JPH07222444A - Dc-dc converter - Google Patents

Abstract

PURPOSE:To obtain the DC-DC converter in which inverter noise and switching loss are reduced to achieve high efficiency. CONSTITUTION:Capacitances 22a, 22b which are used as lossless snubbers are connected respectively in parallel with a first transistor 20a and a second transistor 20b for an inverter 4 which receives a direct current from a DC power supply 1 and which converts the direct current into an alternating current. In addition, inductances 23a, 23d which are used as lossless snubbers are connected respectively in series with a third transistor 20c and a fourth transistor 20d. Thereby, a noise which is generated in the inverter 4 can be reduced, a loss in the individual transistors 20a to 20d is reduced, and the high efficiency of the DC-DC converter is achieved.

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 supplying an AC voltage from a suitable DC power supply to a transformer via an inverter to rectify its output and supply the DC voltage to a required load. DC-DC of the soft switching system that can suppress the time change rate of the voltage / current applied to each switching element of the inverter to reduce noise and also reduce the loss in the switching element to achieve high efficiency. Regarding the converter.

[0002]

2. Description of the Related Art In recent years, a resonance type converter has been developed in which a resonance element is inserted in a part of a converter to make a voltage waveform or a current waveform in a Masahiro wave shape so as to reduce the load on the switching element at the time of switching. There is a phase difference control method as one of the methods for controlling the output voltage of the resonance type converter using the full bridge type inverter. The operation of this system is described in detail in JP-A-63-190556, and a soft switching system DC using this system.
-For the DC converter, see Japanese Patent Application No. 4-201772. This soft switching method is very effective for reducing switching loss and noise generated by switching, but depending on load conditions, a large current may flow in the switch and the auxiliary circuit required for soft switching operation. There is also a drawback that the current flows and the conduction loss increases.

Therefore, this time, as a method for compensating for the drawbacks of the soft switching method, a soft switching type full-bridge DC-DC converter to which a lossless snubber capacitance and a lossless snubber inductance are applied will be described.

First, as an example of a DC-DC converter using an inverter, a resonance type DC-D as shown in FIG.
Take the C converter. This DC-DC converter has a DC power supply 1, a first series connection body composed of a first switch 2a connected to the positive electrode of the DC power supply 1 and a second switch 2b connected to the negative electrode thereof, and It has a third switch 2c connected to the positive electrode and a fourth switch 2d connected to the negative electrode, and has a second series connection body connected in parallel to the first series connection body, and To the fourth switches 2a to 2d, respectively, are connected in anti-parallel to the first to fourth diodes 3a, 3b, 3c,
An inverter 4 having 3d, which receives direct current from the direct current power source 1 and converts it into alternating current, an inductance 5 and a capacitance 6 connected in series at the output side of the inverter 4, and a series connection with the inductance 5 and the capacitance 6. A transformer 7 that is connected and insulated from the output, a rectifier 8 that converts the output of the transformer 7 into a direct current, a load 9 that is connected to the output side of the rectifier 8, and a voltage and load applied to the load 9. It comprises means (not shown) for controlling the on / off timing of the first to fourth switches 2a to 2d according to the setting signal of the current to be supplied. Note that FIG.
In the above, the first to fourth switches 2a to 2d and the diodes 3a to 3d respectively include the first arm 10a,
The second arm 10b, the third arm 10c, and the fourth arm 10d are configured. Further, the rectifier 8 is
The four diodes 11a, 11b, 11c and 11d are configured to perform full-wave rectification of the input voltage. Further, reference numeral 12 denotes a capacitance that smoothes the output voltage of the rectifier 8 and applies it to the load 9.

Next, the conventional DC configured as described above
The operation of the DC converter will be briefly described with reference to FIG. In FIG. 10, (a), (b),
(C) and (d) show ON and OFF periods of the first switch 2a, the fourth switch 2d, the second switch 2b, and the third switch 2c of the inverter 4 shown in FIG. 9, respectively. . As is apparent from FIG. 10, the first switch 2a and the fourth switch 2d are turned on with a phase difference α, and the second switch and the third switch 2c are turned on with a phase difference α. It is supposed to do. Further, the first switch 2a and the second switch 2b, and the third switch and the fourth switch 2d are respectively 180
Turn on alternately with a phase difference of °.

