Power supply circuits

Abstract: A power supply (C) for an x-ray tube filament (10) selectively functions either as a constant voltage source (12) or a constant current source (14). A control circuit (A) includes a voltage controller (56) for enabling the power supply to operate as the constant voltage source in a first time period t₁ to t₂ during which filament temperature (40) is increasing toward a preselected filament operating temperature (42). Once the filament temperature has substantially reached its operating temperature at a time t₂, a timer (52) enables a current control (84) which causes the power supply to supply a constant current across the filament.

Description: This invention relates to an x-ray tube filament power supply circuit and a method of controlling an x-ray tube filament current. The current through the filament of an x-ray tube is one of the parameters that is controlled during an x-ray exposure. Most commonly, the filament is connected in series with a constant current supply. A current limiting or controlling device is placed in series between the constant current source and filament for controlling the amount of current flowing through the filament. The amount of current controls the amount of energy put into the filament, hence its temperature. The temperature affects the rate at which electrons are boiled off, hence the tube current or electron flow between the cathode and anode. If the x-ray tube were operated while the filament is overheated, the patient would be over irradiated and the anode could be damaged. Commonly, the filament is only brought up to temperature for an exposure and is at a reduced temperature between exposures. One of the problems with a constant current source is that it brings the filament up to the selected operating temperature relatively gradually. One solution for bringing the filament up to its operating temperature is described in U.S. Patent No.4,775,992 to Resnick and Dupuis. To bring the filament up to temperature more quickly, a current boost is applied when the current to the filament is first turned on. That is, instead of supplying the normal operating current to the filament, a higher current is provided for a preselected short duration. Commonly, a current limiting device is disposed between the current source and the filament to prevent the filament from being overdriven. That is, the current limiting device keeps the filament from receiving a current that would heat the filament to a temperature at which a tube current is produced that will heat damage the anode. In some x-ray tube power supplies, a voltage source rather than a current source drives the filament. With a voltage source, the power delivered to the filament is proportional to the V²/R, where V is the voltage and R is the filament resistance. Because the filament resistance is low, when the filament is cool and increases as the filament becomes warmer, the actual current flowing through the filament from a constant voltage source is higher initially and drops off towards the steady state operating current as the filament warms. This provides a built-in protection against overheating a filament still hot from the preceding exposure. Although a voltage source brings the filament up to temperature more quickly, it is relatively difficult to control. Typically, the x-ray tube current is relatively high and its resistance relatively low. Moreover, the same current flows through relatively long power supply cables between the power supply and the x-ray tube. The resistance of the cables tends to exceed the resistance of the filament. Because the actual current supply is controlled by the V²/R relationship in which R is the sum of these resistances, the filament current control accuracy is much worse than with a constant source. According to one aspect of the present invention an x-ray tube filament power supply circuit is provided, which comprises: a power supply which is connected to a cathode filament of an x-ray tube and which selectively functions as a constant current source or a constant voltage source; and control means arranged to selectively cause the power supply to function as a constant voltage source to provide a constant voltage across the x-ray tube filament as the x-ray tube filament is being brought up to a selected operating temperature and arranged to cause the power supply to function as the constant current source, after the x-ray tube filament has substantially reached the operating temperature to provide a constant current through the filament to maintain the filament substantially at the selected temperature. According to another aspect of the present invention, there is provided a method of controlling an x-ray tube filament current which comprises supplying a constant voltage across an x-ray tube filament as the filament is being brought up toward a selected operating temperature; and once the x-ray tube filament has substantially reached the operating temperature, providing a constant current through the filament to maintain the filament substantially at the selected temperature. One advantage of the present invention is that it brings an x-ray tube filament up to temperature quickly. Another advantage of the present invention is that it controls x-ray tube filament current accurately. Yet another advantage of the present invention is that it reliably protects the x-ray tube anodes from excessive tube currents caused by overheated filaments. One x-ray tube filament power supply circuit