Patent Publication Number: US-10314120-B2

Title: System for powering dual magnetrons using a dual power supply

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and is a continuation application of U.S. utility patent application Ser. No. 13/954,480 filed Jul. 30, 2013, which claims the benefit of U.S. provisional patent application No. 61/788,500 filed Mar. 15, 2013, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a system and method for controlling a pair of magnetrons that are powered by dual power supplies operating at a common voltage. 
     BACKGROUND OF THE INVENTION 
     Magnetrons may be used to generate radio frequency (RF) energy. This RF energy may be used for different purposes, such as heating items (i.e., microwave heating), or it may be used to generate a plasma. The plasma, in turn, may be used in many different processes, such as thin film deposition, diamond deposition and semiconductor fabrication processes. The RF energy may also be used to create a plasma inside a quartz envelope that generates UV (or visible) light. Those properties decisive in this regard are the high efficiency achieved in converting d.c. (direct current) power to RF energy and the geometry of the magnetron. One drawback is that the voltage required to produce a given power output varies from magnetron to magnetron. This voltage may be determined predominantly by the internal geometry of the magnetron and the magnetic field strength in the cavity. 
     Some applications may require two magnetrons to provide the required RF energy. In these situations, an individual power source has been required for each magnetron. However, two magnetrons of identical design may not have identical voltage versus current characteristics. Normal manufacturing tolerance and temperature differences between two identical magnetrons may yield different voltage versus current characteristics from unit to unit and are subject to change under dynamic operating conditions of their life cycle. As such, each magnetron may have a slightly different voltage. For example, the magnetrons may have mutually different operating curves such that one magnetron may produce a higher power output than the other magnetron. The magnetron having the higher output power may become hotter than the other, resulting in a shorter useful lifespan than the other. In addition, this may cause the power output of the magnetron producing the higher output to render the plasma in its half of the bulb to become hotter than the other, thereby producing an asymmetrical UV output power pattern. 
     Accordingly, what would be desirable, but has not yet been provided, is a system and method for maintaining a constant voltage and current operating point of a dual power supply for powering dual magnetrons. 
     SUMMARY OF THE INVENTION 
     The above-described problems are addressed and a technical solution is achieved in the art by providing a system and method for powering a dual magnetron with a dual power supply. A first power supply supplies a first voltage to a first magnetron. A second power supply supplies a second voltage to a second magnetron. A balancer circuit controls a drive current for altering a magnetic field of the first magnetron and a magnetic field of the second magnetron to maintain the first voltage and the second voltage at a substantially equal voltage. The first voltage and the second voltage may be a substantially constant voltage. 
     In an embodiment, the first power supply provides a first supply current to the first magnetron and the second power supply provides a second supply current substantially equal to the first supply current to the second magnetron to maintain a substantially common operating point between the first magnetron and the second magnetron. 
     In an embodiment, the system may further comprise a first coil driver electrically coupled to the balancer circuit and magnetically coupled to the first magnetron and a second coil driver electrically coupled to the first coil driver and magnetically coupled to the second magnetron. The first coil driver and the second coil driver may be electrically coupled in series. 
     The first coil driver and the second coil driver may receive the drive current. The drive current energizes the first coil driver and the drive current energizes the second coil driver to adjust the magnetic field of the first magnetron and the magnetic field of the second magnetron in opposite directions, respectively, to maintain the first voltage and the second voltage at the substantially equal voltage. 
     In an embodiment, the balancer circuit may further comprise an auxiliary power supply for supplying the drive current. The balancer circuit may further comprise a processing device in signal communication with the first power supply for sensing the first voltage and in signal communication with the second power supply for sensing the second voltage. The processing device may be a digital signal processor. The processing device may supply an error signal to the auxiliary power supply to adjust the drive current. The error signal supplied to the auxiliary power supply may be based on an output of a proportional-integral-derivative (PID) feedback loop or a proportional-integral (PI) servo-loop implemented by the processing device. 
     In an embodiment, the processing device may sense a difference in magnitude of voltage between the first voltage and the second voltage. 
     In an embodiment, the drive current may have a polarity corresponding to a polarity of the difference in magnitude between the first voltage and the second voltage. A magntiude of the drive current may further be based on an instantaneous voltage difference between the first voltage and the second voltage and a rate of convergence between the first voltage and the second voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more readily understood from the detailed description of an exemplary embodiment presented below considered in conjunction with the attached drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a circuit diagram of one embodiment of a system for powering two magnetrons from a dual power supply; 
         FIG. 2  is a flow diagram illustrating an example of one embodiment of a method of powering a system having a first magnetron and a second magnetron; and 
         FIG. 3  is a graph illustrating a conventional magnetron voltage vs. coil current. 
     