In the above operation, the periods (Tb3 to Tb4) shown in (a) and (b) in which the first switch 2a and the fourth switch 2d are simultaneously turned on, and (c) and
The second switch 2b and the third switch 2c shown in (d)
9 are turned on at the same time (Tb6 to Tb7).
Since electric power is supplied from the DC power supply 1 shown in Fig. 10, the output voltage Vab of the inverter 4 is, as shown in Fig. 10 (j),
Only during the above period, the voltage becomes a square wave having positive and negative peak values. Therefore, the first switch 2a and the fourth switch 2d
9 and the phase difference α between the second switch 2b and the third switch 2c can be changed to change the period in which the respective switches 2a to 2d are turned on at the same time. The power supplied to 9 can be controlled. In this case, the phase difference α between the corresponding switches
The relationship between the output voltage Vout and the output voltage Vout is shown in FIG. In this figure, the horizontal axis represents the phase difference α and the vertical axis represents the output voltage Vout to the load 9, and the phase difference α and the output voltage Vo
The relation with ut is the resistance values R1, R2, R3 of the load 9 described above.
It is a graph showing (R1>R2> R3) as a parameter.

Here, in the above-mentioned structure and operation, the first switch 2a whose turn-on is not delayed and the first diode 3a connected in antiparallel to the first switch 10a, and the second switch 2b. And a second arm 1 composed of a second diode 3b connected in parallel and in reverse.
Consider the operation of 0b (the first and second arms are collectively referred to as the left arm). As shown in FIG. 10E, the current Ic1 flowing through the first arm 10a is the time point Tb1 when the ON signal is input to the first switch 2a.
Is negative in. Therefore, at this point in time, the voltage applied to the first switch 2a is only the ON voltage of the first diode 3a and is almost zero. The loss of the switch 2a when the current changes from negative to positive and the current starts to flow in the first switch 2a is zero because it is the product of the voltage and the current at that time. However, at the time point Tb4 when the first switch 2a is turned off, the current Ic1 flowing through the first arm 10a is as shown in FIG.
It is positive as shown in (e). At this time, the operation until the first switch 2a starts turning off and the current becomes zero is shown in FIG. 12. As shown in this figure, the voltage of the switch 2a increases before the current becomes zero. As a result, the first switch 2a is
A loss corresponding to the shaded area will occur. Such an operation is the same for the second arm 10b.

In order to reduce the loss as described above, for example, as shown in FIG. 13A, a configuration including a capacitance 14 and a resistor 15 connected in parallel to a switch 13 such as a transistor, and the same figure. As shown in (b), a circuit called a snubber circuit, which is composed of a capacitance 14, a resistor 15, and a diode 16, which are also connected in parallel to the switch 13, is used. When such a snubber circuit is provided in parallel with the first and second switches 2a and 2b, the rise of the voltage when each of the switches 2a and 2b is turned on is suppressed, and the switching loss at the time of turn-off can be reduced. Met.

However, in the snubber circuit as described above, when the switch 13 shown in FIG. 13 is turned off, the charge accumulated in the capacitance 14 passes through the switch 13 and the resistor 15 when the switch 13 is turned on. As it is discharged, its resistance 15 causes a loss. Since the resistor 15 controls the maximum value of the current at this time, an excessive current flows without the resistor 15,
The switch 13 will be destroyed. Since the loss due to the resistor 15 occurs every time the switch 13 is repeatedly turned on and off, in the inverter 4 shown in FIG. 9, the loss of each of the switches 2a and 2b increases in proportion to the operating frequency of the inverter 4. . In particular, in a resonance converter, 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, the third switch 2c whose turn-on is delayed and the third arm 10c including the third diode 3c connected in anti-parallel to the third switch 2c and the third switch 10c. The operation of the fourth arm 10d (fourth switch 2d and the fourth diode 3d connected in antiparallel thereto) (third arm and fourth arm are collectively referred to as right arm) will be examined.

