Abstract:
Radio-frequency (RF) power supply apparatus for supplying RF power to discharge electrodes of a gas-discharge laser includes a plurality of amplifier modules each having an RF output. A power combining arrangement is provided for combining the amplifier module RF-outputs into a single combined RF-output connected to the discharge electrodes. A DC power supply is connectable to or disconnectable from each of the transistor amplifier modules, separately, to allow current drawn by any of the amplifier modules to be monitored by a single current sensor.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to carbon dioxide (CO 2 ) gas-discharge lasers. The invention relates in particular to CO 2  gas-discharge lasers driven by the combined output of plural RF-transistor power supplies. 
     DISCUSSION OF BACKGROUND ART 
     In a CO 2  gas-discharge laser a lasing gas mixture within a laser housing is energized by a radio-frequency (RF) discharge in the gas mixture struck between a pair of parallel spaced-apart electrodes. In a high power CO 2  laser, for example, a CO 2  slab-laser having an output power of 100 Watts (W) or more, the gas mixture typically includes CO 2 , nitrogen (N 2 ) and helium He, and is at a pressure between about 50 and 150 Torr. RF voltage (RF power) for driving the laser (energizing the gas-discharge) is provided by the combined output of a plurality of RF amplifiers which are connected to a single RF oscillator, the output of which is optionally pre-amplified. Typically, each of the amplifiers includes two RF-transistor amplifier modules in a push-pull arrangement. 
     The RF voltage typically required to excite a gas discharge in a CO 2  slab laser is about 225 volts at about 80 to 100 MHz drive frequency. Current in the discharge for a constant voltage V applied to increases linearly with power delivered into the discharge. The impedance of the discharge decreases as the RF power into the discharge is increased. A CO 2  slab-laser has an efficiency of about 10% for converting RF power into the discharge to laser output power. By way of example, a CO 2  laser having 250 W output requires about 2500 W of RF power at a current of about 11 Amps (A) to be delivered into the discharge. The impedance of the discharge is about 20 Ohms. 
     By way of example, to in order to provide 2500 W of RF power using transistor power modules a minimum of six MOSFET BLF278 modules available from Philips Corporation of Eindhoven, Holland would be required. Outputs of the modules would need to be combined to form a single output that is provided to electrodes generating the laser gas-discharge. 
     A problem that needs to be addressed in combining the outputs of multiple transistor power amplifier modules is current balancing and phase adjustment of the outputs of each of the individual amplifiers. This is required in order to obtain maximum power output into a load (the discharge electrodes) with minimum back reflection. This back reflection exhibits itself as heat dissipated within the transistor modules. 
       FIG. 1  schematically illustrates a prior-art arrangement  10  for current and phase balancing RF power amplifiers in a RF combiner type power supply used in driving a CO 2  diffusion cooled lasers. Here, the output of a RF oscillator  12  is provided to the input of a driver amplifier  14 . The output of the driver amplifier is provided to the input of a 1 to 3 signal splitter (signal divider)  16 . Each of three outputs of the splitter is provided to a corresponding power amplifier stage  18 . The amplifier stages are nominally identical but are separately designated as stages  18 A,  18 B, and  18 C to reflect the fact that there may be subtle differences due to manufacturing tolerance in the amplifier stages and components thereof. These differences lead to a requirement for the above-discussed current and phase balancing. 
     In amplifier stage  18 A, the corresponding signal from splitter  16  is further split into two portions by a signal splitter (signal divider) D 1 . One portion is sent to a transistor amplifier module A 1  and the other portion is sent to a transistor amplifier module A 2 . Amplifier modules A 1  and A 2  are arranged, here, in a push-pull configuration. The amplifier outputs are combined by an impedance matching network Z 1 . In amplifier stage  18 B, the corresponding signal from splitter  16  is further split into two portions by a signal splitter D 2 . The portions are amplified by transistor amplifier modules A 3  and A 4 , and the amplified outputs are combined by a impedance matching network Z 2 . In amplifier stage  18 C, the corresponding signal from splitter  16  is further split into two portions by a signal splitter D 3 . The portions are amplified by transistor amplifier modules A 5  and A 6 , and the amplified outputs are combined by a impedance matching network Z 3 . 
