Abstract:
A high voltage, fast pulse rise/fall time, and high repetition rate pulse generator solves the high pulse repetition rate limitations associated with RF power amplifiers and gap switch type pulse generators. The pulse generator employs a transmission line architecture and structural techniques that allow for continued high voltage, fast rise/fall time, and high repetition pulse rate operation of the pulse generator without impairment of the pulse generator while exceeding performance characteristics achievable with conventional RF power amplifiers and gap switch type pulse generators.

Description:
BACKGROUND 
       [0001]    The invention relates generally to electronic power conversion and more particularly to a solid state high voltage, fast rise/fall time, and high pulse repetition rate pulse generator using a transmission line transformer. 
         [0002]    Plasma generators capable of operating at high voltages with fast pulse rise and fall times, and high pulse repetition rates have generally employed radio frequency (RF) power amplifiers and related technology to accomplish high voltage, high speed and high pulse repetition rate generation and transmission. Such RF power amplifiers are expensive to produce and suffer in reliability due to internal heat build-up during high pulse repetition rate generation. RF amplifiers also undesirably require significant real estate and generally have low electric efficiency. Further, RF power amplifier technology is not particularly suitable for generation of high pulse repetition rates due to thermal losses, among other things. 
         [0003]    Known high voltage pulse generators for the generation of plasma and the like generally employ gap type switches that substantially limit the upper pulse repetition rate as well as the overall system reliability level. These known high voltage pulse generation systems also require a substantial amount of real estate to provide a working system due to structural limitations. 
         [0004]    It would be both advantageous and beneficial to provide a high voltage, fast rise/fall time, high pulse repetition rate pulse generator that overcomes the high pulse repetition rate limitations associated with conventional high voltage pulse generators. It would be further advantageous if the high voltage, fast rise/fall time, high pulse repetition rate pulse generator were capable of continued operation without impairment of the pulse generator during substantially longer time periods than that achievable using conventional high voltage pulse generators. It would be further advantageous if the high voltage, fast rise/fall time, high pulse repetition rate pulse generator occupied substantially less real estate to provide a working system than that required by conventional high voltage pulse generators. 
       BRIEF DESCRIPTION 
       [0005]    Briefly, in accordance with one embodiment, a pulse generator for generating high voltage, fast rise/fall time, high repetition rate pulses comprises:
       a plurality of pulse forming modules, each module comprising a solid-state switch; and   a transmission line transformer configured to generate high voltage, fast rise/fall time, high repetition rate pulses in response to voltage pulses generated via the plurality of pulse forming modules.       
 
         [0008]    According to another embodiment, a system for generating high voltage, fast rise/fall time, high repetition rate pulses comprises:
       a plurality of solid-state switches;   a pulse forming line corresponding to each solid-state switch, each pulse forming line configured to generate a high fast rise/fall time, high repetition rate voltage pulse simultaneously with each other in response to operation of its corresponding solid-state switch; and   a transmission line transformer configured to generate high voltage, fast rise/fall time, high repetition rate pulses in response to the voltage pulses generated via the pulse forming lines.       
 
         [0012]    According to yet another embodiment, a plasma pulse generation system comprises:
       a plurality of solid-state switching pulse forming modules; and   a transmission line transformer configured to generate high voltage, fast rise/fall time, high repetition rate output pulses for the generation of plasma in response to input voltage pulses generated via the plurality of pulse forming modules.       
 
     
    
     
       DRAWINGS 
         [0015]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0016]      FIG. 1  is a system diagram illustrating a high voltage, fast rise/fall time, high repetition rate pulse generator according to one embodiment of the invention; 
           [0017]      FIG. 2  illustrates one of the Blumlein pulse forming lines depicted in  FIG. 1  modified with alphabetical characters representing different parts of the Blumlein pulse forming line; 
           [0018]      FIG. 3  is a diagram illustrating construction of a Blumlein pulse forming line according to one embodiment of the invention; and 
           [0019]      FIG. 4  is a diagram illustrating construction of a transmission line transformer according to one embodiment of the invention. 
       
