JP3722561B2 - Power supply device for magnetic resonance imaging apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to a power supply apparatus suitable for various power supply apparatuses required for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field that require high power.
[0002]
[Prior art]
The current waveform required for excitation of the gradient coil of the MRI apparatus also requires a sinusoidal waveform depending on the diagnosis location, but in general, the rise time and fall time as shown in FIG. It is a trapezoidal waveform in both positive and negative directions under 600 US. A high-frequency PWM switching power amplifier using a power MOSFET (field effect transistor) is used as a gradient magnetic field generating power supply system of a current small-capacity MRI apparatus for realizing such a current waveform. This method is superior in terms of high efficiency, energy saving, and space saving compared to a linear power amplifier in which a conventional transistor is operated linearly. In a power supply device using such a switching power supply, high-frequency switching with a withstand voltage of several hundred volts and a current capacity of several tens of A is possible.
[0003]
However, in order to obtain an image useful for diagnosis in a short time in an MRI apparatus, it is necessary to generate a higher magnetic field and realize high-speed control. For this reason, as a magnetic field power supply for the MRI apparatus, a large current power supply of about 4 times the voltage and twice the current is required, but the high frequency PWM switching power amplifier using the power MOSFET has limitations on the performance of the MOSFET. Because of this, it is difficult to meet this requirement.
[0004]
In contrast to this MOSFET, an IGBT (insulated gate bipolar transistor) module as a power switching element has a high withstand voltage and can handle a large current, and is advantageous in terms of maintainability and cost. However, the upper limit of the operating frequency is 20 kHz compared to a MOSFET. Therefore, it is difficult to increase the response speed and reduce the ripple current. Therefore, the present inventors proposed a parallel current amplifier circuit that increases the apparent operating frequency by connecting a conventional circuit using IGBT in parallel and providing a phase difference, and also supports a large capacity. (Japanese Patent Application No. 7-21989, etc.). FIG. 9 shows a configuration diagram of the output circuit of the gradient magnetic field power supply in the case of two parallels by this method.
[0005]
This circuit includes two sets of single-phase full-bridge PWM current amplifier circuits A and B connected in parallel to a gradient magnetic field (GC) coil 10 (a series circuit model of an inductor Lgc and a resistor RLgc). The phase full bridge PWM current amplifier circuit is composed of four IGBT1 to IGBT4 (IGBT5 to IGBT8), soft recovery diodes D1 to D4 (D5 to D8) connected in reverse parallel thereto, and two sets of IGBT and diode D. LCR filters (inductors La1, La2, Lb1, Lb2, capacitors Ca1, Ca2, Cb1, Cb2, resistors) for eliminating current ripples provided between the arms Aa1, Aa2, Ab1, Ab2 and the load 10 Rca1, Rca2, Rcb1, Rcb2). Here, the arms Aa1 and Ab1 are called the first phase, the arms Aa2 and Ab2 are called the second phase, the arms Aa1 and Aa2 are called the left arm, and the arms Ab1 and Ab2 are called the right arm.
[0006]
In this power supply system, the phase difference between the output voltages of the two sets of single-phase full-bridge PWM current amplifier circuits is set to 180 ° to reduce the ripple of current flowing through the GC coil 10.
[0007]
Now, when the load circuit system (filter and inductive load) of FIG. 9 is represented as shown in FIG. 10, each filter current iLa1, iLa2, iLb1, iLb2 and gate drive signals Va1, Va2 of the switching elements in such a power supply system are represented. , Vb1 and Vb2 are as shown in FIG. That is, the phase difference between the current iLa1 flowing through the first set of filter reactors La1 and the current iLa2 flowing through the second set of filter reactors La2 is 180 °, and the combined output load current (current flowing through the GC coil) iref is ideal. The ripple becomes zero in the state. In FIG. 11, S11 corresponds to IGBT1, S12 corresponds to IGBT2, S21 corresponds to IGBT5, and S22 corresponds to IGBT6. 11 shows only the left arms Aa1 and Aa2 of the PWM current amplifier circuit shown in FIG. 9, the same applies to the right arms Ab1 and Ab2.
