Abstract:
A transformer-less power supply is provided for ozone generation. The power supply advantageously reduces costs and increases reliability of ozone generators. The power supply provides a first AC voltage from a power source to a resonant circuit and the resonant circuit provides a second AC voltage to the ozone generating unit, the second AC voltage being greater than the first AC voltage. A controller for the power supply that adapts to the resonance of the circuit to provide control with a wide tolerance for the high Q circuit component values of the circuit.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/708,445, filed on Aug. 16, 2005, the entire teachings of which are incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     Ozone is useful for numerous applications that require a high level of oxidation. For example, ozone is useful for disinfection of drinking water and has been used for water treatment since the early 1900s. More recently, ozone has been used for semiconductor device processing. One application for ozone in semiconductor device processing is forming insulating layers on semiconductor wafers by growing insulating films or by oxidizing thin films on the wafer. For example, high deposition rate chemical vapor deposition of high quality SiO 2  can be accomplished by using a TEOS/ozone process.  
         [0003]     Another application for ozone in semiconductor device processing is for cleaning semiconductor wafers and the processing chambers of semiconductor processing equipment. Ozone is particularly useful for removing hydrocarbons from the surface of semiconductor wafers or from processing chambers. Using ozone for cleaning is advantageous because it avoids the use of dangerous chemicals which require costly disposal. In contrast, ozone does not present a toxic waste disposal problem because ozone decays to oxygen without residues.  
       SUMMARY  
       [0004]     Ozone can be generated from oxygen according to a so-called “silent discharge principle.” For instance, ozone can be generated by exposing high purity oxygen to an electrical discharge or an electrical flux. The discharge or flux excites the oxygen molecules, breaking them into their atomic state. The atoms then recombine into a mixture of ozone (O 3 ) and oxygen (O 2 ).  
         [0005]     Ozone (O 3 ) is typically produced by passing oxygen through an ozone cell where it is acted upon by an electrical discharge causing the dissolution and recombination of the oxygen atoms into ozone molecules. The electrical discharge or electrical flux needed for ozone generation is produced by applying a high voltage AC power across opposing plates of the ozone cell. The high voltage AC power is produced from transformer-based power oscillators.  
         [0006]     Disadvantages of a transformer-based power supply (an oscillator) typically include high cost, limited reliability, and limited range of operation. For example, the high cost is typically due to the high-voltage transformer with multiple windings and special potting requirements for cooling and insulation. Limited reliability is typically due to the topology of the self-oscillator, high voltage corona caused by the dependence of the potting quality, and use of single source unique parts. Limited range of operation with respect to the regulated output voltage is typically due to the self-oscillator topology and use of transformer feedback for the transistor&#39;s gate drive.  
         [0007]     The present invention is directed to a method and apparatus for supplying power using a power supply including transformer-less high voltage power oscillators for ozone generation. Embodiments of the present invention can reduce cost, increase reliability and operation range of ozone generators.  
         [0008]     One embodiment includes a power supply having a power source and a resonant circuit coupled to the power source, the power source providing a first AC voltage to the resonant circuit, the resonant circuit providing a second AC voltage for use by an ozone generating unit, the second AC voltage being greater than the first AC voltage. The resonant circuit can apply a substantially resonant voltage to the ozone generating unit in response to the first AC voltage having a frequency substantially close to the resonant frequency of the resonant circuit.  
         [0009]     In some embodiments, the resonant circuit can be a series resonant circuit including a resonant inductor coupled in series with a resonant capacitor. The resonant capacitor can be an individual capacitor, a natural capacitance of the ozone generating unit, or a combination of both an individual capacitor and natural capacitance of the ozone generating unit. The resonant circuit has a q-factor greater than or equal to 10. In other embodiments, the resonant circuit can be a parallel resonant circuit including a resonant inductor coupled in parallel with a resonant capacitor. The resonant capacitor can be an individual capacitor, a natural capacitance of the ozone generating unit, or a combination of both an individual capacitor and natural capacitance of the ozone generating unit.  
         [0010]     The power source can be a half bridge inverter, a full bridge inverter, and/or a switching power source. The switching elements can be MOSFETs, BJTs, IGBTs, and/or any other type of switching elements.  
         [0011]     The power supply can further include a controller providing signals to the power source that cause the power source to modulate the first AC voltage, resulting in the second AC voltage having a desired voltage magnitude. The first AC voltage can be modulated using pulse width modulation and/or frequency modulation. The controller can provide signals to the  10  power source that allows the resonant circuit to operate at its maximum operating resonant frequency. The controller can tune to the maximum operating frequency of the resonant circuit by comparing a sensed input DC current to a set point input current. The controller can control a resonant voltage of the ozone generating unit during self-tuning to the maximum operating frequency of the resonant circuit by comparing a sensed resonant current to a set point resonant current.  
