US3567995A

US3567995A - Current stabilizer circuit for thermionic electron emission device

Info

Publication number: US3567995A
Application number: US751927A
Authority: US
Inventors: Peter O Lauritzen; Cormack E Boucher
Original assignee: Automation Industries Inc
Current assignee: Automation Industries Inc
Priority date: 1968-08-12
Filing date: 1968-08-12
Publication date: 1971-03-02
Anticipated expiration: 1988-03-02

Abstract

A current stabilizer circuit for a thermionic electron emission device is described in which the cathode heating winding of a power supply transformer is connected to the high-voltage winding through a heating current control circuit. The control circuit senses changes in the emission current of an X-ray tube or other electron emission device having a thermionic cathode, and produces a control signal which is applied to a variable impedance means connected in series with the cathode filament to vary the heating current in order to maintain such emission current substantially constant. The control circuit may operate by series regulation or by pulse duration modulation of the cathode heating current to change the average heating current and temperature of the cathode and thereby vary its electron emission in order to stabilize the emission current in spite of changes in the power supply voltage which tend to vary such emission current.

Description

United States Patent [72] Inventors Peter O. Lauritzen;

Cormack E. Boucher, Seattle, Wash. [2]] Appl. No. 751,927 [22] Filed Aug. 12, 1968 [45] Patented Mar. 2, 1971 [73] Assignee Automation Industries, Inc.

Los Angeles, Calif.

[54] CURRENT STABILIZER CIRCUIT FOR THERMIONIC ELECTRON EMISSION DEVICE 2 Claims, 3 Drawing Figs.

[52] U.S.Cl 315/311, 250/97, 250/103, 307/297, 315/302 [51] Int. Cl. 1105g H34 [50] Field of Search 250/97, 99, 103; 307/297; 315/291, 302, 311

[5 6] References Cited UNITED STATES PATENTS 2,617,045 11/1952 Coe 250/97 2,810,838 10/1957 Clappetal 3,327,131 6/1967 Grimmer ABSTRACT: A current stabilizer circuit for a thermionic electron emission device is described in which the cathode heating winding of a power supply transformer is connected to the high-voltage winding through a heating current control circuit. The control circuit senses changes in the emission current of an X-ray tube or other electron emission device having a thermionic cathode, and produces a control signal which is applied to a variable impedance means connected in series with the cathode filament to vary the heating current in order to maintain such emission current substantially constant. The control circuit may operate by series regulation or by pulse duration modulation of the cathode heating current to change the average heating current and temperature of the cathode and thereby vary its electron emission in order to stabilize the emission current in spite of changes in the power supply voltage which tend to vary such emission current.

'RECI'IFIERL CONSTAN T 11c. VOLTAGE 42 CURRENT REG.

SOURCE I v Ic 44 H PATENTEDMAR 2|97I 3567.995 SHEET 1 BF 2 X-RAY TUBE FIG. I IE f 1H s 30 r A ,la

'RECTIFIER& ,icoNsTAN T D.C.VOLTAGE 42 CURRENT c REG.

SOURCE K IR E 7 22 I9 Ic 44 IHv N SWITCH IN LEVEL 2 SWITCHING 1. EVEL. 1 I

VOLTAGE V AT POINT Bl VOLTAGE v AT POINT s PETER O. LAURlTZEb CORMACK E. BOUCHEF INVENTORS BUCKHORN, BLORE, KLAROU/S 7' 8 SPAR/(MAN ATTORNEYS PATENTEDMAR 2mm 3.667.995

SHEET 2 BF 2 PETER O. LAURITZEN CORMACK E. BOUCHER INVENTORS. BY

BUG/(HORN, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS CURRENT STABILIZER CIRCUIT FOR THERMIONIC ELECTRON EMISSION DEVICE BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to the stabilization of emission current in an electron emission device employing a thermionic cathode to maintain such emission current substantially constant, and in particular to an improved stabilizer circuit which compensates for changes in any supply voltage applied across such device which otherwise would tend to vary the emission current. More specifically, the present current stabilizer circuit includes a control circuit connected between the high-voltage cathode winding and the cathode heating winding of a power supply transformer. The control circuit senses changes in emission current and derives a control signal therefrom which is applied to a transistor or other variable-impedance element connected in series with the cathode filament to vary the heating current in an opposite manner to the changes in emission current. In one embodiment the control circuit operates by series regulation of the heating current, while in another embodiment it operates by pulse duration modulation of such heating current.