In the example shown in FIG. 10, the period during which the ON signal of the fourth switch 2d shown in FIG.
At time Tb5 within (b3 to Tb6), the current Ic4 of the fourth arm 10d becomes zero (see FIG. 10 (f)), and after that time Tb5, a negative current flows. That is, the fourth diode 3 connected in anti-parallel in the fourth arm 10d
Power is passed to d. Then, at time Tb6, as shown in FIG.
As shown in 0 (b), the ON signal to the fourth switch 2d disappears, and as shown in FIG.
c turns on. Then, the current that has flowed through the fourth diode 3d until then is commutated to the third switch 2c, and the fourth diode 3d is reverse biased and turned off. However, at this time, the fourth diode 3d cannot be instantly turned off, and a current called a recovery current flows through the diode until the junction capacitance of the PN junction is charged. Therefore, while this recovery current is flowing, the third arm 10 shown in FIG.
c and the fourth arm 10d are equivalent to a short-circuited state, an excessive current flows to increase the loss of the switch, and a large noise is generated to adversely affect the control system or the like. Third and fourth switches 2c,
It sometimes destroyed 2d. Such an operation is exactly the same when the third switch 2c is turned off. Since the loss due to the above phenomenon (recovery phenomenon of the diode) occurs each time the switches 2c and 2d are repeatedly turned on and off, the loss of the switches 2c and 2d in the inverter 4 shown in FIG. It increases in proportion to the operating frequency. In particular, in a resonance type converter, it is common to increase the operating frequency in order to reduce the size and weight of the device, resulting in a very large switch loss.

[0012]

As described above, in the phase difference control in the conventional DC-DC converter,
Operation of the first and second arms (left arm) 10a, 10b in which turn-on of the inverter 4 shown in FIG. 9 is not delayed, and operation of third and fourth arms (right arm) 10c, 10d in which turn-on is delayed. Is different from the left arm 10a, 10b snubber circuit (see FIG. 13)
Switching loss due to the increase of the right arm 1
In 0c and 10d, there is a problem that the switches 2c and 2d are destroyed or switching loss increases due to arm short circuit due to the recovery current of the diodes 3c and 3d connected in antiparallel to the switches 2c and 2d.
In addition, since the time change rate of the voltage applied to each switch and the time change rate of the current flowing through each switch are large,
In some cases, the generated noise becomes large and adversely affects the control system.

As a method for solving the above problems, Japanese Patent Application No. 4-201772 discloses a soft switching system DC-DC.
I wrote about converters. This soft switching method is a very effective method for reducing switching loss and noise generated by switching,
Depending on the load conditions, a large current flows through the main switch of the inverter and the auxiliary circuit required for soft switching operation, resulting in large conduction loss and reduced efficiency.

Therefore, the present invention addresses the above-mentioned problems and can reduce the noise generated by the inverter under any load condition, and also reduce the loss in the switching element to improve the efficiency. An object of the present invention is to provide a soft-switching DC-DC converter capable of performing the above-mentioned operation.

[0015]

In order to achieve the above object, a DC-DC converter according to the present invention is connected to a DC power source, a first switch connected to the positive electrode of the DC power source and its negative electrode. It has a first series connection body composed of a second switch and is composed of a third switch connected to the positive electrode and a fourth switch connected to the negative electrode, and connected in parallel to the first series connection body. It has a second series connection body, and has first to fourth diodes respectively connected in antiparallel to the first to fourth switches, receives direct current from the direct current power source, and converts it to alternating current. An inverter, a transformer that is connected in series with the inductance and capacitance that are connected in series on the output side of the inverter and that is insulated from the output, a rectifier that converts the output of this transformer into a direct current, and this rectifier A load connected to the output side of the switch and means for controlling the on / off timings of the first to fourth switches according to the voltage applied to the load and the setting signal of the current flowing through the load. DC configured
In the DC converter, the capacitances used as lossless snubbers are connected in parallel to the first and second arms (left arm) of the inverter, and the lossless snubber inductances are connected in series to the third and fourth arms (right arm). It is provided in.

Further, in the above circuit configuration, the lossless snubbers of the left and right arms are replaced (that is, the left arm is provided with the lossless snubber inductance and the right arm is provided with the lossless snubber capacitance), and at the same time, the turn-on of the right arm is delayed. Even if the left arm is used and the left side is used as the arm whose turn-on is delayed, the control is completely equivalent.