     The outputs of impedance matching networks Z 1 , Z 2 , and Z 3  are combined by a RF Output Power Combiner  20 . The combined outputs are applied to live electrode  24  of an electrode pair  22  (discharge electrodes) comprising electrode  24  and a grounded electrode  26 , spaced apart and parallel to each other. The electrodes are located within a laser housing (not shown) including the lasing gas mixture. An impedance matching network (IMN)  28  matches the output impedance of combiner  20  with the impedance of the discharge electrodes. 
     Transistor amplifier modules A 1 - 6  are powered by DC voltage from a DC power supply  30 . The DC power supply delivers power to each of the transistor amplifier modules A- 6  through one of 6 corresponding current sensors CS 1 - 6  respectively. A preferred current sensor is a Hall-effect sensor. A Hall-effect current sensor produces an output voltage in proportion to the current flowing through it. Such a sensor can handle a wide range of currents from sub-amperes to hundreds of amperes in a package compatible with printed circuit board technology. 
     Current and phase balancing is accomplished by adjusting selectively variable impedance circuits B 1 - 6  connected to a respective input of transistor amplifier modules A 1 - 6 . The circuits here are each in the form of a variable shunt (parallel) capacitance. Adjusting the impedances of circuits B 1 - 6  adjusts the input power and phase of inputs into the transistor amplifier modules A 1 - 6 , which in turn varies the amount of DC current drawn by the transistor amplifier modules from DC power supply  30 . This, in turn again, varies the output power and the phase of the output of transistor amplifier modules, and, correspondingly, varies the amplitude and phase of the RF outputs of impedance matching networks Z 1 - 3 . 
     The impedances of the variable impedance circuits B 1 - 6  are adjusted until the amplitude and phase of each of the inputs to power combiner  20  are equal or nearly equal. The amplitude and phase can be monitored with the aid of an oscilloscope and temporary connections, as in known in the art. The adjustments are necessary to compensate for variations, within manufacturing tolerances, of components of amplifier stages  18 A-C. Having the same current and phase out of the output from the amplifier stages is important for achieving maximum RF power delivery into the gas discharge created by electrode-pair  22 . 
     It should be noted here that only sufficient description of apparatus  10  has been provided for understanding the current and phase balancing of the inputs combined by power combiner  20 . Detailed descriptions of RF power combiners, and impedance matching networks for the same, are provided in U.S. Pat. No. 7,755,452 and U.S. Pat. No. 7,970,037, each assigned to the assignee of the present invention, and the complete disclosure of each of which is hereby incorporated herein by reference. 
     While the above-described current and phase balancing method is perfectly adequate for achieving the desired optimization of power transfer to the discharge electrodes in a finished CO 2  laser, the method has certain shortcomings from a manufacturing point-of-view, particularly regarding the time required for, and the corresponding cost, of the balancing operation. This time required is relatively long because there is a cross-talk between the amplifiers which makes the balancing operation an iterative process, with a series of re-adjustments required of each variable impedance circuit to converge on a balance point. Considerable skill and experience is required of an operator to master the iterative process. Another shortcoming is the time and cost required for installation current sensors in each of connections between the transistor amplifier modules and the DC power supply. There is a need for a simpler and less time-consuming method current and phase balancing for combined amplifier outputs. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, radio-frequency (RF) power supply apparatus for supplying RF power to discharge electrodes of a gas-discharge laser comprises a first plurality of transistor amplifier modules each thereof having an RF output. A power combining arrangement is provided for combining the transistor amplifier module RF outputs into a single combined RF output connected to the discharge electrodes. A DC power supply is provided for supplying a DC voltage to each of the first plurality of transistor amplifier modules. The DC power supply is connectable to or disconnectable from each of the transistor amplifier modules, separately, via a corresponding first plurality of electrical switches. A first current-sensor is located in an electrical path between the DC power supply and the first plurality of electrical switches for monitoring current drawn by any one of the first plurality of transistor amplifier modules when that transistor amplifier module is connected to the DC power supply. 
