JP3205631B2 - DC-DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter for transmitting an AC voltage from a DC power supply via an inverter to a transformer, rectifying the output and supplying the DC voltage to a load. In addition to reducing the rate of change of the voltage applied to each switching element to reduce noise,
The present invention relates to a DC-DC converter of a soft switching system which achieves high efficiency by reducing loss in the switching element.

[0002]

2. Description of the Related Art In recent years, converters have been developed in which a resonant element is inserted into a part of a converter to make a voltage waveform or a current waveform sinusoidal, thereby reducing the load on the switching element during switching. There is a phase difference control method as a method of controlling the output voltage of such a resonance type converter,
As a conventional DC-DC converter using this type of control system, there is one described in Japanese Patent Application Laid-Open No. 63-190556.

As shown in FIG. 9, this DC-DC converter includes a DC power supply 1, a first switching element 2a connected to the positive electrode + of the DC power supply 1, and a second switching element connected to the negative electrode-. It has a first series connection consisting of an element 2b and comprises a third switching element 2c connected to the positive electrode + and a fourth switching element 2d connected to the negative electrode-in parallel with the first series connection. The first direct-current power supply 1 includes first to fourth diodes 3a to 3d having a second series-connected body connected thereto and anti-parallel-connected to the first to fourth switching elements 2a to 2d, respectively. An inverter 4 for converting a direct current into an alternating current, an inductance 5 and a capacitance 6 connected in series on the output side of the inverter 4, and an inductance 5 and a capacitance A transformer 7 for insulating the serially connected output, the rectifier 8 to convert the output of the transformer 7 in the DC
And the load 9 connected to the output side of the rectifier 8 and the first to fourth switching elements 2 a to 2 d according to the setting signal of the voltage applied to the load 9 and the current flowing through the load 9.
For controlling the timing of turning on and off the power supply (not shown)
And had.

In FIG. 9, the first to fourth switching elements 2a to 2d and the diodes 3a to 3d respectively include a first arm 10a and a second arm 10a.
b, a third arm 10c, and a fourth arm 10d. The rectifier 8 is configured to perform full-wave rectification of the input voltage by the four diodes 11a, 11b, 11c, and 11d. Further, reference numeral 12 denotes the capacitance of a high-voltage cable for applying the output voltage of the rectifier 8 to the load 9, and functions as a capacitance for smoothing the output voltage from the rectifier 8.

Next, the conventional DC constructed as described above is used.
The operation of the DC converter will be briefly described with reference to FIG. In FIG. 10, (a), (b),
(C) and (d) are the ON and OFF periods of the first switching element 2a, the fourth switching element 2d, the second switching element 2b, and the third switching element 2c of the inverter 4 shown in FIG. Is shown. And
As is clear from FIG. 10, the first switching element 2a and the fourth switching element 2d are turned on with a phase difference α, and the second switching element 2b and the third switching element 2c are also shifted. They are turned on with a shift of the phase difference α. Further, the first switching element 2
a and the second switching element 2b, and the third switching element 2c and the fourth switching element 2d are alternately turned on with a phase difference of 180 °.

In the above operation, the periods (Tb3 to Tb4) in which the first switching element 2a and the fourth switching element 2d shown in (a) and (b) are simultaneously turned on, and (c) and (d) The second switching element 2b shown in FIG.
Since power is supplied from the DC power supply 1 shown in FIG. 9 only during a period (Tb6 to Tb7) in which the third switching element 2c is simultaneously turned on, the output power waveform Vt of the inverter 4 is shown in FIG. As described above, a square wave having positive and negative peak values of the voltage only during the above period is obtained.

Therefore, the first switching element 2a
Phase difference α between the second switching element 2b and the third switching element 2d.
When the phase difference α with respect to c is changed, the period during which the switching elements 2a to 2d are simultaneously turned on can be changed, and the power supplied to the load 9 shown in FIG. 9 can be controlled. FIG. 11 shows the relationship between the phase difference α between the corresponding switching elements and the output voltage Vt in this case. In FIG. 11, the horizontal axis represents the phase difference α, and the vertical axis represents the output voltage Vt to the load 9, and the relationship between the phase difference α and the output voltage Vt is represented by the resistance values R1, R2, R3 of the load 9.
6 is a graph showing a predetermined curve using (R1>R2> R3) as a parameter.

