JP4290408B2 - X-ray high voltage device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a DC-DC converter having a soft switching circuit that reduces a loss rate of the switching element by reducing a rate of change of a voltage applied to the switching element when the switching element of an inverter is turned off. The present invention relates to a high-efficiency DC-DC converter with reduced circuit loss and an inverter type X-ray high-voltage apparatus using the same.
[0002]
[Prior art]
An X-ray imaging apparatus and an X-ray computed tomography apparatus (hereinafter referred to as “X-ray imaging apparatus”) that apply a DC high voltage from an X-ray high-voltage apparatus to an X-ray tube and irradiate the subject with X-rays emitted from the X-ray tube. X-ray image diagnosis apparatuses such as X-ray CT apparatuses) are used very often in the field of medical image diagnosis apparatuses. In recent years, these X-ray diagnostic imaging apparatuses have required so-called high load factor X-ray generators that output a large amount of X-rays over a long period of time in order to cope with new imaging methods.
In particular, in the X-ray CT apparatus, features such as “a wide range of scanning is possible in a short time” and “continuous data in the body axis direction can be obtained, thereby enabling generation of a three-dimensional image” Spiral CT called helical scan or spiral scan is popular.
[0003]
In order to realize such a helical scan, it is necessary to continuously rotate the scanner turntable that supports the X-ray tube and the X-ray detector. To that end, it is necessary to continuously connect the X-ray tube mounted on the scanner turntable. Thus, a means for supplying power is required. As this means, a power transmission mechanism consisting of a slip ring and a brush is used. In order to apply a high voltage (hereinafter referred to as a tube voltage) to the X-ray tube together with at least the X-ray tube on the scanner turntable. The high-voltage generator is mounted, and the high-voltage generator is supplied with electric power for generating required X-rays from the X-ray tube via the power transmission mechanism.
[0004]
Thus, since the high voltage generator is mounted on the scanner turntable and rotated at a high speed, it is desirable that the weight be as light as possible. For this reason, an inverter type X-ray high voltage device that can reduce the size and weight of the high voltage transformer of the high voltage generator and reduce the pulsation of the tube voltage is used for the X-ray high voltage device.
[0005]
This inverter type X-ray high voltage device has a function of a DC-DC converter that sends an AC voltage to a transformer from an appropriate DC power source via an inverter and adjusts its output to supply the DC voltage to a required load. The converter converts a commercial AC power source into a DC voltage with a converter, converts the DC voltage into an AC voltage with a frequency higher than the commercial power source frequency with an inverter, and boosts the high-frequency AC voltage with a high-voltage transformer. The boosted AC high voltage is rectified to a DC high voltage by a high voltage rectifier, and the DC high voltage is applied to an X-ray tube to generate X-rays. In order to reduce the size of the high voltage transformer and reduce the pulsation of the tube voltage waveform, the inverter operating frequency is increased to 20 kHz or higher.
[0006]
As mentioned above, spiral CT has a long scan time due to continuous scanning, and in recent years, the scan time is getting faster and faster because there is no motion artifact and the heart can be diagnosed. There is a tendency, and one scan time of 0.5 seconds or less has been demanded.
Therefore, in order for one scan time to correspond to 0.5 seconds, the scanner must be rotated once in 0.5 seconds. Therefore, per unit time in inverse proportion to the scanner rotation time as compared with the conventional device that is slower than that. The X-ray dose must be increased.
[0007]
That is, in order to obtain a good tomographic image with little granular noise, a large amount of X-ray dose can be obtained by flowing a large amount of current flowing between the anode and cathode of the X-ray tube (hereinafter referred to as tube current) in inverse proportion to the scanner rotation speed. To generate X-rays to the subject, and the tube current becomes larger than the conventional one. For this reason, an X-ray tube with a large capacity is required due to an increase in tube current due to faster scanning time and longer imaging time due to spiral scanning, and an inverter that supplies power to this X-ray tube An X-ray high-voltage device is required to have a high output.
