Abstract:
Circuits for shortening a rise time of a load include a main pulse transformer coupled to a load, an auxiliary pulse transformer coupled to the load, and a diode connected between the auxiliary pulse transformer and the main pulse transformer. The main pulse transformer delivers a main pulse to the load. The auxiliary pulse transformer delivers an auxiliary pulse to charge the capacitance associated with the load. The diode isolates the auxiliary pulse transformer from the main pulse. By charging the capacitance in this manner, a rise time of the main pulse is shortened and ringing of the main pulse on the load is reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The present invention relates to systems and methods for pulsing a high impedance load. More particularly, the present invention relates to systems and methods for compensating the rise times and ringing or overshoot responses associated with pulsed high impedance loads. 
   2. Background and Relevant Art 
   Pulse generators are devices that are used, for example, to generate an energy pulse that is delivered or applied to a load. Pulse generators deliver an energy pulse to loads such as, for example, spectrometers, accelerators, radar transmitters, high impedance electron guns, klystrons, as well as other equipment that require high voltage pulses. In this type of equipment, the energy pulses generated by the pulse generators should be fast and powerful. 
   In order to reduce the voltages actually generated by the pulse generators, pulse transformers are used to deliver the energy pulse the loads. The pulse transformers have a turn ratio that can provide the voltages required by the load. In many situations, the voltage delivered to the load can be quite large. For example, a pulse transformer may be used to deliver a voltage on the order of 30 kV (kilovolt) to 100 kV. Typical high impedance loads range from 1500 ohms to 30 kilohms. 
   One problem associated with delivering a pulse to a high impedance load through a pulse transformer is that significant ringing may result in the waveform. The ringing is partially associated with the rise time. When a fast rise time is needed, the target voltage of the pulse is often overshot and ringing results as the circuitry attempts to compensate. In other words, high impedance loads that are driven through a pulse transformer often experience ringing when trying to achieve fast rise times. 
   Another reason that makes it difficult to shorten rise times and reduce ringing is related to stray capacitance associated with the load. Stray capacitance adversely affects the rise time of the pulse on the load. As the stray capacitance increases, the rise time increases. The impedance of the load in combination with the stray capacitance can thus affect the ability of a pulse transformer based system to generate pulses that have a sufficiently fast rise time and minimal ringing. 
   BRIEF SUMMARY OF THE INVENTION 
   These and other limitations are overcome by embodiments of the present invention, which relates to systems and methods for delivering a pulse to a high impedance load. Embodiments of the present invention are able to improve rise times and at the same time reduce ringing associated with pulsing high impedance loads. 
   High impedance loads are often associated with a stray capacitance that can adversely affect the rise time and ringing of a pulse. One embodiment of the invention generates an auxiliary pulse that charges the stray capacitance of the load. When the main pulse is delivered to the load, the effect of the stray capacitance is reduced or eliminated because the capacitance has been charged by the auxiliary pulse. As a result, the rise time of the main pulse decreases and ringing is reduced. 
   In one embodiment, the load is connected to an auxiliary pulse generator through a diode and a pulse transformer. The auxiliary pulse generator delivers a pulse to the load before or at the beginning portion of the pulse delivered by the main pulse generator. The auxiliary pulse generator effectively charges or partially charges the capacitance related to the load and thereby reduces the effects of the capacitance on the pulse applied by the main pulse generator. 
   The pulse delivered by the auxiliary pulse generator can be adapted or adjusted to improve the rise time of the main pulse and/or reduce ringing. The duration of the auxiliary pulse, which is typically less than the duration of the pulse delivered by the main pulse generator can be adjusted. The magnitude of the auxiliary pulse can also be adjusted. The timing of the auxiliary pulse with respect to the main pulse can also be adjusted. Typically, the auxiliary pulse is delivered before the main pulse or at a beginning portion of the main pulse. Thus, the auxiliary pulse may or may not overlap with the main pulse. 
   Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to describe the manner in which the above-recited and other advantages and features of the invention cam be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1  illustrates a pulse generator that delivers a main pulse to a load through a pulse transformer and also illustrates capacitance related to the load; 
       FIG. 2  illustrates the rise time and ringing on the waveform associated with the pulse delivered to the load in  FIG. 1 ; 
       FIG. 3  illustrates one embodiment of compensation circuitry that improves the rise time of a main pulse and reduces or eliminates ringing on the waveform; 
       FIG. 4  illustrate an exemplary schematic of compensation circuitry that includes an auxiliary pulse transformer that is used to deliver a pulse to charge the capacitance associated with the load being pulsed; 
       FIG. 5A  illustrates an example of a waveform associated with a load when both a main pulse and an auxiliary pulse are delivered; 
       FIG. 5B  illustrates an example of a waveform associated with a load when only a main pulse is delivered; and 
       FIG. 6  illustrates an exemplary method for using an auxiliary pulse transformer to increase the rise time of a main pulse and/or reduce the ringing of the main pulse delivered to a load through a main pulse generator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates to systems and methods for delivering a high voltage pulse to a load and more particularly to circuits that both shorten the rise times of a pulse and reduce the ringing of the pulse on the load. Embodiments of the invention use an auxiliary pulse transformer to charge stray capacitance that is associated with the load being pulsed through a main pulse transformer. The stray capacitance is quickly charged by the auxiliary pulse and results in faster rise times for the main pulse and reduced or eliminated ringing when the main pulse is applied to the load. 
     FIG. 1  is a schematic diagram that illustrates one embodiment of a circuit for delivering a pulse to a load. In this example, a pulse generator  102  delivers a main pulse through the pulse transformer  106  to the load  110 . The transformer  106  is a 1:N transformer and is typically configured to step up the voltage of the main pulse generated by the pulse generator  102 . Thus, N is usually greater than 1. For example, the pulse generator  102  may generate a pulse on the order of 1 kilovolt that is transformed, when N=30, to 30 kV across the load. One of skill in the art can appreciate that other voltages, or turn ratios can be used to generate and deliver the main pulse to the load. 
   The capacitance  108  represents the stray capacitance associated with the load  110 . As previously stated, loads are often associated with a stray capacitance that can adversely affect the rise time and/or the ringing associated with the pulse. The rise time and overshoot or ringing response associated with the load  110  is illustrated in  FIG. 2 . In  FIG. 2 , the portion  202  of the plot  200  represents a relatively slow rise time of the pulse on the load and the portion  204  of the plot  200  illustrates ringing that can occur on the load  110 . Both of these characteristics are undesirable, particularly when the pulse duration of the main pulse is short. 
     FIG. 3  illustrates an exemplary system that implements one embodiment of the present invention.  FIG. 3  illustrates a pulse generator  302  that applies a pulse to a load  308  through a pulse transformer  304 . The pulse generator  302  generates a main pulse that is used by the load for various functions related to the load. The pulse generator  302  and the pulse transformer  304  function similar to the system illustrated in  FIG. 1 . The capacitance  306  represents stray capacitance that is associated with or related to the load  306 . As previously stated, the load may be a spectrometer, an accelerator, a radar transmitter, a high impedance electron guns, a klystron, and the like. 
   Effects associated with the stray capacitance  306 , such as slower rise times and ringing on the waveform, are reduced or eliminated by the compensation circuitry  312 . In  FIG. 3 , an auxiliary pulse, generated by the pulse generator  310 , is applied through the compensation circuitry  312 . The auxiliary pulse charges the capacitance  306  such that the rise time of the pulse delivered by the pulse generator  302  shortens and the ringing of the main pulse is reduced. In addition, the compensation circuitry  312  provides isolation from the main pulse delivered by the pulse generator  302  such that the load  308  is pulsed effectively and is not adversely affected by the compensation circuitry  312  or the auxiliary pulse generator  310 . 
     FIG. 4  is a schematic diagram that further illustrates one embodiment of the present invention.  FIG. 4  illustrates an example of the compensation circuitry  416 , which includes a pulse transformer  412  and a diode  410 . The pulse generator  414  generates an auxiliary pulse that charges the stray capacitance  406  associated with the load  408 . Once the capacitance  406  is charged (or partially charged) the pulse generator  402  then delivers a main pulse to the load  408  through the pulse transformer  404 . Because the capacitance  406  has been at least partially charged by the auxiliary pulse generated by the pulse generator  414 , the load  408  experiences a faster rise time and less ringing. 
     FIG. 5A  illustrates an exemplary waveform resulting when a main pulse and an auxiliary pulse are applied to a load on the order of 50 kilohms that is associated with a stray capacitance on the order of 50 picofarads.  FIG. 5B  illustrates an exemplary waveform resulting when only a main pulse is applied to the same load and stray capacitance. One of skill in the art, however, can appreciate that this example is not limiting and that embodiments of the present invention can be applied to various other loads and associated stray capacitances. The load, for example, can be greater or less than 50 kilohms and the associated capacitance can also be more or less than 50 picofarads. 