    
    
     It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a circuit diagram of one embodiment of a system  100  for powering two magnetrons from a dual power supply. In particular,  FIG. 1  shows a power supply  10  and a power supply  12 , such as a pair of high-voltage low ripple d.c. power modules. For example, the power supplies  10 ,  12  may each include a solid state high voltage power module capable of 0.84 amp output at 4.5 KV. The power supplies  10 ,  12  may be designed to provide a constant current output (or approximately constant current). Other amounts of current and power are also within the scope of the invention. The power supplies  10 ,  12  may be coupled to corresponding cathodes of magnetrons  14 ,  16  along corresponding high potential lines  18 ,  20 . In an embodiment, corresponding filaments  22 ,  24  of the magnetrons  14 ,  16  may be coupled to corresponding filament transformers  26 ,  28  that provide the necessary current for filament heating. The primaries of the filament transformers  26 ,  28  may be powered from an AC (alternating current) source (such as 100 to 200 volts). The cathode terminal may also be shared with one of the filament terminals. 
     The high potential lines  18 ,  20  provide power supply signal voltages, HVA and HVB, respectively, to be sensed and adjusted according to an embodiment. The power supply output voltages, HVA and HVB, are sensed by corresponding voltage dividers  30 ,  32  that each provide reduced output voltages on signal lines  50 ,  52  proportional to the power supply signal voltages, HVA and HVB. The reduced output voltages on signal lines  50 ,  52  are provided as inputs to a balancer circuit  34 . 
     In one embodiment, the balancer circuit  34  may comprise a processing device  36 . In one embodiment, the processing device  36  may be a digital signal processor. The processing device  36  is coupled to an auxiliary power module  38 . An output signal line  40  is configured to provide a coil drive current, ICOIL, to a pair of series connected coil drivers  42 ,  44  each magnetically coupled to corresponding magnetrons  14 ,  16 . The windings of the coil drivers  42 ,  44  are driven in opposite directions to provide opposing magnetic fields to the magnetrons  14 ,  16  to reduce a difference in the power supply output voltages, HVA and HVB, on the high potential lines  18 ,  20  of the power supplies  10 ,  12  to a substantially equal voltage. In one embodiment, the power supply output voltages, HVA and HVB, may be substantially constant. 
     In  FIG. 1 , the balancer circuit  34  may be utilized to adjust the voltage in the magnetrons  14 ,  16 . More specifically, the balancer circuit  34  is configured to control a coil drive current, ICOIL, supplied to the coil driver  42  associated with the first magnetron  14  and the coil drive current, ICOIL, supplied to the coil driver  44  associated with the second magnetron  16 . The coil drive current, ICOIL, has the effect of altering a magnetic field of the first magnetron  14  and a magnetic field of the second magnetron  16  to maintain the power supply output voltages, HVA and HVB, from the power supplies  10 ,  12  on the high potential lines  18 ,  20  at a substantially equal voltage. 
     In an embodiment, the balancer circuit  34  may be further configured to maintain the signal voltages, HVA and HVB, at a substantially constant voltage. The balancer circuit  34  is further configured to drive the coil drivers  42 ,  44  with a coil drive current, ICOIL, of equal magnitude but opposite polarity when the respective coils of the coil drivers  42 ,  44  are in opposite directions. As a result, a magnetic field of the first magnetron  14  and a magnetic field of the second magnetron  16  tend to oppose each other to drive any difference in voltage between the signal voltages, HVA and HVB, of the power supplies  10 ,  12  on the high potential lines  18 ,  20  to zero. The first power supply  10  is further configured to provide a first supply current, HIA, to the first magnetron  14  and the second power supply  12  is further configured to provide a second supply current, HIB, substantially equal to the first supply current, HIA, to the second magnetron  16  to maintain a substantially common operating point (i.e., of the voltage-current characteristics) between the first magnetron  14  and the second magnetron  16 . 
     The auxiliary power module  38  is configured to supply the coil drive current, ICOIL, under the control of the processing device  36 . The processing device, in turn, is configured to sense an error signal, V_Error, on inputs  46 ,  48  of the balancer circuit  34  to adjust the coil drive current, ICOIL. The error signal, V_Error, is provided on output signal lines  50 ,  52  of voltage dividers  30 ,  32  to sense a difference in magnitude of voltage between the signal voltages, HVA and HVB, of the power supplies  10 ,  12  on the high potential lines  18 ,  20 . The coil drive current, ICOIL, has a polarity corresponding to a polarity of the difference in magnitude between the first voltage, HVA, and the second voltage, HVB. The coil drive current magnitude and polarity are derived from the error signal, V_Error, by the balancer circuit  34  when the processing device  36  is configured to simulate a proportional-integral-derivative (PID) feedback loop or a proportional-integral (PI) servo-loop. 
