Abstract:
An RF impedance-matching transformer for matching the output impedance of an RF amplifier to the discharge of a gas-discharge laser includes upper and lower dielectric plates arranged face-to-face and bonded together. A primary U-shaped strip winding is embedded in the bonded surface of one of the dielectric plates. A secondary strip-winding is formed on an exposed surface of the upper dielectric plates. A ground-plane electrode formed on an exposed surface of the lower dielectric plate. An electrical connector connects one end of the secondary strip-winding to the ground-plane electrode via a via-hole extending through the dielectric plates. The other end of the secondary strip-winding can be connected to the laser.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims priority from U.S. Provisional Application No. 60/810,538, filed on Jun. 2, 2006, by Robotham et al., titled “Transformer for Impedance-Matching Power Output of RF Amplifier to Gas-Laser Discharge.” U.S. Provisional Application No. 60/810,538 is hereby incorporated herein by reference in its entirety. 
     
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to radio-frequency (RF) gas-discharge lasers and, in particular, to a transformer for matching the output of an RF amplifier to the gas discharge in such a laser. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    The cost of solid-state RF power supplies used for CO 2  lasers is approximately equal to the cost of the laser head. It is well known to those skilled in the art that, as the RF laser excitation frequency of the power supply is increased, the amount of RF power that can be coupled into the laser&#39;s gas discharge can be increased to a higher value without developing arcs within the discharge volume. Arcs within the discharge are detrimental to CO 2  laser performance. It is also well known that, as the RF frequency is increased, the discharge can be operated at higher pressures while still maintaining a uniform discharge. Both of these higher frequency benefits enable higher laser output power to be obtained from a given-size laser head. 
         [0004]    Unfortunately, as the RF frequency of the solid-state power supply is increased, the design, assembly and cost of the power supply increases. Laser manufacturers are forced to make compromises between benefits in decreased laser size for a given laser power output, versus the disadvantages associated with design, assembly and higher cost associated with the use of higher-frequency solid-state power supplies. 
         [0005]    Typically, frequencies in the VHF band (e.g. 30 to 300 MHz) are utilized in sealed-off, RF excited, diffusion cooled CO 2  lasers, with the frequencies between 30 MHz and 100 MHz being most common. 
         [0006]    RF power supplies for the above-discussed lasers usually include a master oscillator and at least one stage of amplification. One of the more challenging tasks in designing RF power supplies for CO 2  lasers is the matching between the final RF amplifier and the laser discharge. The final RF amplifier may require impedances as low as 5-10 Ohms, while the transmission line carrying RF power to the discharge has an impedance that is typically 50 Ohms. The laser discharge impedance is in the order of 50 Ohms after ignition for laser power of 100 W. The power drops for higher power CO 2  lasers. This match must be efficient such that minimum RF power is lost within the impedance transformation. In addition, the impedance of the un-lit discharge is much higher than for the lit discharge. Consequently, there is a large mismatch prior to the discharge being lit. Additionally, the match must be able to withstand the possible high voltages generated during the process of igniting the discharge. Since to light the discharge requires a higher voltage than to keep it running after ignition, the ignition is usually performed with a high voltage pulse or a series of fast pulses. Transmission line transformers are inherently broadband so that they can deliver the high voltage “spikes” and they are also very efficient under CW operation. Consequently, they are presently the preferred choice for this application. As discussed in detail below, the present invention provides a transformer design that maintains these characteristics and provides additional benefits. 
         [0007]    Another challenging task is matching the relatively high output impedance (i.e. typically 50 ohms) of the electronic oscillator circuitry feeding into the relatively low input impedance (i.e. typically several ohms) of the input to the first stage of the RF power amplifier chain. The present invention can also be used to address this challenge. 