Further, as means for controlling the on / off timings of the first to fourth switches, the first to fourth switches of the above-mentioned first to fourth switches are set in accordance with the voltage applied to the load and the setting signal of the current flowing in the load. A frequency phase determination circuit for controlling the operating frequency and the phase difference may be provided. In each of the above means, it is effective to configure the X-ray high voltage device by using the load as the X-ray tube.

[0018]

In the DC-DC converter configured as described above, in the first and second arms (left arm) in which the lossless snubber capacitance of the inverter is provided and the turn-on is not delayed, the voltage applied to the switch at the time of turn-on is the diode. The on-voltage of is almost zero,
The loss at that time is zero because it is the product of voltage and current.
Further, by connecting a capacitance as a lossless snubber in parallel to the first and second switches of the inverter, the time change rate of the voltage applied to each switch due to charging and discharging of the lossless snubber capacitance is small,
Soft switching with less noise can be realized.

On the other hand, in the third and fourth arms (right arm) where the turn-on of the inverter is delayed, the values of the inductance and capacitance connected in series to the output of the inverter are L and C, respectively, and the lossless snubber inductance is large. Let La be La (the values of the lossless snubber inductances provided on the third and fourth arms are the same), the series resonance frequency fr of this circuit is

Fr = 1 / 2π√ (L + La) × C, but if the operating frequency f _{0 of the} inverter is set to this fr or a value close to this, the current will be in the advanced phase in the right arm where the turn-on is delayed. , The lossless snubber inductance causes the third and fourth switches to turn on with zero current,
On the other hand, turn-off is realized when the current and voltage are both zero.

Further, if the operation mode as described above can be maintained, the recovery phenomenon can be eliminated and the loss at the time of switching can be further reduced, and the noise reduction can be realized.

Using the above method, Japanese Patent Application No. 4-2
Compared with the soft switching type DC-DC converter described in No. 01772, it is possible to realize a DC-DC converter with higher efficiency even under a wider load condition or phase difference, and to make up for its drawbacks.

[0023]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 shows a resonance type DC-D according to the present invention.
It is a circuit diagram which shows the Example of a C converter. This resonance type DC-DC converter serves as a power converter that sends an AC voltage from an appropriate DC power supply to a transformer via an inverter to rectify its output and supply the DC voltage to a required load. As shown in, the DC power supply 1 and the inverter 4
And an inductor 5, a capacitor 6, a transformer 7, a rectifier 8, an X-ray tube 17 as a load, a phase determination circuit 18 and a phase control circuit 19, and the resonance type inverter X-ray height It is called a voltage device.

The DC power source is, for example, a battery, and the inverter 4 receives DC from the DC power source 1 and converts it to AC, and serves as a first switch connected to the positive electrode of the DC power source 1. Of the transistor 20a and a transistor 20b as a second switch connected to the negative electrode thereof, and a transistor 20c as a third switch connected to the positive electrode and a fourth connected to the negative electrode. A second series-connected body, which is connected in parallel to the first series-connected body, and which includes the transistor 20d as a switch for each of the transistors 20a-2.
0d and first to fourth diodes respectively connected in anti-parallel. In addition, each of the above transistors 20a to 20
Each d is turned on by passing a base current. Then, the first transistor 20a and the first diode 3a form the first arm 10a.
The second arm 10b by the second transistor 20b and the second diode 3b.
0c and the third diode 3c, the third arm 10c
The fourth transistor 20d and the fourth diode 3d
And constitute the fourth arm 10d.

The right arm of the inverter 4 is provided with lossless snubber inductances 23c and 23d. Further, an inductance 5 is connected to the output side, and a capacitance 6 is connected in series to the inductance 5. The lossless snubber inductances 23c and 23d, the inductance 5 and the capacitance 6 form a resonance circuit. 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 isolates it from the output.
The rectifier 8 converts the output voltage from the transformer 7 into a direct current, and has four diodes 11 as shown in FIG.
a to 11d. Further, an X-ray tube 17 is connected as a load to the output side of the rectifier 8. Reference numeral 12 is an X-ray tube 17 for smoothing the output voltage of the rectifier 8.
Is a smoothing capacitance for applying to.