     In a preferred embodiment of the invention there is a second plurality of transistor amplifier modules, each thereof having an RF output, with the RF outputs of the second plurality of transistor modules combined into the single combined RF output by the power combining arrangement. Each of the second plurality of transistor amplifier modules being connectable to or disconnectable from the DC power supply by a corresponding second plurality of electrical switches. A second current-sensor is located in an electrical path between the DC power supply and the second plurality of electrical switches for monitoring current drawn by any one of the second plurality of transistor amplifier modules when that transistor amplifier module is connected to the DC power supply. 
     The inventive circuit arrangement allows for current and phase balancing in the power combining arrangement with only a pair of the transistor amplifier modules connected to the power supply at any one time, while being monitored by the two current sensors. This avoids the above discussed cross-talk problem between plural pairs of sensors in prior-art apparatus while saving cost by reducing the total number of sensors required to two, regardless of the number of transistor amplifier module pairs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
         FIG. 1  schematically illustrates a prior-art circuit in which the RF output of three amplifier stages is combined, each amplifier stage including two transistor amplifiers in a push-pull configuration the transistor amplifiers being powered by DC current delivered from a single power supply through six corresponding current sensors, with six selectively variable impedance circuits being provided for adjusting inputs to the transistor amplifiers for balancing the amplitude and phase of outputs of the amplifier stages prior to combining. 
         FIG. 2  schematically illustrates one preferred embodiment of a circuit in accordance with the present invention in which the RF output of three amplifier stages is combined, similar to the circuit of  FIG. 1 , but wherein there are only two current sensors arranged to be separately connectable to each of the amplifier stages during the amplitude and phase balancing operation. 
         FIG. 3  schematically illustrates another preferred embodiment of a circuit in accordance with the present invention in which the RF output of three amplifier stages is combined, similar to the circuit of  FIG. 2 , but wherein the amplifier stages each include only one transistor module, there are only three selectively variable impedance circuits, and there is only one current sensor connected to a DC power supply and arranged to be separately connectable to each of the amplifier stages during the amplitude and phase balancing operation. 
         FIG. 4  schematically illustrates yet another preferred embodiment of a circuit in accordance with the present invention in which the RF output of four (two pairs) of amplifier stages is combined, similar to the embodiment of  FIG. 3 , but wherein there are two (a pair of) current sensors connected to the DC power supply, with each pair separately connectable to an amplifier-stage pair during the amplitude and phase balancing operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Continuing with reference to the drawings, wherein like features are designated by like reference numerals,  FIG. 2  schematically illustrates one preferred embodiment  40  of a circuit in accordance with the present invention in which the RF output of three amplifier stages is combined. Circuit  40  is similar to circuit  10  of  FIG. 1  with exceptions as follows. In circuit  40  there are only two current sensors CS 1  and CS 2  in two connections  42  and  44  to DC power supply  20 . Suitable current sensors are Hall-effect current sensors, such as model number AE5758 manufactured by Allegro MicroSystems Inc., of Irvine, Calif. Connections  42  and  44  are connectable, via switches S 1 , S 2 , S 3 , S 4 , S 5  and S 6 , to all of the amplifier stages  18 A-C. One of the connections is made (via the appropriate switch) to one of the transistor amplifier modules in the amplifier stage and the other connection (via the appropriate switch) to the other of the transistor amplifier modules in the amplifier stage. 
     In actual operation by a user, all of switches S 1 - 6  are closed, such that all three amplifier stages are powered. In a current and phase balancing operation, only one amplifier can be connected to the power supply at a time. In  FIG. 2 , amplifier stage  18 C is depicted as being connected to the power supply with switches S 5  and  6  closed, while amplifier stages  18 A and  18 B are disconnected from the power supply by opening switches S 1  and S 2 , and switches S 3  and S 4 . As each of the amplifier stages include two amplifier modules in a push-pull configuration, both switches associated with an amplifier stage must be closed in order for the amplifier stage to operate. 
