JP2588786B2 - X-ray power supply - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an X-ray power supply device using a series resonance type inverter.

[Problems to be solved by conventional technology and invention]

In recent years, an inverter type X-ray power supply device using a switching element such as a transistor or an FET has been put into practical use.
FIG. 6 is a diagram for explaining a conventional inverter type X-ray power supply device. 1 is a DC input power supply, 2 is an inverter, 3 is a step-up transformer represented by an equivalent circuit, 4 is a high-voltage rectifier, and 5 is a load (X-ray tube). The step-up transformer 3
It is represented by the exciting inductance L _O , the leakage inductance L _L , and the winding distribution capacitance C _P converted to the primary side. When the inverter 2 is high frequency, leakage inductance L _L, winding distributed capacitance C _P affects the operation of the inverter 2, the inverter 2 is series resonance with a series resonant circuit of the leakage inductance L _L and winding distributed capacitance C _P Move to the operation of the inverter. In this operation mode, the winding distributed capacitance C _P contributes only to voltage alternately reversed at the operating frequency f _s, the circuit circulating current is not supplied to the load i _S, reactive current increases in other words. This circulating current i _S causes switching loss and conduction loss of the switching element and resistance loss of the winding of the boosting transformer 3. This resistance loss increases as the frequency of the circulating current i _S increases, due to the covering effect of the winding material. This circulating current i
_S is, by its nature, proportional to the magnitude of the output voltage rather than the magnitude of the output power, and tends to increase as the output voltage, that is, the tube voltage, increases in the X-ray power supply device. By the way, in an X-ray power supply, for example, 10 kW
For example, 500 W (tube voltage of 100 kV) for observing X-ray fluoroscopic images on a CRT screen, and an imaging mode for supplying large power of (tube voltage 100 kV, tube current 100 mA) at low duty for several seconds.
V, tube current of 5 mA) requires a fluoroscopic mode that continuously supplies low power for a long time. In the X-ray power supply device using the series resonance type inverter, in the imaging mode, the power loss of the inverter 2 and the step-up transformer 3 increases as a sum depending on the output power and a portion depending on the circulating current.
Since the duty of the operation time is small, the temperature rise of the inverter 2, the step-up transformer 3 and the like is averaged. Therefore, the inverter 2, the step-up transformer 3 and the like are designed to have more margin than the normal design, so that the size and economy can be reduced. It is trying to make it. However, the power loss in the fluoroscopy mode is small depending on the output power, but is small because the tube voltage is high, and the circulating current is almost the same as in the imaging mode. Temperature rise cannot be ignored.

[Means for solving the problem]

SUMMARY OF THE INVENTION In order to eliminate the above-mentioned disadvantages, the present invention provides a series-resonant inverter in which a series resonant circuit composed of a resonant inductance and a resonant capacitance is connected equivalently to the output of an inverter composed of at least a pair of switching elements. In the X-ray power supply device for rectifying and outputting the voltage between both ends of the resonance capacitance, in order to perform normal X-ray imaging, the switching element is used in an imaging mode in which large power is supplied for a short time and with a low duty. The pulse width modulation or the phase modulation is performed at a substantially fixed frequency equal to or lower than the series resonance frequency, and the switching element is substantially fixed in a fluoroscopic mode in which small power is continuously supplied for a long time in order to observe an X-ray fluoroscopic image. An X-ray power supply device characterized in that pulse frequency modulation is performed at a frequency equal to or lower than the frequency.

[Action]

According to such an X-ray power supply device, in the imaging mode, the switching element is pulse-width-modulated or phase-modulated at a substantially fixed frequency equal to or lower than the series resonance frequency, and in the fluoroscopic mode, the switching element is pulsed at a frequency equal to or lower than the substantially fixed frequency. Since the frequency is modulated, the ripple in the imaging mode can be reduced, the effective value of the series resonance current in the fluoroscopic mode can be reduced, and the power consumption can be reduced even in the fluoroscopic mode. Since the current is small in the fluoroscopic mode, the ripple does not increase even if the frequency decreases.