The phase determination circuit 18 and the phase control circuit 19 are provided for the voltage applied to the X-ray tube 17 and the X-ray tube 1.
7 is a means for controlling the on / off timings of the first to fourth transistors 20a to 20d according to the setting signal of the current flowing through the phase setting circuit 18, and the phase determination circuit 18 sets the tube voltage setting signal S1 and the tube current setting signal. The operation phase of each of the transistors 20a to 20d is determined by the signal S2, and the phase control circuit 19 outputs a control signal for operating each of the transistors 20a to 20d in accordance with the output signal S3 from the phase determination circuit 18 (not shown). It is output by the X-ray exposure signal S4 input from the controller. Reference numerals 21a to 21d denote transistors 20a to 20d, respectively, according to the control signal output from the phase control circuit 19.
The drive circuit which drives d is shown.

Here, in the present invention, the first and second arms 10a and 10b of the inverter 4 have a capacitance 22 used as a lossless snubber.
a and 22b are connected in parallel, and inductances 23c and 23d as lossless snubbers are connected in series to the third and fourth arms 10c and 10d.

Next, the resonance type DC-configured as described above.
The operation of the DC converter will be described. First, the main circuit components (DC power supply 1, inverter 4, inductance 5, capacitance 6, transformer 7, rectifier 8, X-ray tube 17) in the resonance type DC-DC converter shown in FIG.
The equivalent circuit is as shown in FIG. That is, each of the transistors 20a to 20d of the inverter 4 has the first switch 2a and the second switch 2 respectively, as shown in FIG.
b, the third switch 2c, and the fourth switch 2d, and the X-ray tube 17 is represented by the load 9. Therefore, this Figure 2
The principle of operation 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 left arm, that is, the first arm 10a and the second arm 10b in which the turn-on of the inverter 4 is not delayed. When the inverter frequency f ₀ is operated near the series resonance frequency fr of the circuit, the current waveform of the left arm is, as shown in FIGS. 10 (e) and (g), the diode 3a connected in anti-parallel to each arm at turn-on, as shown in FIGS. And 3b, a current flows, while when turned off, a current flows through the switches 2a and 2b. Therefore, in the left arm, turn-on (ZVS / ZCS turn-on) occurs at zero voltage and zero current.
Further, turn-off (ZVS turn-off) is realized at zero voltage. Further, in FIG. 3, a period in which both the first switch 2a and the second switch 2b are off, that is, in FIG.
The period of (b) to (c) is called dead time Td,
At this time, the capacitances 22a and 22b used for the lossless snubbers are effectively used for both the switches 2a and 2b by appropriately selecting the values of the capacitances 22a and 22b that complete the charging and discharging during the dead time Td. Soft switching with less noise that suppresses the voltage-time change rate can be realized.

4A to 4D show that the switch 2a is turned off (b) from the state (a) in which the first switch is on, and the second switch is turned on after the dead time period (b, c). The mode until 2b is turned on is shown. First,
In FIG. 4A, the first switch 2a is turned on,
At this time, the voltage Vc1 = 0 of the first capacitance 22a
Volts, voltage of the second capacitance 22b Vc2 = E
It is a bolt. Next, FIG. 4B shows the first switch 2
a indicates the time when a is turned off, and at this time, both the first and second switches 2a and 2b are off. During this time, the voltage Vc1 of the first capacitance 22a gradually rises from zero volt, while conversely the voltage Vc of the second capacitance 22b is increased.
2 gradually decreases while maintaining the relationship of Vc1 + Vc2 = E.

Next, in FIG. 4 (c), charging and discharging of the capacitances 22a and 22b are completed, the voltage Vc1 of the first capacitance 22a is E volt, and the voltage Vc2 of the second capacitance 22b is 0 volt. Change to
The second diode 3b becomes conductive. After that, FIG. 4 (d)
Then, when an ON signal is given to the second switch 2b and the polarity of the current Ir is inverted from positive to negative, the second switch 2b is naturally commutated to the second switch 2b and turned on.

Contrary to this state, the operation of turning off the second switch 2b and turning on the first switch 2a is performed by switching the first switch 2a and the second switch 2b as shown in FIG. )-(D). In addition, FIG.
FIG. 4E shows how the voltages Vc1 and Vc2 of the first and second capacitances 22a and 22b change during the operation of (d).