     As in the circuit of  FIG. 1 , selectively variable impedance circuits B 1 - 6 , here in the form of a shunt capacitor, are provided for adjusting inputs to transistor amplifier modules A 1 - 6  respectively for the balancing inputs to power combiner. For the Philips NXP BLF278 (MOSFET) amplifier modules referenced above, the shunt capacitors preferably have a value between about 3 and 15 picofarads (pF). The NXP BLF278 MOSFET is rated up to 400 W of continuous output power at 100 MHz. This transistor module requires 48 volts DC from power supply  30 . Using this MOSFET, for all amplifier modules A 1 - 6 , the output of power combiner  20  could be up to 2400 W, which is sufficient RF power to drive a laser having 240 W output. 
     Connections to amplifier stage  18 A-C are depicted and described as being makeable or breakable by mechanically operated switches S 1 -S 6  for clarity of description. It has been found in practice, however, to be more convenient and less costly to substitute removable links or fuses for the switching operation, with a fuse (link) being removed to open a connection and replaced to close a connection. The term “switch”, as used in this description and the appended claims, is intended to include such removable links or fuses. For the power levels exemplified above, these removable links can be typically low cost and low height-profile mini-fuses, such as LITTELFUSE, part number 0891, with a low-profile fuse-socket, such as model 3557-2 both available from Keystone Electronic Corporation, of Astoria, N.Y. 
     In one example of a current and balancing operation in the circuit of  FIG. 2 , a computer simulation of the circuit, using a commercially available circuit analysis software, such as PSPICE available from Cadence Design Systems Inc, of San Jose, Calif., is conducted to establish nominal values (based on nominal component specifications for amplifier stages  18 A-C), for the variable impedance circuits B 1 - 6 . Then, switches S 3 - 6  are opened leaving S 1  and S 2  closed, such that only amplifier stage  18 A is connected to DC power supply  30 . In an iterative, or trial and error, manner, the impedance (reactance) values of circuits B 1  and B 2  are varied until equal currents (measured by current sensors CS 1  and CS 2 ) are being drawn by transistor amplifier modules A 1  and A 2 . The phase of the output RF signals from A 1  and A 2  is also observed. If the outputs are not in phase, the phases are equalized by re-adjusting the reactance values of circuits B 1  and B 2  around the nominal values calculated by the software simulation. This will usually disturb the previously established current-balance. Fine adjustment of the reactance of circuits B 1  and B 2  is continued iteratively until an optimum balance of current and phase has been established. 
     Next, switches S 1  and S 2  are opened to disconnect amplifier stage  18 A from the power supply, and switches S 3  and S 4  are closed to connect amplifier stage  18 B to the DC power supply (via the current sensors). Then the reactance of circuits B 3  and B 4  is adjusted to balance the currents being drawn by amplifier modules A 3  and A 4  and the phase of the amplifier module outputs as performed for amplifier stage  18 A. The currents of amplifier  18  A and amplifier  18 B are compared. The currents should be equal such that the RF power fed into the output power combiner by amplifier stage  18 B matches that from stage  18 A. If the currents are not equal, iterative adjustments of the reactance of circuits B 1  and B 2 , and B 3  and B 4  (with only one amplifier stage at a time connected to the power supply) until an acceptable phase and current balance has been established for the four amplifier currents and the amplitude and phase of inputs to power combiner  20 . 
     Once this acceptable balance has been achieved, switches S 1 , S 2 , S 3 , and S 4  are opened to disconnect amplifier stages  18 A and  18 B from the DC power supply and switches S 5  and S 6  are closed to connect amplifier stage  18 C to the DC power supply. An iterative process as described above is carried out, with readjustment if necessary to circuits B 1 , and B 2 , and B 3  and B 4 , until an acceptable balance has been established for currents drawn by the amplifier stages, and for the phases of inputs to power combiner  20 . 
     Once the acceptable balance has been achieved with each of the amplifier stages  18 A-C adjusted individually, all of switches S 1 - 6  are closed to connect all three amplifier stages to the DC power supply. The RF power output and phase from stages  18 A-C is measured at the inputs to the RF power combiner. If the powers and phases are equal within a specified tolerance, then current and phase balancing is completed. If either current or phase balancing falls without the specified tolerances, the balancing can be further adjusted as described above. Further fine-tuning could be achieved by adjusting component values in one or more of impedance matching circuits Z 1 - 3 . As laborious a task as this may seem from the description provided above, it has been found to be significantly less time-consuming than the current and phase balancing process in the prior-art apparatus of  FIG. 1 , wherein cross-talk between the amplifier stages complicates the iterative balancing procedure. 