[0008]
Therefore, if a current flowing through the GC coil 10 is detected and feedback control is performed on the current, a control system that follows the target value is established.
[0009]
[Problems to be solved by the invention]
However, in the parallel circuit configuration as shown in FIG. 9, a sneak current (circulating current, cross current) is generated between the first phase A and the second phase B, and a new short circuit between the phases or an excessive current imbalance is generated. Problems can arise. Such a sneak current cannot be suppressed by the current feedback control detected by the GC coil. To suppress this, it is conceivable to perform feedback control for detecting the inductor current of the filter of each arm. By controlling the inductor currents of the individual filters, the current imbalance of the inductor currents of the first-phase and second-phase filters is suppressed, and the generation of sneak currents can be prevented. For this feedback control, the digital control method is more advantageous than the analog method from the viewpoint of temperature drift suppression and high accuracy in addition to the problem of phase imbalance.
[0010]
By performing digital feedback control of the filter current in this way, it becomes possible to control the load current and suppress the sneak current of the filter current of each phase. For this purpose, it is necessary to detect each filter current with high accuracy. . In particular, as shown in FIG. 11, since the filter currents (iLa1, iLa2) include a triangular wave current having a larger amplitude than the load current iLgc, sampling points (k-1, k, k + 1,.・ ・), The detected values must be kept from large variations.
[0011]
However, in actual switching control, the upper and lower switches at the time of switching of each arm, for example, the down time during which both the upper and lower switches are turned off as shown in FIG. It is necessary to provide Td (dead time), and by providing such a dead time, 1) the variation in current at the sampling point increases, and the value of the filter current cannot be accurately detected. 2) ON for the dead time. Since the duty period of the pulse width modulation pulse (PWM pulse) does not become a desired value, for example, 50%, that is, the output voltage (voltage applied to the GC coil) does not increase even if the average current of the filter is increased. The problem of creating an area occurs.
[0012]
For this reason, as shown in FIG. 13, there is a step in the relationship between the filter current and the output voltage, exhibiting non-linearity, and the feedback control of the filter current becomes difficult. As a result, the gradient magnetic field coil current waveform does not become an ideal waveform as shown in FIG. 8A, and is distorted as shown in FIG. 8B, or the response is slow as shown in FIG. This adversely affects the MRI image.
[0013]
Therefore, the present invention enables a highly responsive control with a good response even if a dead time is provided in a power supply device including a single-phase PWM switching current amplifier using a switching element such as an IGBT. An object of the present invention is to provide a power supply device with a high response and a low ripple current for a magnetic field.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, the power supply device of the present invention detects the filter current in the PWM current amplifier circuit when digitally controlling the PWM current amplifier circuit which is a switching power supply, and sets the PWM pulse width based on the polarity of this current. The control generates a dead time, detects the average value of the output current, and makes it possible to reduce the influence on the output voltage.
[0015]
That is, the power supply apparatus for a magnetic resonance imaging apparatus according to the present invention includes a switching power supply connected to a magnetic field generation coil of a magnetic resonance imaging apparatus as a load, a detected value of a current flowing through the magnetic field generation coil, and a current command value. The PWM control means for performing pulse width modulation control on the duty of the switch constituting the switching power supply so that the difference between the detected value and the current command value becomes zero, and the current of the filter circuit provided in the switching power supply is detected. Filter current detection means and polarity determination means for determining the polarity of the filter current detected by the filter current detection means, and the PWM control means include means for increasing or decreasing the duty based on the determination result of the polarity determination means. Yes. Such a power supply device of the present invention is suitably applied to a power supply device in which a plurality of switching power supplies are connected in parallel.
[0016]
Specifically, the PWM control means controls a plurality of switches with a predetermined dead time, and when the polarity of the filter current is positive, the duty is increased by the dead time, and the polarity of the filter current is negative. In some cases, the duty is controlled to be reduced by the dead time.