         [0012]     Embodiments of the invention also include a power supply for ozone generation. Other embodiments of the invention may be applied for supplying power for generation of any reactive gases.  
         [0013]     Advantages of the embodiments of the invention include reduced cost and increased reliability and operation range of ozone generators by eliminating the need for a transformer.  
         [0014]     Using a high Q resonant circuit (Q≧10 typically for an ozone generator) instead of a transformer implies that the circuit resonant frequency peak is narrow. Since its center frequency depends on circuit elements with tolerances often wider than the resonance peak width, control of such a circuit can be a problem. A circuit to control high Q resonant circuits allows realization of the advantages above in both ozone generators and in resonant power supplies for other applications. 
     
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0015]     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0016]      FIG. 1  is a diagram illustrating a typical ozone generator;  
         [0017]      FIG. 2  is a diagram that illustrates a transformer-based power supply used in an ozone generator according to the prior art;  
         [0018]      FIG. 3  is a diagram illustrating a power supply having a transformer-less power oscillator for ozone generation in a single ozone cell according to one embodiment;  
         [0019]      FIG. 4  is a diagram illustrating a power supply having a transformer-less power oscillator for ozone generation in a single ozone cell according to a particular embodiment;  
         [0020]      FIG. 5A  shows a detailed schematic of one embodiment of a frequency modulation controller;  
         [0021]      FIG. 5B  shows a detailed schematic of one embodiment of a pulse-width modulation controller;  
         [0022]      FIG. 6  shows a graph showing the relationship between set point power and resonant frequency;  
         [0023]      FIG. 7  is a diagram illustrating a power supply having multiple transformer-less power oscillators for ozone generation across multiple ozone cells according to one embodiment; and  
         [0024]      FIGS. 8A and 8B  are diagrams illustrating a power supply having a transformer-less power oscillator for ozone generation in a single ozone cell according to other particular embodiments. 
     
    
     DETAILED DESCRIPTION  
       [0025]      FIG. 1  is a diagram illustrating a typical ozone generator  100 . The ozone generator  100  includes a bank of ozone generating units, referred to herein as ozone cells  110   a  . . .  110   n . Oxygen (O 2 ) is supplied to each ozone cell  110  through an oxygen inlet  120  for conversion into a mixture of ozone (O 3 ) and oxygen (O 2 ). The resulting ozone mixture flows out of the ozone generator  100  through an ozone outlet  130 .  
         [0026]     Components of the ozone cell  110  typically include opposing electrode plates (not shown) and a dielectric barrier (not shown). The dielectric barrier is positioned against one of the electrode plates, forming a channel between the dielectric barrier and the opposing electrode plate. In operation, oxygen (O 2 ) passing through the channel is acted upon by an electrical discharge causing the dissolution and recombination of the oxygen atoms into ozone molecules. To cause the electrical discharge or flux, high voltage AC power is applied across the opposing electrode plates of each ozone cell  110 .  
         [0027]     The high voltage AC power is provided by a bank of power oscillators  140   a  . . .  140   n  with each oscillator  140  supplying power to a respective ozone cell  110 . The power oscillators  140  are coupled to a common DC power supply  150  that can convert single-phase or three-phase AC line voltage  152  into a regulated DC voltage (Vdc). Each oscillator  140 , in turn, converts the regulated DC voltage (Vdc) into high voltage AC power that is supplied to a corresponding/respective ozone cell  110 , resulting in the electrical discharge or electrical flux needed for ozone generation. An exemplary embodiment of the ozone cell  110  can be found in U.S. Pat. No. 5,932,180, the entire contents of which are incorporated herein by reference.  
         [0028]     Generally, the power oscillators  140  are implemented using transformers to generate high voltage AC power.  FIG. 2  is a diagram that illustrates a transformer-based power supply  200  used in an ozone generator according to the prior art. The illustrated power supply  200  consists of a DC power supply  210  and two additional stages: (1) a buck converter  220  for regulation of output power and (2) a self oscillating push-pull converter  230  that includes a transformer  232  to generate the high voltage AC power across the ozone cell  110 .  
         [0029]      FIG. 3  is a diagram illustrating a power supply  300  having a transformer-less power oscillator  310  for ozone generation in a single ozone cell  110  according to one embodiment. The power oscillator  310  includes a power source  320  coupled to a resonant circuit  330 . The resonant circuit  330  is coupled, in turn, to the ozone cell  110 . The power source  320  can be a switching power source.  