[0008]
As described above, when the output current of the inverter increases, the switching loss of the power semiconductor switching element constituting the inverter increases. That is, the switching element of the inverter is applied to the switching element during a turn-on operation for switching the switching element from the non-conductive state to the conductive state and a turn-off operation for switching the switching element from the conductive state to the non-conductive state. The switching loss occurs due to the product of the voltage to be applied and the current flowing through the switching element. This switching loss increases as the operating frequency of the inverter increases, and means for cooling the switching element is required. This cooling means becomes large in a high-power X-ray high-voltage apparatus, which leads to an increase in the size of the apparatus and makes it difficult to mount it on a scanner turntable as described above. Thus, for example, Patent Document 1 discloses an inverter-type X-ray high-voltage device that reduces the switching loss.
[0009]
[Patent Document 1]
Japanese Patent Laid-Open No. 6-25251.
[0010]
In the technology disclosed in this publication, in order to prevent a reverse voltage exceeding the rating from being applied to each switching element constituting the inverter, a diode is arranged in parallel with the switching element so that the polarity is reversed. When the switching element is connected and turned on and off, a current is passed through the diode (hereinafter, this current is referred to as an auxiliary current, and a circuit through which this auxiliary current flows is referred to as an auxiliary circuit) and applied to the switching element. The switching method is to switch the voltage to approximately zero (hereinafter, this switching method is referred to as a soft switching method). Although this method can significantly reduce switching loss, the auxiliary current is supplied from a DC input power source of the inverter via an inductor (hereinafter referred to as an auxiliary inductor). In the X-ray high voltage apparatus, since the inverter current also increases, the current flowing through the auxiliary inductor also increases, and the loss of the auxiliary inductor cannot be ignored. Therefore, as a method for solving this, there is a method described in Non-Patent Document 1, for example.
[0011]
[Non-Patent Document 1]
Non-Patent Document 1 RWDe Doncker, et al: “The Auxiliary Resonant Commutated Pole Converter”, IEEE-IAS (1990), pp.1228-1235.
[0012]
This is because a bidirectional semiconductor switch is connected in series with the auxiliary inductor (hereinafter, this bidirectional switch is referred to as an auxiliary switch), and the auxiliary switch is opened and closed to provide the auxiliary current only when necessary, that is, the inverter. It is made to flow only when the switching element is turned on and off. By operating the auxiliary circuit in this way, it is not necessary to constantly pass a current through the auxiliary inductor, so that the loss of the auxiliary inductor is greatly reduced.
[0013]
[Problems to be solved by the invention]
However, in the inverter-type X-ray high-voltage apparatus using the DC-DC converter, the current flowing through each element of the power semiconductor switching element (hereinafter referred to as a switch) constituting the inverter depends on the load condition (X-ray tube). It depends on the applied tube voltage and the tube current flowing in the X-ray tube) and the inverter operating conditions (in the example disclosed in Japanese Patent Laid-Open No. 6-292361, the phase difference of the inverter). Therefore, when the period Δt during which the auxiliary current flows is constant, the inverter switch current may increase unnecessarily or the soft switching operation may not be possible depending on the load condition and the inverter operating condition.
[0014]
In addition, a method of presetting an optimal Δt for each of the load conditions and the inverter operating conditions is conceivable. However, the transient operation of the X-ray high-voltage device (at the rise of the tube voltage) and the steady operation (at the tube voltage of It is not possible to cope with variations in operation due to differences in the setting voltage maintaining period), temperature, component characteristics, and the like. Therefore, Δt is set to be long so that a certain amount of current flows so that soft switching is reliably performed. The fact that Δt is excessively long means that the current flowing through the switching element flows more than necessary. As a result, there is a lot of waste in terms of the conduction loss of the switch of the inverter circuit (loss when the switch is in the conduction state) and the conduction loss of the auxiliary inductor. In particular, when applied to an X-ray high-voltage apparatus, since a long-time output of high power is required, heat generation of an inverter switching element and an auxiliary inductor becomes a serious problem.
[0015]
Therefore, an object of the present invention is to reduce the loss of a DC-DC converter having a soft switching circuit that reduces the loss rate of the switching element by reducing the rate of change of the voltage applied to the switching element when the switching element of the inverter is turned off. It is an object of the present invention to provide a highly efficient DC-DC converter and an inverter type X-ray high voltage apparatus using the same.