         [0008]    The most mature RF impedance matching transformer technology is the use of wire wound on ferrite cores. This technology dates back to the middle 1950&#39;s and is commonly used at lower RF frequencies (i.e. below 80 MHz) as ferrite transformers tend to be lossey at higher RF frequencies (i.e. above 80 MHz). At high RF power levels (say, above 300 W) and for frequencies above 80 MHz, the loss within the ferrite creates a thermal problem and, therefore, adds further design complexity and cost to the RF supply. Many users of CO 2  lasers having up to approximately 100 W of output power usually desire to have the laser&#39;s RF power supply mounted directly on the laser head. The totally self-contained laser and power supply allows the user to avoid dealing with a co-axial cable connecting the laser head to a remotely located RF power supply. This desire is especially strong in applications that require the laser to be mounted on a robotic arm. 
         [0009]    Cooling the ferrite transformers within RF power supplies mounted directly on laser heads is especially difficult when air cooling is desired. In addition to the loss at higher RF frequencies, ferrite transformers tend to have larger height, width and depth dimensions than other components on the printed circuit board (PCB). 
         [0010]    The electrical characteristics of ferrite transformers vary from unit to unit so as to require special sorting before being used in a PCB assembly. The sorting results in special tuning steps required during power supply assembly. The sorting, assembly tuning and thermal management raise the final cost of the laser and are major disadvantages of this technology. 
         [0011]      FIG. 1A  schematically shows a co-axial cable  100  that includes an outer conductor  102  and an inner conductor  104  with a dielectric  106  separating the two conductors.  FIG. 1B  is a schematic illustration of a 1-to-4 step-up co-axial transformer frequently used in RF power supplies to drive sealed-off, diffusion cooled CO 2  laser discharges. Such transformers are presently used in commercially available 100 MHz power supplies driving 20 W to 100 W CO 2  wave-guide laser discharges.  FIG. 1C  shows a schematic of the physical electrical connections between two co-axial cables of equal length L to form the 1-to-4 step-up transformers shown in  FIG. 1B . 
         [0012]    The advantages of the co-axial transformers over the ferrite transformer approach are lower cost, lower RF losses and the capability of higher frequency operation. Unfortunately, the co-axial transformer technology shares some of the same disadvantages associated with ferrite transformer technology. These disadvantages are: the need to mount, restrain, and connect the transformer onto the PCB; the completed transformer has a relatively large height dimension when compared to the other components on the PCB; and its electrical characteristics are strongly related to position and manner of connection to the PCB. The last issue is the one of most concern. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention enables the use of a combination of buried micro-strip and coupled micro-strip technology to achieve high RF frequency step-up or step-down transformers. The disclosed technology overcomes most of the disadvantages of the well-established ferrite and semi-rigid co-axial transmission line transformers technology with reduced over-all cost, increased repeatability, and increased reliability. 
         [0014]    A transformer in accordance with one embodiment of the present invention comprises first and second dielectric plates each having an upper surface and a lower surface. The lower surface of the first dielectric plate is non-conductively bonded to the upper surface of the second dielectric plate. A primary transformer winding in the form of an electrically conductive strip is embedded in the upper surface of the second dielectric plate. A secondary transformer winding in the form of an electrically conductive strip is formed on the upper surface of the first dielectric plate. A ground plane electrode is formed on the lower surface of the second dielectric plate. An electrical connector connects the secondary transformer winding to the ground plane electrode via a via-hole extending through the first and second dielectric plates. 
         [0015]    In one example of the inventive transformer for providing an about one-to-two impedance matching ratio (e.g., a step-up ratio), the primary transformer winding has one, generally U-shaped turn, and has a first strip-width. The secondary transformer winding has about two, coplanar turns, and has a second strip-width that is less than one-half of the first strip-width. The primary and secondary windings are arranged face-to-face with the secondary transformer winding overlapping the primary transformer winding. The terms “about one turn” and “about two turns” as used above mean that the number of turns need not be complete turns or that the strip length may be more than needed to form the stated number of turns. In this example, a higher step-up ratio may be achieved using more than about two turns cooperative with the U-shaped primary winding. By way of example, a secondary winding having about two secondary turns may be used to achieve a step-up ratio of about one-to-four. The strip width of the secondary winding in this case would be less than one-quarter of the primary strip-width. 