[0017]
When the dead time is provided, the output voltage of the switching power supply becomes smaller or larger by the dead time, and an accurate average output current cannot be detected at the time of sampling. The fluctuation of the output voltage varies depending on the polarity of the filter current. Therefore, by controlling the PWM pulse width according to the polarity of the filter current, the output voltage does not decrease or increase by the amount of dead time even if the dead time is provided, and the average value of the filter current can be accurately detected. The relationship between the filter current and the output voltage can be linearized.
[0018]
As a result, the current waveform distortion as shown in FIG. 8B and the response delay problem as shown in FIG. 8C can be avoided, and a high-quality MRI image effective for diagnosis can be obtained.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the power supply device for a magnetic resonance imaging apparatus of the present invention will be described in more detail with reference to embodiments shown in the drawings.
[0020]
FIG. 1 is an overall circuit block diagram of a power supply device using a PWM control system according to an embodiment of the present invention. This power supply device is connected in parallel to a DC power supply E in the same manner as the power supply device of FIG. Full-bridge PWM current amplification circuits A and B (only A is shown in the figure) that are switching power supplies, detectors CS1 and CS2 that detect the filter current of each arm, and a filter that determines the polarity of these filter currents Based on the current discriminating circuit 20, the PWM pulse width of each switch (IGBT1 to IGBT4 in this case) based on the detected value and polarity of the filter current, the PWM control circuit 30 that generates the PWM pulse, and the PWM control circuit 30 generate And a pulse amplification circuit 40 that amplifies the PWM pulse, and the outputs of the PWM current amplification circuits A and B are applied to the GC coil 10 as a load.
[0021]
The PWM current amplifier circuits A and B are basically the same as the single-phase full-bridge PWM current amplifier circuit of FIG. 9, and corresponding elements are denoted by the same reference numerals. That is, the PWM current amplifying circuit A includes four sets of IGBT1 to IGBT4 and diodes D1 to D4 connected in antiparallel to these IGBTs, and between the arms Aa1 and Ab1 composed of two sets of IGBTs and diodes and the GC coil 10. And LCR filters (inductors La1 and Lb1, capacitors Ca1 and Cb1, resistors Rca1 and Rcb1) respectively connected to each other. The filter reactor (inductors La1 and Lb1) absorbs an instantaneous potential difference, and the CR filter smoothes the voltage ripples at both ends of the GC coil 10 with capacitors (capacitors Ca1 and Cb1), and the resistors Rca1 and Rcb1 filter. Suppresses the current resonance phenomenon caused by the reactor and the smoothing capacitor.
[0022]
Although not shown in FIG. 1, the other PWM current amplifier circuit B has the same configuration as the illustrated PWM current amplifier circuit A.
[0023]
The detectors CS1 and CS2 and the polarity discrimination circuit 20 for detecting the filter current include a pair of current detectors CS11 and CS12 and these current detectors CS11 and CS12 as shown in FIG. 2, for example, for the filter current of the left arm Aa1. Are connected to transistors Q1 and Q2, respectively. As the current detector, a known current detector such as a Hall element or CT can be used. Vcc is the power supply voltage of the circuit 20. The polarity discriminating circuit 20 having such a configuration inputs the filter current iLa1 detected by the current detectors CS11 and CS12 when discriminating the polarity of the filter current iLa1. Since a positive voltage is output, the transistor Q1 conducts and outputs an ON signal to its output. Further, when iLa1 is negative, a positive voltage is output to the polarity at the end of winding of CS12, so that transistor Q2 conducts and outputs an ON signal to its output.
[0024]
Similarly, the polarity of the filter current iLb1 of the right arm and the filter current of the other single-phase full-fridge PWM current amplifier circuit B are discriminated, and these signals are used for the PWM pulse width calculation.
[0025]
The PWM control circuit 30 inputs the detected value of the current flowing through the GC coil 10 and the current command value, and switches the PWM current amplification circuits A and B so that the difference between the detected value and the current command value becomes zero. In addition to PWM control, the PWM pulse width is corrected according to the detected filter current and its polarity as described above. Such a PWM control circuit 30 (PWM pulse calculation and PWM pulse generation) can be configured by a known microcomputer, and the pulse width is calculated by using the polarity discrimination signal of the filter current, and according to this. It is generated by distributing the pulses.