     FIG. 5A  illustrates an example where the auxiliary pulse  501  has been generated by the auxiliary pulse generator and applied to the load through an auxiliary pulse transformer. The duration of the auxiliary pulse  501  is less than the duration of the pulse  505  and functions to at least charge or partially charge the stray capacitance associated with the load. In contrast,  FIG. 5B  illustrates an example where the auxiliary pulse illustrated in  FIG. 5A  is not applied to the load. 
   Together,  FIGS. 5A and 5B  illustrate several benefits of the auxiliary pulse. The rise time  504  in  FIG. 5A  is faster than the rise time  506  illustrated in  FIG. 5B . In addition, the ringing  502  on the waveform shown in  FIG. 5A  is less than the ringing  508  on the waveform illustrated in  FIG. 5B . 
   The timing of the main pulse with respect to the auxiliary pulse can also vary. With further reference to  FIGS. 4 ,  5 A, and  5 B, the main pulse delivered through the pulse transformer  404  and the auxiliary pulse delivered through the auxiliary pulse transformer  412  begin at substantially the same instant in time in this example. One of skill in the art can appreciate however, that the auxiliary pulse delivered to the load  408  through the auxiliary transformer  412  can begin before the main pulse or after the beginning of the main pulse. In addition, the auxiliary pulse may end before the main pulse begins in another embodiment. Usually, the auxiliary pulse overlaps a beginning portion of the main pulse. The timing of the auxiliary pulse can be controlled by a computer or other control circuitry or by setting a delay in the auxiliary pulse transformer. In each situation where the timing of the auxiliary pulse is changed, however, the rise time may also be changed. 
   In addition,  FIGS. 5A and 5B  illustrate that the length of the auxiliary pulse is short in comparison to the length of the main pulse even though the duration of the main pulse may be measured, for example, in the microsecond range. An exemplary purpose of the auxiliary pulse, as stated, is to charge the stray capacitance such that the main pulse has a fast rise time and reduced ringing. 
   Because the auxiliary pulse has an impact on the rise time of the main pulse and on the overshoot or ringing response of the main pulse, the auxiliary pulse can be adjusted to maximize the rise time and/or reduce ringing of the pulse. The auxiliary pulse can be adjusted, as discussed above, by controlling the timing and/or the duration of the auxiliary pulse. 
   The auxiliary pulse can further be adjusted by controlling the magnitude of the auxiliary pulse. The magnitude of the auxiliary pulse applied to the load can be varied, for example, by controlling the turn ratio in the windings of the auxiliary pulse transformer or by changing the magnitude of the pulse generated by the auxiliary pulse generator. 
   Thus, the turn ratio of the auxiliary pulse transformer, the duration of the auxiliary pulse, the timing of the auxiliary pulse, can all be adjusted to, in one embodiment, minimize or shorten the rise time and reduce ringing or overshoot. These characteristics of the auxiliary pulse can be controlled, in one embodiment, by adjusting the auxiliary pulse generator and/or the auxiliary pulse transformer or through signals that control the auxiliary pulse transformer. The optimum characteristics of the auxiliary pulse duration may be determined empirically for a given load, especially if the stray capacitance is unknown. 
     FIG. 6  illustrates an exemplary method for applying a pulse to a waveform such that the rise time is shortened and ringing on the waveform is reduced. An auxiliary pulse is generated and applied  602  to a load. One purpose of the auxiliary pulse is to charge the secondary capacitance associated with the load. The auxiliary pulse is applied through a pulse transformer and a diode. 
   Next, a main pulse is applied  604  to the load. Because the stray capacitance is charged by the auxiliary pulse, the main pulse has a faster rise time and ringing in the pulse, as represented in the waveform of the pulse on the load, may also be reduced. Then, the auxiliary pulse is terminated  606  while the main pulse continues until it is also terminated. The auxiliary pulse is typically only needed during a first portion of the main pulse. After that time, the auxiliary pulse is no longer needed as the main pulse is established and effects of the stray capacitance have been reduced by the auxiliary pulse. Although this example assumes that the auxiliary pulse begins before the main pulse and overlaps at least a first portion of the main pulse, the relative beginning points of the two pulses can vary as previously described. 
   The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.