Here, in the above configuration and operation, a first switching element 2a whose turn-on is not delayed and a first diode 3a connected in anti-parallel to the first switching element 2a.
The operation of the second arm 10b including the second arm 10a and the second switching element 2b and the second diode 3b connected in anti-parallel to the second switching element 2b will be considered.

As shown in FIG. 10 (e), the current I1 flowing through the first arm 10a is
It is negative at time Tb1 when the ON signal to a is input. Therefore, at this time, the first switching element 2
The voltage applied to a is only the ON voltage of the first diode 3a and is almost zero. Then, when the current changes from negative to positive and the current starts to flow through the first switching element 2a, the loss of the switching element 2a is zero because it is the product of the voltage and the current at that time. However, at time Tb4 when the first switching element 2a is turned off,
The current I1 flowing through the first arm 10a is as shown in FIG.
It is positive as shown in (e).

FIG. 12 shows the operation until the first switching element 2a starts to turn off and the current becomes zero. As shown in FIG. 12, before the current becomes zero, the voltage of the switching element 2a becomes zero. Begin to increase, the first switching element 2a generates a loss corresponding to the shaded region by the current and the voltage. Such an operation is the same for the second arm 10b.

In order to reduce the above-mentioned loss, for example, a configuration in which a capacitance 14 and a resistor 15 are connected in series as shown in FIG. 13A, or a configuration in which the capacitance 14 and the resistor 15 are connected as shown in FIG. A circuit called a snubber circuit having a configuration in which the switching element 13 and the diode 16 are combined is used in parallel with the switching element 13 such as a transistor.

When such a snubber circuit is provided in parallel with the first and second switching elements 2a and 2b, the rise of the voltage when each of the switching elements 2a and 2b is turned off is suppressed, and the voltage at the time of turning off is suppressed. The switching loss can be reduced.

However, in the snubber circuit as described above, when the switching element 13 shown in FIG. 13 is turned off, the electric charge accumulated in the capacitance 14 turns on the switching element 13 when the switching element 13 is turned on. Discharged through the resistor 15, the resistor 1
5 causes a loss. Since the resistance 15 controls the maximum value of the current at this time, the resistance 15
Otherwise, an excessive current flows, and the switching element 13 is destroyed.

Since the loss caused by the resistor 15 occurs every time the switching element 13 repeats turn-on and turn-off, the loss of each of the switching elements 2a and 2b in the inverter 4 shown in FIG. Increase in proportion. In particular, such DC-
In DC converters, it is common to increase the operating frequency in order to reduce the size and weight of the device, resulting in a very large switching loss.

Next, in the configuration and operation shown in FIGS. 9 and 10, a third arm 10c including a third switching element 2c whose turn-on is delayed and a third diode 3c connected in anti-parallel thereto, and Fourth switching element 2d and fourth diode 3 connected in anti-parallel to this
Consider the operation of the fourth arm 10d consisting of d and d. In the example shown in FIG. 10, the periods Tb3 to Tb6 during which the ON signal of the fourth switching element 2d shown in FIG.
At time Tb5, the current I4 of the fourth arm 10d becomes zero (see FIG. 10 (f)), and a negative current flows after time Tb5. That is, a current flows through the fourth diode 3d connected in antiparallel in the fourth arm 10d. Thereafter, at time Tb6, there is no ON signal to the fourth switching element 2d as shown in FIG. 10B, and the third switching element 2c starts to turn on as shown in FIG. As a result, the current that has been flowing through the fourth diode 3d is commutated to the third switching element 2c, and the fourth diode 3d is reverse-biased and turned off.