〔Example〕

FIG. 1 is a diagram for explaining one embodiment of the present invention. In the figure, reference numeral 1 denotes a DC input power supply obtained by rectifying and smoothing a battery or a commercial AC power supply, and is usually 100 VDC to 30 VDC.
0V. Reference numeral 2 denotes a bridge-type inverter including four switching elements 6 to 9 and feedback diodes 10 to 13 connected in anti-parallel to the switching elements 6 to 9. 3 is a voltage required for converting the AC output voltage of the inverter 2 to 100 kV, for example.
This is a step-up transformer that steps up the voltage. The leakage inductance L _L of the step-up transformer 3 forms a series resonance circuit with the secondary winding distributed capacitance C _S together with the resonance inductance 14 provided as necessary. The secondary winding of the step-up transformer 3 is grounded at the midpoint and connected to the full-wave high-voltage rectifier 4. The DC output of the high-voltage rectifier 4 is connected between the anode of the X-ray tube as the load 5 and the filament. The filament power supply circuit of the X-ray tube is omitted because it is not directly related to the present invention. Usually, switching of the filament and adjustment of the filament power are performed in the imaging mode and the fluoroscopic mode. The resistors 15 and 16 are voltage dividers for detecting an X-ray tube voltage. The output voltage of the voltage divider is compared with the voltage of the tube voltage setting reference power supply 18 in the error amplifier 17 to generate an error signal. Here, the entire control circuit can be switched between the photographing mode and the fluoroscopic mode by the interlocked mode changeover switches 20 and 21, and the connection of the mode changeover switch in the figure is on the photographing mode side. The reference oscillator 22 determines the oscillation frequency based on the time constant of the resistor combined with the capacitor 23. For example, when the resistor 24 is selected by the mode changeover switch 21, the oscillator oscillates at 40 kHz, which is twice the inverter operating frequency of 20 kHz. The output of the reference oscillator 22 is supplied to a pulse width modulation circuit 25 and a maximum pulse width generation circuit 26. The pulse width modulation circuit 25 generates a pulse of 40 kHz pulse width modulated by a well-known method by comparing an error signal and a triangular wave, for example. The pulse width control signal is selected by the mode switch 20 and the AND circuit
Added to 27, 28. The AND circuits 27 and 28 further have a maximum pulse width signal of the maximum pulse width generation circuit 26 and a 40 kHz
A two-phase distribution signal from a flip-flop 29 for generating a 20-kHz two-phase signal from the signal is added. These signals cause the outputs of the AND circuits 27 and 28 to alternately
An ON signal having a phase of 20 kHz and a maximum pulse width of, for example, 20 μs is generated. These ON signals are applied to the control poles of the switching elements 6 to 9 via signal insulation means such as a pulse transformer or an optical isolator (not shown), and the switching elements are turned on at a fixed frequency of 20 kHz.

First, the control of the photographing mode will be described. In the photographing mode, the detection voltage of the tube voltage is compared with the voltage of the reference power supply 18 by the error amplifier 17 by the control circuit configuration, and the pulse width of the ON signal, that is, the duty is adjusted, so that the tube voltage becomes a constant voltage. Be transformed into FIG. 2 shows a computer simulation result at the time of output of 10 kW in such a shooting mode. The conditions are as follows: DC input power supply voltage is DC250V, resonance inductance is 15μH, resonance capacitance is 1.9μF
(The series resonance frequency is 30 kHz) and the on-pulse width is 20 μs. The horizontal axis in FIG. 2 is time, and a simulation period of 200 μs to 500 μs is displayed. Waveform 1 is a waveform diagram showing a two-phase pulse width control signal with positive and negative polarities, and waveform 2
The waveform diagram of a high pressure rectifier current I _O, the waveform 3 is a waveform diagram of resonance current I _l through the primary side of the step-up transformer 3, the waveform 4 respectively show waveform diagrams of the output voltage V _l of the inverter 2. The voltage or current (Sc) per division of each waveform is shown in the lower frame of FIG.
ale / div) and simulation period (200μs ~ 500μ)
This shows that the average value of the high-voltage rectified current I _{O in} s) is 102 mA, and the effective value of the resonance current I _l is 101 A. That is, at the time of output of 100 kV and 102 mA, the effective value of the primary current of the step-up transformer 3 is 101 A. In this shooting mode, 20kH
Since the operation is performed at the fixed frequency z, the ripple of the output voltage can be sufficiently reduced.