The current stabilizer circuit of the present invention may be employed with an X-ray tube having a filament cathode but may also be employed with other electron emission devices such as electron beam welders, cathode ray tubes, electron microscopes which employ thermionic cathodes and are operated in a temperature limited thermionic emissionmode so that the cathode temperature controls the amount of anode-to-cathode emission current. i

The current stabilizer circuit of the present invention has several advantages over previous stabilizer circuits such as that shown in US. Pat. No. 2,617,045 of M. R. Coe and US. Pat. No. 2,627,035 of J. Ball which employ separate heater transformers to supply heating current to the cathode filament of the X-ray tube. These heater transformers are quite bulky and expensive since they must be insulated for the high voltage applied to the cathode of the X-ray tube. The present circuit avoids the need for such a highly insulated filament transformer by providing the heating winding as a secondary winding on the same transformer core as the high-voltage cathode winding and connecting the control circuit between such high voltage winding and such heating winding.

The stabilizer circuit of the present invention has the advantage of providing an electron emission apparatus of smaller size, light weight, whose emission current is maintained substantially constant over a wide range of anode-to-cathode voltage adjustments. The present circuit is simpler and more reliable than previous current stabilizer circuits and operates in a fast, accurate manner. Also the pulse duration modulation embodiment of the present circuit has the additional advantage of dissipating very little power since the transistors in the control circuit operate as switches. As a result no special provisions for heat removal are needed, which would be difficult since the circuit is not connected to ground potential except through the high-voltage secondary winding of the power supply transformer so that any heat conduction path to the chassis or other grounded heat sink would have to be highly insulated to prevent current leakage. This connection has the advantage that the current stabilizer circuit and the high-voltage cathode winding can be contained within the same insulated housing with no external connections, to provide the small size and light weight.

It is therefore one object of the present invention to provide an improved current stabilizer circuit for an electron emission device having a thermionic cathode, which maintains the emission current of such device substantially constant in spite of variations in power supply voltage.

Another object of the present invention is to provide an improved current stabilizer circuit which is of small size and light weight and which operates in a simple, reliable and accurate manner.

Still another object of the invention is to provide an improved X-ray apparatus including a current stabilizer circuit which is connected to ground only through the high-voltage secondary winding of a power supply transformer and whose supply potential is added to the high voltage on such winding to enable such circuit to be contained within the same insulated housing as the high-voltage secondary winding of such transformer with no external connections.

A further object of the invention is to provide an improved current stabilizer circuit of low power dissipation which employs pulse duration modulation to vary the cathode heating current of an electron emission device in order to maintain the emission current of such device substantially constant in spite of voltage variations at the power supply transformer.

An additional object of the present invention is to provide an improved current stabilizer circuit for an X-ray apparatus in which a control circuit is employed between the cathode heating transformer winding and the high-voltage cathode winding of the same power supply transformer in order to maintain the emission current of the X-ray tube substantially constant while enabling the anode-to-cathode voltage of the X-ray tube to be varied over a wide range.

