.g. for service or maintenance, may change the resonance frequency due to differences or tolerances in capacitance; and the resonance frequency may also change with the voltage at which the ozone generating device is operated since the corona is a non-linear phenomenon. It would therefore be advantageous to have a power supply apparatus which operates at the actual resonance frequency of the resonance circuit and which adapts its frequency of operation to the actual resonance frequency of the resonance circuit. 
         [0006]    Ozone generating devices may be operated at voltage levels in the range of several kV, at frequencies of several kHz and at power levels of several kW. The power supply apparatus may have a high voltage transformer with a high voltage second coil as its output. When designing high voltage and high frequency transformers special considerations should be paid to the design of in particular the high voltage coil to avoid arcing between windings of the high voltage coil and between the windings and other objects near the coils. Arcing itself may damage the high voltage coil and other components, but arcing will create ozone which may have undesired effects on the equipment and the environment. It would therefore be advantageous to have a high voltage transformer with a high voltage coil where arcing between windings of the high voltage coil is reduced or even avoided. 
         [0007]    On a commercial and industrial scale ozone is produced from oxygen, O 2 , in a gas containing oxygen. The oxygen-containing gas can be atmospheric air or oxygen-enriched gas. Methods exist for extracting oxygen from atmospheric air to produce oxygen-enriched gas. Ozone can be produced from oxygen mainly by two methods, one comprising irradiating the oxygen with ultra violet light, the other comprising a corona discharge device. Providing oxygen-enriched gas and producing ozone from oxygen are processes that consume energy and the consumptions of energy and other resources of the two processes are comparable. 
         [0008]    In some applications where ozone is used a predetermined yield of ozone is needed or prescribed, or the required yield of ozone may change. A simple and straightforward way of adjusting the yield is to adjust only the electric power of the ozone-generating apparatus and leaving the flow or supply of oxygen-containing gas constant, or vice versa. This is not optimized for minimizing the consumption of resources comprising oxygen-containing gas and power supplied from the power supply apparatus, and the desired yield may possibly not result or may even be impossible to obtain. 
       OBJECT OF THE INVENTION 
       [0009]    It is an object of the invention to provide a power supply apparatus having an inductive output impedance for supplying electric power to a capacitive load where it is ensured that the resonance circuit formed by the inductive output impedance and the capacitive load impedance is operated at the resonance frequency and. 
         [0010]    It is also an object of the invention to provide a method of operating an ozone generating apparatus in order to minimize the consumption of resources comprising oxygen-containing gas and power supplied from the power supply apparatus. 
         [0011]    It is a further object of the present invention to provide a high voltage transformer with reduced risk of arcing between windings of the high voltage coil and which is suitable for handling voltages in the kV range, frequencies in the kHz range and power levels in the kW range. 
       SUMMARY OF THE INVENTION 
       [0012]    The invention provides a power supply apparatus for supplying electric power to a capacitive load having a capacitive load impedance. The apparatus comprises 
         [0013]    a transformer with a first coil and a second coil, 
         [0014]    a positive half-period driver and a negative half-period driver arranged to alternatingly supply positive half-periods of voltage and negative half-periods of voltage, respectively, to the first coil,
       the second coil is connectable to the capacitive load so as to form an electric resonance circuit having a resonance frequency, and to supply electric voltage to the load, and       
 
         [0016]    a device for determining zero crossings of the voltage supplied to the first coil and for causing alternation between positive and negative half-periods of voltage supplied to the first coil at the zero crossings of the voltage supplied to the first coil, wherein the device for determining zero crossings comprises a third coil on the transformer. 
         [0017]    An effect of this is that alternation between positive and negative half-periods of voltage supplied to the first coil is controlled by the actual resonance frequency of the resonance circuit formed by the second coil of the transformer and the capacitive load. 
         [0018]    Another effect is that electric switching noise from the switching elements is avoided since switching is done at times with no or very low voltage across the switching elements. 
         [0019]    Such a power supply apparatus is useful for supplying electric power to a capacitive load having a capacitive load impedance such as an ozone generating device, and in particular an ozone generating device in which a suitable combination of frequencies and voltages that are sufficiently high to produce a corona discharge in the ozone generating device. 
         [0020]    Other examples of capacitive loads include, without limiting the invention thereto: 
         [0021]    reactors for the destruction or disintegration of substances or gases. Examples of gases that are considered to have a negative effect on the environment if released are Halon 1301 and other gases having fire extinguishing properties, SF 6  and other gases used e.g. for their electrical properties, and gases used in cooling apparatuses; 
         [0022]    piezo-electric transducers used e.g. for generating ultrasound in a medium for cleaning of objects immersed into the medium; 
         [0023]    electro-luminescent devices such as electro-luminescent films for use in LCD screens and in signs; and 
         [0024]    devices for producing light arcs or corona discharges. Such devices are used egg for producing ozone from an oxygen-containing gas. 
