Patent Publication Number: US-8981799-B2

Title: Current measuring circuit

Description:
This application is based upon and claims the benefit of priority from Japanese patent application No. 2008-299435, filed on Nov. 25, 2008, the disclosure of which is incorporated herein in its entirety by reference. 
     TECHNICAL FIELD 
     The present invention relates to a current measuring circuit suitable for measuring a current flowing through a high-voltage site to which a high voltage is applied. 
     BACKGROUND ART 
     A traveling-wave tube, a klystron, and the like are electron tubes used to amplify and oscillate a radio frequency signal through the interaction between an electron beam emitted from an electron gun and a high-frequency circuit. For example, as shown in  FIG. 1 , traveling-wave tube  1  is configured so as to include: electron gun  10  that emits electron beam  50 ; helix electrode  20  that is a high-frequency circuit that causes electron beam  50  emitted from electron gun  10  to interact with a radio frequency signal (microwave); collector electrode  30  that acquires electron beam  50  outputted from helix electrode  20 ; and anode electrode  40  that extracts electrons from electron gun  10  and, at the same time, guides electron beam  50  emitted from electron gun  10  to spirally-shaped helix electrode  20 . Electron gun  10  includes: cathode electrode  11  that emits thermal electrons; and heater  12  that supplies cathode electrode  11  with thermal energy for emitting thermal electrons. 
     Electron beam  50  emitted from electron gun  10  is accelerated by a potential difference between cathode electrode  11  and helix electrode  20 , introduced into helix electrode  20 , and progresses through the inside of helix electrode  20  while interacting with a radio frequency signal inputted from one terminal of helix electrode  20 . Electron beam  50  having passed through the inside of helix electrode  20  is acquired by collector electrode  30 . At this point, a radio frequency signal amplified by the interaction with electron beam  50  is outputted from another terminal of helix electrode  20 . 
     Power supply  60  includes: helix power circuit  61  that supplies cathode electrode  11  with helix voltage Vhel that is a negative direct voltage with respect to potential HELIX of helix electrode  20 ; collector power circuit  62  that supplies collector electrode  30  with collector voltage Vcol that is a positive direct voltage with respect to potential H/K of cathode electrode  11 ; and heater power circuit  63  that supplies heater  12  with heater voltage Vh that is a negative direct voltage with respect to potential H/K of cathode electrode  11 . Helix electrode  20  is normally connected to a case of traveling-wave tube  1  and is grounded inside power supply  60 . 
     While  FIG. 1  shows a configuration example of traveling-wave tube  1  including one collector electrode  30 , traveling-wave tube  1  may alternatively be configured so as to include a plurality of collector electrodes  30 . In addition, while  FIG. 1  shows a configuration in which anode electrode  40  and helix electrode  20  are connected inside power supply  60 , a case is also possible where anode voltage Va that is a positive direct voltage with respect to potential H/K of cathode electrode  11  is supplied to anode electrode  40 . 
     Helix voltage Vhel, collector voltage Vcol, and heater voltage Vh can be generated by, for example, a configuration that includes a transformer, an inverter that converts an externally supplied direct voltage into an alternating voltage and that supplies the alternating voltage to a primary winding of the transformer, and a rectifier circuit that converts an alternating voltage outputted from a secondary winding of the transformer into a direct voltage. 
     Meanwhile, a conceivable method of measuring a current flowing through a high-voltage site such as cathode electrode  11 , collector electrode  30 , heater  12 , and the like in traveling-wave tube  1  shown in  FIG. 1  involves inserting an ammeter in series between a measurement object electrode and a power circuit that supplies a predetermined direct voltage to the electrode. 
     However, with the method using an ammeter, since high voltage is also applied to the ammeter that measures currents flowing through cathode electrode  11 , collector electrode  30 , and heater  12  which operate at high voltage (several kVs to several tens of kV), measures such as insulating the ammeter need to be taken so as to ensure that the measurement operation is conducted in a safe manner. In addition, since an ammeter is normally used only when testing traveling-wave tube  1  or power supply  60 , currents cannot be constantly monitored. 
     In consideration thereof, a method is conceivable in which current sensor  70  including a hall element or the like is fixed to, for example, wiring connecting cathode electrode  11  and helix power circuit  61 , whereby flux generated when a current flows through the wiring is detected and the detected flux is converted into a current value. 
     However, current sensor  70  that detects a current using flux is disadvantageous in that current sensor  70  is also unintentionally affected by a peripheral magnetic field which makes it difficult to detect a small current. In addition, the fact that current sensor (hall element)  70  is generally expensive raises the cost of the entire high-frequency circuit system including traveling-wave tube  1  and power supply  60 . 