BRIEF DESCRIPTION OF DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a schematic diagram of a current stabilizer circuit in accordance with the present invention which can be operated either in a series regulation mode or a pulse duration modulation mode;

FIG. 2 is a detailed circuit diagram of the preferred embodiment of the current stabilizer circuit of the present invention which operates by pulse duration modulation; and

FIG. 3 shows the waveforms of electrical signals produced during the operation of the circuit of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS As shown in FIG. 1, an X-ray tube 10 or other electron emission device, has its anode 12 and filament cathode 14'- connected to high-voltage transformers I6 and 18, respectively, which provide an AC power supply for such tube of highpeak voltage. Of course the power supply can also be a single high-voltage transformer having a grounded center tap on its secondary winding for supplying both the positive and negative high voltage at the opposite ends of such winding. ln order to vary the anode-to-cathode voltage across the X-ray tube 10, a variable auto transformer 19 may be provided in the primary winding circuits of the high voltage transformers l6 and 18 whose primary windings 20 and 22, respectively are connected in parallel across an AC power supply 24, which may be conventional volts 60 c.p.s. AC powerline. The anode 12 of the X-ray tube is connected to one terminal of a highvoltage secondary winding 26 on transformer 16, while the cathode 14 is connected through the control circuit of the present invention to one terminal of a high voltage secondary winding 28 on transformer 18. The other terminals of the

secondary windings

26 and 28 are grounded. The polarity of the

secondary windings

26 and 28 is such that for one-half cycle a positive voltage is applied to the anode and simultaneously a negative voltage is applied to the cathode to provide a total peak voltage of, for example, approximately kilovolts across the X-ray tube, which causes a current to flow between the cathode and anode of such tube of about 2 to 3 milliamperes.

Another secondary winding 30 forming the heating winding is provided on the high-voltage end of transformer 19 with its opposite ends connected together through

rectifiers

32 and 34 and their common anode terminal is connected to one terminal of the cathode filament 14, while the other terminal of such filament is connected to a centertap on such heating winding. When the variable auto transformer 19 is adjusted to change the voltage applied between the anode and cathode of the X-ray tube, it causes a corresponding change in heating voltage across secondary winding 30. This tends to vary the heating current and the electron emission of the cathode which causes an undesirable change in emission current of the X-ray tube. The current stabilizer circuit of the present invention maintains this emission current substantially constant in spite of changes in power supply voltage due to adjustment of auto transformer 19 and other factors such as changes in emission characteristics of the filament caused by aging of the X- ray tube or tube replacement. It should be noted that while the X-ray tube shown is a diode, it may be a triode or any other tube which operates with temperature limited thermionic emission, rather than space charge-limited emission, so that the filament temperature alone controls the amount of emission current. This enables variations in heating current to cause corresponding changes in the emission current.

The current stabilizer circuit of the present invention includes a signal controlled variable impedance connected in series with filament 14. The variable impedance may be in the form of a transistor 36 of the NPNv type having its collector connected to one terminal of the cathode filament 14 at point B and its emitter connected to the anodes of rectiflers32 and 34 at point A. The base of transistor 36 is connected to the output of a control amplifier 38 whose input is connected through a coupling resistor 40 to the "high-voltage terminalof transformer winding 28. A source of substantially constant current 42 is also connected to the same high-voltage terminal of transformer winding 28 at a current summing point 44 to provide a current comparator means as hereafter discussed. Current source 42 provides a DC reference current (I of predetermined value from which the high-voltage current (I cathode winding 28 is subtracted to produce a correction current (1 which is applied as a control voltage input signal to the input of amplifier 38. The amplifier 38 applies the control signal corresponding to such correction current, to the base of the transistor 36 in order to vary the impedanceof such transistor and change the heating current flowing through filament 14. Such a control signal is produced when the current (1, in the high-voltage secondary winding 28 corresponding to the X-ray tube emission current (I differs from that of the reference current (I supplied by current source 42.

The DC power supply for the amplifier 38 and the current source 42 is provided by a rectifier and voltage regulator circuit 46, having its input connected across a third secondary winding 48 provided on transformer 18 at the high-voltage end of cathode winding 28. The regulated DC output voltage of the regulator circuit 46 is applied ,to the constant current source 42 and to the common terminal of the cathode filament Find the center tap of the heating winding 30. This point C at the output of the voltage regulator circuit 46 is effectively the signal ground of the circuit.