in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic illustration of the x-ray filament control circuit; Figure 2 is a diagrammatic illustration of the operation of the control circuit of Figure 1 and the resultant effect on x-ray tube filament temperature, hence tube current; and Figure 3 is a more detailed diagram of the x-ray tube filament control circuit shown in Figure 1. With reference to Figure 1, a control means A such as a timer selectively controls (i) when a cathode filament 10 of an x-ray tube B is held at a low stand-by current, (ii) when the filament is being heated, and (iii) when the filament is being held at a selected operating temperature which produces a selected current between the cathode filament and an anode of the x-ray tube. The control means A controls a power supply C connected in series with the filament 10 such that the power supply acts as a constant voltage source 12 when the filament is being warmed or brought up to temperature and as a constant current source 14 when the x-ray tube B is projecting a beam 16 of radiation across a patient receiving region to an x-ray sensitive medium 18. An active current limiting means 20 limits the current flow through the x-ray tube filament. With reference to Figure 2, the control means A connects a switch means 30 to a low current stand-by power supply 32 when the x-ray tube is in stand-by, denoted in Figure 2 as times t o to t₁. During an initial pre-exposure period between t₁ and t₂, the switch means 30 is connected with the constant voltage source 12. The current through the filament 10 is proportional to V²/R, where V is the voltage of the constant voltage source 12 and R is the resistance through the filament. The resistance of the filament varies with temperature. It has a relatively low resistance when cold and a higher resistance when warm. This causes the current through the filament to be a maximum 34 at time t₁ decaying generally exponentially 36 toward a steady state operating current 38. In this manner, a current boost is caused following time t₁. It is to be appreciated that if the x-ray tube has been operated recently and the filament is still relatively warm, the initial current spike at 34 is lower. In this manner, the size of the current boost is self regulating in accordance with filament temperature. This prevents the filament from being driven beyond the selected operating temperature regardless of whether the filament is warm or cold. The initial current boost 34 causes the temperature to rise relatively rapidly 40 towards a selected operating temperature 42 relative to a slower heating rate 44 of a constant current source. At time t₂ when the filament current and temperature has substantially reached their steady state operating conditions, the control means A causes the switching means 30 to connect the constant current source 14 in series with the filament. The control means A may include a filament current sensor, a timer, or the like. The constant current source produces a constant current of the magnitude of the steady state current 38. By holding the current substantially constant, the temperature of the filament is held substantially constant at the selected operating temperature 42. Of course, the control means A may include an appropriate control to bring the filament to any one of a plurality of selected operating temperatures, such as by switching in one of a plurality of constant current sources or adjusting the level of the constant current source. This enables the x-ray tube to be operated at a selectable tube current (mA). With reference to Figure 3, the control means A includes an operator panel 50 on which the operator selects exposure parameters, such as the tube current, exposure duration, and operating voltage for the x-ray tube. An appropriate enable pulse is sent to a timer 52 such that at time t₁, an enable pulse is provided on a pre-exposure initiate output 54. The pre-exposure output causes an adjustable current source 14′ to function as an effective constant voltage source. A voltage tap 60 and a common tap 62 measure a voltage across the filament 10. More specifically, the voltage is measured across a transformer 64 which is connected across the filament, thus measuring the filament voltage indirectly. A voltage control means 70 selectively adjusts the current level of the current source 14′ such that the voltage across the filament is held constant. The constant voltage control means 70 in the illustrated embodiment includes a current to voltage converting means 72 which converts the filament current that corresponds to the selected tube current at the selected tube voltage to a corresponding selected filament voltage. A comparing means 74 compares the monitored voltage with the selected voltage from the voltage converting means 72 and produces an output signal in accordance with the difference therebetween. In this manner, the level of the current generated by the current source 14′ is controlled such that the filament voltage is held constant, i.e. functions as an effective constant voltage source. A current sensing tap 80 is connected on the opposite side of a resistor 82 that is in series with the current source 14′ and effectively in series with the filament 10. The voltage across the resistor 82 is proportional to the current through the filament 10, hence acts as a filament current feedback signal. A current control means 20' controls the current source 14′ to maintain the current substantially at the selected