However, at this time, the fourth diode 3
d cannot be turned off instantaneously, and a current called a recovery current flows through the diode 3d until the junction capacitance of the PN junction is charged. Therefore, while the recovery current is flowing, the third arm 10 shown in FIG.
c and the fourth arm 10d are equivalent to a short-circuited state, so that not only an excessive current flows to increase the switching loss, but also the third and fourth switching elements 2c and 2d.
d was sometimes destroyed. Such an operation is the same when the third switching element 2c is turned off.

[0017]

As described above, in the phase difference control in the conventional DC-DC converter,
First and second arms 10 of inverter 4 shown in FIG.
a, 10b and the third and fourth arms 10c, 1
0d, the first and second arms 1
0a and 10b, the switching loss due to the snubber circuit (see FIG. 13) increases and the third and fourth arms 10a and 10b
In c and 10d, the switching elements 2c and 2d are connected by the arm short-circuit due to the recovery current of the diodes 3c and 3d connected in anti-parallel to the switching elements 2c and 2d.
Was destroyed.

In addition, each of the switching elements 2a to 2
d, the switching elements 2a to 2
Since the time change rate of the current flowing through d is large, the generated electromagnetic interference noise increases, which may adversely affect the control system. In particular, when such a DC-DC converter is used in a power supply device such as an X-ray CT device (X-ray computed tomography device) as an X-ray device, an inverter 4 that switches hundreds of volts and hundreds of amps is used. It is necessary to detect a small signal (several microvolts) to construct a diagnostic image in the immediate vicinity, and the effect of the electromagnetic interference noise becomes very large,
There was a strong need for improvement.

The present invention addresses such a problem and has a small rate of change in voltage applied to each switching element of the inverter, thereby reducing electromagnetic interference noise, and at the same time, reducing the loss in the switching element to increase efficiency. It is an object of the present invention to provide a soft-switching DC-DC converter that can be achieved.

[0020]

An object of the present invention is to provide a DC power supply having a neutral point and a first DC power supply connected to a positive electrode of the DC power supply.
A first series connected body composed of a second switching element connected to a negative switching element and a negative switching element, and a third switching element connected to the positive electrode and a fourth switching element connected to a negative electrode. A DC power supply having a second series-connected body connected in parallel to the first series-connected body and having first to fourth diodes connected in anti-parallel to the first to fourth switching elements, respectively; An inverter for converting DC from AC to AC, an output circuit of the inverter including at least a transformer connected to an output side of the inverter, a rectifier connected to the transformer and converting the output to DC, and the rectifier And a load connected in parallel with the capacitance, and a voltage applied to the load and a setting signal of a current flowing through the load. A DC-DC converter having means for controlling the ON / OFF timing of the first to fourth switching elements, wherein the first to fourth switching elements are connected in parallel to the first to fourth switching elements, respectively. 4 and the first of the inverters
And / or between the connection point of the second switching element and the neutral point of the DC power supply, and / or between the connection point of the third and fourth switching elements and the neutral point of the DC power supply. , A phase control circuit for controlling the on / off phases of the first to fourth switching elements, and a first of the first and second switching elements connected to the phase control circuit. A switching element to be turned on and a mask circuit for setting the first on time of the operation of the switching element to be turned on first of the third and fourth switching elements to １ ／ of the on time after the second time are provided. This is achieved by:

[0021]

The DC-DC converter thus configured is connected between the connection point of the first and second switching elements of the inverter and the neutral point of the DC power supply, and the connection of the third and fourth switching elements. By connecting an inductance as an auxiliary circuit to one or both of the point and the neutral point of the DC power supply, the current flowing through each arm of the inverter becomes negative when turned on, and becomes positive when turned off. The operation mode can always be maintained.

The first to fourth switching elements of the inverter are connected in parallel with each other as capacitances as lossless snubber circuits, so that the switching elements are charged and discharged by the lossless snubber circuits during a dead time period. A soft switching element having a small voltage rate of change over time can be realized.