[0026]
Next, the operation of the power supply device having such a configuration and the PWM pulse width control in the PWM control circuit 30 will be described. In order to simplify the description, FIG. 3 shows a circuit including only one set of PWM current amplification circuits (A or B) and a GC coil of the power supply circuit device of FIG. In FIG. 3, switch S11 corresponds to IGBT1 or IGBT5, switch S12 corresponds to IGBT2 or IGBT6, switch S21 corresponds to IGBT3 or IGBT7, and switch S22 corresponds to IGBT4 or IGBT8. The diodes D11 to D22 correspond to the diodes D1 to D4 or D5 to 8, respectively.
[0027]
In this power supply apparatus, as shown in the switching sequence of FIG. 4, the switches S11 and S22 are simultaneously turned on and off, and the switches S12 and S21 are simultaneously turned on and off opposite to the switches S11 and S22. At this time, the PWM control circuit 30 controls the PWM pulse width and controls the ON time (duty) of these switches S11 to S22. In order to prevent a short circuit due to variations in characteristics of the switching elements, a dead time Td in which all the switches are turned off is provided. The output current iL in the PWM current amplifier circuit of FIG. 3 corresponds to the filter current detected by the filter current detectors CS1 and CS2, and has a current waveform that changes as the switch is turned on or off, and the output voltage Vab corresponding thereto. Is obtained.
[0028]
Here, in the state where the output current iL crosses zero as shown in FIG. 4, even if there is a dead time, the PWM duty corresponds to the output voltage Vab as it is. That is, the waveform of the output current iL changes with the falling of the switches S12 and S21 in the negative state, and changes with the falling of the switches S11 and S22 in the positive state, so the output voltage reflects the PWM duty as it is. It becomes. Therefore, the current detection accuracy at each sampling point is not changed, and the relationship between the filter current and the output voltage is linear.
[0029]
On the other hand, when the output current iL does not cross zero and is always positive as shown in FIG. 5A, the waveform of the output current iL is dominated by the rise and fall of the switches S11 and S22 having a long on-time. . Therefore, if there is a dead time Td in such a case, the average value of the output voltage Vab becomes smaller than the PWM duty by the dead time, and the relationship between the filter current and the output voltage also exhibits nonlinear characteristics as shown in the positive region of FIG. Present. Further, the filter current detected at the sampling point also has a value different from the average current, which causes a large error in detection.
[0030]
In this way, when the output current iL is always positive, the average value of the output voltage Vab becomes smaller by the amount of dead time than the PWM duty. Therefore, the PWM duty is increased by that amount, that is, the ON time of the switches S11 and S22 is increased. The PWM pulse width is controlled so as to be longer. That is, as shown in FIG. 5B, when it is determined that the polarity of the filter current at the sampling point k-1 one sampling before is positive, the switch S12 and S21 are turned off earlier, After the dead time Td is provided, the switches S11 and S22 are turned on. As a result, the on periods of the switches S12 and S21 are shortened by the dead time, and the on periods of the switches S11 and S22 are lengthened by the dead time. PWM control is performed as follows.
[0031]
On the other hand, when the output current iL does not cross zero and is always negative, as shown in FIG. 6A, the waveform of the output current iL is dominated by the rising and falling of the switches S12 and S21 having a long on-time. Is done. Accordingly, if there is a dead time Td in such a case, the average value of the output voltage Vab becomes larger than the PWM duty by the dead time, and the relationship between the filter current and the output voltage also has a nonlinear characteristic as shown in the negative region of FIG. Will come to present. Also, the sampling point has a value different from the average current, which causes a large error in detection.
[0032]
Accordingly, when the output current iL is always negative, as shown in FIG. 6B, when it is determined that the polarity of the filter current at the sampling point of k-1 before one sampling is negative, the switch S12, S21 is turned off and the switches S11 and S22 are turned on after the dead time Td is set, the switches S12 and S21 are turned on by the dead time, and the switches S11 and S22 are turned on. Make it shorter by. That is, when the output current iL is always negative, the average value of Vab is larger than the PWM duty by the dead time, so that the PWM duty is decreased by that amount, that is, the on-time of the switches S11 and S22 is shortened. Control the pulse width.