     The magnitude and polarity of the coil drive current, ICOIL, is based on an instantaneous voltage difference between signal voltages, HVA and HVB, of the power supplies  10 ,  12  on the high potential lines  18 ,  20  and a rate of convergence between the signal voltages, HVA and HVB, of the power supplies  10 ,  12  on the high potential lines  18 ,  20 . 
       FIG. 2  is a flow diagram illustrating an example of one embodiment of a method  200  of powering a system having a first magnetron  14  and a second magnetron  16 . At block  205 , a balancer circuit  34  provides a coil drive current, ICOIL, to a first coil driver  42  magnetically coupled to a first magnetron  14 . At block  210 , the balancer circuit  34  provides the coil drive current, ICOIL, to a second coil driver  44  electrically coupled to the first coil driver  42  and magnetically coupled to the second magnetron  16 . At block  215 , the balancer circuit  34  adjusts the coil drive current, ICOIL, to the first coil driver  42  and the second coil driver  44  for altering a magnetic field of the first magnetron  14  and a magnetic field of the second magnetron  16  to maintain a first voltage, HVA, supplied by a first power supply  10  to the first magnetron  14  and the second voltage, HVB supplied by a second power supply  12  to the second magnetron  16  at a substantially equal voltage. Substantially equal is defined to be a difference in voltage between the first voltage, HVA, and the second voltage, HVB, of about ±10 volts or less. 
     The substantially equal voltage may be a substantially constant voltage. The coil drive current, ICOIL, may energize the first coil driver  42  and the coil drive current, ICOIL, may energizes the second coil driver  44  to adjust the magnetic field of the first magnetron  14  and the magnetic field of the second magnetron  16  in opposite directions, respectively, to maintain the first voltage, HVA, and the second voltage, HVA, at the substantially equal voltage. 
     The coil drive current, ICOIL, supplied to the first magnetron  14  and the second magnetron  16  may be of the same magnitude but of opposite polarity. The first power supply  10  may be further configured to provide a first supply current, HIA, to the first magnetron  14  and the second power supply  12  may be further configured to provide a second supply current, HIB, substantially equal to the first supply current, HIA, to the second magnetron  16  to maintain a substantially common operating point between the first magnetron  14  and the second magnetron  16 . 
     The coil drive current, ICOIL, supplied by the balancer circuit  34  may be adjusted based on an error signal, V_error, based on, for example, sensing a difference in magnitude of voltage between the first voltage supplied, HVA, by the first power supply  10  to the first magnetron  14  and a second voltage supplied, HVB, by the second power supply  12  to the second magnetron  16 . In one embodiment, the balancer circuit  34  may adjust the coil drive current, ICOIL, based on determining an instantaneous voltage difference between the first voltage, HVA, and the second voltage, HVB, and a rate of convergence between the first voltage, HVA, and the second voltage, HVB. 
     More particularly, in one embodiment, software control to operate the processing device  36  of the system  100  of  FIG. 1  may employ a drive subroutine to continuously sample the operating anode voltages applied to the two magnetrons, HVA, and HVB, respectively. The drive subroutine may operate the processing device  36  to furnish an appropriate amount of coil drive current, ICOIL, to the coil drives  42 ,  44  in a specific direction to achieve a balance of the two voltages HVA, and HVB, at substantially all times and under substantially all operating conditions. 
     At system startup, the two magnetron voltages, HVA and HVB, may be sampled. When the two magnetron voltages, HVA and HVB, have reached their respective peak operating level to within less than a peak voltage magnitude of variation over a certain number of the most recent sampling periods (e.g., 100V (volts) of variation in the last 5 sampling periods), then the balancing current routine begins. A difference in monitored magnetron voltage, V_error, is calculated. The output current, ICOIL, of the auxiliary power module  38 , is controlled with a command from the processing device  36  to alter the magnitude of current in a range of from 0 A (amps) to ±3 A. The current amplitude and polarity of auxiliary power module  38 , is adjusted under program control to balance the two voltages, HVA and HVB. The polarity of the current amplitude of the auxiliary power module  38  may be positive for a positive difference in voltages of V_error and vice versa, as monitored. 
     The appropriate amount of current to be supplied to the coil drivers  42 ,  44  at any instant during the balancing process depends on both the instantaneous voltage difference, V_error, and the rate of convergence between the two voltages, HVA and HVB. It should be provided at levels such that the difference diminishes to zero in a controlled manner with the least amount of oscillations. This is accomplished by commonly known feedback control techniques, such as proportional-integral-derivative (PID) feedback or proportional-integral (PI) feedback. In one example, a settling time to 1% differential of the mean of the two voltages should be targeted, e.g., 100 msec. In an example, a balancing accuracy of voltage differential between the two magnetrons  14 ,  16 , should be maintained at 10V or less. 