[0016]
[Means for Solving the Problems]
The above object is achieved by the following means.
(1) An inverter that converts a DC voltage into an AC voltage and a semiconductor switching element capable of bidirectional conduction control only during a predetermined period when the semiconductor switching element that constitutes the inverter is turned off are turned on and applied to the switching element of the inverter. Zero voltage switching means for turning off the switching element of the inverter by setting the voltage being substantially zero, a transformer connected to the output side of the inverter, a rectifier for converting the output of the transformer to direct current, and the rectifier In the DC-DC converter comprising a load connected to the output side of the inverter, the zero voltage switching means includes a current detection means for detecting a current flowing through the switching element of the inverter, and a current detected by the current detection means. Turn that turns off the switching element of the inverter Providing a first control means for controlling the period for conducting the bidirectional conduction controllable semiconductor switching devices to match the full target current value.
[0017]
(2) The first control means of (1) calculates a conduction period of the semiconductor switching element capable of bidirectional conduction control in synchronization with the operating frequency of the inverter.
[0018]
(3) The DC-DC converter of (1) or (2) controls the turn-on and turn-off timing of the switching element of the inverter according to the voltage applied to the converter load and the setting signal of the current flowing to the load. Second control means.
[0019]
(4) The inverter having the zero voltage switching means of (1) is a first series composed of a first switching element connected to the positive electrode of the DC power supply and a second switching element connected to the negative electrode of the first switching element. And a second series connection body connected in parallel to the first series connection body, comprising a connection body and a third switching element connected to the positive electrode and a fourth switching element connected to the negative electrode. And first to fourth diodes and capacitors connected in reverse parallel to the first to fourth switching elements, respectively, and the first and second switching elements are turned off when the first and second switching elements are turned off. A first auxiliary current supply means for supplying a current in a forward direction; and the third and fourth diodes when the third and fourth switching elements are turned off. And a second auxiliary current supply means for supplying a current in the direction, the DC power supply comprising a first and a second DC power supply connected in series and the connection point being a neutral point. The first auxiliary current supply means includes a first inductor and a first bidirectional switch between a connection point of the first and second switching elements and a neutral point of the DC power supply. The second auxiliary current supply means includes a second inductor between a connection point of the third and fourth switching elements and a neutral point of the DC power supply. It is configured by connecting a series connection of second bidirectional switches.
[0020]
(5) An X-ray high voltage apparatus is configured using an X-ray tube as a load of the DC-DC converter described in (1) to (4) above.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a circuit diagram showing a first embodiment of a DC-DC converter and an X-ray high voltage apparatus using the same according to the present invention. The circuit of FIG. 1 includes a rectifier circuit 2 that converts an AC voltage from a single-phase or three-phase commercial power supply 1 into a DC voltage, smoothing capacitors 3, 4, 5, and 6 that smooth the output voltage of the rectifier circuit, An inverter 42 that converts the DC voltage smoothed by the capasta into an AC voltage having a frequency higher than that of the commercial power supply, a high-voltage transformer 25 that boosts the output voltage of the inverter, and an output voltage of the high-voltage transformer A high voltage rectifier 26 for converting to a DC voltage, a tube voltage detector 27 for detecting a voltage (tube voltage) of an X-ray tube 28 that emits X-rays when an output DC high voltage of the high voltage rectifier is applied, and this A phase shift PWM (pulse width modulation) signal generation circuit 32 for generating a signal for controlling the operation phase of the inverter so that the tube voltage detected by the tube voltage detector matches the target tube voltage 31; Applied to switching element Soft switching circuit (hereinafter, referred to as auxiliary circuit) to reduce loss of the switching element to reduce the rate of change in pressure consists of auxiliary current control circuit 60 for generating a control signal.