When the operation of the inverter is started, the operation of the switching element which is turned on first of the first and second switching elements and the operation of the switching element which is turned on first of the third and fourth switching elements of the inverter is started. By halving the first ON time, a current equal in the positive and negative directions can flow through each of the inductances used as the auxiliary circuit, and the peak value of the current flowing through the first to fourth switching elements can be suppressed. DC-DC that can use small switching elements
A converter can be realized.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing one embodiment of a DC-DC converter according to the present invention. This DC-DC converter is a power converter that sends an AC voltage from a DC power supply via an inverter to a transformer, rectifies its output, and supplies a DC voltage to a load, as shown in FIG. DC power supply 1, inverter 4, inductance 5 and capacitance 6, transformer 7, rectifier 8, and X as load
It has a wire tube 17 and a phase control circuit 19, and is called an inverter type X-ray high voltage device.

The DC power supply 1 is, for example, a secondary battery. In FIG. 1, two power supplies E / 2 are shown symmetrically for the sake of convenience. The inverter 4 receives direct current from the direct current power source 1 and converts it into alternating current. The inverter 4 is connected to a transistor 20 a as a first switching element connected to the positive electrode + of the direct current power source 1 and to the negative electrode − of the direct current power source 1. Transistor 20 as a second switching element
b, and a transistor 20c as a third switching element connected to the positive electrode +
And a second series-connected body composed of a transistor 20d as a fourth switching element connected to the negative electrode and connected in parallel to the first series-connected body, and anti-parallel connected to the transistors 20a to 20d, respectively. And first to fourth diodes 3a to 3d.

The above transistors 20a to 20d
Are turned on by flowing a base current. And the first transistor 2
0a and the first diode 3a, the first arm 10a
With the second transistor 20b and the second diode 3b
And the second arm 10b is connected to the third transistor 20c.
And the third diode 3c constitute a third arm 10c, and the fourth transistor 20d and the fourth diode 3d constitute a fourth arm 10d.

The output side of the inverter 4 is connected to an inductance 5, and a capacitance 6 is connected to the inductance 5 in series. The inductance 5 and the capacitance 6 form a resonance circuit. A primary winding of a transformer 7 is connected in series to the inductance 5 and the capacitance 6, and the transformer 7 boosts the output voltage from the inverter 4 and insulates the output from the output.

The rectifier 8 converts the output voltage from the transformer 7 into a direct current by full-wave rectification, and includes four diodes 11a to 11d as shown in FIG. Furthermore,
An X-ray tube 17 is connected to the output side of the rectifier 8 as a load. In addition, 12 is a capacitance for smoothing the output voltage of the rectifier 8.

The phase control circuit 19 turns on and off the first to fourth transistors 20a to 20d in accordance with the voltage applied to the X-ray tube 17 and the setting signal of the current flowing through the X-ray tube 17. The operation phase of each of the transistors 20a to 20d is determined by the tube voltage setting signal S1 and the tube current setting signal S2, and the base current at which the transistors 20a to 20d operate is input from a controller (not shown). The X-ray irradiation signal S3 is output.

Here, in the present invention, the first to fourth transistors 20a to 20d of the inverter 4 include:
Lossless snubber capacitances 22a to 22d used as a lossless (lossless) snubber circuit are connected in parallel, respectively, and the first and second transistors 20a, 20a,
20b and the neutral point (potential E / 2) of the DC power supply 1, and the third and fourth transistors 20c and 20c.
Between the connection point d and the neutral point of the DC power supply 1, inductances 23a and 23b are connected as auxiliary circuits, respectively. A mask circuit 24 is connected to the output side of the phase control circuit 19, and its output terminal is connected to the first and fourth terminals.
Are connected to the bases of the transistors 20a and 20d, respectively.

Next, the operation of the DC-DC converter thus configured will be described. First, D shown in FIG.
The main circuit components (DC power supply 1, inverter 4, inductance 5, capacitance 6, transformer 7, rectifier 8, X-ray tube 17) of the C-DC converter are equivalent circuits as shown in FIG. That is, the transistors 20a to 20d of the inverter 4 are represented as a first switching element 2a, a second switching element 2b, a third switching element 2c, and a fourth switching element 2d, respectively, as shown in FIG. , X-ray tube 17 is designated as load 9. Therefore, the operation principle of the main circuit component will be described with reference to FIGS. 3 and 4 using the equivalent circuit shown in FIG.

