JP2016149834A - Dc/dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a rush current at actuation with a simple configuration.SOLUTION: A DC/DC converter 1 includes: a transformer T1 including primary windings W11-W13 and secondary windings W21-W23; a first bridge circuit 10 which converts a DC input voltage Vin into a first AC voltage by first switching elements SW1-SW6 to supply to the primary windings; a second bridge circuit 20 which converts and outputs a second AC voltage, supplied from the secondary windings into a DC output voltage Vout, by second switching elements SW7-SW12; and a control circuit 30 which outputs first drive signals G1-G6 for switching each first switching element and second drive signals G7-G12 for switching each second switching element. The control circuit sets a duty ratio of the first and second drive signals at activation to be larger than a duty ratio of the first and second drive signals at constant voltage control.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a DAB (Dual Active Bridge) DC / DC converter.

  Conventionally, a DAB type DC / DC converter is known as a DC / DC converter capable of increasing power conversion efficiency in both directions (see, for example, Patent Document 1). The DC / DC converter includes a transformer having a primary winding and a secondary winding, a first bridge circuit, a second bridge circuit, and an inductor connected between the transformer and the second bridge circuit. And a control circuit for outputting a first drive signal and a second drive signal.

  The first bridge circuit converts the input DC input voltage into a first AC voltage according to the first drive signal, and supplies the first AC voltage to the primary winding of the transformer. The second bridge circuit converts the second AC voltage supplied from the secondary winding of the transformer into a DC output voltage according to the second drive signal, and outputs the DC output voltage. In this DC / DC converter, the output current is controlled by controlling the phase difference between the first drive signal and the second drive signal, and the output voltage is controlled to be constant. The input current and output current are determined by the phase difference, the inductance value of the inductor, and the voltage applied to the inductor. In a state where the output voltage is stable, a potential difference between the input voltage and the output voltage is applied to the inductor.

  When the soft start is performed, generally, the output power is set to a phase difference at which the output power becomes zero and started, and then the output voltage is gradually increased by gradually increasing the phase difference. However, with this method, the output voltage gradually increases, but a large inrush current flows through the switching element of the first bridge circuit. The reason is that immediately after startup, the output voltage is zero, so that all of the input voltage is applied to the inductor.

  This characteristic is not preferable when a fuel cell or the like having a characteristic that does not allow inrush current is connected as an input power source. Therefore, the start-up method is to start up with a small potential difference of the inductor by applying the input voltage after precharging the output side by using the voltage of the battery, system (AC), etc. Can be considered. Thereby, the inrush current at the time of starting can be suppressed.

US Pat. No. 5,027,264

  However, when a precharge circuit is provided, the number of parts increases and control for the precharge circuit is required, which complicates the configuration.

  Therefore, an object of the present invention is to provide a DC / DC converter that can suppress an inrush current at startup with a simple configuration.

A DC / DC converter according to an aspect of the present invention includes:
A transformer having a primary winding and a secondary winding;
A first bridge circuit that converts an input DC input voltage into a first AC voltage and supplies the first AC voltage to the primary winding by a switching operation of the plurality of first switching elements;
A second bridge circuit that converts a second AC voltage supplied from the secondary winding of the transformer into a DC output voltage and outputs the DC voltage by a switching operation of a plurality of second switching elements;
A first driving signal for switching each first switching element on or off and a second driving signal for switching each second switching element on or off are output, and during constant voltage control, A control circuit that controls a phase difference between the first drive signal and the second drive signal so that an output voltage approaches a target voltage,
The control circuit sets a duty ratio of the first and second drive signals at start-up to be larger than a duty ratio of the first and second drive signals at the constant voltage control. .

In the DC / DC converter,
The first and second bridge circuits are three-phase full bridge circuits,
The control circuit may set the duty ratio at the time of startup to be larger than 2/3, and set the duty ratio at the time of the constant voltage control to 2/3 or less.

In the DC / DC converter,
The first and second bridge circuits are two-phase full bridge circuits,
The control circuit may set the duty ratio at the time of startup to be larger than ½ and set the duty ratio at the time of the constant voltage control to be ½ or less.

In the DC / DC converter,
The control circuit may set the start-up duty ratio to be smaller than a value at which the output power output from the second bridge circuit becomes zero.