[0022]
The inverter 42 includes a first series connection body including a first switching element 7 connected to the positive electrode of the rectifier circuit 2 and a second switching element 8 connected to the negative electrode thereof, and is connected to the positive electrode. A diode is connected in antiparallel to the third switching element 9 and the fourth switching element 10 connected to the negative electrode and the first to fourth switches (hereinafter referred to as main switches). In addition, capacitors 15 to 18 used as lossless (lossless) snubber circuits are connected in parallel to constitute a full bridge type inverter circuit. The inverter includes an auxiliary circuit for reducing the switching loss of the first to fourth main switches, and includes a first series connection including the first and second switches 7 and 8 of the inverter 42. Between the connection point of the body and the neutral point of the smoothing capacitors 3 and 4 and between the connection point of the second series connection body composed of the third and fourth switches 9 and 10 and the smoothing capacitors 5 and 6. Two auxiliary switching elements 19 to 22 facing the inductor 23 (24) are connected as auxiliary circuits to either or both of the neutral points. The on / off timing of the auxiliary switching elements 19 to 22 (hereinafter referred to as sub-switches) of the auxiliary circuit is determined by the output signal of the phase shift PWM signal generation circuit 32 (PWM signal of SW1 to SW4). Generated by the auxiliary current control circuit 60 from the detection values of the current detection circuits 11 to 14 for detecting the current flowing through the first to fourth main switches and the target value 50 of the current for turning off the first to fourth switches. Controlled by the generated signal.
[0023]
The PWM signals of the output signals SW1 to SW4 of the phase shift PWM signal generation circuit 32 are amplified by the drive circuits 33 to 36 and input to the first to fourth main switches, and the signal of the auxiliary current control circuit 60 is the drive circuit The signals are amplified by 38 to 41 and input to the sub switches 19 to 22, and the main switch and the sub switch are controlled to be turned on / off. Insulated gate bipolar transistors IGBTs, metal oxide semiconductor field effect transistors MOS-FETs, bipolar transistors, etc. are suitable for power semiconductor switching elements as these main switches and sub switches, but on / off control is possible. Any switching element may be used as long as it is an element.
[0024]
In the DC-DC converter configured as described above, if the phase shift PWM control described in Non-Patent Document 2 is applied and the appropriate values of the auxiliary inductors 23 and 24 are selected, the inverter 42 starts from the first. The fourth switches 7 to 10 can always be turned on while the current is flowing through the antiparallel diode, and can be turned off while the current is flowing in the forward direction of the switch. A soft switching operation that effectively uses the lossless snubber cap pasters 15 to 18 can be realized during a dead time period in which all of the switches are non-conductive.
[0025]
[Non-Patent Document 2]
Ishiyaku Shuppan Co., Ltd .: Medical Radiology Science Course 13, Radiological Equipment Engineering, Figure 2-120 on page 89.
The auxiliary switching elements 19 to 22 as sub switches are provided to prevent an unnecessarily large current from flowing through the auxiliary inductors 23 and 24. The conduction loss of the main switches 7 to 10 and the auxiliary inductors 23 and 24 are provided. It is controlled so as not to be useless in terms of conduction loss.
[0026]
As shown in FIG. 2, the sub switches 19 to 22 are turned on for a certain period Δt before and after the on / off timings of the first to fourth switches. The peak value of the current flowing through the auxiliary inductors 23 and 24 is proportional to the ON period Δt of the sub switch, and this current is an increment of the current flowing through the first to fourth main switches.
[0027]
In the embodiment of FIG. 1, the increment of current flowing through the first to fourth main switches is minimized even if the load condition and the inverter operating condition are changed, and the increment of loss due to the auxiliary current is minimized. Therefore, the current detection circuits 11 to 14 and the auxiliary current control circuit 60 are provided in the conventional circuit.
[0028]
The sub switches 19 to 22 detect the cutoff current when the main switches 7 to 10 are turned off by the current detectors 11 to 14, and thereby increase or decrease the on-time Δt of the sub switches 19 to 22. Detection of the cut-off current when the main switch is turned off is detected by current detectors 11-14. The current values of the main switches 7 to 10 detected by the current detectors 11 to 14 are input to the auxiliary current control circuits 43 to 46, respectively.
Each auxiliary current control circuit samples and holds the detected current value by the sample and hold circuit 47 in synchronization with the OFF timing of the PWM signal of the corresponding main switch.
As a result, the sampled and held detection value becomes a current value when the main switch is turned off, and is input to the Δt determination circuit 49.