From the above, by appropriately selecting the values of the capacitances 22a and 22b so that charging / discharging is completed during the dead time Td shown in FIG. 4 (e), lossless for both switches 2a and 2b. Soft switching that effectively utilizes the capacitances 22a and 22b used as snubbers can be realized.

Next, the operation of the third arm 10c and the fourth arm 10d (right arm) provided with the lossless snubber inductance which delays the turn-off will be described with reference to FIGS. 5A to 5D show modes from the state in which the third switch 2c is turned on to the state in which the fourth switch 2d is turned on. As shown in FIGS. 10 (f) and 10 (h), in the conventional right arm, the current is in the advance phase, so the current immediately before the third switch 2c is turned off is the third current as shown in FIG. 5 (a). It was flowing through the diode 3c. Here, if the value of the series resonance inductance 5 is L and the value of the lossless snubber inductance is La, and the sum (L + La) of these is set to the same value as the series resonance inductance 5 in the conventional circuit, the third switch is still the same as the conventional one. Immediately before 2c turns off, the current flows through the third diode 3c.

Next, the third switch is turned off (see FIG. 5).
(B)) When the fourth switch 2d is turned on after the dead time (FIG. 5 (c)), the current flowing through the third diode 3c gradually decreases, while the fourth switch 2d
The current flowing through the diode 3c gradually increases from zero, and thirdly, the diode 3c naturally turns off (FIG. 5 (d)). The time change rate of the current at this time is determined by the power supply voltage E and the value La of the lossless snubber inductance. As described above, in FIG. 5B, in the third switch 2c, since the current flows through the third diode 3c, the voltage applied to the third switch 2c is zero, and the third switch 2c is turned off at zero voltage / zero current ( ZVS, ZCS) is realized. Also, FIG.
In (c), the current flowing through the fourth switch 2d gradually increases from zero after the fourth switch is turned on,
Turn-on (ZCS) is realized when the current is zero.
On the contrary, the operation in which the fourth switch 2d is turned off and the third switch 2c is turned on is also shown in FIG.
It is exactly the same as (d). Due to the lossless snubber inductance as described above, zero current turn-on in the third switch and fourth switch, while zero voltage
Zero current turn-off is achieved. Further, in the above-mentioned operation mode, when each switch is turned off, a voltage is applied to the lossless snubber inductance, so that the anti-parallel diode 3c or 3d does not receive a voltage as rapidly as in the conventional case. The recovery phenomenon does not occur, and it is effective in reducing loss and noise during switching. In addition, if the above method is used, compared to the soft switching type DC-DC converter described in Japanese Patent Application No. 4-201772, the efficiency of D is higher even under a wider load condition or phase difference.
A C-DC converter can be realized and its drawback can be compensated.

Next, returning to the embodiment shown in FIG. 1, the operation of this embodiment will be described with reference to the timing diagram shown in FIG. First, the X-ray tube 17 as a load shown in FIG.
When the tube voltage and the tube current to be applied to the tube voltage are determined, the tube voltage setting signal S1 corresponding to the tube voltage and the tube current setting signal S2 corresponding to the tube current are input to the phase determination circuit 18. In the phase determination circuit 18, a load resistance value is obtained from the tube voltage signal S1 and the tube current signal S2, and the relationship between the load resistance value and the tube voltage is used in the graph shown in FIG.
The operating phase difference α of each of the transistors 20a to 20d of the inverter 4 is determined. Then, the output signal S3 corresponding to this phase difference is output from the phase determination circuit 18 and sent to the phase control circuit 19. Then, the phase control circuit 19 inputs the output signal S3 and creates an operation signal for turning on and off the respective transistors 20a to 20d.

Next, when the X-ray exposure signal S4 is input to the phase control circuit 19 from a controller (not shown) in FIG. 1, the transistors 20a to 20d of the inverter 4 are supplied.
Signals for turning on and off are output to the corresponding drive circuits 21a to 20d, respectively.
The a to 21d drive the respective transistors 20a to 20d according to the operation signal output from the phase control circuit 19. As a result, as shown in FIGS. 6A to 6D, the first transistor 20a and the second transistor 20b are alternately turned on with a phase difference of 180 °, and the third transistor 20c and the fourth transistor 20c are turned on. The transistor 20d is alternately turned on while maintaining the phase difference of 180 °, but is controlled to operate with a certain phase difference α depending on the load condition.