         [0016]    The features and advantages of the various aspects of the present invention will be more fully understood and appreciated upon consideration of the following detailed description of the invention and the accompanying drawings, which set forth illustrative embodiments in which the concepts of the invention are utilized. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1A  shows a conventional co-axial transmission line cable. 
           [0018]      FIG. 1B  is a schematic drawing illustrating an impedance transformer for use with a co-axial cable transmission line. 
           [0019]      FIG. 1C  is a schematic drawing illustrating the physical electrical connections for 1 1-to-4 steep-up co-axial transformer. 
           [0020]      FIGS. 2 and 2A  are cross section drawings illustrating an embodiment of an impedance matching transformer in accordance with the present invention. 
           [0021]      FIG. 3  is a top view drawing illustrating the FIGS.  2 / 2 A transformer. 
           [0022]      FIG. 4  provides a 3-dimensional illustration of the transformer of FIGS.  2 / 2 A and  3 . 
           [0023]      FIG. 5  is a schematic drawing illustrating a co-axial cable transmission line equivalent circuit for the 1-to-4 step-up transformer shown in FIGS,  2 ,  2 A,  3  and  4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIGS. 2 and 2A  schematically illustrate in cross-section an embodiment of a buried micro-strip/coupled micro-strip impedance matching transformer  200  in accordance with the present invention for driving a CO 2  laser discharge. The transformer  200  includes an electrical conductor  202 , such as copper, deposited on the bottom of a high thermal conductivity, low RF loss dielectric, printed circuit board (PCB) material  204 , such as, for example, R04350B manufactured by Rogers Corporation. The copper  202  serves as an electrical ground plane and for making good thermal contact to a chill plate (not shown) to conduct heat away from the transformer  200 . The dielectric material  204  upon which the metal electrical ground plane  202  is deposited has a thickness “A”. 
         [0025]    A second dielectric layer  206  of thickness “B” is copper plated on both sides and etched on both sides to obtain patterns of thin metal layers that form both the primary winding  208  and the secondary winding  210  of the transformer  200 . This second dielectric layer  206  is thin so as to provide good inductive coupling between the primary  208  and secondary  210  tracks. The primary  208  is formed in the shape of a wide horse-shoe or U-shape, while the secondary  210  overlays the primary  208  on the opposite side of the dielectric  206  and consists of the desired number of overlapping traces. The number of secondary traces determines the turns ratio and, therefore, the impedance transformation. The actual number of secondary turns is only limited by the width of the primary  208  such that suitable coupling may be established between the two metal patterns separated by the thin dielectric  206  of thickness B. This dielectric layer  206 , with the finished metal traces forming the primary circuit  208  and the secondary circuit  210  is then laminated onto the first dielectric layer  204  of thickness A. Rogers Corporation supplies the required printed circuit dielectrics, RO4350B and RO4450, and the adhesives used to laminate these two dielectric layers together. The Rogers material is used as an example in this disclosure, but those skilled in the art will appreciate that other high performance dielectric manufacturers material&#39;s may be substituted. 
         [0026]    As shown in  FIG. 2 , the lamination of the two dielectric layers  204 ,  206  results in a PCB structure of height “C” including the thickness of the interlayer binding material  212 . At the back end of the primary pattern, a connector is provided for connecting a dc power supply voltage, normally 48 volts dc. 
         [0027]    The “A” and “B” dimensions of the dielectric  204  an  206 , respectively, and the configurations of the primary  208  and secondary  210 , such as length, width and thickness, are determined with the use of RF circuit design software tools to obtain the desired impedances and the overall impedance transformation for a desired application. As shown in  FIGS. 2 and 2A , a via hole  214  from one end of the secondary coil  210  down to the ground plane  202  is provided to obtain an electrical connection between the two electrical conductors. The other end  210   a  of the secondary coil  210  serves as the RF output  216  to drive the CO 2  laser discharge. 
