JP2719746B2 - X-ray power supply control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control circuit for an X-ray power supply.

2. Description of the Related Art As this type of X-ray power supply inverter, there is a series resonance type inverter which positively utilizes the distributed capacity of a high voltage transformer. FIG. 5 is a diagram for explaining a conventional X-ray power supply using the series resonance type inverter.
Reference numeral 1 denotes a DC input power supply obtained by rectifying and smoothing a battery or a commercial AC power supply, and usually has an input fluctuation of about ± 10% with respect to a rated input voltage. Reference numeral 2 denotes a bridge-type inverter including switching elements 3 to 6 such as FETs, IGBTs, and transistors, and feedback diodes 7 to 10 connected to the switching elements 3 to 6 in anti-parallel. A transformer 11 boosts the AC output voltage of the inverter 2 to a required voltage. A resonance inductance 12 including a leakage inductance of this transformer connected in series to the primary winding N1,
A series resonance circuit is formed by the resonance capacitance 13 including the distributed capacitance of the secondary winding N2 connected in parallel to the secondary winding N2. The winding start of the secondary winding N2 of the transformer 11 is grounded, and the winding high voltage terminal is connected to positive and negative voltage doubler rectifier circuits 14, 15. Positive voltage doubler rectifier 1
4 is a doubler capacitor 16 and diodes 17, 1
8 and a filter capacitor 19. The negative voltage doubler rectifier circuit 15 includes a voltage doubler capacitor 20, diodes 21 and 22, and a filter capacitor 23.
Reference numeral 24 denotes an X-ray tube. The output of the positive voltage doubler rectifier circuit 14 is connected to the anode A, and the output of the negative voltage doubler rectifier circuit 15 is connected to the cathode K. The filament current supply circuit of the X-ray tube 24 is omitted because it is not directly related to the present invention. Resistance 2
Reference numerals 5 and 26 denote voltage dividers for detecting the anode voltage of the X-ray tube as the X-ray tube voltage. When the X-ray tube voltage is strictly detected, the voltages of both the anode and the cathode are detected and added. The detected voltage of the voltage divider is compared with the voltage of the X-ray tube voltage setting voltage source 28 in the error amplifier 27 to generate an error signal. 29 is twice the operating frequency of the inverter 2,
For example, if the inverter frequency is 15 KHz, 30
An oscillator that oscillates at KHz. Here, the frequency of the inverter is set below the resonance frequency of the series resonance circuit, and the resonance frequency of the series resonance circuit is 1.5 to 2.5 times the frequency of the inverter. The output pulse of the oscillator 29 divides the oscillation frequency by half and sets the phases of the switching elements 3 and 4 and the switching elements 5 and 6 to 180 °.
A flip-flop 30 for shifting, a maximum pulse width circuit 31 for determining the maximum on-time of the switching element, that is, a pause time, and a pulse width modulation circuit (hereinafter referred to as PWM) for generating a pulse width modulation signal in accordance with the error signal. Circuit 32). The outputs of the flip-flop 30, the maximum pulse width circuit 31, and the PWM circuit 32 are 2
And AND gates 33 and 34. According to these signals, the outputs of the AND gates 33 and 34 are alternately set to two.
A pulse signal equal to the inverter operating frequency f _S of the phase is generated. These pulse signals are applied to the control poles of the switching elements 3 to 6 via signal insulation means (not shown) such as a pulse transformer and an optical isolator to control the on time of the switching elements. With the inverter and the feedback loop configured as described above, the X-ray tube voltage is stabilized corresponding to the set voltage.