In the DC / DC converter,
The control circuit may perform the constant voltage control after the output voltage reaches a predetermined value after the output voltage reaches an upper limit value at the start-up, and after the output voltage reaches a predetermined value. .

In the DC / DC converter,
The control circuit may increase the phase difference from the angle at which the output power output from the second bridge circuit becomes zero to 90 ° during the startup.

In the DC / DC converter,
The control circuit may increase the phase difference up to 90 ° before the output voltage reaches the upper limit value.

In the DC / DC converter,
The control circuit may set the phase difference to 90 ° immediately after startup.

In the DC / DC converter,
The control circuit may gradually decrease the duty ratio and gradually decrease the phase difference after the output voltage reaches the upper limit value at the start-up.

In the DC / DC converter,
The control circuit may perform the constant voltage control while gradually decreasing the duty ratio after the output voltage reaches the upper limit value at the time of startup.

In the DC / DC converter,
The duty ratio of the first drive signal is a high period / cycle of the first drive signal for switching the first switching element on the high side,
The duty ratio of the second drive signal may be a high period / cycle of the second drive signal for switching the second switching element on the low side.

  According to the present invention, since the duty ratio at the start of the first and second drive signals is set larger than the duty ratio at the time of constant voltage control, the period during which the input voltage is applied to the primary winding at the start Can be shortened. Thereby, since the electric current which flows into a primary winding at the time of starting can be made small, an inrush current can be suppressed. Such duty ratio control can be realized by changing the control of the control circuit without adding a circuit element. Therefore, the inrush current at the time of starting can be suppressed with a simple configuration.

1 is a circuit diagram showing a schematic configuration of a three-phase DC / DC converter according to a first embodiment. FIG. (A) is a timing diagram of the 1st and 2nd drive signal at the time of constant voltage control, (b) is a timing diagram of the 1st and 2nd drive signal at the time of starting. It is a figure which shows the relationship between the duty ratio of the 1st and 2nd drive signal, and the peak current of a 1st switching element. It is a figure which shows the relationship between the 1st drive signal in case a duty ratio is 60%, and the voltage between the both ends of a primary winding. It is a figure which shows the relationship between the 1st drive signal in case a duty ratio is 66.6%, and the voltage between the both ends of a primary winding. It is a figure which shows the relationship between the 1st drive signal in case a duty ratio is 70%, and the voltage between the both ends of a primary winding. It is a figure for demonstrating the starting sequence of the DC / DC converter of FIG. It is a figure for demonstrating the starting sequence of the DC / DC converter which concerns on 2nd Embodiment. It is a figure for demonstrating the starting sequence of the DC / DC converter which concerns on 3rd Embodiment. It is a circuit diagram which shows schematic structure of the DC / DC converter for two phases which concerns on 5th Embodiment.

  Embodiments of the present invention will be described below with reference to the drawings. These embodiments do not limit the present invention.

(First embodiment)
FIG. 1 is a circuit diagram showing a schematic configuration of a three-phase DC / DC converter 1 according to the first embodiment. As shown in FIG. 1, the DC / DC converter 1 includes a transformer T1, a first bridge circuit 10, a second bridge circuit 20, a control circuit 30, and inductors L1 to L3.

  The three-phase transformer T1 has primary windings W11, W12, W13 and secondary windings W21, W22, W23. Primary winding W11 is magnetically coupled to secondary winding W21. Primary winding W12 is magnetically coupled to secondary winding W22. Primary winding W13 is magnetically coupled to secondary winding W23.

  Primary winding W11 has one end connected to node N11 and the other end connected to node N12. Primary winding W12 has one end connected to node N12 and the other end connected to node N13. Primary winding W13 has one end connected to node N13 and the other end connected to node N11.

  Secondary winding W21 has one end connected to node N21 and the other end connected to node N22. Secondary winding W22 has one end connected to node N22 and the other end connected to node N23. Secondary winding W23 has one end connected to node N23 and the other end connected to node N21.

  The first bridge circuit 10 is a three-phase full bridge circuit. The first bridge circuit 10 converts the DC input voltage Vin input between the input terminals T11 and T12 into a first AC voltage by a switching operation of the six first switching elements SW1 to SW6, thereby converting the transformer. Supply to the primary windings W11, W12, W13 of T1. The first switching elements SW1 to SW6 are, for example, N-type MOS transistors.