In the equivalent circuit of FIG. 2, attention is paid to the first arm 10a and the second arm 10b of the inverter 4. The current flowing through the inductance 23a to the DC power supply 1 is defined as Ia, and the first and second arms 10
The current output from a and 10b to the transformer 7 side is Ir. In this state, when the first switching element 2a is on, the current I flowing through the first switching element 2a is
1 is represented by I1 = Ia + Ir (1).

Since the first switching element 2a and the second switching element 2b are turned on and off with a duty cycle of about 50%, the current I in the steady state is
The waveform of “a” becomes a triangular wave as shown in FIG.
When the switching element 2a is turned off (FIG. 3 (a)
Reference) The current Ia becomes the maximum value Ia (max). That is,
From the above equation (1), the current I1 (0) at the time of turn-off is as follows: I1 (0) = Ia (max) + Ir (0) (2) Here, Ir (0) means the current Ir at the time of turn-off.

At this time, the slope of the current Ia depends on the value La of the inductance 23a and the potential E / 2 at the neutral point of the DC power supply 1.
The maximum value Ia (max) is also determined by L
a and E / 2. Therefore, it is possible to always set the magnitude of the current I1 (0) at the time of turn-off to a certain value or more. That is, Ia (max) can be set so that I1 (0) = Ia (max) + Ir (0)> (constant value) (3).

With this setting, the current flowing through each arm 10a, 10b at the time of turn-on in each switching element 2a, 2b is negative (as anti-parallel connected to each switching element 2a, 2b) as described below. The current flows through the respective diodes 3a and 3b). At this time, as shown in FIGS. 3A and 3B, between the time when the first switching element 2a is turned off and the time when the second switching element 2b is turned on,
A dead time period Td in which both the switching elements 2a and 2b are off is set.

As described above, the dead time period T
During d, soft switching that effectively utilizes the capacitances 22a and 22b as the lossless snubber circuit shown in FIG. 2 can be realized. In this regard, FIGS.
(D) turns off the first switching element 2a from the state (a) of the first switching element 2a (b),
The mode until the second switching element 2b is turned on (d) after a predetermined dead time (b) to (c) is shown.

First, in FIG. 4A, only the first switching element 2a is on, and as shown in FIG. 3E, the current Ia increases almost linearly regardless of the polarity of the current Ir. I do. Further, the slope depends on the value La of the inductance 23a and the power supply potential E / 2. At this time, the voltage Vc1 of the first capacitance 22a is 0 volt,
Vc2 = E volts of the capacitance 22b.

Next, in FIG. 4B, both the switching elements 2a and 2b are turned off. In this case, FIG.
As shown in (e), the current Ia = Ia (max). According to the setting of the maximum value Ia (max) and the above equation (3), the current I1 (0) of the switching element 2a at the time of turn-off is sufficiently large. It can be a positive current. Therefore, the capacitance 22a is charged and the capacitance 22b is discharged by the resonance phenomenon of the inductance 23a and the other inductance 5 as the auxiliary circuit and the capacitances 22a and 22b as the lossless snubber circuit shown in FIG.

Next, in FIG. 4C, the charging and discharging of the capacitances 22a and 22b are completed, the voltage Vc1 of the first capacitance 22a changes from 0 to E volt, and the voltage Vc2 of the first capacitance 22b changes to E. → It changes to 0 volts,
The second diode 3b conducts. At this time, the current Ia flowing through the inductance 23a starts to decrease due to the voltage of -E / 2.

Thereafter, in FIG. 4D, an ON signal is applied to the second switching element 2b, and the current (Ia + I
When the polarity of r) reverses from positive to negative, the second switching element 2b is replaced with the second switching element 2b as shown in FIGS.
Proceed in the same way as. The changes in the voltages Vc1 and Vc2 of the first and second capacitances 22a and 22b during the operations of FIGS. 4A to 4D are as shown in the graph of FIG.