Next, when the respective transistors 20a to 20d of the inverter 4 start to operate by the above control, the resonance current Ir as shown in FIG. 6 (i) flows through the transformer 7 shown in FIG. A set tube voltage is applied to the X-ray tube 17 and a tube current flows. At this time, the currents Ic1 and Ic2 flowing through the first arm 10a and the second arm 10b (left arm) provided with the lossless snubber capacitance that does not delay the turn-on are shown in FIGS. 6 (d) and 6 (e). As described above, according to the operating principles described in FIGS. 3 and 4, the respective transistors 20a and 20b have a negative value at turn-on and a positive value at turn-off, and have zero voltage / zero current turn-on and It can be seen that zero voltage turn-off is achieved.
Further, the time change rate of the voltage applied to each of the transistors 20a to 20d can be reduced due to the influence of the lossless snubber capacitance, and the generation of noise can be suppressed.

On the other hand, in the third arm 10c and the fourth arm 10d (right arm) provided with the lossless snubber inductance which delays the turn-on, the respective arms 10c and 10d are operated according to the operation principle described in FIG.
It can be seen that the current has a zero value at turn-on and a negative value at turn-off, and the first and second switches realize zero current turn-on and zero voltage / zero current turn-off. In addition, the time change rate of the voltage applied to each of the transistors 20c to 20d becomes small due to the influence of the lossless snubber inductance, and the generation of noise is suppressed. In addition, the output voltage waveform of the inverter is affected by the lossless snubber inductances 23c and 23d.
It is different from the conventional square wave shown in 0 (j).

FIG. 7 is a circuit diagram showing a second embodiment of the present invention. In this embodiment, the main circuit structure is exactly the same as that of the embodiment shown in FIG. 1, but the timing control means for controlling the on / off of the first to fourth transistors 20a to 20d of the inverter 4 serves as a load. A frequency phase control circuit 24 is provided for controlling the operating frequencies and phases of the first to fourth transistors 20a to 20d according to the setting signals of the voltage applied to the X-ray tube 17 and the current flowing in the load. is there. The frequency determination circuit 25 is provided in parallel with the phase determination circuit 18 on the upstream side of the frequency phase control circuit 24.
Is provided. The frequency determining circuit 25 determines the operating frequency of each of the transistors 20a to 20d according to the tube voltage setting signal S1 and the tube current setting signal S2. As a result, the frequency / phase control circuit 24 outputs the output signal S3 from the phase determination circuit 18 and the frequency determination circuit 2
5 according to the output signal S5 from
X-ray exposure signal S that inputs a signal for controlling the frequency and phase at which a to 20d operate from a controller (not shown)
4 operates to output. The phase determination circuit 18 and the frequency determination circuit 25 are respectively shown in FIG.
Further, the relationship shown in the graph of FIG. 8 can be easily constituted by a table memory, a relationship generator, an operational amplifier, or the like. In the case of this embodiment, the inverter 4
The on / off timing of each of the transistors 20a to 20d can be controlled by not only the phase difference but also the frequency control, and more efficient operation can be realized especially when the frequency is increased.

The present invention is not limited to the above embodiments, but can be applied within the scope of the invention. For example, the switching element used for the inverter 4 may be a GTO instead of a transistor, and elements such as MOSFET, IGBT, SI transistor, and SI thyristor can be applied to further increase the frequency. The DC power source 1 of the inverter 4 may be a battery or a commercial power source rectified. The load is not limited to the X-ray tube and may be another general load target. Furthermore, the inductance 5 is
The leakage inductance of the transformer 7 and the wiring inductance can also be used. The smoothing capacitance 12 is
When the load is an X-ray tube, the capacitance of the high voltage cable can also be used.

