US3315124A - Transistorized constant emission current regulator using a resonant transformer in the power supply - Google Patents

Description

3,315 GULATOR IN THE POWER SUPPLY 1964 p l 1967 H. T. BOEKER TRANSISTORIZED CONSTANT EMISSION CURRENT RE usmc A RESONANT TRANSFORMER Filed Aug. 14.

I N VEN TOR.

HA OLD T. BOEKER BY ATTORNEY United States Patent TRANSISTORIZED CONSTANT EMISSION CUR RENT REGULATOR USING A RESONANT TRANSFORMER IN THE POWER SUPPLY Harold T. Boeker, Brookfield, Wis., assignur to General Electric Company, a corporation of New York Filed Aug. 14, 1964, Ser. No. 389,598 13 Claims. (Cl. 315106) This invention relates to current control and more particularly to an improved automatic tube current control for electron tubes such as X-ray and electron beam tubes.

In X-ray and electron beam generator equipment it has been found to be advantageous to utilize a resonant transformer instead of an iron core transformer in the power supply in order to realize the advantages of minimum size and weight, optimum configuration for ultra-high voltage applications and improvement in the voltage output waveshape. Past methods employed to achieve tube current control in such systems typically utilized servo motor control techniques. In such systems a reactor was connected in series with the tube filament, the inductance of the reactor being varied by controllably positioning a slug in the air gap, usually by means of stainless steel gear systems driven by a servo motor. Such servo control systems, having mechanical components, were sluggish in response to changes in load conditions. In addition, backlash in the gear systems and hunting problems made stabilization of the control system ditficult. Moreover, such systems usually required frequent maintenance since foreign particles could easily enter the gearing system to bind the steel gears. As a result, these mechanical problems adversely atfected the accuracy and reliability of such control systems.

The use of solid state circuitry in X-ray power supplies and electron beam generators makes possible an improvement in the control of the supply of power to the tube filament to thereby control the tube current. The employment of inexpensive and reliable solid state, rather than mechanical components having moving parts, permits the avoidance of many of the problems caused by the inherent mechanical limitations described above.

Accordingly, it is an object of this invention to provide, in X-ray and electron beam tube power supplies, an improved low cost and easily stabilized tube current control system having a fast response to changes in load conditions.

The servo motor control apparatus heretofore employed were inherently large in size and in weight. This was due to the large size of the component parts and also to the elaborate measures required to electrostatically isolate the high voltage portions of the resonant transformer from the current control apparatus. As a result, substantial amounts of power were expended in the operation of the servo motor control apparatus.

It is accordingly another object of this invention to provide a compact current control system for X-ray and electron beam tube power supplies, whereby large amounts of power may be controlled with a very small expenditure thereof.

Briefly stated and in accordance with the invention, in a resonant transformer power supply for an electron tube having an emitting electrode and accelerating electrodes, there is provided a system for controlling the tube current of said electron tube. Said system comprises an amplifier which is operative to detect the potential at the accelerating electrode most remote from said emitting electrode, said detected potential being determined by said tube current, and to produce an error signal corresponding to the deviation of the detected potential from a preselected level.

Pulsating means, comprising an inverter, is operative to pulsate a voltage applied to said emitting electrode by a source of DC. potential at a frequency determined by the magnitude of said determined tube current, the inverter output terminals being connected in series With said emitting electrode and said source of DC. potential. Electrostatic shielding means are provided to electrostatically isolate said source of DC. potential, said emitting electrode and said inverter which are all interconnected with the high voltage end of the high voltagewinding of said resonant transformer. By controlling the frequency of the pulses of current flowing through said emitting electrode, said inverter is operative to control the magnitude of the tube current thereby produced.

The novel features of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may be best understood by referring to the following description and accompanying drawing.

In the figure there is shown in schematic form, an X-ray tube current regulation system comprising an illustrative embodiment of the invention. In the following description it is understood that an X-ray tube is shown in the figure for illustrative purposes, and that a similar tube such as an electron beam tube, may be substituted therefor without requiring any significant modification in the accompanying circuitry.

In this circuit there is shown an X-ray tube 10 and a two stage D.C. amplifier 11 comprising potentiometer 12, transistors 13 and 14, and having a reference voltage determined by zener diode 15 connected in the emitter circuit of transistor 14. A pulse generator 16 is shown to include a unijunction transistor 17 and a series RC charging circuit comprising a resistor 18, a capacitor 19 and the primary winding 20 of a pulse transformer 21. An oscillator 22 com-prises a gate controlled rectifier 23 and includes a resistor 24 and a capacitor 25 in series arrangement, and the primary winding 26 of a signal transformer 27.