A shunt capacitor 50 is connected between the output of "the voltage regulator 46 and the coupling resistor 40 so that it forms a filter circuit with such coupling resistor for smoothing or averaging the correction 'current (I to provide a slowly v varying DC control current at the input of amplifier 38. This control current provides an input level for the amplifier which changes by an amount corresponding to the difference between high-voltage winding current (I and the reference current (I The output signal of the amplifier38 which is employed as the control signal applied to transistor 36,'may be the slowly varying DC signal produced on capacitor 50 to provide series regulation of the heating current or it may be the form of pulses whose width varieswith the value of such slowly varying DC signal to provide pulse duration modulation (PDM) of such heating current, depending upon the position of a switch 52. The switch 52 is a two position switch which in the PDM position shown connects point A to the input of the control amplifier 38 through an AC coupling network including a rechanges the heating current flowing through filament 14 in a manner opposite to the changes in the current flowing through secondary winding 28 corresponding to the anode-to-cathode emission current of the X-ray tube 10. Thus, decreases in emission current (I cause a decrease in the current (I in the high voltage winding 28 under the reference current (I which produces increase in correction current (I that is transmitted through the

filter network

40 and 50 to produce a slowly varying DC control signal. This control signal is applied to the base of transistor 36 at the output of the control amplifier rendering such transistor more conducting and increases the heating current flowing in filament l4. This increase in heating current raises the filament temperature which in turn increases the anode-to-cathode emission current in the tube until the current (1 flowing in secondary winding 28 again becomes substantially equal to the reference current (I provided by current source 42, so that the correction current decreases to a small value, thereby stopping further decreases in the heating current. As a result the emission current returns to its previous value, which is set'at a predetermined current by adjustment of the value of the reference current within the currentsource 42. It should be noted that the forward bias voltage for the control amplifier 38 is provided by the small amount of correction current (I which normally flows to charge the filter capacitor 50 to such bias voltage.

In the PDM position of the switch 52 in the circuit of FIG. 1, the transistors in the control amplifier 38 and the variable impedance transistor 36 are operated as switches, with transistor 36 being quiescently biased ON or conducting. In FIG. 3 nega tive half cycles 58 of the full wave rectified sine wave output signal of the heating current supplied through

diodes

32 and 34 to point A are shown with respect to the signal ground point C, as are the other signals in this FIG. The AC signal 58 is transmitted through the AC coupling

network including elements

54 and 56 to the input of control amplifier 38. The AC signal 58 flowing through the

AC coupling elements

54 and 56 periodically switches control amplifier 38 OFF to render transistor 36 nonconducting and produce

heating voltage pulses

60 or 62 at point B which terminate when signal 58 crosses switching levels'64 or 66. This switching OFF occurs when the sum of the current through resistor 54 and the current through resistor 40 causes the direction of current flow at the input of the amplifier 38 to be away from the amplifier. When the sum of the currents through

resistors

54 and 40 causes the net current to flow towards the amplifier input38 the transistor 36 is switched ON and rendered conducting to apply the instan taneous value of voltage 58 across the filament 14. The particular switching level 66 or 64 is determined by the control voltage signal produced on capacitor 50 by the correction current (I transmitted from the summing point 44. As a result heating voltage pulses 62 of narrower width Y are produced at point B when the control current through 40 is reduced and generates the switching level 66. Similarly heating pulses 60 of greater width X are produced when the control current through 40 is relatively larger causing the switching level to move to 64. This pulse duration modulation operation varies the average heating current of the filament which changes the temperature and electron emission of such filament to maintain the emission current constant. Pulse duration modulation has the added advantage that the control amplifier and transistor 36 dissipate less power, since they are operated as switches.