By the way, medical X
Wire devices generally require a constant power rating. For example,
In an X-ray apparatus rated at a nominal 30 kW, there are various X-ray tube voltages. Here, three types of 150 kV, 100 kV, and 60 kV are used.
To explain the species, when the voltage is 150 kV, the current is 2
An output of approximately 30 kW is required in the entire rated voltage range, such as a current of 320 mA at a voltage of 100 mA and a voltage of 100 kV, and a current of 500 mA at a voltage of 60 kV. When designing a power supply having such a constant power rating, three constants of the step-up ratio n of the transformer, the inductance L for resonance, and the capacitance C for resonance are important. For example, input voltage 420
In order to obtain the above rated output at ~ 550V and the inverter frequency 15kHz, for 150kV, n = 150 ×
N = 6 for 10 ³ ÷ 4 ÷ 420 ≒ 89, 60 kV
0 × 10 ³ ÷ 4 ÷ 420 ≒ 36. That is, n is 36
-89. When n is reduced, the current on the low-voltage rated side is increased due to load characteristics, and an output of 60 kV and 500 mA can be achieved with a small switching current.
The output of V200mA stops. When n is increased, the current on the high-voltage rated side increases due to load characteristics, and
Output of 0kV200mA becomes easy, but 60kV50mA
No output of 0 mA is output. From these balances, the peak value of the resonance current, that is, the switching current, is normally reduced within the DC input voltage range, and the boosting ratio n of the transformer and the resonance voltage are set so that the switching element having the smallest possible current capacity can be used from the economical viewpoint. Three constants, inductance L and resonance capacitance C, are selected. One example was selected in this way, n = 70, L = 27μH , C P = 0.
8 μF. C _P is a converted value on the primary side of the transformer, and the value of the capacitor 13, that is, the value on the secondary side C _S = C _P ÷
n ² ≒ 163 pF. The resonance frequency of the series resonance circuit is 34.2 k, which is higher than the inverter frequency of 15 kHz.
Hz. 6 to 8 are current waveform diagrams of the switching element at each voltage rating. FIG. 6 shows a current waveform of 60 kV at a DC input voltage of 420 V and 550 V, respectively.
FIG. 7 is a current waveform diagram of a switching element of mA, FIG. 7 is 100 kV 320 m at a DC input voltage of 420 V and 550 V.
FIG. 8 is a current waveform diagram of the switching element of A, FIG. 8 is 150 kV 200 mA at DC input voltages of 420 V and 550 V.
3 is a current waveform diagram of the switching element of FIG. As is apparent from these current waveform diagrams, the peak value 325 A of the switching element current of 60 kV and 500 mA at the DC input voltage of 550 V is 15
This is 2.3 times the peak value 142A of the current of the switching element of 0 kV 200 mA. Therefore, 60kV5
Since the current rating of the switching element is determined by 00 mA, a switching element having a large current capacity, that is, an uneconomical switching element must be used.

SUMMARY OF THE INVENTION In order to eliminate the above-mentioned drawbacks, the present invention provides an inverter having at least a pair of switching elements, which is equivalently connected in series with a resonance inductance and a resonance capacitance. In a control circuit of an X-ray power supply for connecting a resonance circuit and rectifying a voltage between both ends of the resonance capacitance and applying the rectified voltage to an X-ray tube,
An error amplifier for amplifying a deviation between the X-ray tube voltage setting signal and the X-ray tube voltage detection signal; and an inverter driving frequency corresponding to the rise of the X-ray tube voltage setting signal, the resonance frequency of the series resonance circuit. A voltage controlled oscillator that changes so as to approach the frequency from below, a PWM circuit that modulates the output of the voltage controlled oscillator with the output signal of the error amplifier to generate a pulse width modulation signal of an inverter driving pulse,
Means for controlling the on-time of the switching element in accordance with the output signal of the M circuit.

1 to 4 are diagrams for explaining one embodiment of the present invention. FIG. 1 is a circuit diagram, and FIGS. 2 to 4 are current waveform diagrams of a switching element. In this embodiment, X
When the tube voltage is set to 150 kV, the set voltage of the X-ray tube voltage setting voltage source 28 is set to 7.5 V, and the gain of the voltage controlled oscillator 35 is set to 10 kHz / V. Reference numeral 36 denotes a buffer amplifier having a gain of 1 and receiving the voltage of the X-ray tube voltage setting voltage source 28. The diode 40 in series with the output of the buffer amplifier 36 is used to exclusively output the output of the buffer amplifier. That is, the cathode voltage of the diode 40 can be higher than that of the X-ray tube voltage setting voltage source 28, but cannot be lower. A voltage source 37 of 3 V and a resistor 38 having a resistance value r are connected in series to the input of the voltage controlled oscillator 35.
A resistor 39 having a resistance value of 9r is connected between the input of the voltage controlled oscillator 35 and the output of the buffer amplifier 36.
8 and the resistor 39 constitute a 1/10 voltage dividing circuit. X
When the voltage of the tube voltage setting voltage source 28 is 3 V (setting voltage 60 k
V), the diode 40 is reverse-biased by the voltage source 37 and turned off, and the input voltage of the voltage controlled oscillator 35
That is, the control voltage becomes 3V. Since the gain of the voltage controlled oscillator 35 is 10 kHz / V, the frequency f _o of the output of the voltage controlled oscillator 35 is 30 kHz, which is divided by フ リ ッ プ フ ロ ッ by the flip-flop 30, and the frequency f _S of the inverter is 15 kHz. . Next, when the voltage of the X-ray tube voltage setting voltage source 28 is 5 V (setting voltage 100 kV), the output of the buffer amplifier 36 becomes 5 V, and the voltage controlled oscillator is formed by the voltage dividing circuit of the resistors 38 and 39 and the voltage source 37. 35, that is, the control voltage is 3+ (5-3) / 10 = 3.
2 [V]. Therefore, the frequency f _o is 32kHz next to the output of the voltage controlled oscillator 35, flip-flop 30
, The frequency f _S of the inverter becomes 16 kHz. Next, when the voltage of the X-ray tube voltage setting voltage source 28 is 7.5 V (setting voltage 150 kV), the output of the buffer amplifier 36 becomes 7.5 V, and the voltage dividing circuit of the resistors 38 and 39 and the voltage source 37 , The input voltage of the voltage controlled oscillator 35, that is, the control voltage is 3+ (7.5-3) / 10
= 3.45 [V]. Therefore, the voltage controlled oscillator 35
The frequency f _o of the output of the next 34.5 kHz, is 1/2 divided by the flip-flop 30, the frequency f _S of the inverter becomes 17.25KHz. Here, the maximum duty lock circuit 41 is a conventional maximum pulse width circuit 3.
This is the same as the purpose of 1 but locks the maximum duty constant, for example, 80% even if the frequency changes.
The same triangular wave signal as that of the PWM circuit 32 is received from the triangular wave signal generation circuit 42, and a constant maximum duty signal is generated by comparison with the voltage of the maximum duty voltage source 43. As described above, the control circuit of the present invention brings the frequency of the inverter closer to the series resonance frequency of the resonance inductance and the capacitance in response to the increase in the setting of the tube voltage. As a result, when the setting of the tube voltage is high, the operating frequency of the inverter approaches the series resonance frequency, and the voltage of the resonance capacitance increases due to the resonance action, so that the step-up ratio n of the transformer may be small. When the setting of the tube voltage is low, the operating frequency moves away from the series resonance frequency to a lower value, and becomes apparently inductive, so that the inverter current can be limited. FIGS. 2 to 4 show main circuit currents of the control circuit, a transformer having a step-up ratio of 50, and an inverter having a constant of resonance inductance 40 μH, capacitance 0.8 μF, and series resonance frequency of 28.1 kHz. FIG. 2 shows 60 kV5 at 420 V and 550 V DC input voltage.
FIG. 3 shows a current waveform of a switching element of 00 mA, and FIG.
FIG. 4 shows a current waveform of a switching element of 0 mA, 150 kV at 200 V and 550 V DC input voltage.
It is a current waveform diagram of a switching element of mA. As is apparent from these current waveform diagrams, the peak value of the current of the switching element is 550 V for the conventional input and 60 kV for the output.
From 325A at 00mA to 241A according to the present invention under the same conditions, it is reduced to 74.2% of the conventional.