  The first switching element SW1 has one end (drain) connected to one input terminal T11, the other end (source) connected to the node N11, and a control terminal (gate) to which the first drive signal G1 is supplied. And).

  The first switching element SW2 has one end (drain) connected to the node N11, the other end (source) connected to the other input terminal T12, and a control terminal (gate) to which the first drive signal G2 is supplied. And).

  The first switching element SW3 has one end (drain) connected to one input terminal T11, the other end (source) connected to the node N12, and a control terminal (gate) to which the first drive signal G3 is supplied. And).

  The first switching element SW4 has one end (drain) connected to the node N12, the other end (source) connected to the other input terminal T12, and a control terminal (gate) to which the first drive signal G4 is supplied. And).

  The first switching element SW5 has one end (drain) connected to one input terminal T11, the other end (source) connected to the node N13, and a control terminal (gate) to which the first drive signal G5 is supplied. And).

  The first switching element SW6 has one end (drain) connected to the node N13, the other end (source) connected to the other input terminal T12, and a control terminal (gate) to which the first drive signal G6 is supplied. And).

  One end of the inductor L1 is connected to the node N21. One end of the inductor L2 is connected to the node N22. One end of the inductor L3 is connected to the node N23.

  The second bridge circuit 20 is a three-phase full bridge circuit. The second bridge circuit 20 is supplied with the second windings W21, W22, and W23 of the transformer T1 via the inductors L1 to L3 by the switching operation of the six second switching elements SW7 to SW12. The AC voltage is converted into a DC output voltage Vout and output between output terminals T21 and T22. The second switching elements SW7 to SW12 are, for example, N-type MOS transistors.

  The second switching element SW7 has one end (drain) connected to one output terminal T21, the other end (source) connected to the other end of the inductor L1, and a control to which the second drive signal G7 is supplied. And a terminal (gate).

  The second switching element SW8 has one end (drain) connected to the other end of the inductor L1, the other end (source) connected to the other output terminal T22, and a control to which the second drive signal G8 is supplied. And a terminal (gate).

  The second switching element SW9 has one end (drain) connected to one output terminal T21, the other end (source) connected to the other end of the inductor L2, and a control to which the second drive signal G9 is supplied. And a terminal (gate).

  The second switching element SW10 has one end (drain) connected to the other end of the inductor L2, the other end (source) connected to the other output terminal T22, and a control to which the second drive signal G10 is supplied. And a terminal (gate).

  The second switching element SW11 has one end (drain) connected to one output terminal T21, the other end (source) connected to the other end of the inductor L3, and a control to which the second drive signal G11 is supplied. And a terminal (gate).

  The second switching element SW12 has one end (drain) connected to the other end of the inductor L3, the other end (source) connected to the other output terminal T22, and a control to which the second drive signal G12 is supplied. And a terminal (gate).

  A capacitor C1 is connected between the output terminals T21 and T22.

  The parasitic capacitances of the secondary windings W21 to W23, the inductors L1 to L3, and the second switching elements SW7 to SW12 constitute a resonance circuit for performing zero volt switching.

  The control circuit 30 includes first drive signals G1 to G6 that turn on or off the first switching elements SW1 to SW6, and second drive signals that turn on or off the second switching elements SW7 to SW12. G7 to G12 are output.

  FIG. 2A is a timing diagram of the first and second drive signals G1 to G12 at the time of constant voltage control, and FIG. 2B is a timing diagram of the first and second drive signals G1 to G12 at the time of activation. FIG.

  As shown in FIG. 2A, during the constant voltage control, the first and second drive signals G1 to G12 are rectangular waves having a duty ratio of about 50%, for example. Each of the first drive signals G2, G4, G6 is an inverted signal of the corresponding first drive signal G1, G3, G5. Each of the second drive signals G8, G10, and G12 is an inverted signal of the corresponding first drive signal G7, G9, and G11.

  The phase difference between the first drive signals G1, G2 and the first drive signals G3, G4 is about 120 °. The phase difference between the first drive signals G1, G2 and the first drive signals G5, G6 is about 240 °.

  The phase difference between the second drive signals G7 and G8 and the second drive signals G9 and G10 is about 120 °. The phase difference between the second drive signals G7 and G8 and the second drive signals G11 and G12 is about 240 °.

  The control circuit 30 controls the phase difference φ between the first drive signals G1 to G6 and the second drive signals G7 to G12 so that the output voltage Vout approaches the target voltage during constant voltage control.