         [0025]    The device for determining zero crossings may sense the voltage itself, but in high voltage applications this may not be feasible, and the device may then comprise a separate coil on the transformer. This ensures that the sensed voltage is in phase with the voltages in the coils, whereby it is ensured that alternating between positive and negative half-periods of voltage supplied to the first coil is actually done at the zero crossings of the voltage supplied to the first coil. 
         [0026]    In an embodiment each of the positive and negative half-period drivers is arranged to feed a voltage through an inductive element to the first coil for a duration of no more than one quarter of a period corresponding to a predetermined highest resonance frequency. The inductive element reduces high frequency content of the voltage supplied to the first coil whereby electromagnetic interference is also reduced. 
         [0027]    In an embodiment the duration of the voltage fed through the inductive element is controllable to durations between zero and one quarter of a period corresponding to the predetermined highest resonance frequency. This is useful for controlling and varying the power supplied to the capacitive load. This maximum duration is the first half of a half-period, where voltage builds up, and the second half of the half-period is then used for the voltage to decrease. 
         [0028]    In an embodiment the resonance frequency is higher than the audible frequency range for humans. This ensures that sound caused by alternating between positive and negative half-periods of voltage supplied to the first coil is inaudible. 
         [0029]    In an embodiment the positive and negative half-period drivers each comprises an electronic switching element such as a solid state semiconductor switch or a vacuum tube. 
         [0030]    In an embodiment where an ozone generating device is connected to the second coil of the power supply apparatus to form an ozone-generating apparatus, the apparatus can be operated according to a method comprising controlling the power supplied from the power supply apparatus to the ozone generating device to a predetermined power level; supplying a flow of oxygen-containing gas to the ozone generating device; and controlling the flow of oxygen-containing gas so as to obtain a predetermined concentration of ozone from the ozone generating device. 
         [0031]    In an embodiment the power supply apparatus includes a transformer comprising a core, a low voltage coil on the core, and a high voltage coil on the core, where the high voltage coil has a plurality of insulating carrier substrates stacked in an overlaying arrangement, each carrier substrate carrying an electrically conductive trace with end portions, the trace forming one or more turns around the core, and a connector pad connecting an end portion of the trace on one substrate to an end portion of a trace on an overlaying substrate. 
         [0032]    Traditional transformers have two or more layers with several turns in each layer where an outer layer is wound around an inner layer, and physically adjacent turns in adjacent layers can be separated electrically by several turns. This requires very good insulation between layers in order to avoid arcing between layers. A high voltage transformer according to the invention has the advantage that the maximum voltage between physically adjacent turns of the high voltage coil is limited to the voltage difference between two electrically adjacent turns. This minimizes the risk of arcing between turns, whereby a long lifetime of the coil can be expected. Further, the high voltage coil of such a transformer can have a short length measured along the core, whereby it can be made compact, and it can be manufactured with a high degree of precision compared to coils that are wound from a length of wire. The coil can be manufactured as one unit, and if needed the entire coil can easily be exchanged, and individual substrates carrying one or more turns can also be exchanged. Coils can be composed of as many substrates as needed according to the actual application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set. 
           [0034]      FIG. 1  shows schematically a first embodiment of a power supply apparatus of the invention, 
           [0035]      FIG. 2  illustrates the timing of the first and second half-period drivers in the embodiment in  FIG. 1 , 
           [0036]      FIG. 3  is a cross section through a high voltage transformer used in the embodiment in  FIG. 1 , and 
           [0037]      FIGS. 4 and 5  each shows a substrate carrying an electrically conductive trace for use in the transformer in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0038]    In  FIG. 1  is shown a power supply apparatus  100  with a load  300  having a load impedance with a capacitive component C and possibly also a resistive component. The load  300  is therefore referred to as a capacitive load and is illustrated as a capacitor. The load  300  can be any capacitive load such as an ozone generating device. The power supply apparatus  100  comprises a transformer  110  with a first coil  120  and a second coil  130 . The first coil  120  has a center tap  121 , which is connected to an inductive coil  150  and a switching element  151 . The switching element  151  can be operated under the control of a controller  160  to open and close and thereby establish and disestablish a connection between the inductive coil  150  and a DC supply voltage. Switching elements  170  and  180  at respective ends of the first coil  120  are also operated under the control of the controller  160  to establish and disestablish connections to ground. The switching elements  151 ,  170  and  180  are preferably solid state semiconductor switching elements such as CMOS transistors, SCR&#39;s or other fast switching elements. In some applications it might be considered to use vacuum tube switching elements. The second coil  130  of the transformer  110  has an impedance with an inductive component L and possibly also a resistive component R. Thereby the complex impedance Z is of the form Z=R+jωL. The capacitive load  300  is detachably connected to the second coil  130  of the transformer  110  to form a resonance circuit with a resonance frequency f r  determined by the capacitive component C of the capacitive load and the inductive component L of the second coil  130  of the transformer  110  in accordance with the formula f r =½π√{square root over (LC)}. The transformer  110  also has a third coil  140  connected to the controller  160 . 