[0033]
In FIGS. 4 to 6, the on-time (pulse width) of each switch is shown to be substantially the same for the sake of simplicity, but in actuality, the output current iL is always positive in FIG. The on-time of the switches 11 and S22 is longer than the on-time of the switches S12 and S21, and the on-time of the switches S12 and S21 is longer than the on-time of the switches 11 and S22 in FIG.
[0034]
As described above, the PWM control circuit 30 calculates the pulse width using the polarity determination signal of the filter current, and distributes the pulse width according to the calculation to generate the PWM pulse. The PWM pulse is amplified by the PWM pulse amplification circuit 40 and then input to the gates of the IGBTs 1 to 8 of the PWM current amplification circuits A and B, and the IGBTs are driven to control the output currents of the PWM current amplification circuits. To do.
[0035]
As described above, by controlling the PWM duty based on the polarity of the filter current, the output voltage does not decrease or increase by the dead time even if the dead time is provided, and the average value of the filter current is accurately detected. As a result, the relationship between the filter current and the output voltage can be linearized as shown in FIG. As a result, the distortion of the current waveform as shown in FIG. 8B can be eliminated, and the problem of response delay as shown in FIG. 8C can be avoided, and a high-quality MRI image effective for diagnosis is obtained. be able to.
[0036]
Further, since the output currents of the PWM current amplifier circuits connected in parallel can be controlled with high accuracy, an extremely low ripple output current can be obtained by shifting the phases of the PWM current amplifier circuits A and B by 180 °.
[0037]
In the above-described embodiment, the case where two parallel PWM current amplifier circuits are provided as the switching power supply has been described. However, the present invention can be applied to a power supply device including one PWM current amplifier circuit, and 3 (N pieces). It can also be applied to switching power supplies connected in parallel. In the case of a plurality of parallel power supplies, driving with the phase of each switching power supply shifted by 360 ° / N can reduce ripples. Further, in this embodiment, the PWM current amplifier circuit using IGBT as a switching element has been described as an example, but the present invention includes application to a current amplifier circuit using MOSFET.
[0038]
Furthermore, the power supply apparatus of the present invention can be used as a power supply not only for the GC coil of the MRI apparatus but also for other magnetic field generating coils.
[0039]
【The invention's effect】
As described above, according to the power supply device of the present invention, when PWM control is performed for each switch of the switching power supply with a dead time, the PWM duty is set so that the output voltage does not become smaller or larger by the amount corresponding to the dead time. By controlling, the average value of the filter current can be accurately detected, and the relationship between the filter current and the output voltage can be linearized. As a result, problems of current waveform distortion and response delay can be avoided, and high-quality MRI images effective for diagnosis can be obtained.
[0040]
Further, according to the power supply device of the present invention, since the filter current can be accurately controlled, it can be suitably applied to a power supply device in which switching power supplies are connected in parallel. A ripple power supply can be provided.
[Brief description of the drawings]
FIG. 1 is an overall block diagram showing an embodiment of a power supply circuit provided with PWM control means of the present invention.
FIG. 2 is a diagram showing an embodiment of a filter current polarity discrimination circuit;
FIG. 3 is a simplified diagram showing a part of the power supply circuit of FIG. 1;
FIG. 4 is a diagram showing a switching sequence when the output current iL of the circuit of FIG. 3 crosses zero and has a dead time.
5A and 5B are diagrams showing a switching sequence when the output current iL of the circuit of FIG. 3 is always positive and there is a dead time, where FIG. 5A shows a conventional PWM control, and FIG. 5B shows a PWM control according to the present invention. FIG.
6 is a diagram showing a switching sequence when the output current iL of the circuit of FIG. 3 is always negative and there is a dead time, where (a) shows a conventional PWM control, and (b) shows a PWM control according to the present invention. FIG.
FIG. 7 is a diagram showing a relationship between a filter current and a load output voltage in PWM control according to the present invention.