A detailed circuit diagram of a current stabilizer circuit employing such pulse duration modulation is shown in FIG. 2, in which the same reference numerals have been employed to designate elements similar to those in FIG. 1 and for this reason such elements will not be discussed. The control amplitier 38 includes an input transistor 68 of NPN type, having its base connected to resistor 40, its emitter connected to the anode of a Zener diode 70 in the voltage regulator 46, and its collector connected through a coupling resistor 72 to the base of a second transistor 74 of PNP type. The emitter of transistor 74 is connected through a resistor 76 to the cathode of another Zener diode 78 in the voltage regulator 46, while its collector is connected through a load resistor 80 to the base of the variable impedance transistor 36. In addition. a third transistor 82 of PNP type is provided in the control amplifier 38 with its base connected to the emitter of transistor 74, its emitter connected to a signal ground conductor 84 which con nects the center tap of the heating winding 30 to the common terminal of the cathode of diode 70 and the anode of diode 78. The collector of transistor 82 is connected through a load resistor 86 to the base of the transistor 36. p

A normally nonconducting Zener diode 87 may be connected across the emitter to collector circuit of transistor 36 to provide overvoltage protection for such transistor and a five ampere fuse 88 may be connected in series with a filament 14 to protect such filament against too high a heating current. A frequency compensation circuit including a diode 90 in series with a resistor 92 may be connected between the emitter of transistor 68 and the common terminal of resistor 40 and capacitor 50.

The rectifier and voltage regulator circuit 46 includes a full wave rectifier bridge formed by four

diodes

94, 96, 98 and 100. The anodes of

diodes

94 and 98 and the cathodes of

diodes

96 and 100 are respectively connected to the opposite terminals of a filter capacitor 102. The

Zener diodes

70 and 78 form part of the voltage regulator circuit with the cathode of diode 78 connected through a coupling resistor 106 to the upper terminal of capacitor 102 and the anode of diode 70 connected to the lower terminal of such capacitor. Capacitor 102 is charged to a DC voltage which is sufficient to cause reverse breakdown of the

Zener diodes

70 and 78. As a result a substantially constant voltage of about 10.7 volts is produced across the

Zener diodes

70 and 78 and applied as the supply voltage of the control amplifier 38. Of course the absolute values of voltage in the entire circuit including that on the Zener diodes vary with the AC voltage on the high-voltage secondary winding 28 to which they are connected. A coupling resistor 104 is connected between the upper terminal of such filter capacitor and the cathodes of

diodes

96 and 100.

The common connection at the anode of Zener diode 78 and the cathode of Zener diodes 70 is connected to the base of a transistor 108 of PNP type, forming the constant current source 42. The collector of transistor 108 is connected to the current summing point 44 and its emitter is connected through a variable resistor 110 to the cathode of Zener diode 78. The Zener diode 78 has a reverse breakdown voltage of 5.1 volts and is normally conducting to provide this voltage across the series circuit including the variable resistor 110 and the emitter-to-base junction of transistor 108, which maintains the emitter-to-collector current of such transistor substantially constant. The reference current (I flowing in the collector of transistor 108 may be varied by adjusting variable resistor 110. Thus the setting of this resistor determines the reference current and indirectly the emission current of the X-ray tube. It should be noted that the emission current of the X-ray is on the order of 2 to 3 milliamperes, while the heating current flowing through transistor 36 is ordinarily about 4 amperes RMS, with ampere peak values. Therefore transistor 36 through which the heating current flows must be a high-power transistor, but the

other transistors

68, 74, 82 and 108 of the control circuit may be of conventional type.

Typical values of the components employed in the circuit of FIG. 2 are as follows:

transistor 36 type 2N377l transistor 68

type 2N2473 transistors

74, 82 type 2N4030 transistor 108

type 2N3962 resistors

40, 72, 92 2.2 k!) capacitor 50 50 pf.

resistor 76 10 k!) 47 k 200 k [2 k!) l uf 5 kQ(variableI 100 k0.

resistor 54 resistor resistor 86 capacitor 56 resistor ll0 resistor [04 resistor I06 2 KO.

capacitor 102 15 uf.

diode type lN400l diode 87

type 1N3024 diodes

32 and 34 type MR1 l2] diode 78 5. 1 volts Zener diode 70 5.6 volts Zener.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above described preferred embodiment of the present invention without departing from the spirit of the invention. For example, pulse duration modulation can also be achieved through the

filter circuit

40 and 50 by causing such filter circuit to be less efficient in smoothing the correction signal, since the X- ray tube functions as a rectifier to provide half way rectified pulses of correction voltage which can be used in a similar manner to the negative pulses transmitted through the AC coupling network including resistor 54 and capacitor 56, thereby eliminating the need for these components. Therefore the scope of the present invention should only be determined by the following claims.