Next, the control of the fluoroscopic mode will be described. In the fluoroscopy mode, the mode changeover switches 20 and 21 are reversely connected as shown. That is, the AND circuit 27,
The pulse width control signal to 28 is switched to the + V level and fails. By switching the mode switch 21,
The resistor 24 is switched to a series circuit of a phototransistor 30 as a variable resistor and a maximum frequency adjusting resistor 31. The phototransistor 30 is controlled by a light emitting diode 32 driven by a current determined by an error signal voltage of the error amplifier 17 and a resistor 33. When the voltage of the error signal increases, the light emitting diode current increases, the resistance value of the phototransistor 30 decreases, and the oscillation frequency of the reference oscillator 22 increases. That is, the reference oscillator 22 operates as a voltage controlled oscillator controlled by the error signal. The resistor 31 determines the upper limit of the oscillation frequency when the phototransistor 30 is saturated. In this embodiment, the switching frequency f _s is equal to the series resonance frequency f _r = 30 kHz.
z or less, and switching frequency of shooting mode 20k
Select the oscillation frequency to be 36kHz so that it is below Hz. This 3
The 6 kHz pulse is frequency-divided by the distribution signal, and becomes a maximum 18 kHz signal to the switching element. Maximum pulse width is approximately half cycle and 1 cycle of the series resonance frequency f _r is desirable,
1 cycle of the series resonance frequency f _r in this embodiment is about 33.3μs
Therefore, it is set to about 20 μs as in the shooting mode. With the control circuit in such a fluoroscopic mode, the inverter 2 performs constant voltage control in the frequency modulation mode. When the output voltage is lower than the set voltage, the frequency is increased to compensate for this. Conversely, when the output voltage is higher than the set voltage, the frequency is decreased. In such an X-ray power supply, as shown in FIG. 3, the relationship between the switching frequency and the output voltage is such that the output voltage peaks before and after the series resonance point, and the output voltage decreases when the resonance point is separated from the resonance point. Is well known. In this embodiment, to operate the inverter 2 in the following series resonance frequency f _r in phantom mode. Even when operating at such a low frequency, since the output current is small, the ripple voltage of the output voltage does not increase. FIG. 4 shows a simulation result when outputting 100 KV 5.99 mA in the control method of this embodiment. The way to read the figure is the same as in Fig. 2, but the time axis is 0.70
ms to 1.00 ms. The effective value of the primary side resonance current is 3
6.1A. On the other hand, FIG. 5 shows a simulation result in which substantially the same output is generated by the 20 kHz pulse width control. In this example, to output 100 kV 4.66 mA, the effective value of the primary side current is 75.1 A, which is more than twice the present invention,
This proves the effect of the present invention. As a control method of the photographing mode, the on-pulse width of the switching elements 6 to 9 is fixed, and the on-phase difference of the set of the switching elements 6 and 7 and the on-phase difference of the set of the switching elements 8 and 9 are controlled. Output control is also possible.

As described above, according to the present invention, it is possible to reduce the effective value of the series resonance current in the fluoroscopic mode and to cope with a small power loss even in the fluoroscopic mode. Transformers and the like can be designed compact and economical. Further, the present invention can be applied to a half-bridge inverter and a center tap type inverter in addition to the full-bridge inverter.

〔The invention's effect〕

As described above, the present invention can reduce the effective value of the series resonance current in the fluoroscopy mode. Therefore, it is possible to cope with a small power loss even in the fluoroscopy mode,
Efficiency is improved, and inverters and transformers can be designed compact and economically.

[Brief description of the drawings]

1 to 5 are diagrams for explaining one embodiment of the present invention, and FIG. 6 is a diagram for explaining a conventional X-ray power supply device. DESCRIPTION OF SYMBOLS 1 ... DC input power supply 2 ... Inverter 3 ... Step-up transformer 4 ... High voltage rectifier 5 ... Load, 6-9 ... Switching element 10-13 ... Feedback diode, 14 ... Resonance inductance 15, 16, 24, 31, 33: Resistance, 17: Error amplifier 18: Reference power supply, 20, 21, Mode switch 22: Reference oscillator, 23: Capacitor 25: Pulse width modulation circuit, 26: … Maximum pulse width generation circuit 27, 28… AND circuit 29… Flip-flop 30… Phototransistor 32… Light emitting diode

Claims (1)

(57) [Claims] A series resonance type inverter is equivalently connected to the output of an inverter composed of at least a pair of switching elements to form a series resonance circuit composed of a resonance inductance and a resonance capacitance. In the X-ray power supply device that rectifies and outputs the voltage between both ends, in the imaging mode in which large power is supplied for a short time and at a low duty in order to perform normal X-ray imaging, the switching element is substantially fixed at a series resonance frequency or lower in an imaging mode. In the fluoroscopic mode, in which small power is continuously supplied for a long time in order to observe the X-ray fluoroscopic image while performing pulse width modulation or phase modulation with the frequency, the switching element is pulse-frequency modulated at a frequency less than the above substantially fixed frequency. An X-ray power supply device comprising: