Patent Publication Number: US-6340912-B1

Title: Solid state magnetron switcher

Description:
This Application is a C-I-P of Ser. No. 09/109,608 filed Jul. 2, 1998, abandoned. 
    
    
     BACKGROUND 
     The present invention relates generally to magnetron devices, and more particularly, to a solid state magnetron switcher employing insulated gate bipolar junction transistors for switching high voltage pulses among multiple magnetrons. 
     The prior art approach to switching high voltage pulses among multiple magnetrons is disclosed in U.S. patent application Ser. No. 08/652,889, filed May 23, 1996, entitled “Thyratron Switched Beam Steering Array”, assigned to the assignee of the present invention. The approach disclosed in this patent application provides for a reasonably fast switching scheme with relatively small compactness. However, this approach cannot be directly commanded OFF and relies on a power supply pulse to be discontinued. This approach requires and generates hundreds of watts of heat. It was found that noise emissions using this approach were relatively high and the effects were difficult to deal with. 
     Prior art switching devices such as those disclosed in the above-cited patent application, have utilized thyratrons as switches instead of IGBT devices. Thyratrons require extensive filament and reservoir power which produces high temperature heat loads. Thyratrons emit a large amount of plasma noise when switched which can adversely affect operation of the magnetrons. Also, these devices cannot be commanded Off and must rely on the power supply high voltage pulse to be discontinued. 
     Therefore, it would be an advantage to rapidly switch several microwave tubes On and Off while driven from a single high voltage power supply. Accordingly, it is an objective of the present invention to provide for an improved solid state magnetron switcher for switching high voltage pulses among multiple magnetrons. 
     SUMMARY OF THE INVENTION 
     To accomplish the above and other objectives, the present invention provides for a solid state magnetron switcher that switches high voltage pulses among multiple magnetrons. The present invention switches high voltage to multiple magnetrons thereby gating them ON and OFF. The present invention provides for a small, light-weight, efficient, and fast switching mechanism that offers both ON and OFF control. 
     More particularly, the solid state magnetron switcher comprises a plurality of magnetrons, one insulated gate bipolar junction transistor (IGBT) high voltage switch coupled to each magnetron, a trigger circuit coupled to each IGBT high voltage switch, an isolation transformer coupled to the trigger circuit, and a floating resistor coupled across each of the IGBT high voltage switches. 
     The present invention provides a versatile solution to drive multiple magnetrons with a single high voltage pulse power supply. For a system requiring the use of an incoherent RF source, such as a magnetron, this solution is ideal for meeting the stringent objectives of rapidly switching a microwave tube on and off several times a second with long millisecond RF pulse characteristics. Presently, no other approach exists that provides command OFF ability and high current and large pulse width capabilities when driven by a single high voltage power supply. 
     The first advantage of the present invention over the prior art approach is that the high voltage power supply does not need to be turned off in order to extinguish each magnetron pulse. The IGBT devices are ON/OFF controllable devices. 
     The second advantage of the present invention is that IGBT devices require minimal drive power to operate on the order of milliwatts, whereas the prior art approach requires hundreds of watts which produces excess heat. The low power consumption here will allow a much smaller and lighter high voltage isolation transformer to be used. 
     The third advantage of the present invention is that the IGBT devices can be switched on prior to the high voltage power supply, which offers a smoother voltage rise time on the magnetron instead of the voltage pedestal effect normally encountered when the magnetron goes from low to high level oscillation. The low level oscillation refers to a level at which the magnetron draws a minimal amount of current but maintains itself in a conduction mode. This allows the magnetron to be less sensitive to the pedestal voltage rise time used to place the magnetron at its full power operation. Also, by switching prior to the high voltage pulse, the switching losses in the IGBT device are greatly reduced. 
     The present invention may be advantageously used with one or more magnetron devices having high PRF. The principles of the present invention may be used in instances where magnetrons must have small size and weight. 
     The solid state magnetron switcher of the present invention offers command OFF capability, requires only milliwatts of drive power, and is very efficient. Furthermore, the solid state magnetron switcher of the present invention is also smaller and lighter than the prior art devices. 
     A key aspect of the present invention is to place each magnetron into low level oscillation by using the correct value of resistance between the high voltage power supply and the magnetrons. This minimizes the voltage across each IGBT device. 
     The IGBT switches can be switched between magnetrons at a maximum rate of several KHz (i.e., 10-20 KHz) limited by pulse width, number of magnetrons, average power available, and switching speed of the IGBT devices. Interpulse switching times between array elements primarily depend on the limitations of the high voltage power supply. The technological approach of the present invention offers fast ON and OFF pulse commands with large pulse widths (i.e., microseconds—seconds) typically around several milliseconds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 illustrates an exemplary solid state magnetron switcher in accordance with the principles of the present invention implemented using two magnetrons; 
     FIG. 1 a  illustrates the switcher of FIG. 1, but using one magnetron; 
     FIG. 1 b  illustrates the switcher of FIG. 1, but using a plurality of magnetrons; 
     FIG. 2 illustrates post-trigger IGBT timing that may be used in the switcher of FIG. 1; 
     FIG. 3 illustrates pre-trigger IGBT timing that may be used in the switcher of FIG. 1; 
     FIG. 4 illustrates a detailed circuit diagram of the present solid state magnetron switcher; 
     FIG. 5 is a graph illustrating magnetron voltage versus time using the present solid state magnetron switcher; 
     FIG. 6 is a graph illustrating IGBT voltage versus time using the present solid state magnetron switcher; and 
     FIG. 7 is a graph illustrating magnetron current versus time using the present solid state magnetron switcher. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, wherein the same numerals refer to the same elements throughout, FIG. 1 illustrates an exemplary solid state magnetron switcher  10  in accordance with the principles of the present invention. FIG. 1 shows the solid state magnetron switcher  10  used to switch two magnetrons  11 . However, it is to be understood that any number of magnetrons  11  may be switched using the present invention, as depicted in FIG. 1 b,  and that the disclosed embodiment is not to be taken as limiting the scope of the present invention. Further, a single magnetron  11  may alternatively be switched, as depicted in FIG. 1 a.    
     With reference to FIG. 1, the solid state magnetron switcher  10  comprises a plurality of magnetrons  11 , one insulated gate bipolar junction transistor (IGBT) high voltage switch  12  coupled to each magnetron  11 , a trigger circuit  13 , which includes a trigger driver power supply  15 , coupled to each IGBT high voltage switch  12 , a common high voltage power supply (P/S)  18  coupled to each IGBT high voltage switch  12 , a common isolation transformer  16  coupled to the trigger circuit  13 , and a floating resistor  17  coupled across each of the IGBT high voltage switches  12 . The floating resistor  17  thus “rides” at a high voltage and is not at ground potential. Fiber optic cables  14  are used to interconnect the trigger circuit  13  to the trigger driver power supply  15  for each IGBT high voltage switch  12 . 
     The solid state magnetron switcher  10  of the present invention operates to switch high voltage to multiple magnetrons  11  thereby gating them ON and OFF. The solid state magnetron switcher  10  is a small, lightweight, efficient, and fast switching mechanism that offers OFF control as well as ON control. Hence, the ability to rapidly switch several microwave tubes (i.e., the magnetrons  11 ) ON and OFF which are driven from a single high voltage power supply  18  is provided by the present invention. 
     A key aspect of the technical approach employed in the present invention is to place all magnetrons  11  into low level oscillation prior to switching them into full conduction. This maintains minimal voltage across the IGBT switches  12  while allowing only a small percentage of full load current to be drawn (i.e., 1-2%). The gains compared with the conventional thyratron approach used in the prior art will become evident in the following discussion. 
     One advantage over the prior art approach is that the high voltage power supply  18  does not need to be turned off in order to extinguish each magnetron pulse. The IGBT switches  12  are ON/OFF controllable, or triggerable, devices. 
     Another advantage is that IGBT switches  12  require minimal drive power to operate on the order of milliwatts, whereas the prior art approach requires hundreds of watts, which produces excess heat. The low power consumption of the present invention allows a much smaller and lighter high voltage isolation transformer  16  to be used. The IGBT switches  12  can also be driven using trigger driver power supplies  15  comprising batteries  15  (without the need for the isolation transformer  16 ) to provide for isolated power since the power consumption is minimal. This allows for increased isolation during magnetron arcs. 
     Another advantage is that the IGBT switches  12  can be switched on prior to the high voltage power supply  18  which offers a smoother voltage rise time on the magnetrons  11  instead of the voltage pedestal affect that normally occurs when the magnetron  11  goes from low to high level oscillation. This allows each magnetron  11  to be less sensitive to pedestal voltage rise time used to place it at its full power operation. Also, by switching prior to the high voltage pulse, switching losses in the IGBT switches  12  are greatly reduced. 
     There are several switching approaches that may be implemented to switch between magnetrons  11 . Two general approaches are discussed below. The general circuit layout of the magnetron switch  10  is shown in FIG.  1 . The resistance value of the resistor  17  is chosen that allows each magnetron  11  to draw only a minimal amount of current. This provides for a low power drop across the resistor  17  and minimizes the voltage drop across the resistor  17  and the IGBT switch  12 . Keeping the voltage across the IGBT switches  12  to less than 1 kV is a key aspect of the present invention. Since this voltage is maintained at 1 kV or less, small low voltage power resistors may be used instead of large high voltage resistors, thereby providing for a more compact circuit. 
     More than one IGBT switch  12  can be connected in series to hold off more voltage than one 1 kV. However, it becomes difficult to simultaneously switch the switches  12  and have matched dynamic impedances for each of the switches  12 . The voltage across the IGBT switches  12  must be shared equally to insure reliable operation. An alternative is the use of a few switches  12  to share the 1 kV overall voltage (i.e., 500 volts per switch  12  if two IGBT switches  12  are implemented. The voltage must be maintained within its voltage specification. The magnetrons  11  that work at these large millisecond pulse widths generally operate at no more than tens of amps, which is well within the 1200 amp limit. The IGBT switch  12  was selected due to its triggerable ON/OFF, high voltage, and high current characteristics. Other switching devices, such as field effect transistors (FETs), for example, may be used but are considered less robust. However, FETs are limited by their current handling capacity, and thus parallel/series combinations of FETs would be required. 
     The first switching approach or technique is to turn the high voltage power supply  18  ON, either pulsed or continuously (CW), prior to switching the IGBT switch  12 . This approach lends itself to higher pulse repetition frequency (PRF) waveforms where high voltage power supply rise time and instabilities are a concern. FIG. 2 illustrates this timing approach. Two magnetrons  11  triggered sequentially are assumed for this diagram. Voltages and currents shown are for illustration purposes only and are not design requirements. 