     Japanese Patent No. 2711897 discloses a configuration for detecting an operating current of a traveling-wave tube in which a dedicated transformer for current detection (hereinafter referred to as a current-detecting transformer) is included in a power supply. 
     The power supply described in Japanese Patent No. 2711897 includes: a power supply transformer; an inverter that supplies power to a primary winding of the transformer; and a rectifier circuit that rectifies an alternating voltage outputted from a secondary winding of the transformer and generates a power voltage to be supplied to a cathode electrode and a collector electrode of a traveling-wave tube, and is configured such that a primary winding of a current-detecting transformer is inserted in series between the secondary winding of the transformer and the rectifier circuit. In such a configuration, a signal indicating a value substantially equal to a current flowing through the cathode electrode of the traveling-wave tube can be obtained from the secondary winding of the current-detecting transformer. 
     As described above, in the method of detecting a current using a current sensor, there are problems in that a small current is difficult to detect and that in the expensiveness of the current sensor (hall element) raises the cost of the entire high-frequency circuit system including the traveling-wave tube and the power supply. 
     Meanwhile, as described above, since the power supply described in Japanese Patent No. 2711897 is configured such that the primary winding of the current-detecting transistor is inserted in series between the secondary winding of the transformer and the rectifier circuit, a signal outputted from the secondary winding of the current-detecting transistor includes not only a value of the current flowing through the cathode electrode of the traveling-wave tube but also values of currents consumed by the current-detecting transistor and the rectifier circuit. Therefore, in the configuration described in Japanese Patent No. 2711897, it is hard to say that a current flowing through the cathode electrode of the traveling-wave tube is properly measured. 
     In addition, since the power supply described in Japanese Patent No. 2711897 uses a relatively expensive current-detecting transformer capable of also operating at high voltage (several kVs to several tens of kV), the cost of the entire high-frequency circuit system including the traveling-wave tube and the power supply rises. 
     Furthermore, the power supply described in Japanese Patent No. 2711897 is incapable of measuring a current flowing through electrodes other than the cathode electrode. 
     SUMMARY 
     In consideration of the above, an object of the present invention is to provide a current measuring circuit capable of accurately measuring a current flowing through a high-voltage site while preventing an increase in cost. 
     In order to achieve the object described above, a current measuring circuit according to an exemplary aspect of the present invention includes: 
     a current-detecting resistor inserted in series between a high-voltage site and an output terminal of a power circuit that supplies a predetermined direct voltage to the high-voltage site and which generates, between both ends thereof, a potential difference proportional to a current flowing through the high-voltage site; 
     a first resistor, a second resistor, a transistor, and a third resistor used to measure a current flowing through the high-voltage site and which are connected in series between a ground potential and an output terminal of the power circuit; 
     a differential amplifier that controls a current flowing through the transistor so that a potential difference generated between both ends of the third resistor becomes proportional to the potential difference generated between both ends of the current-detecting resistor; and 
     a direct voltage source that generates a direct voltage for operating the differential amplifier. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, which illustrate examples of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a block diagram showing a configuration example of a high-frequency circuit system of background art; 
         FIG. 2  is a schematic diagram showing an example of a current measuring circuit of background art; 
         FIG. 3  is a block diagram showing an exemplary configuration of a current measuring circuit according to the exemplary embodiment; 
         FIG. 4  is a circuit diagram showing a configuration example of a direct voltage source shown in  FIG. 3 ; 
         FIG. 5  is a block diagram showing another exemplary configuration of a current measuring circuit according to the exemplary embodiment; 
         FIG. 6  is a block diagram showing another exemplary configuration of a current measuring circuit according to the exemplary embodiment; 
         FIG. 7  is a block diagram showing another exemplary configuration of a current measuring circuit according to the exemplary embodiment; 
         FIG. 8  is a block diagram showing an exemplary configuration of a high-frequency circuit system according to the exemplary embodiment; and 
         FIG. 9  is a block diagram showing another exemplary configuration of a high-frequency circuit system according to the exemplary embodiment. 
     