filament current. Of course, if the transformer 64 has an other than one to one winding pattern, the current source current would be maintained at a current which is the same ratio relative to the filament current as the turns ratio of the transformer 64. The current control means 20′ in the illustrated embodiment includes a comparing means 84 which compares the selected filament current, or a multiple thereof as determined by the turns ratio of the transformer and the magnitude of the resistor 82, with the sensed filament current and controls the current source 14′ in accordance with the difference therebetween. The timing means 52 enables the constant voltage control means 70 to control the current source 14′ between times t₁ and t₂ and causes the current control means 20' to control the current source 14′ after time t₂. In the illustrated embodiment, the timer 52 includes an output 90 which causes a switching means 92 to connect the current source 14′ with a stand-by control value such that a low current is supplied to the filament when no exposure is imminent. At time t₁, the timer provides an output on output 54 which causes the switching means 92 to connect the filament voltage comparing means 74 with the current source 14′ in a controlling relationship. When the steady state filament operating temperature has been attained, e.g. at time t₂, the timing means 52 provides an output on output 94 which causes the switching means 92 to connect the filament current comparing means 84 to the current source 14′ in a controlling relationship thereto. Alternately, the timing means might use the output 54 to enable comparing means 74 and the output on line 94 to enable the filament current comparing means 84. Of course, the control means may measure the filament temperature or current directly rather than allocating an estimated time.

Claims: An x-ray tube filament power supply circuit characterised in that it comprises: a power supply (C) which is connected to a cathode filament (10) of an x-ray tube (B) and which selectively functions as a constant current source (14) or a constant voltage source (12); and control means (A) arranged to selectively cause the power supply (C) to function as a constant voltage source (12) to provide a constant voltage across the x-ray tube filament as the x-ray tube filament (10) is being brought up to a selected operating temperature and arranged to cause the power supply to function as the constant current source (14), after the x-ray tube filament has substantially reached the operating temperature to provide a constant current through the filament to maintain the filament substantially at the selected temperature. A circuit as claimed in Claim 1, further comprising current limiting means (20) for limiting the current supplied by the constant current source (14) to the filament; and wherein the control means (A) includes switching means (30, 92) for selectively switching one of the constant voltage source (12) and the constant current source (14) into electrical communication with the filament. A circuit as claimed in Claim 2, further including timing means (A, 52) for controlling the switching means such that the constant voltage source is switched into communication with the x-ray tube filament for a first preselected duration (t₁ to t₂) and for connecting the constant current source with the x-ray tube filament for a second preselected duration (t₂ onwards) following the first preselected duration. A circuit as claimed in anyone of Claims 1 to 3, wherein the control means includes filament voltage sensing means (60, 62) for sensing a voltage across the filament and for producing a sensed filament voltage signal indicative thereof and constant voltage controlling means (70) for controlling the power supply such that the sensed voltage is maintained substantially at a preselected voltage. A circuit as claimed in any one of the preceding claims, wherein the control means includes filament current sensing means (80) for sensing current flow through the filament and producing a sensed filament current signal indicative thereof, and means (84) for comparing the sensed filament current signal with a selected filament current and controlling the power supply such that the filament current is held substantially at the selected filament current. A circuit as claimed in Claim 2, comprising a current source means (14′) having a selectively adjustable current level, filament voltage monitoring means (74) for monitoring voltage across the filament and to thereby control the current source means (14′) and filament current monitoring means (84) for monitoring a current through the filament and to thereby control the current source means (14′); and, wherein the switching means (92) selectively controls which of the filament voltage and current monitoring means is connected with the adjustable current source means (14′). A method of controlling an x-ray tube filament current characterised in that it comprises: supplying a constant voltage across an x-ray tube filament (10) as the filament is being brought up toward a selected operating temperature (42); and once the x-ray tube filament has substantially reached the operating temperature, providing a constant current (38) through the filament to maintain the filament substantially at the selected temperature.

Inventors: SAMMON ROBERT J, SAMMON, ROBERT J.
Applicants: PICKER INT INC, PICKER INTERNATIONAL, INC.
Application year: 1991
Main classification: H05G1
IPC codes: H05G1