We claim: 1. A current stabilizer circuit for an electron emission device having a thermionic cathode, comprising:

transformer means including a high-voltage winding having one end connected for applying a high voltage to said cathode to cause emission current to flow in said device, and a heating winding for supplying heating current to said cathode, said heating winding being provided as a secondary winding on the same transformer core as said high-voltage winding; control circuit means connected between said one end of said high-voltage winding and said heating winding for producing a control signal proportional to variations of current in the high-voltage winding corresponding to changes in said emission current; variable-impedance means connected to said heating winding and said control circuit means for varying said heating current in response to said control signal, said variableimpedance means causing the heating current to vary in an opposite manner to the changes in said emission current to compensate for said changes in order to stabilize said emission current and maintain it substantially constant; said control circuit means including a source of substantially constant reference current, comparator means for subtracting said reference current and the current in said high voltage winding to produce a difi'erence current from which the control means derives the control signal, an amplifier having its input connected to the output of the comparator means, and filter means including a shunt capacitor connected across the input of said amplifier for changing the AC difference current to a slowly varying DC input level; and said amplifier also being connected at its input to the heating winding through an AC coupling capacitor and rectifier means to apply pulses of one polarity to the input of said amplifier which switch the amplifier between conduction and nonconduction when the amplitude of said pulses exceeds the DC input level and cause said amplifier to produce corresponding output pulses whose widths are modulated by changes in the value of said DC input level to provide said control signal.

2. A circuit in accordance with claim 1 in which the pulse duration modulated control signal is applied to the base electrode of a transistor having its emitter-to-collector circuit in series with the heating winding, for switching such transistor on and off to vary the amount of average heating current flowing through the cathode filament.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 1 r 995 Dat d March 2 1971 Peter O. Lauritzen and Cormack E. Boucher Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 35, after (I should be --flI in through the high voltage-;

Column 3, line 69, after "may be" should be --i:

Column 5, line 61, after "X-ray" should be --tu1 Signed and sealed this 20th day of July 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents

Claims

1. A current stabilizer circuit for an electron emission device having a thermionic cathode, comprising: transformer means including a high-voltage winding having one end connected for applying a high voltage to said cathode to cause emission current to flow in said device, and a heating winding for supplying heating current to said cathode, said heating winding being provided as a secondary winding on the same transformer core as said high-voltage winding; control circuit means connected between said one end of said high-voltage winding and said heating winding for producing a control signal proportional to variations of current in the high-voltage winding corresponding to changes in said emission current; variable-impedance means connected to said heating winding and said control circuit means for varying said heating current in response to said control signal, said variable-impedance means causing the heating current to vary in an opposite manner to the changes in said emission current to compensate for said changes in order to stabilize said emission current and maintain it substantially constant; said control circuit means including a source of substantially constant reference current, comparator means for subtracting said reference current and the current in said high voltage winding to produce a difference current from which the control means derives the control signal, an amplifier having its input connected to the output of the comparator means, and filter means including a shunt capacitor connected across the input of said amplifier for changing the AC difference current to a slowly varying DC input level; and said amplifier also being connected at its input to the heating winding through an AC coupling capacitor and rectifier means to apply pulses of one polarity to the input of said amplifier which switch the amplifier between conduction and nonconduction when the amplitude of said pulses exceeds the DC input level and cause said amplifier to produce corresponding output pulses whose widths are modulated by changes in the value of said DC input level to provide said control signal.