         [0039]    In  FIG. 2  is illustrated the operation of the power supply apparatus  100  in  FIG. 1 . The resonance circuit formed by the capacitive load  300  connected to the second coil  130  of the transformer  110  has a resonance frequency with a corresponding period T. In a first half-period the controller  160  controls the switching element  151  and the switching element  170  to close, whereby electric current flows from the DC voltage source through the inductive coil  150  and through the center tap  121  into the upper half of the first coil  120  and through the switching element  170  to ground. The inductive coil  150  and the inductive impedance of the first coil  120  of the transformer  110  have the effect that this current does not rise momentarily but exponentially towards an upper asymptote. After a period t the switching element  151  is controlled to open, and due to the inductive impedance in the circuit including the inductive coil  150  the current in the upper half of the first winding  120  continues but is now drawn through the diode  152  rather than from the DC voltage source. The voltage over the switching element  180  decreases at a rate determined by the resonance frequency. After one half-cycle T/2 of the resonance frequency this voltage has decreased to zero the switching elements  170  and  180  are both controlled to change their state so that switching element  170  is opened and switching element  180  is closed, and the next half-cycle begins. Electric current flows from the DC voltage source through the inductive coil  150  and through the center tap  121  into the lower half of the first winding  120  and through the switching element  180  to ground. After another period t the switching element  151  is controlled to open, and the current in the lower half of the first winding  120  continues but is now again drawn through the diode  152  rather than from the DC voltage source. The voltage over the switching element  170  decreases at a rate determined by the resonance frequency. After another half-cycle, i.e. one full cycle, of the resonance frequency this process is repeated. 
         [0040]    The actual resonance frequency determines the time when the voltage over the open one of the switching elements  170  and  180  is zero, which happens after each half-period, which is when the switching of switching elements  151 ,  170  and  180  is made. This time is determined using the third coil  140  on the transformer. The coil  140  senses a voltage which is in phase with the voltage over the open one of the switching elements  170  and  180 , which in particular means that zero crossings occur simultaneously. The voltages sensed by the third coil  140  is input to the controller  160 , and the controller  160  determines zero crossings of the voltage sensed by the third coil  140 , at which times the switching elements are controlled as described above. 
         [0041]    The period t in which the switching element  151  is closed can be varied, and the switching element  151  may be controlled to open e.g. when the current has reached a predetermined level. Hereby e.g. the average value or the RMS value of the voltage on the first and second coils can be controlled, and hereby the power delivered to the load can be varied. The maximum duration of the period t in which the switching element  151  is closed is determined as no more than one quarter of a period T corresponding to a predetermined highest resonance frequency at which the apparatus is designed to operate. 
         [0042]    In case of disconnection of the capacitive load during operation of the apparatus the resonance frequency will increase, which might cause undesired operating conditions, in particular if the switch  151  were allowed to operate at such increased resonance frequencies. In order to avoid such conditions a maximum repetition frequency has been set for the operation of the switch  151 . This maximum repetition frequency corresponds to the predetermined highest resonance frequency at which the apparatus is designed to operate or slightly higher. 
         [0043]    In case of short circuiting of the terminals of the second coil  130  during operation of the apparatus undesired operating conditions might also arise, in particular high currents in the first and second coils of the transformer. The opening of the switching element  151  when the current has risen to a predetermined level limits the current that can be drawn from the second coil, which is useful in case of short circuiting of the terminals of the second coil  130 . 
         [0044]      FIG. 3  shows an embodiment of a high voltage transformer  500  suitable for use in the embodiment in  FIG. 1 . The transformer  500  has a core  501  composed of two preferably identical E cores  502  and  503  with their middle legs touching each other and thus in magnetic contact with each other. Their outer legs are shorter than the middle legs whereby air gaps are formed in each of the outer legs of the core. A first coil  510  is wound on a bobbin  511  and placed around the middle leg. A second, high voltage coil  520  comprising two half-coils with one half-coil placed on either side of the first coil  510 . 