8A and 8B are diagrams showing current waveforms of a GC coil of an MRI apparatus, where FIG. 8A shows an ideal current waveform, FIG. 8B shows a current waveform by a conventional power supply apparatus, and FIG. 8C shows a rising waveform by a conventional power supply apparatus. .
FIG. 9 is a diagram showing a configuration of an output circuit of a gradient magnetic field power supply.
10 is a diagram showing a load circuit system of the circuit of FIG. 9;
11 is a diagram showing a relationship between a filter current and a gate drive signal in the circuit of FIG. 10, and shows a case where there is no dead time.
12 is a diagram showing a relationship between a filter current and a gate drive signal in the circuit of FIG. 10, and shows a case where a dead time is provided.
FIG. 13 is a diagram showing a relationship between a filter current and a load output voltage when a dead time is provided in conventional PWM control.
[Explanation of symbols]
E ・ ・ ・ ・ ・ ・ DC power supply
IGBT1-IGBT8 ・ ・ ・ ・ ・ ・ Insulated gate bipolar transistor (switch)
S11, S12, S21, S22... Switches D1 to D8 ... Soft recovery diodes La1, La2, Lb1, Lb2 ... Inductors (filter circuit)
Ca1, Ca2, Cb1, Cb2 ... Capasta (filter circuit)
Rca1, Rca2, Rcb1, Rcb2 ··· Resistance (filter circuit)
10 .... Gradient magnetic field coil (load)
A, B ... PWM current amplification circuit (switching power supply)
20... Filter current polarity discriminating circuit 30... PWM control means 40... Pulse amplification circuit Td .. Dead time CS1, CS2. Filter current detector

Claims (4)

A switching power supply connected to a magnetic field generating coil of a magnetic resonance imaging apparatus as a load, a detected value of a current flowing through the magnetic field generating coil and a current command value are input, and a difference between the detected value and the current command value In a magnetic resonance imaging apparatus power supply device comprising: PWM control means for performing on / off control of a plurality of switches constituting the switching power supply so as to be zero and performing pulse width modulation control of the duty;
Filter current detection means for detecting the current of the filter circuit provided in the switching power supply, and polarity determination means for determining the polarity of the filter current detected by the filter current detection means,
The PWM control means is provided with a predetermined dead time during the on / off control of the switch, and includes means for increasing or decreasing the duty based on the determination result of the polarity determination means, and when the polarity of the filter current is positive A power supply apparatus for a magnetic resonance imaging apparatus , wherein the duty is increased by the dead time, and the duty is decreased by the dead time when the polarity of the filter current is negative . A plurality of switching power supplies connected in parallel to a magnetic field generating coil of a magnetic resonance imaging apparatus as a load, a detected value of a current flowing through the magnetic field generating coil, and a current command value are input, and the detected value and the current command are input. PWM control means for performing on / off control of the switches constituting the switching power supply so that a difference from the value becomes zero, and performing pulse width modulation control of the duty ;
A filter current detecting means for detecting the current of the filter circuit provided in each of the switching power supply, and a polarity discriminating means for discriminating the polarity of the detected filter current by the filter current detecting means,
It said PWM control means, provided with a predetermined dead time during on-off control of the switches, e Bei means for increasing or decreasing the duty based on a discrimination result of said polarity discriminating means, when the polarity of the filter current is positive Increases the duty by the dead time, and decreases the duty by the dead time when the polarity of the filter current is negative .   3. The magnetic resonance imaging apparatus power supply apparatus according to claim 2, wherein when the number of the switching power supplies connected in parallel to the magnetic field generating coil is N, the phase difference between the switching power supplies is 360 ° / N. A power supply apparatus for a magnetic resonance imaging apparatus, wherein the power supply apparatus is set.   4. The power supply device for a magnetic resonance imaging apparatus according to claim 1, wherein the filter circuit is disposed on both sides of the magnetic field generating coil, and the filter current detection circuit of the switching power source is disposed on both sides. A power supply device for a magnetic resonance imaging apparatus, wherein the current of each of the filter circuits is detected to determine the polarity.

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