Abstract:
A high voltage, high speed, and high repetition rate pulse generator solves the high pulse repetition rate limitations associated with RF power amplifiers. The pulse generator employs resonant techniques to provide current limiting features that allow for continued high voltage, high speed, and high repetition pulse rate operation of the pulse generator without impairment of the pulse generator during both short circuit and open circuit load conditions.

Description:
BACKGROUND 
       [0001]    The invention relates generally to electronic power conversion and more particularly to a high voltage, high speed, high pulse repetition rate pulse generator using soft switching and pulse shaping technologies. 
         [0002]    Generators capable of operating at high voltages, high speeds, 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]    It would be both advantageous and beneficial to provide a high voltage, high speed, high pulse repetition rate pulse generator that solves the high pulse repetition rate limitations associated with RF power amplifiers. It would be further advantageous if the high voltage, high speed, high pulse repetition rate pulse generator were capable of continued operation without impairment of the pulse generator during both short circuit and open circuit load conditions. 
       BRIEF DESCRIPTION 
       [0004]    Briefly, in accordance with one embodiment, a pulse generator for generating high voltage, high speed, high repetition rate pulses is provided. The pulse generator comprises: 
         [0005]    an inverter configured to convert a DC voltage to a high frequency AC voltage; 
         [0006]    a converter configured to operate as an AC current source in response to the AC voltage; and 
         [0007]    a voltage shaping portion configured to generate a high voltage, high speed, high repetition rate voltage pulse in response to an AC input current generated by the AC current source. 
         [0008]    According to another embodiment, a method of generating a high voltage, high speed, high repetition rate voltage pulse comprises: 
         [0009]    converting a DC voltage to a high frequency AC voltage; 
         [0010]    generating an AC current in response to the AC voltage; and 
         [0011]    generating a high voltage, high speed, high repetition rate voltage pulse in response to the AC current. 
         [0012]    According to yet another embodiment, a pulse generator comprises: 
         [0013]    means for converting a DC voltage to a high frequency AC voltage; 
         [0014]    means for generating an AC current in response to the AC voltage; and 
         [0015]    means for generating a high voltage, high speed, high repetition rate voltage pulse in response to the AC current. 
     
    
     
       DRAWINGS 
         [0016]    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: 
           [0017]      FIG. 1  is a simplified circuit diagram illustrating a soft switching, pulse shaping generator according to one embodiment; 
           [0018]      FIG. 2  is a flowchart illustrating exemplary steps of a method for generating a high voltage, high speed, high repetition rate voltage pulse according to one embodiment; 
           [0019]      FIG. 3  is a circuit diagram illustrating in more detail, a soft switching, pulse shaping generator according to one embodiment; 
           [0020]      FIG. 4  is a set of waveforms illustrating operating circuit voltages and currents during steady state operation of the generator shown in  FIG. 3 , according to one embodiment; 
           [0021]      FIG. 5  is a set of waveforms illustrating operating circuit voltages and currents during short circuit load conditions for the generator shown in  FIG. 3 , according to one embodiment; and 
           [0022]      FIG. 6  is a set of waveforms illustrating operating circuit voltages and currents during open circuit load conditions for the generator shown in  FIG. 3 , according to one embodiment. 
       
    
    