         [0028]      FIG. 3  provides a top down view of the 1-to-4 step-up, high frequency, RF impedance matching transformer  200  shown in cross-section in  FIGS. 2 and 2A . The secondary  210  (shown in white), a 2-turn coil in this case, is clearly seen with the output end  210   a  of the coil  210  (i.e. to the right in  FIG. 3 ) serving as the output connector feeding the CO 2  laser&#39;s discharge. The opposite end  210   b  of the 2-turn coil secondary  210  is located near the center of  FIG. 3  (with legend “Via to Ground”) and is connected to the ground plane  202  by a via-hole connector, as discussed above and shown in  FIGS. 2 and 2A . Underneath the secondary coil  210  is the top surface of the high thermal conductivity, low RF loss upper dielectric  206  of thickness B through which the via hole  214  is formed. The via-hole  214  also goes through the lower dielectric of thickness A. 
         [0029]    The wide U-shape, single turn, primary  208  that is deposited on top of the lower dielectric  204  of thickness A is also shown in  FIG. 3  (cross-hatched). The tab  208   a  protruding from the primary  208  at the top of  FIG. 3  is the connector to the DC power supply for the power transistors. This voltage is normally 48V dc. At the bottom of  FIG. 3  are the two contacts  208   b ,  208   c  for connecting the transformer primary to the drains/collectors of two RF power transistors (not shown), which may be operated in a push-pull configuration and in class-C operation for maximum efficiency, as is well know to those skilled in the art. Those skilled in the art will also appreciate that other circuit topologies and class of RF power amplifiers (PA) operation may be used depending on specific design requirements. Two additional tabs  208   d  and  208   e  are shown near the drain/collector connections for connection to a reactive element (not shown) for optional fine tuning. It is noted that the drawing of  FIG. 3  is approximately 2× scale (i.e. approximately 2¼″ long by 1¼″ wide) and was designed for a 100 MHz, 1-to-4 step-up transformer  200  delivering approximately 350 W of CW output power into a discharge. This power is sufficient to drive approximately a 35 W CO 2  laser. The use of this approach with four transistors to double the delivered power would require two of these transformers, etc. 
         [0030]      FIG. 4  presents a 3-D illustration of the 1-to-4 step-up buried micro-strip/coupled micro-strip line transformer  200  of  FIGS. 2 ,  2 A and  3 . For a 4-to-1 step-down version for impedance matching, the high output-impedance of digital semiconductor circuitry to the low input-impedance of a typical power transistors, the connections are reversed. In other words, the 2-turn coil coupled micro-strip  210  becomes the primary and the wide, U-shape buried micro-strip  208  becomes the secondary of the transformer. Consequently for this step down case, the digital signal is fed to the connector that serves as the output to the discharge in the step-up case of  FIG. 4 . For the step-down case, the inputs to the push/pull transistors are provided to the two connectors previously used to connect to the drains/collectors of the transistors. 
         [0031]    A proof of principle 100 MHz, 1-to-4 step-up model was designed, constructed and tested. The DC power into the RF PA stage was 450 W. The RF PA plus the invented transformer transmitted 370 W into a 50 ohm load yielding an efficiency of 82%. The hottest spot on the secondary was found to be 75 C with a chill plate temperature of 20 C. Thermal images (not included herein) of each of the transistors showed no visible signs of drain load unbalance. In achieving this performance, the A and B dimensions of the R04450 material were 0.040 inches and 0.10 inches, respectfully. The thickness of the pre-pregnated layers of R4450B material used to bind the top and bottom dielectric together was 0.012 inches. 
         [0032]      FIG. 5  illustrates a co-axial cable transmission line equivalent circuit for the 1-to-4 step-up transformer  200  illustrated in  FIGS. 2 ,  2 A,  3  and  4 . 
         [0033]    It should be understood that the particular embodiments of the invention described above have been provided by way of example and that other modifications may occur to a person skilled in the art without departing from the spirit and scope of the invention as expressed in the appended claims and their equivalents.