         [0045]      FIG. 4  shows an embodiment of the individual turns of the high voltage transformer in  FIG. 3 . A flat sheet or substrate  600  of an electrically insulating material with a central opening  601  carries an electrically conductive trace  610  forming a loop around the central opening  601 . At the outer end portion  611  the electrically conductive trace  610  has a connector pad  612  on the same side of the substrate  600  as the conductive trace  610 , and at the inner end portion  613  the electrically conductive trace  610  has a connector pad  614  on the opposite side of the substrate  600  with a through-going connection. The conductive trace  610  can have one or more turns around the central opening  601 . 
         [0046]      FIG. 5  shows another embodiment of the individual turns of the high voltage transformer in  FIG. 3 . A flat sheet or substrate  700  of an electrically insulating material with a central opening  701  carries an electrically conductive trace  710  forming a loop around the central opening  701 . The structure in  FIG. 7  is a minor image of the structure in  FIG. 6 , except that at the outer end portion  711  the electrically conductive trace  710  has a connector pad  712  on the opposite side of the substrate  700  with a through-going connection, and at the inner end portion  713  the electrically conductive trace  710  has a connector pad  714  on the same side of the substrate  700  as the conductive trace  710 . The conductive trace  710  can have one or more turns around the central opening  701 . 
         [0047]    In  FIG. 3  each of the half-coils of the high voltage coil  520  is composed by stacking alternating substrates  600  and  700 . When a substrate  600  is placed on top of a first substrate  700  in an overlaying arrangement, the pad  614  will be just above the pad  714 , and the two pads  614  and  714  can be connected electrically, e.g. by soldering. The thus interconnected traces  610  and  710  on their respective substrates will thereby form two turns or loops around the central openings. A second substrate  700  can then be placed on top of the substrate  600  with the pad  712  just above the pad  612 , and the two pads  612  and  712  can be connected electrically in the same manner to form a coil with three turns. In this way several substrates  600  and  700  can be stacked alternatingly to form a coil with any desired number of turns. The high voltage coil  520  of the transformer  500  comprises two half-coils which each are made like this. In  FIG. 3  the high voltage coil  520  with its thus stacked substrates is seen from the edge of the substrates. 
         [0048]    The distance from the electrically conductive traces  610  and  710  to the edge of the substrate should be large enough to prevent arcing between traces on adjacent substrates. 
         [0049]    As mentioned, in an embodiment the ozone generating apparatus described above will be operated at frequencies above the audible range for humans, e.g. in the frequency range 15-25 kHz. This also has the effect that the size of the transformer core can be reduced in comparison to the size required at lower frequencies. 
         [0050]    For high frequency purposes Litz wire is used for the first coil  510 . Litz wire consists of a number of insulated wire strands which may be twisted or woven together. At high frequencies the electric current will flow in a surface layer of a thickness which decreases with increasing frequency—this is the so-called skin effect. At 20 kHz the skin depth is about 0.5 mm in copper. At the air gaps in the outer legs of the transformer the stray magnetic field may influence the first coil  510 . The use of Litz wire reduces the eddy currents in the first coil  510 . 
         [0051]    For high frequency purposes a laminated transformer core or a ferrite core can be used to reduce or eliminate eddy currents in the core. 
         [0052]    The core  502 ,  503  has air gaps in the outer legs. Such a transformer is particularly useful for to supply loads that exhibit negative resistance, such as corona discharge devices used for ozone production in an apparatus of the invention. At the air gaps there will be a magnetic stray field, and there is a distance from the first coil  510  to the air gaps, and two half-coils of the second coil are kept apart so that the windings are kept out of the stray field. At frequencies higher than the audible frequency range for humans and power levels of several kW as are handled in the apparatus of the invention the magnetic field would dissipate considerable power in all metal parts subjected to the stray field, and it is therefore important to keep the stray field and all metallic components separate. This arrangement ensures that. 
         [0053]    In some applications where ozone is used a predetermined yield of ozone is needed or prescribed, or the required yield of ozone may change. In an embodiment each of the flow of oxygen-containing gas and the power supplied from the power supply apparatus to the ozone generating device is controlled so as to obtain a predetermined yield of ozone from the ozone generating device and so as to minimize the consumption of resources comprising oxygen-containing gas and power supplied from the power supply apparatus. The control can be based on a mathematical model of the apparatus and of the process including theoretical and experimental data and may also include actual measurements of relevant parameters for use e.g. in a feedback control system. 
         [0054]    Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.