    
    
     EXEMPLARY EMBODIMENT 
     The present invention will now be described with reference to the drawings. 
     Hereinafter, while a description will be given of an example in which currents flowing through the respective electrodes of traveling-wave tube  1  shown in  FIG. 1  are to be measured, the present invention is not limited to measuring electrodes of traveling-wave tube  1  and is applicable to cases where currents flowing through other high-voltage sites are measured. 
       FIG. 3  is a block diagram showing an exemplary configuration of a current measuring circuit according to the exemplary embodiment. 
       FIG. 3  shows a configuration example of a circuit for measuring a current (hereinafter referred to as a cathode current) flowing through a cathode electrode of the traveling-wave tube shown in  FIG. 1 . In addition,  FIG. 3  shows a configuration which includes, as a power circuit that supplies a predetermined direct voltage to the cathode electrode and a collector electrode of the traveling-wave tube: transformer  110 ; inverter  111  that converts an externally supplied direct voltage to an alternating voltage and supplies the alternating voltage to a primary winding of transformer  110 ; and rectifier circuits  112 ,  113  that convert an alternating voltage outputted from a secondary winding of transformer  110  to a direct voltage. Configurations of a power circuit are not necessarily limited to the configuration shown in  FIG. 3  and any circuit may be used as long as a predetermined direct voltage to be supplied to the respective electrodes can be generated. 
     As shown in  FIG. 3 , current measuring circuit  100  according to a first exemplary embodiment includes current-detecting resistor R 4 , differential amplifier  101 , direct voltage source  102 , transistor  103 , zener diode  104 , and resistors R 1  to R 3 . 
     Current-detecting resistor R 4  is inserted in series between the cathode electrode of the traveling-wave tube that is a high-voltage site whose current is to be measured and an output terminal of a helix power circuit (refer to  FIG. 1 ) that supplies a direct voltage to the cathode electrode, and that generates, between both ends thereof, a potential difference proportional to cathode current Ik. 
     Resistor R 1 , resistor R 2 , transistor  103 , and resistor R 3  are to be used to measure the current flowing through a high-voltage site that is the current measurement object. Resistor R 1 , resistor R 2 , a collector-emitter of transistor  103 , and resistor R 3  are connected in series between ground potential (HELIX: a helix of the traveling-wave tube) and an output terminal (a terminal not connected to the cathode electrode of current-detecting resistor R 4 ) of the helix power circuit. 
     An output signal of differential amplifier  101  is to be inputted to a base of transistor  103 . While  FIG. 3  shows a circuit example in which a bipolar transistor is used as transistor  103 , a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) may be used as transistor  103 . 
     A positive (+) input terminal of differential amplifier  101  is connected to the cathode electrode of the traveling-wave tube. A negative (−) input terminal of the differential amplifier is connected to a connection node of the emitter of transistor  103  and resistor R 3 . Feedback of an emitter voltage of transistor  103  to the negative (−) input terminal enables differential amplifier  101  to control current Imon flowing between the emitter and the collector of transistor  103  so that a potential difference generated between both ends of resistor R 3  becomes proportional to the potential difference generated between both ends of current-detecting resistor R 4  (in the configuration shown in  FIG. 3 , the potential differences are to become equal to each other). 
     Direct voltage source  102  generates a direct voltage that becomes a power voltage for operating differential amplifier  101 . For example, as shown in  FIG. 4 , direct voltage source  102  is generated using, for example, an output voltage of a heater power circuit that supplies heater voltage Eh to a heater of the traveling-wave tube. 
     Normally, a negative direct voltage of around several kVs to several tens of kV with respect to potential H/K of the cathode electrode is supplied to the heater of the traveling-wave tube. Therefore, a direct voltage to be supplied to differential amplifier  101  as a power voltage can be generated using the alternating voltage outputted from the secondary winding of transformer  120  included in the heater power circuit. 
     Direct voltage source  102  shown in  FIG. 4  includes diode D 1 , diode D 2 , capacitor C 1 , inductor L 1 , and capacitor C 2 . Direct voltage source  102  generates a direct voltage higher than potential (H/K) of the cathode electrode (close to the ground potential) by rectifying an alternating voltage outputted from the secondary winding of transformer  120  included in the heater power circuit with diode D 1 , diode D 2 , and capacitor C 1 , boosting the rectified voltage, and smoothing the voltage boosted by inductor. L 1  and capacitor C 2 . Moreover, diode D 3 , diode D 4 , inductor L 2 , and capacitor C 3  shown in  FIG. 4  are a rectifier circuit that converts an alternating voltage outputted from the secondary winding of transformer  120  into a direct voltage. 
     Zener diode  104  is connected to the emitter-collector of transistor  103  and prevents damage to transistor  103  by limiting the voltage between the emitter and the collector of transistor  103  to or under a certain value. Zener diode  104  is unnecessary if excessive voltage can be prevented from being applied between the emitter and the collector of transistor  103  by other means. 
     In such a configuration, as described above, transistor  103  is connected in series to resistor R 1 , resistor R 2 , and resistor R 3 , and connected between the helix of the traveling-wave tube and the output terminal of the helix power circuit. Therefore, a current flowing between the emitter and the collector of transistor  103  becomes equal to the current flowing through resistors R 1 , R 2 , and R 3 . In addition, the helix power circuit supplies constant helix voltage Vhel to the cathode electrode. 
     Therefore, if the emitter-collector voltage of transistor  103  is denoted by Vmon, then a relation expressed by
 