     The second approach or technique is to switch the IGBT switch  12  prior to pulsing or turning the high voltage power supply  18  on. This approach minimizes the total time for which the IGBT switch  12  must hold off high voltage and therefore increases its reliability. This feature allows the engaged magnetron  11  to rise from zero oscillation to full conduction smoothly and without the pedestal voltage and its instantaneous current loading of the high voltage power supply  19  (e.g., 100 ma to 13 amps) when switched. FIG. 3 illustrates this timing approach. Two magnetrons  11  triggered sequentially are assumed for the timing diagram of FIG.  3 . 
     In the examples shown in FIGS. 2 and 3, the low level oscillation draws enough current to develop up to a 1 kV voltage drop across a network that includes the IGBT switch  12  and the resistor  17 . This is an example voltage drop, and other magnetron tubes may require higher (2 kV, for example) voltages which would require multiple series IGBT switches  12  driven in parallel, for example. This becomes an optimization tradeoff between the power dissipated by the resistor  17  verses voltage held off by the IGBT switch  12 . Other limitations imposed by the design such as total current available from the high voltage power supply  18  and low level tube radiation may influence the design. It is to be understood that the use of 1 kV magnetrons  11  is not to be taken as limiting the present invention. 
     The resistor  17  has two basic functions. One function is to place the magnetron  11  into oscillation so that when the additional voltage (i.e., 1 kV) is applied, the rise time is not an issue. The resistor  17  drops enough voltage so that the magnetron  11  only oscillates at about 1-2% of full power level. The other function preformed by the resistor  17  is that the voltage across the IGBT switches  12  that hold-off voltage to the other magnetrons  11  that are not currently pulsed will not exceed 1 kV. Otherwise, as in the examples above, the switches  12  would have to hold off the full voltage, here 23 kV, and therefore could not be used. 
     In this design, the IGBT switches  12  simply rise and float at high voltage. This requires a simple high voltage isolation transformer  16  to provide trigger power to the switch  12 . As mentioned previously, this power is minimal and can be triggered using fiber optic cables  14   s  and a pulse generator (implemented in the trigger circuit  13 ), for example. 
     An exemplary modeling circuit corresponding to the solid state magnetron switcher  10  is shown in FIG.  4 . FIG. 4 depicts a single magnetron  11  and its switching logic comprising the switcher  10 . The modeling circuit of FIG. 4 is basic and does not require modeling of the isolation transformer  16  or the fiber optic cables  14  shown in FIG.  1 . FIG. 5 is a graph showing the voltage on a magnetron  11  when triggered as shown in FIG. 2 (post-trigger). FIG. 6 is a graph showing the voltage across the IGBT switch  12  of FIG.  4 . To increase reliability, the IGBT switch  12  can be triggered as shown in FIG. 3 (pre-trigger) prior to the high voltage and therefore need only hold off voltage when other magnetrons  11  are pulsed. In a pre-triggered situation, the double pulse shown in FIG. 6 would not exist. Only a constant voltage pulse will result when other switches  12  are triggered. FIG. 7 shows the current waveform of the magnetron  11  of FIG. 4 when it is driven using the solid state magnetron switcher  10 . 
     The IGBT switching approach implemented in the solid state magnetron switcher  10  is small in size, light in weight, is highly efficient, has fast switching speed, has ON/OFF capability, and uses long RF pulse widths (&gt;10&#39;s of milliseconds). Although this solution applies to magnetrons  11  specifically, since there is no modulating anode or grid as with other types of RF/microwave tubes, the present invention may be adapted to other tubes if appropriate. 
     The principle of operation of the magnetron switcher  10  is as follows. The preferred implementation of the magnetron switcher  10  uses continuous repetitive pulsing, although more complex waveforms may be used. In this case, each channel is sequentially triggered using the trigger source  13  and fiber optic cables  14  which then trigger the gates of the IGBT switches  12  and enable conduction. The voltage drop across the resistor  17  essentially goes to zero and therefore applies an additional 1 kV to the magnetron  11 . 
     Since magnetrons  11  change current demand rapidly with small changes in voltage (once in the oscillation region), this results in full power oscillation. Consequently, changing the voltage from −22 kV to −23 kV will increase the current draw from 100 ma (low level oscillation) to  13 A (full power). 
     The design of the magnetron switcher  10  allows a single high voltage power supply  18  to be used and thereby reduce overall system size and weight. High voltage pulses are present on cathodes of all magnetrons  11  simultaneously through the resistors  17  but are connected directly to only one magnetron  11  at any particular time by the triggered IGBT switch  12 . 
     Triggering of the IGBT switch  12  before the high voltage power supply  18  reduces switching losses, provide a more gradual current demand on the power supply  18 , and provide a smooth voltage rise to the magnetrons  11 . 
     The magnetron switcher  10  may include various protection devices such as pulse spark gaps, varistors, and snubbers. The spark gaps can be specified with a higher breakdown than the operating voltage but lower than the maximum rating of the IGBT switch  12 . This helps protect the IGBT switch  12  during magnetron arcs. It is believed that the rating of the IGBT switch  12  is sufficient to handle momentary arc currents that typically exist for less than a microsecond. Other protection devices may be implemented to insure reliability. These devices are not shown across the IGBT switch  12  so as not to confuse the drawing. 
     The solid state magnetron switcher  10  of the present invention provides the smallest and lightest design approach available with currently available technology. Design parameters for the solid state magnetron switcher  10  are as shown in Table 1. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Peak IGBT switch voltage 
                 1.4 kV 
               