[0029]
The detected current value is compared with a target value of the turn-off current set in advance by the Δt determining circuit 49 to obtain a period Δt for turning on the sub switch for supplying current to the main switch to be turned off. Further, the Δt determining circuit 49 calculates the timing for turning on the sub switch corresponding to the main switch to be turned off from the on timing of the PWM signal corresponding to the main switch to be turned off in the next cycle and the Δt, and from this timing A signal for turning on the sub switch is amplified by the drive circuit only during the period of Δt to turn on the sub switch.
[0030]
When the cut-off current is lower than the target value, the current flowing through the auxiliary inductor is increased by increasing Δt, as shown in (e) to (g) of FIG. On the contrary, when the cutoff current is larger than the target value, Δt is shortened as shown in (e ′) to (g ′) of FIG.
[0031]
Since the period during which soft switching is possible is a period in which current flows in the positive direction of the main switch when the main switch is turned off, the target value of the cutoff current is determined so as to satisfy this condition. This target value may be set by inputting a value corresponding to the shooting condition from the outside each time shooting is performed, or a value corresponding to each condition is stored in the memory of the auxiliary current control circuit 60 and is not shown in the figure. It may be possible to cope with a photographing condition command from the operation unit.
[0032]
As shown in Fig. 4, the above Δt control can be performed by using a classical control theory such as proportional / derivative / integral control as long as it can be controlled so that the cutoff current of the main switch becomes a target value. But it ’s okay.
[0033]
In the above, the detected value of the main switch current is obtained by sampling and holding in synchronization with the PWM signal. Therefore, the sampling frequency for main switch current detection is equal to the switching frequency of the main switch.
Therefore, since current detection does not require sampling at a speed higher than the switching frequency of the main switch, the current detection sampling method of the present invention is easy to adapt even when the switching frequency of the main switch increases.
[0034]
In the above embodiment, the current flowing through each main switch is detected by the current detector. However, the present invention is not limited to this, and the current of each main switch is supplied to the high voltage transformer 25. A method may be used in which the current flowing and the current flowing in the auxiliary inductors 23 and 24 are detected and the difference between them is obtained by calculation. Any current detector that can detect the current flowing through the main switch, such as a method using a current transformer using a Hall element, a method using a shunt resistor, or a method of detecting the magnetic flux of a transformer But you can.
[0035]
Furthermore, in the above-described embodiment, the control of the sub switch is described in the case where the result in the previous cycle is fed back. However, Δt is calculated using the past main switch current such as the previous two or the previous one. You may make it control.
[0036]
Moreover, although the case where the inverter circuit system is applied to an X-ray high-voltage device has been described, it may be applied to a translink DC-DC converter circuit or a translink DC-AC converter.
[0037]
The case where two sets of two semiconductor switching devices connected in series are used for the sub switch has been described, but any bidirectional switch can be used as long as the current flowing in both directions can be cut off at an arbitrary time. It may be used.
[0038]
In addition, in the above embodiment, an auxiliary circuit for realizing soft switching in which an inductor and a bidirectional switch are connected in series is connected to both the first series connection body and the second direct connection body. Depending on the current waveform of the main switches 7 to 10, either one may be used as an auxiliary circuit with only the inductor 23 or 24. If soft switching is possible even if one of the auxiliary circuits is removed, this may be done.
[0039]
Furthermore, in the present embodiment, an example in which the phase shift PWM method is used for the control of the inverter is shown. However, the present invention is not limited to this control. For example, only the operation frequency of the inverter and the simple pulse width are controlled. Any control can be used as long as the auxiliary circuit shown in the above embodiment can be applied.
[0040]
As described above, the minimum auxiliary current necessary for realizing the soft switching can be supplied to each of the first to fourth switches 7 to 10, and the power conversion efficiency is further improved. Note that the circuit in Fig. 1 always operates efficiently under all load conditions when applied to an X-ray high-voltage device that has a very wide load range (changes by 10 to 4 times as the load resistance). It is very effective in that it is possible.
[0041]
【The invention's effect】
As described above, according to the present invention, the auxiliary current for minimizing the rate of change of the voltage applied to the switching element when the switching element of the inverter is turned off is minimized. The loss of the (soft switching circuit) is greatly reduced, whereby a high-efficiency DC-DC converter and an inverter type X-ray high-voltage device using the same can be provided.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a first embodiment of a DC-DC converter and an X-ray high voltage apparatus using the same according to the present invention.