         [0030]     In operation, the power source  320  converts a regulated DC voltage (Vdc) from a DC voltage source  210  into a first AC voltage that is supplied to the resonant circuit  330 . Preferably, the first AC voltage from the power source  320  has a frequency substantially close to the resonant frequency of the resonant circuit  330 . In response, the resonant circuit  330  applies a substantially resonant second AC voltage to the ozone cell  110  causing an electrical discharge or flux within the ozone cell  110 . Thus, by coupling the resonant circuit  330  to the power source  320 , the power supply  300  is able to provide high voltage AC power (a second AC voltage) needed for ozone generation in the ozone cell  110  without the use of a transformer.  
         [0031]     With reference to  FIG. 3 , a controller  340  provides control signals to the power source  320  that cause the power source  320  to modulate the frequency and/or duty cycle of the first AC voltage resulting in the resonant circuit  330  providing a substantially second AC resonant voltage having a desired magnitude to the ozone cell  110 . In some embodiments the second resonant AC voltage can be 4.5 kVpk at 30 kHz.  
         [0032]     In operation, the controller  340  compares a reference current REF with a sensed input current at the power source  320  and sends control signals (gate control signals) to the power source  320  to make adjustments to the operating frequency or duty cycle of the power source  320  to obtain the desired magnitude. The first AC voltage can be modulated by the controller  340  using pulse-width modulation and/or frequency modulation. In some embodiments, the controller  340  can be configured to sense voltage, current, or a combination thereof to determine and control the desired resonant voltage.  
         [0033]      FIG. 4  is a diagram illustrating a power supply  400  having a transformer-less power oscillator  404  for ozone generation in a single ozone cell  110  according to a particular embodiment. In the illustrated embodiment, the resonant circuit  420  is a series resonant circuit including a resonant inductor  422  coupled in series with a resonant capacitor  424  The ozone cell  110  is coupled in parallel with the resonant capacitor  424 . The resonant capacitor  424  can be a separate individual capacitor, the natural capacitance of the ozone cell  110 , or a combination thereof. In the illustrated embodiment, the power source  410  is a half bridge inverter including two switching elements  412   a ,  412   b  connected in series. The switching elements  412   a ,  412   b  can be MOSFETs, BJTs, IGBTs and/or any other type switching elements known in the art. The electrical connection between the switching elements  412   a ,  412   b  is connected to the resonant circuit  420 . The power source  410  can also be a full bridge inverter as shown in  FIGS. 8A and 8B .  
         [0034]     In operation, a DC power supply  210  supplies a regulated DC voltage (Vdc) to the power source/half bridge inverter  410 . Control signals from the controller  340  are provided to a gate driver  540  ( FIGS. 5A and 5B ) that causes the switches  412   a ,  412   b  to turn on and off resulting in the half bridge inverter  410  supplying the first AC voltage having a frequency substantially close to the resonant frequency of the series resonant circuit  420 . Particularly, the first AC voltage applied to the resonant circuit  420  can be square wave pulses with a controlled duty cycle. The control signals can also change the duty cycle of the half bridge inverter  410  to alter the magnitude of the second resonant AC voltage applied to the ozone cell  110 . In response to receiving the first AC voltage from the half bridge inverter  410 , the series resonant circuit  420  provides a resonant or substantially second resonant AC voltage across the ozone cell  110  such that an electrical discharge or flux is provided within the cell to effect conversion of oxygen (O 2 ) to ozone (O 3 ). Particularly, the resonant circuit  420  converts the applied square wave pulses with a controlled duty cycle to a high voltage sine wave of controlled amplitude. According to one embodiment, the frequency and magnitude of the second resonant AC voltage is approximately 4.5 kVpk at 30 kHz.  
         [0035]     The ratio of ozone (O 3 ) to oxygen (O 2 ) depends on the amount of power supplied to the ozone cells  110 . The power applied to the ozone cell  110  increases in proportion to the voltage applied to the ozone cell  110  and is regulated by the controller  340  in accordance with the reference signal REF as described above. Thus, by changing the operating frequency or duty cycle of the half bridge inverter  410 , the controller  340  can alter the concentration of ozone. Further, the resonant frequency changes with even a small variation in inductance and capacitance. Thus, the resonant circuit  420  should have a high Q factor (greater than or equal to 10) to eliminate the need for transformer. Therefore, the controller  340  should be independent of the resonant component variation.  
         [0036]      FIGS. 5A and 5B  show a detailed schematic of embodiments of a controller  500 . The major components of the controller  500  include a pulse-width modulated integrated circuit (PWM IC)  510 , a first operational/error amplifier  520 , a second operational/error amplifier  530 , a gate driver circuit  540 , a first resistor  550 , and a second resistor  560 .  