               
                 Switching time 
                 2 microseconds 
               
               
                 Peak switching current 
                 1200 Amps 
               
               
                 Multiple switching elements 
                 No Limit, waveform dependent 
               
               
                 Trigger pulse 
                 On and Off command capable 
               
               
                 PRF (pulse repetition freq.) 
                 &lt;20 KHz 
               
               
                 Typical pulse width range 
                 100 μs-100 ms 
               
               
                 Duty cycle 
                 50% typical, limited to IGBT heating &lt; 
               
               
                   
                 5 kW 
               
               
                   
               
            
           
         
       
     
     Thus, the present invention provides for a magnetron switcher  10  that rapidly switches between multiple magnetrons  11  with ON/OFF capability while minimizing the volume and weight of the circuit. This design provides the most versatile solution for driving multiple magnetrons  11  with a single high voltage pulse power supply  18 . For a system using an incoherent RF source such as a magnetron  11 , this solution is ideal for meeting the stringent objectives of rapidly switching magnetrons  11 , or other microwave tubes, on and off several times a second with long millisecond RF pulse characteristics. Presently, no other approach exists with command OFF ability as well as high current and large pulse width capabilities when driven by a single high voltage power supply  18 . Further, many magnetron tubes can be operated simultaneously, and specific ones can be selected for operation. 
     Thus, improved solid state switching apparatus for switching high voltage pulses among multiple microwave tubes or magnetrons has been disclosed. It is to be understood that the described embodiment is merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.