[0042]

As described above, the present invention is configured as described above. The first and second arms (left arm) of the inverter are connected in parallel with a capacitance as a lossless snubber, and the third and fourth arms (right arm). ) Is connected in series with an inductance as a lossless snubber, so that in the left arm, zero voltage / zero current turn-on and zero voltage turn-off are performed by charging / discharging the lossless snubber capacitance during the dead time period Td. In the right arm, zero current turn-on and zero voltage / zero current turn-off are realized by the effect of the lossless snubber inductance. As described above, it is possible to realize soft switching that can reduce noise with high efficiency.
Further, in particular, as in the X-ray CT apparatus, it is necessary to detect a minute signal (about several microvolts) for composing a diagnostic image in the vicinity of the inverter circuit, such as switching several hundred volts and several hundred amperes. When applied to such a device, the effect of the present invention that can reduce noise that adversely affects not only the control system but also the image is extremely large.

Further, in the second embodiment shown in FIG. 7, by providing the frequency phase control circuit for controlling the operating frequency and phase of each switch of the inverter, efficient operation can be achieved under any load condition. Can be realized.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing an equivalent circuit of a main circuit constituent part in the resonance type DC-DC converter shown in FIG.

FIG. 3 is a timing diagram for explaining the operation principle of the main circuit configuration unit.

FIG. 4 is an explanatory diagram showing operations of first and second switches in the main circuit configuration unit.

FIG. 5 is an explanatory diagram showing operations of a third switch and a fourth switch in the main circuit configuration unit.

FIG. 6 is a timing diagram for explaining the operation of the embodiment shown in FIG.

FIG. 7 is a circuit diagram showing a second embodiment of the present invention.

FIG. 8 is a graph showing the relationship between the inverter frequency and the output voltage using the phase as a parameter.

FIG. 9 is a circuit diagram showing a conventional resonance type DC-DC converter.

FIG. 10 is a timing diagram for explaining the operation of the conventional example.

FIG. 11 is a graph showing the relationship between the phase difference and the output voltage in the phase difference control, using the load resistance as a parameter.

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

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

[Explanation of symbols]

 1 DC power supply 2a 1st switch 2b 2nd switch 2c 3rd switch 2d 4th switch 3a 1st diode 3b 2nd diode 3c 3rd diode 3d 4th diode 4 Inverter 5 Inductance 6 Capacitance 7 Transformer 8 Rectifier 9 Load 10a First arm 10b Second arm 10c Third arm 10d Fourth arm 12 Smoothing capacitance 17 X-ray tube 18 Phase determination circuit 19 Phase control circuit 20a to 20d Transistor 21 Drive circuit 22a Lossless Capacitance as snubber 22b Capacitance as lossless snubber 23c Inductance as lossless snubber 23d Inductance as lossless snubber 24 Frequency phase control circuit 25 Frequency determination circuit

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Kazuhiko Sakamoto 1-1-14 Kanda, Chiyoda-ku, Tokyo Inside Hitachi Medical Co., Ltd.

Claims (4)

[Claims] 1. A first series connection body comprising a DC power supply, a first switch connected to the positive electrode of the DC power supply, and a second switch connected to the negative electrode thereof, and connected to the positive electrode. A third switch and a fourth switch connected to the negative electrode, and a second series connection body connected in parallel to the first series connection body, and each of the first to fourth switches has a reverse connection. A full bridge type inverter having first to fourth diodes connected in parallel, receiving direct current from the direct current power source and converting it to alternating current, and an inductance and a capacitance connected in series at the output side of this inverter, A transformer that is connected in series with the inductance and capacitance to insulate the output, a rectifier that converts the output of this transformer into DC, and a load that is connected to the output of this rectifier. In the DC-DC converter configured to have means for controlling on / off timings of the first to fourth switches according to a setting signal of a voltage applied to the load and a current flowing in the load, the inverter The first and second switches are each connected in parallel with a capacitance used as a lossless snubber, and the inverters third and fourth switches are each provided with an inductance as a lossless snubber in series. And a DC-DC converter. 2. The DC-DC converter according to claim 1, wherein the load is an X-ray tube. 3. As a means for controlling the on and off timings of the first to fourth switches, the first to fourth switches of the first to fourth switches are controlled in accordance with a voltage applied to the load and a setting signal of a current flowing through the load. The DC-DC converter according to claim 1 or 2, further comprising a control circuit for controlling an operating frequency and a phase difference. 4. The DC-DC converter according to claim 1, wherein the load is an X-ray tube.