 V hel= Imon ×( R 1 +R 2 +R 3)+ Vmon   (1)
 
is true.
 
     In addition, as described above, feedback of an emitter voltage of transistor  103  to the negative (−) input terminal enables differential amplifier  101  shown in  FIG. 3  to control current Imon flowing between the collector and the emitter of transistor  103  so that a potential difference generated between both ends of resistor R 3  becomes proportional to a potential difference generated between both ends of current-detecting resistor R 4  (in this case, the potential differences are to become equal to each other). 
     Therefore, equation (2) below is true for cathode current Ik and current Imon flowing between the collector and the emitter of transistor  103 .
 
 Ik× 4 R=Imon×R 3  (2).
 
     Since resistance values of resistor R 3  and current-detecting resistor R 4  are fixed, it is now understood that current Imon flowing through resistors R 1 , R 2 , and R 3  and between the collector and the emitter of the transistor shown in  FIG. 3  varies in accordance with current Ik flowing through current-detecting resistor R 4 . 
     Therefore, by measuring the voltage of connection node (Ik MON) of resistor R 1  and resistor R 2  with respect to potential (HELIX: ground potential) of the helix of the traveling-wave tube, a value of cathode current Ik can be calculated from the measured value. 
     Specifically, if a voltage of the connection node of resistor R 1  and resistor R 2  is denoted by V 12 , then
 
 V   12   =R 1× Imon   (3)
 
is true.
 
     Therefore, from equations (2) and (3) above, cathode current Ik can be calculated by
 
 Ik =( R 3/( R 4× R 1)) V   12   (4).
 