The above-described operation is the same for the third arm 10c and the fourth arm 10d of the inverter 4 shown in FIG.

From the above, the current I1 at the time of turn-off is obtained.
The value La of the inductance 23a as an auxiliary circuit is determined so that (0) becomes equal to or more than the value required for charging and discharging the first and second capacitances 22a and 22b, and charging during the dead time shown in FIG. By appropriately selecting the values of the capacitances 22a and 22b at which the discharge is completed, FIG.
It is possible to realize a soft switching element that effectively uses the capacitances 22a to 22d as lossless snubber circuits for all the switching elements 2a to 2d shown in FIG.

Next, returning to the embodiment shown in FIG. 1, the operation of this embodiment will be described with reference to a time chart shown in FIG. First, when the tube voltage and the tube current to be applied to the X-ray tube 17 as the load shown in FIG. 1 are determined, a tube voltage setting signal S1 corresponding to the tube voltage and a tube current setting signal S2 corresponding to the tube current are generated. Input to the phase control circuit 19. In this phase control circuit 19, a load resistance value is obtained from the tube voltage setting signal S1 and the tube current setting signal S2, and an inverter is determined from the load resistance value and the tube voltage by using the relationship of the graph shown in FIG. 4 transistors 20a to 20
The operation phase difference α of d is determined. Then, an operation signal for turning on and off the transistors 20a to 20d is generated.

Next, when an X-ray irradiation signal S3 is input from the controller (not shown in FIG. 1) to the phase control circuit 19, the transistors 20a to 20
A signal for turning on and turning off d is output, and drives each of the transistors 20a to 20d.

As a result, as shown in FIGS. 5A to 5D, the first transistor 20a and the second transistor 20b are turned on alternately with a phase difference of 180 °, and the fourth transistor 20d and the fourth transistor 20d are turned on. 3 transistor 20c is also 180 °
, The phase difference between the time when the first transistor 20a turns on and the time when the fourth transistor 20d turns on is α, and the second transistor 20b
Are controlled so that the phase difference from when the third transistor 20c is turned on until the third transistor 20c is turned on is shifted as α.

FIG. 6 shows the waveform of the current Ia of the inductance (auxiliary circuit) 23a when the operation of the inverter 4 starts. When the operation of the inverter 4 starts, the inductance 2
The current Ia of 3a does not flow. For this reason, the maximum current value Ia (max) of the inductance 23a becomes almost twice the steady state value as shown in FIG. 6C due to the first ON signal of the first transistor 20a. Next, the current Ia of the inductance 23a decreases due to the off signal of the first transistor 20a. However, the current Ia of the inductance 23a becomes a triangular wave having a deviation in the positive direction from the steady state.

As a result, as shown in FIG. 6D, a sufficient current in the negative direction does not flow through the first arm 10a as compared with the steady state, and soft switching may not be realized. Further, an excessive current flows in the positive direction, and a transistor (switching element) having a large current rating is required. The same applies to the inductance 23b and the fourth arm 10d.

FIGS. 7 and 8 show a method for preventing the above problem.
Will be described together.

FIG. 7 shows the drive signal B1 'of the first transistor 20a and the waveform of the current flowing through the inductance (auxiliary circuit) 23a. When the X-ray emission signal S3 is output as shown in FIG. 7A, the output B1 of the phase control circuit 19 is output as shown in FIG. 7B. The mask circuit 24 which receives the signal B1 outputs the drive signal B1 'for the first transistor 20a with the first ON period during which the operation is started as shown in FIG. .

As a result, as shown in FIG. 7E, the current flowing through the inductance 23a at the start of the operation of the inverter 4 has a shorter increasing time of the triangular wave, and the inductance 2a
The current maximum value Ia (max) of 3a decreases. Accordingly, a sufficient current flows in the negative direction. Further, as shown in FIG. 7F, a sufficiently negative current flows through the first arm 10a, and soft switching can be realized with a margin. Also, no excessive current flows in the positive direction, and there is no need to use a transistor with a large current rating.