       [0023]    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 
       [0024]      FIGS. 1 and 2  are first described to provide a background helpful to better understanding the discussion associated with  FIG. 3  that is described below. Looking now at  FIG. 1 , a simplified circuit diagram illustrates a basic soft switching, pulse shaping circuit architecture  10  according to one embodiment. Circuit  10  includes a trans-conductance amplifier  12  that functions as a current source in response to an input voltage pulse  13 . 
         [0025]    A MOSFET device  14  in combination with a clamping diode  16  operate together as a pulse shaping circuit to generate a square or rectangular shaped voltage pulse across a desired load  18 . Circuit  10  is robust against short circuit and open circuit loads due to the current source that drives the pulse shaping circuit. 
         [0026]    The gate input drive of MOSFET device  14  is controlled in a manner that allows the MOSFET device  14  to turn on in a soft switching mode. Soft switching reduces switching losses associated with the MOSFET switching device  14  as the switching frequency is increased. The combination of soft switching and pulse shaping allows the circuit  10  to operate as a high voltage, high speed, high repetition rate pulse generator that is robust against both open circuit and short circuit loading. 
         [0027]      FIG. 2  is a flowchart  20  illustrating exemplary steps of a method for generating a high voltage, high speed, high repetition rate voltage pulse according to one embodiment. The method commences by providing a primary DC input voltage, as represented in block  22 . The DC input voltage is then passed through an inverter to generate a high frequency (e.g. 25 MHz) AC voltage, as represented in block  24 . The AC voltage is converted into a current source to generate AC current, as represented in block  26 . The current source allows the pulse generator to function in a robust manner by providing a means of current limiting that is not achievable when using a pure voltage source to drive a dynamic load  18  such as depicted in  FIG. 1 . The AC current then drives a voltage shaping circuit to generate a square or rectangular output voltage pulse, as represented in block  28 . 
         [0028]      FIG. 3  is a circuit diagram illustrating in more detail, a soft switching, pulse shaping generator  30  according to one embodiment. The circuit architecture of generator  30  allows the generator  30  to generate a high voltage, high speed, and high repetition rate voltage output pulse. 
         [0029]    Generator  30  can be seen to include an inverter  40 , a converter  50  and a pulse shaping portion  60 . The inverter  40  includes a first tank circuit including capacitor C 1  and inductor L 1  that together have a natural resonant frequency. The inverter  40  also includes an upper soft switch  42  connected at one end to a positive DC voltage source  32  and connected at its opposite end to capacitor C 1  as shown in  FIG. 3 . Upper soft switch  42  operates in a soft switching mode via a gate drive element  43  to control the on-off switching operation. The clamping diode  44  that limits the voltage across the lower switch  46  could be a parasitic body diode of the switch  42 . Inverter  40  further includes a lower soft switch  46  connected at one end to a generator ground  34  and connected at its opposite end to capacitor C 1  as also shown in  FIG. 3 . Lower soft switch  46  operates in a soft switching mode via a corresponding gate drive element  47  to control the on-off switching operation. The clamping diode  48  that limits the voltage across the upper switch  42  could be a parasitic body diode of the switch  46 . Upper soft switch  42  and lower soft switch  46  are configured such that upper soft switch  42  is turned on when lower soft switch  46  is turned off and vice versa. Each switch  42 ,  46  operates according to one embodiment, in a soft switching mode at a high frequency rate that is equal to or higher than the natural resonant frequency of the tank circuit formed by capacitor C 1  and inductor L 1 , such as, for example, 25 MHz. 
         [0030]    The converter  50  can be seen to also have a tank circuit (second tank circuit) including capacitor C 2  and inductor L 2 . The first tank circuit of the inverter  40  and the second tank circuit of the converter  50 , in one embodiment, are configured such that the resonant frequency of C 1  and L 1  together have the same resonant frequency as a combination tank circuit including C 1  combined with C 2  and L 1  combined with L 2 . This configuration allows substantially all of the energy stored in the first tank circuit including capacitor C 1  and inductor L 1  to be transferred to the second tank circuit including capacitor C 2  and inductor L 2  during the switching process. This configuration also allows for current doubling such that the peak current flowing through inductor L 2  is substantially twice the peak current flowing through inductor L 1  during the switching process. Third switch  52  operates, according to one embodiment, in a soft switching mode via gate drive  53  and in combination with diode  54 , diode  56  and the second tank circuit including capacitor C 2  and inductor L 2  to generate an AC current through inductor L 2 . The diode  54  could be a parasitic body diode of switch  52  although a Zener diode is preferred. The load, RL in  FIG. 3 , then receives its power from the energy that is stored by inductor L 2  and is isolated from the DC voltage source by the inverter  40  and converter  50 . This isolation feature advantageously allows the generator  30  to drive a dynamic load that can change between a short circuit and an open circuit and can include driving a dynamic load during steady state operation in a region ranging anywhere between the short circuit and open circuit conditions. 
         [0031]    Generator  30  further includes a pulse shaping portion  60  that is configured to efficiently drive a dynamically changing load and also to generate a square wave or rectangular voltage pulse in response to the energy flowing through inductor L 2 . Pulse shaping portion includes a soft switch  62  and operates in a soft switching mode via gate drive element  64 . Soft switch  62  functions in combination with diode  66  and diode  68  to generate a square or rectangular voltage pulse during the switching process. The foregoing resonant frequency switching process advantageously provides pure voltage switching at high frequencies (MHz range) in the presence of dynamically changing load conditions. 
         [0032]      FIG. 4  is a set of waveforms illustrating operating circuit voltages and currents during steady state operation of the generator  30  shown in  FIG. 3 , according to one embodiment. The top waveform illustrates a high voltage (approximately 1000 volts), high frequency (MHz range), and high repetition rate output voltage pulse generated by the generator  30 . The middle waveform illustrates the peak current doubling achieved by the generator  30 . The bottom waveform illustrates the AC current flowing through the inverter, converter and pulse shaping switches during normal steady state operation of the pulse generator  30 . 
         [0033]      FIG. 5  is a set of waveforms illustrating operating circuit voltages and currents during short circuit load conditions for the generator  30  shown in  FIG. 3 , according to one embodiment. The short circuit operating waveforms illustrate the operational capabilities of the pulse generator  30 , even during short circuit loading at the output. The waveforms demonstrate the pulse generator  30  continues to operate without any adverse effects due to the current limiting features. 
         [0034]      FIG. 6  is a set of waveforms illustrating operating circuit voltages and currents during open circuit load conditions for the generator  30  shown in  FIG. 3 , according to one embodiment. The open circuit operating waveforms illustrate the operational capabilities of the pulse generator  30 , even during open circuit loading at the generator output. The waveforms demonstrate the pulse generator  30  continues to operate, even during open circuit loading without any adverse effects due to the current limiting features. 
         [0035]    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.