An inverter 28 comprising a gate controlled rectifier 29 includes a capacitor 30, a saturable current transformer 31 having a square loop characteristic, and a resistor 32 in series arrangement. Cathode 33 of rectifier 29 is connected to an intermediate point C on a winding 34 of transformer 31. A source of DC. potential which serves as the DC. voltage supply for inverter 28 comprises an end winding 45 disposed proximate to the high voltage end 41 of the secondary winding 62 of a resonant transformer 46, rectifiers 47 and 48, and output capacitor 49. Inverter 28, connected in series with filament 35 of X-ray tube 10 and said source of DC. potential provided across capacitor 4-9, is operative to produce pulses of current in filament 35 at a rate equal to the switching frequency of rectifier 29. The pulse duration of these current pulses is determined by the core characteristics of transformer 31. As a result, there is produced a flow of current between filament 35 and target 36 of X-ray tube 10 when filament 35 is negative with respect to target 36.

Inverter 28, filament 35 and the source of DC. potential comprising capacitor 49 are located within an electrostatic shield 37 suitably formed to electrostatically shield said members located therein. High voltage end 41, which swings from positive high voltage to negative high voltage, filament 35 and shield 37 are interconnected. Space 39, which is located between electrostatic shield 37 and outside wall 40 of a pressure vessel 38, is filled with insulating gas under pressure to provide a wall of insulation therebetween. High voltage winding 62 is disposed in space 39 with the gas therein providing adequate insulation between high voltage end 41 and wall 40. Extension 66 of X-ray tube 10, which is connected to wall 40 of pressure vessel 38, is at substantially ground potential. A coupling transformer 27, operative to couple the output of oscillator 22 to inverter 28, is disposed in gas-filled space 39 with. primary winding 26 located adjacent the inside face of wall 40 and secondary winding 43 located under electrostatic shield 37 arranged in such a manner as not to interfere with the magnetic fiux produced by primary winding 26. Secondary winding 43 is disposed at a suitable distance away from winding 26 so that the spacing therebetween is sufficient to provide adequate magnetic coupling between the primary and secondary windings 26 and 43 and at the same time provide the required electrostatic insulation under the particular operating conditions. A source of D.C. potential 44 serves as a DC voltage supply for amplifier 11, pulse generator 16 and oscillator 22.

Potentiometer 12 which is operative to sample the potential at low voltage end 63 of high voltage winding 62, is connected to end 63 through biasing resistor 64. Thus, as X-ray tube current flows through potentiometer 12, a voltage is produced thereacross which is proportional to said tube current. A preselected portion of this voltage is applied by means of wiper arm 65, to the input of amplifier 11 at base 50 of transistor 13. Diodes 51 and 52 function to compensate for variations in the potential drop across the base emitter portion of transistor 13 due to temperature variations therein. A zener reference voltage maintains transistor 14 non-conductive until the sampled input voltage applied to the input of amplifier 11 and thus amplified, sufficiently exceeds the voltage of reference 15 and causes transistor 14 to conduct. The output of amplifier 11 is connected to pulse generator -16 by means of a diode 53 and controls the repetition frequency of the trigger pulses produced by pulse generator 16. Since the collector to emitter path of transistor 14 is in parallel with capacitor 19, transistor 14, when conducting, is operative to controllablly shunt a suitable portion of the charging current for capacitor 19 and hence control the pulse repetition frequency of pulse generator 16. The output trigger pulses of pulse generator 16 are suitably applied to the pulse oscillator 22 to control the repetition frequency of gating pulses produced by oscillator 22.

When inverter 28 is just starting up, there will be no current flowing in X-ray tube 10 and hence there will be no voltage across potentiometer 12, and base 50 will therefore be at ground potential, thus causing both transistors 13 and 14 to be non-conductive.

Pulse generator 16, comprising unijunction transistor 17, has its emitter 54 connected to the junction of resistor 18 and capacitor 19. Transistor 17 has its base 55 connected to the positive terminal of source 44 through load resistor 56 and its base 57 connected to the negative terminal thereof. Transistor 17 is rendered conductive when the voltage on emitter 54 reaches a. preselected level corresponding to the firing potential of transistor 17. This level is determined by the magnitude of source 44 and the operating characteristics of transistor 17. Since resistor 18 and capacitor 19 have fixed values, the time necessary for the charge on capacitor 19 to reach this firing potential is inversely proportional to the magnitude of the charging current through capacitor 19. When transistor 17 is thus rendered conductive, capacitor 19 discharges via a discharge path comprising primary winding 20 of pulse transformer 21, capacitor 19, capacitor 58 and base 55 to base 57 diode path to ground. The time variant discharge current flowing through winding 20 thereby produces a voltage pulse thereacross to constitute the output trigger pulse.