FIG. 8 shows a drive signal B4 'for the first transistor 20d and a current waveform flowing through the inductance (auxiliary circuit) 23b. When the X-ray emission signal S3 is output as shown in FIG. 8A, the output B4 of the phase control circuit 19 is output as shown in FIG. 8B. The mask circuit 24 which receives the signal B4 outputs the drive signal B4 'for the fourth transistor 20d with the first ON period during which the operation is started as shown in FIG. .

As a result, as shown in FIG. 8 (e), the current flowing through the inductance 23b at the start of the operation of the inverter 4 has a shorter increasing time of the triangular wave, and
The current maximum value Ib (max) of 3b decreases. Accordingly, a sufficient current flows in the negative direction. As shown in FIG. 8F, a sufficiently negative current flows through the fourth arm 10d, and soft switching can be realized with a margin. Also, no excessive current flows in the positive direction, and there is no need to use a transistor with a large current rating.

Next, when the transistors 20a to 20d of the inverter 4 start operating under the above control, a resonance current It flows to the transformer 7 shown in FIG. A set tube voltage is applied to the tube 17 and a tube current flows. At this time, the currents I1 to I1 flowing through the first arm 10a to the fourth arm 10d
Referring to FIG. 4, as shown in FIGS. 5 (e) to 5 (h), according to the operation principle described in FIGS. 3 and 4, all the transistors 20a to 20d take a negative value at the time of turn-on. At the time of turn-off, it can be seen that it takes a positive value. Further, each of the transistors 20
The time change rate of the voltage applied to a to 20d is small.

In the time chart shown in FIG. 5, the dead time between the transistors 20a to 20d is omitted for convenience.

In the above-described embodiment, the inductances 23a and 23b as auxiliary circuits are connected to the first and second arms 10 and 10.
a, 10b side and third and fourth arms 10c, 10d
Although the case where it is provided on both sides is shown, it may be provided on only one side when the load range or the range of the phase difference to be controlled is narrow. In addition, although the inductance 5, the capacitance 6, and the transformer 7 are connected to the output circuit of the inverter 4, the inductance 5 and the capacitance 6 are not necessarily required.

FIGS. 7 and 8 show the case where the first on-time of the operation start of the first and fourth transistors 20a and 20d is reduced to half (1/2). However, the present invention is not limited to this. First to fourth transistors 20a to 20d
Of the transistors that turn on first, the ON time of the first operation start may be halved (（) for the purpose of preventing the transistors 20a to 20d from being destroyed. Further, although the on / off cycle of the first to fourth transistors 20a and 20d is shown as being constant, the phase is controlled by the phase control circuit 19 and the on / off cycle of the first to fourth transistors 20a and 20d is controlled. The off cycle may be changed.

Further, in the above description, the inverter 4
Transistor 20a as a switching element constituting
-20d is used, but the present invention is not limited to this, and GTO may be used.
Switching elements such as IGBTs, SI transistors, and SI thyristors may be used. The DC power supply 1
It may be a battery or a rectified commercial power supply.
Further, the load is not limited to the X-ray tube 17, but may be another general load.

[0058]

As described above, according to the present invention, between the connection point of the first and second switching elements of the inverter and the neutral point of the DC power supply, and between the connection point of the third and fourth switching elements. By connecting an inductance as an auxiliary circuit to one or both of the connection point and the neutral point of the DC power supply, the current flowing through each arm of the inverter becomes negative when turned on, and becomes positive when turned off. Operation mode can always be maintained. In addition, by connecting the capacitance to the first to fourth switching elements of the inverter in parallel as a lossless snubber circuit, the time of the voltage applied to each switching element by charging and discharging the lossless snubber capacitance during the dead time period Soft switching with a small change rate can be realized. Therefore, there is an effect that the rate of change of the voltage applied to the switching element is small, the noise can be reduced, and the loss in the switching element can be reduced to increase the efficiency.