As described above, according to the present invention, in the control circuit of the X-ray power supply provided with the series resonance circuit, the current rating of the switching element used can be greatly reduced as compared with the conventional one, so that the current capacity can be reduced. This makes it possible to use a switching element having a small size, which is economical. Further, since the effective current of the circuit is reduced, switching loss and copper loss of the transformer are reduced, and efficiency at the time of low voltage output is improved.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram for explaining an embodiment of the present invention.

FIG. 3 is a diagram for explaining an embodiment of the present invention.

FIG. 4 is a diagram for explaining one embodiment of the present invention.

FIG. 5 is a diagram for explaining a conventional X-ray power supply control circuit.

FIG. 6 is a diagram for explaining a conventional control circuit for an X-ray power supply.

FIG. 7 is a diagram for explaining a conventional control circuit for an X-ray power supply.

FIG. 8 is a diagram for explaining a conventional control circuit for an X-ray power supply.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DC input power supply 2 ... Inverter 3-6 ... Switching element 7-10 ... Diode 11 ... Transformer 12 ... Resonance inductance 13 ... Resonance capacitance 14, 15 ... Double voltage rectifier circuit 16 ... Double voltage capacitor 17, 18 ... Diode 19: Filter capacitor 20: Doubler capacitor 21, 22, Diode 23: Filter capacitor 24: X-ray tube 25, 26: Resistance 27: Error amplifier 28: X-ray tube voltage setting voltage source 29: Oscillator 30: Flip-flop Reference Signs List 31 maximum pulse width circuit 32 PWM circuit 33, 34 AND gate 35 voltage controlled oscillator 36 buffer amplifier 37 voltage source 38, 39 resistor 40 diode 41 maximum duty lock circuit 42 triangular wave signal generation circuit 43 ... Maximum duty voltage source

Claims (1)

(57) [Claims] An output of an inverter comprising at least a pair of switching elements is equivalently connected to a series resonance circuit composed of a resonance inductance and a resonance capacitance, and the voltage between both ends of the resonance capacitance is rectified. An X-ray power supply control circuit applied to the X-ray tube includes: an error amplifier that amplifies a deviation between an X-ray tube voltage setting signal and an X-ray tube voltage detection signal; Correspondingly, a voltage-controlled oscillator that changes the drive frequency of the inverter to approach the resonance frequency of the series resonance circuit from below, and modulates the output of the voltage-controlled oscillator with the output signal of the error amplifier to generate a pulse of the inverter drive pulse. A PWM circuit for generating a width modulation signal, and means for controlling an ON time of the switching element by an output signal of the PWM circuit X-ray power supply control circuit, characterized in that it comprises a.