It is evident that the repetition frequency of the trigger pulses thus produced by pulse generator 16 will be inversely proportional to the period of time required for capacitor 19 to charge up to the firing potential of transist or 17. Since the collector to emitter conduction path of transistor 14 is connected in shunt with capacitor 19,

the magnitude of charging current through capacitor 19 will vary inversely with the level of conduction of transistor 14, which, as pointed out above, is determined by the magnitude of the tube current flowing through potentiometer 12. Thus, the repetition rate of the trigger pulse output of pulse generator 16 will vary inversely as the magnitude of the tube current.

During the initial warmup period, due to the absence of tube current, transistor 14 will be non-conductive and hence no part of the charging current flowing through resistor 18 will be diverted through transistor 14 by means of diode 53. During this period, therefore, capacitor 19 will be charging up at a maximum rate and consequently the trigger pulse rate of pulse generator 16 will also he at a maximum.

Oscillator 22 comprises a series charging circuit comprising primary winding 26 of coupling transformer 27, and the cathode to 'an-ode conduction path of gate controlled rectifier 23. When transistor 17 is rendered conductive, the trigger pulse thereby produced across primary winding 20 is applied by secondary winding 59 across the gate and cathode terminals 60 and 61 respectively to thereby turn rectifier 23 ON. Capacitor 25 at this point has been charged up through resistor 24. Capacitor 25 now discharges via :a discharge path comprising the anode to cathode diode path of rectifier 23 and Winding 26 to ground, thereby causing this discharge circuit to oscillate. Upon reversal of current in this discharge path, rectifier 23 is returned to the blocking state. The short pulse of current thus produced by this oscillatory action generates a voltage pulse across primary winding 26 to produce a corresponding pulse on secondary winding 43 due to the magnetic coupling existing therebetween. The pulse thus produced on Winding 43 constitutes the gating pulse which controls the switching operation of inverter 28 as described below.

The function of rectifiers 47 and 48 and capacitor 49 is to rectify and filter the AC. voltage obtained from end winding 45 to thereby provide a DC. supply for the operation of inverter 28.

Consider one cycle in the operation of inverter 28. Initially in the cycle, point A is at substantially zero potential. When the trigger pulse is applied to gate 65, thereby turning rectifier 29 ON, substantially the full line voltage appearing across capacitor 49 will be applied across portion N of winding 34, quickly driving the core of transformer 31 to negative saturation. Due to such saturation, capacitor 30 will charge rapidly toward the supply potential, i.e., the potential at point B, and the then increasing load current will tend to drive the core toward positive saturation. Between positive and negative saturation, due to autotransformer action, point A on capacitor 30 will be charged above the potential at point B, to a level determined by the magnitude of the load current flowing through N When positive saturation is reached, portion N will be essentially short circuited and the potential across capacitor 30 will reverse bias rectifier 29 thereby turning it OFF. The RC time constant of the charging circuit comprising capacitor 30, winding 34 and resistor 32 will determine the period of time that rectifier 29 is reverse biased. These components are therefore chosen so that the reverse bias will be applied to rectifier 29 for a period of time sufficient to render it non-conductive until it is again similarly triggered in a subsequent cycle.

Clearly, the frequency of the pulses of current in filament 35 thus produced by inverter 28 Will be controlled by the repetition frequency of the gating pulses applied thereto. The gating pulse rate is, in turn, controlled by the trigger pulse repetition rate. Thus, as filament 35 warms up and the tube current accordingly increases, the frequency of inverter 28 will correspondingly be reduced until the preselected stable operating conditions are achieved. It is evident, therefore, that the feedback circuitry described above is operative to compensate for variations in operating conditions to thereby maintain a preselected stable level of current flow in X-ray tube While the invention has been described by reference to a particular embodiment thereof, it will be understood that numerous modifications may be made by those skilled in the art without departing from the invention and it is, therefore, aimed in the appended claims to cover all such equivalent variations as fall within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. Apparatus for controlling the tube current of an electron tube having an emitting electrode and accelerating electrodes, said tube current being determined by the emission characteristics of said emitting electrode and the potential on the respective accelerating electrodes comprising, a voltage applied to said emitting electrode, pulsating means for pulsating said voltage applied to said emitting electrode, means for determining the magnitude of said tube current, and frequency control means operative in response to said determining means to control the frequency of said pulsating means in accordance with the magnitude of said determined current.

2. Apparatus as in claim 1 wherein the magnitude of said tube current is determined by the magnitude of the potential on the accelerating electrode most remote from said emitting electrode.