Further, the first on-time of the operation start of the switching element which is turned on first of the first and second switching elements and the switching element which is turned on first of the third and fourth switching elements is halved. By doing so, it is possible to realize a converter that allows an equal current to flow in the positive and negative directions to each inductance used as an auxiliary circuit, eliminates overcurrent of each switching element of the inverter, and does not need to use a transistor with a large current rating. There is also an effect.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a DC-DC converter according to the present invention.

FIG. 2 is an equivalent circuit diagram of a main circuit component of the DC-DC converter according to the first embodiment;

FIG. 3 is a time chart for explaining the operation principle of the main circuit component.

FIG. 4 is a diagram for explaining operation modes of a first switching element and a second switching element in the main circuit configuration unit.

FIG. 5 is a time chart for explaining the operation of the circuit shown in FIG. 1;

6 is a diagram showing ON / OFF timing of a first transistor in the circuit shown in FIG. 1, a waveform of a current flowing through an inductance of the first transistor, and the like.

7 is a diagram showing a drive signal of a first transistor in the circuit shown in FIG. 1, a waveform of a current flowing through an inductance of the first transistor, and the like.

8 is a diagram showing a drive signal of a fourth transistor in the circuit shown in FIG. 1, a waveform of a current flowing through an inductance of the fourth transistor, and the like.

FIG. 9 is a circuit diagram of a conventional DC-DC converter.

FIG. 10 is a time chart for explaining the operation of a conventional DC-DC converter.

FIG. 11 is a graph showing a relationship between a phase difference and an output voltage in a conventional phase difference control type DC-DC converter, using load resistance as a parameter.

FIG. 12 is a diagram showing a turn-off waveform when a snubber circuit is not used.

FIG. 13 is a diagram illustrating an example of a conventional snubber circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DC power supply 3a-3d ... 1st-4th diode 4 ... Inverter 5 ... Inductance 6, 12 ... Capacitance 7 ... Transformer 8 ... Rectifier 10a-10d ... 1st-4th Arm 17 X-ray tube (load) 19 Phase control circuit 20a to 20d First to fourth transistors (first to fourth transistors)
4th switching element) 22a to 22d: lossless snubber capacitance 23a, 23b: inductance (auxiliary circuit) 24: mask circuit

Continuation of front page (56) References JP-A-63-190556 (JP, A) JP-A-2-131364 (JP, A) JP-A-6-22551 (JP, A) (58) Fields investigated (Int) .Cl. ⁷ , DB name) H02M 3/335 H02M 3/28 H05G 1/20

Claims (2)

(57) [Claims] 1. A DC power supply having a neutral point, a first series connection body including a first switching element connected to a positive electrode of the DC power supply, and a second switching element connected to a negative electrode of the DC power supply. A first switching element connected to the first switching element, a third switching element connected to the positive electrode, and a fourth switching element connected to the negative electrode; a second switching element connected in parallel to the first switching element; An inverter having first to fourth diodes respectively connected in antiparallel to the fourth switching element and converting DC from the DC power supply into AC, and at least a transformer connected to the output side of the inverter. An inverter output circuit, a rectifier connected to the transformer and converting its output to DC,
The capacitances connected to the rectifier, the loads connected in parallel with the capacitances, and the first to the above-mentioned first signals according to the setting signals of the voltage applied to the loads and the current flowing through the loads.
In a DC-DC converter having means for controlling ON / OFF timing of a fourth switching element, the first to fourth lossless snubbers connected in parallel to the first to fourth switching elements, respectively. A capacitance between a connection point of the first and second switching elements of the inverter and a neutral point of the DC power supply, and a connection point of a third and fourth switching element and a neutral point of the DC power supply. An inductance connected to any one or both of them, a phase control circuit for controlling the on / off phase of the first to fourth switching elements, and the first and the second connected to the phase control circuit. The operation start of the first switching element of the second switching element and the operation start of the first switching element of the third and fourth switching elements. DC-DC converter characterized by comprising a mask circuit for the eye on time to 1/2 of a second and subsequent on-time. 2. The DC-DC converter according to claim 1, wherein the load is an X-ray tube.