FIG. 2 is a diagram showing a relationship between main switch and sub switch on / off timings and auxiliary current when a conventional circuit is used;
FIG. 3 is a diagram showing the relationship between the on / off timing of the main switch and the sub switch and the auxiliary current when the circuit according to the present invention is used.
FIG. 4 is a control block diagram for calculating a period during which an auxiliary current flows.
[Explanation of symbols]
1: Commercial power supply (single-phase / three-phase), 2: Rectifier circuit, 3-6: Smoothing capacitor, 7-10: Semiconductor switching element (main switch), 11-14: Current detector, 15-18: Lossless snubber Capacitor, 19-22: Semiconductor switching element (sub switch) capable of bidirectional conduction control, 23, 24: Auxiliary inductor, 25: High voltage transformer, 26: High voltage rectifier, 27: Tube voltage detector, 28: X Line tube, 29: Tube voltage detection value, 31: Tube voltage target value, 32: Phase shift PWM signal generation circuit, 33 to 36: Drive circuit for main switch SW1 to 4, 38 to 41: Drive for sub switches SW5 to SW8 Circuit, 42: Inverter circuit, 43 to 46: Auxiliary current control circuit for sub switches SW5 to SW8, 47: Sample & hold circuit, 48: Main switch turn-off timing detection circuit, 49: Δt (sub switch conduction period) decision Circuit, 50: Main switch turn-off current target value, 60: Auxiliary current control A road

Claims (6)

An inverter that converts DC voltage to AC voltage;
Zero voltage switching means having a bidirectional switch that conducts only for a predetermined period so as to turn off the main switch by setting the voltage applied to the main switch constituting the inverter to substantially zero;
A transformer connected to the output side of the inverter;
A rectifier that converts the output of this transformer to direct current;
In an X-ray high voltage device comprising an X-ray tube connected to the output side of this rectifier,
In the zero voltage switching means,
Current value acquisition means for acquiring a value of a current flowing through the main switch ;
A first time period for controlling the bidirectional switch to conduct so that the current value acquired by the current value acquisition means when the main switch is turned off matches a preset turn-off target current value for turning off the main switch . And an X-ray high voltage apparatus. The X-ray high voltage apparatus according to claim 1,
The inverter is connected to the first positive electrode connected to the positive electrode of the power source that supplies the DC voltage and a first main body connected to the negative electrode of the power source, and to the positive electrode. A second series connection body connected in parallel to the first series connection body consisting of a third main switch and a fourth main switch connected to the negative electrode, and the first to fourth main switches Having first to fourth diodes and capacitors connected in reverse parallel to each other,
The power source is a power source in which two DC power sources are connected in series, and the connection point of the two DC power sources is a neutral point,
The zero voltage switching means includes a first inductor and a first bidirectional switch connected between a connection point of the first and second main switches and a neutral point of the DC power supply. An auxiliary circuit, or a second auxiliary circuit having a second inductor and a second bidirectional switch connected between a connection point of the third and fourth main switches and a neutral point of the DC power supply An X-ray high voltage apparatus having at least one of them. The X-ray high voltage apparatus according to claim 1,
The X-ray high voltage apparatus according to claim 1, wherein the current value acquisition means is current detection means for detecting a current flowing through the main switch. The X-ray high voltage apparatus according to claim 1,
The current value acquisition means includes means for detecting a bidirectional switch current flowing through the bidirectional switch, means for detecting a transformer current flowing through the transformer, a value of the bidirectional switch current, and the transformer current. A value of a current flowing through the main switch is obtained by calculating a value. The X-ray high voltage apparatus according to claim 1,
The X-ray high voltage apparatus characterized in that the current value acquisition means acquires a value of a current flowing through the main switch in synchronization with an operating frequency of the inverter. The X-ray high voltage apparatus according to claim 1,
An X-ray high voltage apparatus comprising: second control means for controlling the turn-off target current value in accordance with a voltage applied to the X-ray tube and a setting signal for a current passed through the X-ray tube. .

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