3. Apparatus as defined in claim 1 wherein said fre quency control means comprises signal coupling means to connect the output of said frequency control means to the input of said pulsating means.

4. Apparatus as defined in claim 3 wherein said frequency control means comprises oscillator means, said pulsating means being operative at a frequency corresponding to said oscillator frequency.

5. Apparatus as defined in claim 4 wherein said coupling means comprisese transformer means having primary and secondary windings, said primary winding being the output of said oscillator means and said secondary winding being the input of said pulsating means.

6. Apparatus as in claim 4- wherein said oscillator means is operative to render said pulsating means operative at a frequency determined by the magnitude of said determined current.

7. Apparatus as in claim Sin combination with a source of DC. potential wherein said pulsating means comprises inverter means, said emitting electrode, said source of DC. potential and the output terminals of said inverter means being in series arrangement with said emitting electrode and said inverter means being operative to control the frequency of the pulses of current flowing in said emitting electrode to thereby control said tube current.

8. Apparatus for controlling the tube current of an electron tube having an emitting electrode and accelerating electrodes, a resonant transformer including a secondary winding having one end thereof at a high potential with respect to a ground reference point, said tube current being determined by the emission of said emitting electrode and the potential applied to said accelerating electrodes by said secondary winding, a voltage applied to said emitting electrode, means for pulsating said voltage applied to said emitting electrode, means for determining the magnitude of said tube current, frequency control means operative in response to said tube current determining means to control the frequency of said voltage pulsating means in accordance with the magnitude of said determined current, and shielding means to electrostatically shield the portions of said power supply situated at said high potential from said ground reference point.

9. Apparatus as defined in claim 8 whrein said shielding means comprises, an electrostatic shield surrounded by an enclosure, said voltage pulsating means being within said electrostatic shield.

10. Apparatus as defined in claim 9 wherein said shielding means includes electrostatic insulating material in the space between said shield and said enclosure, said emitting electrode and said resonant transformer secondary winding being disposed within said space between said shield and said enclosure.

11. Apparatus as defined in claim 10 wherein said emitting electrode and said high potential end of said resonant transformer secondary winding are connected to said shield, said enclosure being connected to said ground reference point.

12. Apparatus as defined in claim 11 wherein said frequency control means comprises oscillator means and signal coupling means, said signal coupling means being operative to connect the output of said frequency control means to the input of said pulsating means, said sig' nal coupling means including transformer means having primary and secondary windings, said primary winding being disposed within said space between said shield and said enclosure and said secondary winding being located within said electrosatic shield.

13. Apparatus as defined in claim 7 wherein said pulsating means includes inverter means comprising, a gate controller rectifier having the anode connected to the positive terminal of said source of DC. potential, the cathode being connected to one terminal of said secondary winding of said resonant transformer and the gate electrode connected to the other terminal of said secondary winding, a saturable current transformer having a square loop core characteristic, a first capacitor connected between said anode and one end terminal of said current transformer winding, current conducting means connected between said cathode and an intermediate point on the winding of said current transformer, a resistor connected between the other end terminal of said current transformer winding and one output terminal of said inverter means, the other output terminal of said inverter means connected to the negative terminal of said D.C. source, said emitting electrode being connected in series with the output terminals of said inverter means and said source of"D.C. potential.

References Cited by the Examiner UNITED STATES PATENTS 2,850,676 9/1958 Kan et al 328267 X 2,945,160 7/1960 Burk 315-106 3,072,822 1/1963 Holmes 315-107 3,275,883 9/1966 Watters 315-406 FOREIGN PATENTS 1,144,508 2/1963 Germany.

JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner.

Claims (1)

1. APPARATUS FOR CONTROLLING THE TUBE CURRENT OF AN ELECTRON TUBE HAVING AN EMITTING ELECTRODE AND ACCELERATING ELECTRODES, SAID TUBE CURRENT BEING DETERMINED BY THE EMISSION CHARACTERISTICS OF SAID EMITTING ELECTRODE AND THE POTENTIAL ON THE RESPECTIVE ACCELERATING ELECTRODES COMPRISING, A VOLTAGE APPLIED TO SAID EMITTING ELECTRODE, PULSATING MEANS FOR PULSATING SAID VOLTAGE APPLIED TO SAID EMITTING ELECTRODE, MEANS FOR DETERMINING THE MAGNITUDE OF SAID TUBE CURRENT, AND FREQUENCY CONTROL MEANS OPERATIVE IN RESPONSE TO SAID DETERMINING MEANS TO CONTROL THE FREQUENCY OF SAID PULSATING MEANS IN ACCORDANCE WITH THE MAGNITUDE OF SAID DETERMINED CURRENT.

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