     With current measuring circuit  100  according to the present exemplary embodiment, a current flowing through a high-voltage site such as a cathode electrode of a traveling-wave tube can be measured by simplified circuitry. In addition, since current measuring circuit  100  according to the present exemplary embodiment can be configured using generic parts such as a resistor, a differential amplifier, and a transistor, an increase in cost can be reduced to a minimum. 
     Moreover, in the description above, while a configuration example has been shown in which a direct voltage to be supplied to differential amplifier  101  is generated as shown in  FIG. 4  using an output voltage (alternating voltage) of a heater power circuit as direct voltage source  102 , heater voltage (direct voltage) Vh generated by the heater power circuit can be directly supplied as the power voltage to differential amplifier  101 . 
     In this case, as shown in  FIG. 5 , it will suffice that resistor R 1 , resistor R 2 , the transistor, and resistor R 3  connected in series be connected between the helix of the traveling-wave tube and an output terminal of the heater power circuit (an output terminal of rectifier circuit  114 ), and the positive (+) input terminal of the differential amplifier be connected to an output terminal of the helix power circuit (a terminal not connected to the cathode electrode of current-detecting resistor R 4 ). 
     Since direct voltage source  102  becomes unnecessary in a configuration in which heater voltage (direct voltage) Vh generated by the heater power circuit is supplied to differential amplifier  101 , circuit size can be reduced compared to current measuring circuit  100  shown in  FIG. 3  and  FIG. 4  and the cost of current measuring circuit  100  can be further reduced. 
     In addition, while a configuration example in which a current flowing through the cathode electrode of a traveling-wave tube has been shown in the description above, in addition to the measurement of a current flowing through the cathode electrode, the current measuring circuit according to the exemplary embodiment can be realized with a similar configuration when measuring a current flowing through the collector electrode or the heater. 
     For example, when measuring current Icol flowing through the collector electrode, as shown in  FIG. 6 , current-detecting resistor R 4  is inserted in series between the collector electrode and an output terminal of a collector power circuit (refer to  FIG. 1 ) that supplies a predetermined direct voltage to the collector electrode, and resistor R 1 , resistor R 2 , the transistor, and resistor R 3  connected in series are connected between the helix (ground potential) of the traveling-wave tube and an output terminal of the collector power circuit. Then, current Imon flowing between the collector and the emitter of transistor  103  need only be controlled so that a potential difference generated by differential amplifier  101  between both ends of resistor R 3  becomes proportional to a potential difference generated between both ends of current-detecting resistor R 4 . 
     In this case, it will suffice if direct voltage source  102  generates a direct voltage for operating differential amplifier  101  by adding secondary winding  115  for outputting an alternating voltage for generating a direct voltage higher than (having a difference of around several kVs to several tens of kV) collector voltage Vcol and for converting an alternating voltage outputted from secondary winding  115  into a direct voltage using known rectifier circuit  116 . 
     On the other hand, when measuring current Ih flowing through the heater, as shown in  FIG. 7 , current-detecting resistor R 4  is inserted in series between the heater and an output terminal (output terminal of rectifier circuit  114  shown in  FIG. 7 ) of the heater power circuit (refer to  FIG. 1 ) that supplies a predetermined direct voltage to the collector electrode, and resistor R 1 , resistor R 2 , the transistor, and resistor R 3  connected in series are connected between the helix (ground potential) of the traveling-wave tube and the output terminal of the heater power circuit. Then, in the same manner as described above, only current Imon flowing between the collector and the emitter of transistor  103  needs be controlled so that a potential difference generated by differential amplifier  101  between both ends of resistor R 3  becomes proportional to a potential difference generated between both ends of current-detecting resistor R 4 . 
     In this case, supplying heater voltage Vh outputted from the heater power circuit to differential, amplifier  101  as a power voltage will suffice. 
     In addition, as described above, current measuring circuit  100  according to the present exemplary embodiment can be configured using small generic parts such as a resistor, a differential amplifier, and a transistor. Therefore, for example, current measuring circuit  100  according to the present exemplary embodiment can be mounted on any device in a high-frequency circuit system including a traveling-wave tube and a power supply that supplies a predetermined direct voltage to respective electrodes of the traveling-wave tube. 
     Specifically, current measuring circuit  100  according to the present exemplary embodiment may include power supply  60  that respectively supplies a predetermined direct voltage to the cathode electrode, the collector electrode, and the heater of traveling-wave tube  1  as shown in  FIG. 8  or may be incorporated into traveling-wave tube  1  as shown in  FIG. 9 . 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those ordinarily skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.