     It should be noted here that while variable impedance (variable reactance) circuits B 1 - 6  are each depicted as a shunt-connected (parallel-connected) selectively variable capacitor, other forms of selectively variable reactance circuit may be used without departing from the spirit and scope of the present invention. These include a series-connected selectively variable capacitor of more complex circuits including capacitive or inductive elements. 
       FIG. 3  schematically illustrates another preferred embodiment  50  of a circuit in accordance with the present invention, in which the RF output of three amplifier stages  19 A,  19 B, and  19 C is combined. Circuit  50  is similar to circuit  40  of  FIG. 2 , but in circuit  50 , the amplifier stages each include only one transistor amplifier module, with modules designated as modules A 1 , A 2 , and A 3 , respectively. There are only three selectively variable impedance circuits, designated B 1 , B 2 , and B 3  respectively, and there is only one current sensor CS 1  arranged to be separately connectable via switches S 1 , S 2 , and S 3  respectively to each of the amplifier stages during a current and phase balancing operation. 
     In one example of a current and balancing operation in the circuit of  FIG. 3 , a computer simulation of the circuit is conducted to establish nominal values for the variable impedance circuits B 1 - 3 , as discussed above with reference to operation of the circuit of  FIG. 2 . Switch S 1  is then closed and switches S 2  and S 3  are opened. The current drawn from the DC power supply by transistor amplifier module A 1  is measured by the current sensor CS 1 . Shunt reactance B 1  is adjusted to operate amplifier A 1  in the desired output power range, which is directly proportional to the DC current drawn by the amplifier. 
     Next the phase at the output of transistor amplifier A 1  is measured and compared with the phase of the input signal at a test point T located between the output of driver amplifier  14  and the input to signal splitter  16 . This phase comparison can be made, for example, by contacting one lead of a dual trace oscilloscope at test point T and a test point T 1  located ahead of output power combiner  20 . The phase shift imposed by the amplifier, if any, can be seen by a displacement of two waveforms displayed on the oscilloscope screen. Points T 2  and T 3  are provided for making similar phase comparisons with amplifiers A 2  and A 3 , respectively. 
     The above procedure is repeated for the other two amplifiers (separately), adjusting selectively variable capacitors B 2  and B 3  such that the current drawn by each amplifier A 1 , A 2 . and A 3  is the same, and phase-shifts indicated by measurement, between test point T and test points T 1 , T 2 , and T 3  are the same. This completes the current and phase balancing procedure. 
     The general arrangement of  FIG. 3  can be extended to include more than three amplifier modules. However, when more than three amplifier stages are needed to provide a desired power into the discharge, an even number of amplifier stages is preferred, arranged as depicted in  FIG. 4 . Here, a circuit  60  has four amplifier stages  19 A,  19 B,  19 C, and  19 D, including transistor amplifier modules A 1 , A 2 , A 3 , and A 4  respectively. DC power supply  30  powers transistor amplifier modules A 1  and A 3  through current sensor CS 1 . The power supply powers transistor modules A 2  and A 4  through current sensor CS 2 . 
     This arrangement is similar to the arrangement of  FIG. 2 , with an exception that the transistor amplifier modules are not arranged in a push-pull configuration. A preferred current and phase balancing procedure is essentially that described above with reference to  FIG. 2 . Amplifier stages  19 A and  19 B can be balanced first, with switches S 1  and S 2  closed, and switches S 3  and S 4  open. Then amplifier stages  19 C and  19 D can be balanced, with switches S 1  and S 2  open, and switches S 3  and S 4  closed. 
     The present invention is described above with reference to embodiments including an exemplary number of amplifiers and to preferred circuit components. Those skilled in the art will recognize that a greater number of amplifiers may be combined, or different components used, without departing from the spirit and scope of the present invention as defined by the claims appended hereto.