    
    
       [0020]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0021]      FIG. 1  is a system diagram illustrating a high voltage, fast rise/fall time, high repetition rate pulse generator  10  according to one embodiment of the invention. As used herein, high speed means pulses having a fast rise time and a fast fall time. Pulse generator  10  comprises a plurality of Blumlein pulse forming lines  12 . Each Blumlein pulse forming line  12  is driven via a switching/pulse charging circuit  14  comprising a high voltage solid state switch  16  such as, for example, a high voltage switching MOSFET to form a pulse forming module  30 . Pulse generator  10  can be seen to comprise a plurality of such pulse forming modules  30 . These pulse forming modules  30  are substantially identical to one another according to one embodiment of the invention. 
         [0022]    Pulse generator  10  also comprises a transmission line transformer  40  that is configured within pulse generator  10  to multiply input voltage pulses generated via the plurality of Blumlein pulse forming lines  12  into high voltage pulses without degradation of pulse rise and fall times. Because the Blumlein pulse forming lines  12  are driven by solid state switches  16 , pulse generator  10  advantageously generates high voltage, high speed pulses at a much higher repetition rate and with higher pulse rise and falls times and higher pulse voltage magnitudes than can be achieved when using conventional gap switch pulse generators commonly employed in the prior art. One workable embodiment was found to achieve a pulse repetition rate exceeding 100 kpps, a pulse rise/fall time of about 10 nanoseconds, a high voltage magnitude of at least 30 kV, and a pulse width of about 20 nanoseconds. 
         [0023]    The use of solid state based switches  16  further advantageously allows pulse generator  10  to be constructed in a manner that is substantially smaller, substantially more efficient, and substantially more reliable than conventional gap switch pulse generators commonly employed in the prior art. 
         [0024]    With continued reference to  FIG. 1 , each switching/pulse charging circuit  14  further comprises a diode  18  and an inductor  20 . A switching operation of each switch  16  causes each corresponding inductor  20  to develop a differential pulse voltage and also provides high frequency isolation between switch  16  ground connections and the inner conductor  22  and outer shield  24  of each corresponding Blumlein pulse forming line  12  as a DC voltage source  26  is switched on and off. Because each Blumlein pulse forming line comprises distributed capacitance characteristics, a differential voltage having a magnitude approximately twice the DC input voltage  26  level is stored across this Blumlein pulse forming line distributed capacitance during each switching cycle. Diode  18  operates to prevent undesired backcharging of the pulse voltage during the switching cycle, allowing creation of the desired pulse voltage signal. 
         [0025]    Each line of the transmission line transformer  40  and each of the Blumlein pulse forming lines  12  are constructed of the same type of coaxial cable according to one embodiment of the invention. The transmission line transformer  40 , according to one embodiment, is configured generate a high voltage, high speed, high repetition rate voltage pulse having a magnitude equivalent to the total number N of pulse forming modules  30  times the magnitude of a voltage pulse generated via a single pulse forming module  30 . 
         [0026]    The high voltage, high speed, high repetition rate pulse generator  10  offers several advantages over the conventional gap switch pulse generators that are known in the art. Some of these advantages include 1) the use of modular design providing ease of manufacture and high reliability, 2) substantially perfect matching between the Blumlein pulse forming line and its corresponding transmission line transformer for high efficiency, 3) the capability of using different length lines to construct the transmission line transformer, 4) propagation time delays that can be easily compensated by switch gate signals, 5) configuration of switches that each drive a pair of lines forming a floating transmission line, 6) switch ground connections that are isolated with chokes/inductors, 7) a high efficiency topology in which very minimal energy is wasted within the structure of the transmission line transformer itself, 8) superior current sharing features, 9) switching speeds determined only by transmission line and solid-state switch characteristics, 10) compact topology requiring minimal real-estate, 11) a voltage pulse amplitude that is proportional to an input voltage amplitude, and 12) a pulse repetition rate that is limited only by magnetic material and charging circuit characteristics. 
         [0027]      FIGS. 2 and 3  together illustrate construction of a Blumlein pulse forming line  12  according to one embodiment of the invention.  FIG. 2  depicts a single Blumlein pulse forming line  12  as also depicted in each pulse forming module  30  of  FIG. 1 . Blumlein pulse forming line  12  is represented in  FIG. 2  as a pair of coaxial transmission line elements  42 ,  44 , each having an inner conductor  22  and an outer shield  24  encapsulating the inner conductor  22  along the entire length of the corresponding coaxial line  42 ,  44 . A signal input end of coaxial line  42  is represented as ‘A’, while a corresponding open circuit end of coaxial line  44  is represented as ‘E’. Inner conductor  22  of coaxial line  42  is shorted to inner conductor  22  of coaxial line  44 . The shield  24  of coaxial line  42  is represented as ‘B’ at the signal input end of coaxial line  42 . The outer shield  24  of coaxial line  42  is represented as signal output node ‘C’ at the end opposite the signal input end ‘A’ of coaxial line  42 ; while the outer shield  24  of coaxial line  44  is represented as signal output node ‘D’ at the end opposite the open circuit end ‘E’ of coaxial line  44 . 
         [0028]      FIG. 3  is a diagram illustrating the physical construction of a Blumlein pulse forming line  12  suitable for use in the pulse generator  10  depicted in  FIG. 1 , according to one embodiment of the invention. Blumlein pulse forming line  12  comprises a single length of coaxial type transmission cable  46  that is formed into a U-shape transmission element. The single inner conductor  22  is shorted together at its exposed ends represented as ‘A’ which corresponds to the shorted inner conductors depicted in  FIG. 2 . The outer shield  24  of the U-shape transmission element is separated at the center, along the straight portions of the U-shape transmission element such that the U-shape transmission element is characterized by two sub-elements  48 ,  50 . The first transmission sub-element  48  comprises the two straight portions, each having a length ‘L’, and having the outer shield of each portion shorted together at the point of separation, as depicted in  FIG. 3 . The second transmission sub-element  50  comprises the remaining U-shape portion, having its outer shield shorted together at the point of separation, as also depicted in  FIG. 3 . This physical Blumlein pulse forming line  12  then has one input ‘A’ that corresponds with the conductor  22  input ‘A’ depicted in  FIGS. 2 and 3 , and another input ‘B’ that corresponds with the outer shield  24  input ‘B’ depicted in  FIGS. 1 and 2 . The first transmission sub-element  48  has a first output ‘C’ at the point where the outer shields are shorted together, corresponding with output ‘C’ in  FIG. 2 . The second transmission sub-element  50  has a second output ‘D’ at the point where the ends of the outer shield are shorted together, corresponding with output ‘D’ in  FIG. 2 . The U-shape portion ‘E’ of transmission sub-element  50  corresponds with the open circuit end ‘E’ in  FIG. 2 . 
         [0029]    Blumlein pulse forming line  12  advantageously provides excellent impedance matching characteristics when it comprises the same type of transmission lines, i.e. coaxial cable, as the transmission line transformer  40 . The physical construction of a Blumlein pulse forming line  12  described above with reference to  FIG. 3  also advantageously has a minimal number of high voltage joints, a feature that reduces occurrences of undesirable corona discharge. 
         [0030]      FIG. 4  is a diagram illustrating the physical construction of a transmission line transformer  40  that advantageously functions in a manner that substantially eliminates undesired parasitic/secondary or common mode propagation errors according to one embodiment of the invention. The left portion of  FIG. 4  depicts an axial view of the transmission line transformer  40 , while the right portion of  FIG. 4  depicts a perspective view of the transmission line transformer  40 . Transmission line transformer  40  includes a high-mu magnetic core  60  according to one embodiment. Each individual transformer  40  transmission line L 1 -Ln is wound around the magnetic core  60  to form a torroid  62  in accordance with a suitable construction rule. Line L 1 , for example, can pass straight through the magnetic core  60  without being wound around the core  60 . Line L 2 , for example, can be wound around the magnetic core  60  once to form a single turn of the torroid  52 . Line L 3 , for example, can be wound around the magnetic core  60  twice to form two turns of the torroid  62 , and so on. Table 1 below illustrates a winding rule according to one embodiment that was found suitable by the present inventors to provide desired working results. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Torroid Winding Rule 
               