     The process may be repeated continuously on a real-time basis with updated voltage difference values and corresponding drive currents in an ongoing nested loop to maintain balance between the two magnetrons during the entire period of active operation, including warm-up, stabilization, and dynamic response to operational changes. 
     The approximate amount of the required steady state coil drive current has been established for various voltage differentials between the magnetrons  14 ,  16  and is discussed hereinbelow.  FIG. 3  is a graph illustrating a conventional magnetron voltage vs. coil current. In the graph, the operating anode voltage is approximately 4.45 kV at 840 mA with no coil drive. The magnetron voltage may change with different magnetron current levels. Other magnetrons may operate at somewhat different voltages. 
     The gain of voltage vs. coil current is approximately 100V/A, varying somewhat with manufacturing tolerances of a magnetron. Since the two drive coils  42 ,  44  are driven with the same current but in opposite directions, then, according to  FIG. 3 , a 1 A drive coil current may bring two magnetrons with a 200 V differential of voltage of operation to the same operating voltage. The smaller the differential, the smaller the amount of current that is required to bring the magnetron operating voltage HVA and HVB to within an acceptable tolerance. The magnitude and polarity of the required coil drive current may change from one operating point to another, since operating temperature changes over time or operating parameters vary over time, depending on the current states of the V-I characteristic curves of the two magnetrons  14 ,  16 . 
     Control speed and the transient response of a servo loop subroutine implemented within the processing device  36  may be optimized with empirical testing. A selected initial coil drive current, ICOIL, can be programmed into the processing device  36 . Convergence may be achieved on a real-time basis by means of incremental changes in current amplitude to a final value corresponding to the initial voltage differential between the pair of magnetrons  14 ,  16  in operation when no coil drive current is applied. A lookup table may be employed to select a starting drive coil current for a given voltage difference, V_error. This lookup table may be employed only as a point of reference, because the table may vary to some degree with magnetron operating parameters and their variation over lifetime usage. The final DC current value may be reached when the magnetron voltage difference monitored becomes zero. The amount of incremental change in coil drive current to be programmed may correspond to a given rate of convergence that determines the settling speed and transient response of the coil drive current. This may be optimized empirically. 
     After a certain amount of nominal drive current is determined to bring about a balance between two magnetrons to a certain point of operation, the subroutine may continue to monitor updated voltages and to calculate a new error voltage, V_error. V_error may be employed to further correct the coil drive current for any new changes in V_error that arises with varying operating conditions. For instance, a magnetron voltage differential may reappear as the magnetrons warm up. Another example is when the operating magnetron current level is varied by a user where a magnetron voltage difference may appear. The coil drive current may then be readjusted accordingly to restore balance after the change. 
     In an example, input voltages HVA and HVA for a pair of magnetrons  14 ,  16  may be monitored. The routine waits a period of time until full operating voltages HVA and HVA are established (e.g., within a tolerance of each other) up to a maximum of a time period (e.g., 60 seconds). If the voltages HVA and HVA are still not stable, then the balancing calculation nevertheless is initiated. The difference in voltage between the two magnetrons  14 ,  16 , V_error may then be calculated as:
 
 V _error={ HVA ( V out_ DC , Engine ‘ A ’)}−{ HVB ( V out_ DC , Engine ‘ B ’)}
 
     Approximately for every 200 V of “V_error”, a balancer coil drive current (IOUT_BALANCER) may be set to about +1 A or −1 A (signed value). To adjust V_error downward in absolute value, the following calculations may be performed:
 
Current required to Balance=1000 mA/200 V=5 mA/V
 
 V _error Error Proportion( V _Error_ P )=( V _error/8)
 
Output current New Calculation( I _Out_New)=( V _Error_ P )×5 mA/V
 
     The final output current to write to the processing device  36 :
 
 I OUT_BALANCER=( I _Out_Old)+( I _Out_New)
 
     After each final calculation of “IOUT_BALANCER”, it may be saved as the I_Out_Old for the next calculation. 
     It is to be understood that the exemplary embodiments are merely illustrative of the invention and that many variations of the above-described embodiments may be devised by one skilled in the art without departing from the scope of the invention. It is therefore intended that all such variations be included within the scope of the following claims and their equivalents.