         [0037]      FIG. 5 A  shows one embodiment of a frequency modulated controller  500 ′, In operation, the operational amplifier/error amplifier  520  compares the sensed DC input current  522  with the set point DC current  524 . The resistors  550 ,  560  control the frequency of the PWM IC  510 . The output of the error amplifier  520  controls the current flowing through the resistor  550  by pulling it up or down and thus controls the frequency of the controller  510 . The controller  500 ′ includes an auto tuning circuit that ensures the initial frequency generated by the error amplifier  520  is the maximum operating frequency of the resonant circuit  420  ( FIG. 4 ).  
         [0038]     The tuning circuit includes a resistor  526 , a capacitor  528 , and a small offset voltage at the sensed input of the error amplifier  520 . In operation, when the tuning circuit powers up, the DC current set point  524  slowly increases from zero to its set point through a delay created by the resistor  526  and capacitor  528 . In that time, the offset voltage at the error amplifier  520  ensures that the frequency generated by the error amplifier is the maximum operating frequency of the circuit. The maximum resonant frequency is determined by considering the maximum tolerance on the resonant circuit elements and the capacity of the switching devices.  
         [0039]      FIG. 6  shows a graph showing the relationship between the set point power and the resonant frequency. As shown, as the set point power increases, the pulse-width modulation frequency starts reducing from its maximum value toward maximum power. That is, pulse-width modulation frequency walks over the resonant curve to achieve the maximum power.  
         [0040]     It is important to control the ozone cell  110  voltage because the ozone cell  110  voltage can rise to a very high voltage during auto-tuning of the frequency for maximum power. Thus, the controller  500 ′ includes a second operational amplifier/error amplifier  530 . The error amplifier  530  controls the resonant voltage of the ozone cell  110  by comparing the sensed resonant current  532  to the set point resonant current  534 .  
         [0041]     The resonant current can also be controlled by using pulse-width modulation.  FIG. 5B  shows one embodiment of a pulse-width modulation controller  500 ″. The operation of the pulse-width modulation controller  500 ″ is similar to the operation with respect to the frequency modulated controller  500 ′ as described above.  
         [0042]      FIG. 7  is a diagram illustrating a power supply  600  having multiple transformer-less power oscillators  404   a  . . .  404   n  for ozone generation across multiple ozone cells  110   a  . . .  110   n  according to one embodiment. In the illustrated embodiment, the regulated DC voltage (Vdc) (e.g. approximately 400V) is provided by a known full bridge high frequency converter  610 . The high frequency converter  610  includes a rectifier stage  612 , a full bridge switching stage  614 , a transformer stage  616 , and a filter stage  618 . Other circuits known to those skilled in the art can also be implemented to provide the regulated DC voltage. The power oscillators  404   a  . . .  404   n  are coupled to a corresponding/respective ozone cell  110   a  . . .  110   n  to provide the high voltage AC power. Each oscillator  404  includes a power source  410  coupled to a resonant circuit  420 . In the illustrated embodiment, the power sources  410  are half bridge inverters implemented using MOSFET switching devices  412   a ,  412   b . Other switching devices known to those skilled in the art may also be utilized. Also, mixed implementations of half-bridge oscillators, full-bridge oscillators, and other known devices may be employed. The operation of the illustrated embodiment is similar to the operation described with respect to  FIGS. 1 and 4 .  
         [0043]      FIGS. 8A and 8B  are diagrams illustrating a power supply  700  having a transformerless power oscillator for ozone generation in a single ozone cell  110  according to other particular embodiments. In both embodiments, the power source  710  is implemented as a full bridge converter with four switching elements  712   a ,  712   b ,  712   c ,  712   d  coupled as shown.  
         [0044]     As shown in  FIG. 8A , a voltage supply  210  supplies regulated DC voltage (Vdc) to the full bridge converter  710 . The full bridge converter  710  is coupled to a series resonant circuit  720  having a resonant inductor  722  coupled in series with a resonant capacitor  724 . The resonant circuit  720  is coupled, in turn, to an ozone cell  110  as shown.  
         [0045]     As shown in  FIG. 8B , a current supply  730  supplies a regulated DC current (Idc) to the full bridge converter  710 . The full bridge converter  710  is coupled to a parallel resonant circuit  740  having a resonant inductor  742  coupled in parallel to a resonant capacitor  744 . The resonant circuit  740  is coupled, in turn, to an ozone cell  110  as shown.  
         [0046]     In either embodiment, the resonant capacitor can be a separate individual capacitor or can be the natural capacitance of the ozone cell  110  or combination of both an individual capacitor and natural capacitance of the cell.  
         [0047]     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.