             
          
           
               
                   
                 Turns 
                 ⊖ 
               
               
                   
                   
               
             
          
           
               
                   
                 L1 
                 0 
                 0 
               
               
                   
                 L2 
                 1 * m, where 
                 4 * 180°/[m(n − 1)], where 
               
               
                   
                   
                 m is an integer 
                 n is an integer 
               
               
                   
                 . . . 
                 . . . 
                 . . . 
               
               
                   
                 Lj, where 
                 (j − 1) * m 
                 4 * 180°(j − 1)/[m(n − 1)] 
               
               
                   
                 j is an integer 
               
               
                   
                 . . . 
                 . . . 
                 . . . 
               
               
                   
                 Ln 
                 n * m 
                 4 * 180°/n 
               
               
                   
                 Σ 
                 n(n − 1)m/2 
                 360° 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    The inputs of the lines L 1 -Ln are separated to minimize parasitic coupling while the tails are in close proximity to allow easy interconnection according to one embodiment. According to one aspect of the invention, the length of each of the lines L 1 -Ln are kept as small as practical to minimize line losses. According to another aspect of the invention, each of the lines L 1 -Ln are tightly wound on a high-mu core  60  such that each loop is guaranteed to have substantially the same voltage drop. 
         [0032]    A significant advantage provided by the pulse generator  10  using the foregoing torroidal construction techniques is the capability to calibrate gate drive signals to the solid-state switches  16  in a manner that ensures that every pulse generated by the pulse generator  10  arrives at the output/load at substantially the same point in time. Another significant advantage provided by the foregoing torroidal construction techniques is the increased capability to preserve the integrity of the pulses generated by the pulse generator  10 . 
         [0033]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.