  As the phase difference φ increases, the output power output from the second bridge circuit 20 increases. When each element of the DC / DC converter 1 is ideal and the duty ratio is 50%, the output power output from the second bridge circuit 20 becomes zero when the phase difference φ is 0 °, It becomes maximum when the phase difference φ is 90 °. When the phase difference φ is negative, output power is output from the first bridge circuit 10 to the input terminal. That is, the DC / DC converter 1 can be used as a bidirectional DC / DC converter.

  As shown in FIG. 2B, at the time of startup, the first and second drive signals G1 to G12 are rectangular waves having a duty ratio of, for example, about 70%.

  The control circuit 30 sets the duty ratio of the first and second drive signals G1 to G12 at the time of start-up larger than the duty ratio of the first and second drive signals G1 to G12 at the time of constant voltage control. Specifically, the control circuit 30 sets the duty ratio at the time of startup to be larger than 2/3 (about 66.6%), and sets the duty ratio at the time of constant voltage control to 2/3 or less.

  The duty ratio of the first drive signals G1 to G6 is the “high period / cycle” (for one cycle) of the first drive signals G1, G3, G5 for switching the first switching elements SW1, SW3, SW5 on the high side. “Ratio of high period) and“ low period / period ”(ratio of low period to one period) of the first drive signals G2, G4, G6 for switching the first switching elements SW2, SW4, SW6 on the low side. ).

  The duty ratio of the second drive signals G7 to G12 is the “high period / cycle” of the second drive signals G8, G10, G12 for switching the second switching elements SW8, SW10, SW12 on the low side, and This is the “low period / cycle” of the second drive signals G7, G9, G11 for switching the second switching elements SW7, SW9, SW11 on the high side.

  The control circuit 30 sets the duty ratio at the time of start-up smaller than a value at which the output power output from the second bridge circuit 20 becomes zero. The value at which the output power output from the second bridge circuit 20 becomes zero varies depending on the load or the like, but is approximately 75% when there is almost no load, for example.

  FIG. 3 is a diagram illustrating a relationship between the duty ratios of the first and second drive signals G1 to G12 and the peak currents of the first switching elements SW1 to SW6.

  As shown in FIG. 3, when the duty ratio is 50% or more and less than 66.6%, the peak current is almost constant. When the duty ratio is 66.6% or more, the peak current monotonously decreases as the duty ratio increases. As described above, for example, when the duty ratio is about 75% or more, the output power output from the second bridge circuit 20 is zero.

  4 to 6 are diagrams illustrating the relationship between the first drive signals G1, G3, and G5 and the voltages V1, V2, and V3 across the primary windings W11, W12, and W13. In FIG. 4, the duty ratio is 60%, in FIG. 5, the duty ratio is 66.6%, and in FIG. 6, the duty ratio is 70%.

  As shown in FIGS. 4 and 5, when the duty ratio is 60% and 66.6%, there is no period in which the first drive signals G1, G3, and G5 are simultaneously high. In this case, each voltage V1, V2, V3 between both ends of the primary windings W11, W12, W13 becomes a positive input voltage Vin in a period of about 33.3% of the cycle, and is about 33.3% of the cycle. It becomes a negative input voltage Vin in the period, and becomes zero in a period of about 33.3% of the period.

  On the other hand, as shown in FIG. 6, when the duty ratio is 70%, there is a period T1 in which the first drive signals G1, G3, and G5 are simultaneously high. In this period T1, the first switching elements SW1, SW3, SW5 are turned on, and the first switching elements SW2, SW4, SW6 are turned off. Accordingly, in the period T1, the voltages V1, V2, and V3 become zero.

  Thereby, each voltage V1, V2, V3 becomes a positive input voltage Vin in a period of 30% (= 100% -70%) of a cycle, and becomes a negative input voltage Vin in a period of 30% of a cycle, It becomes zero in 40% of the period.

  The period T1 in which the first drive signals G1, G3, and G5 are simultaneously high becomes longer as the duty ratio increases. Therefore, the period during which the voltages V1, V2, and V3 become the positive or negative input voltage Vin, that is, the period during which the input voltage Vin is applied to the primary windings W11, W12, and W13 is shortened as the duty ratio increases. .

  That is, by setting the duty ratio to be larger than 66.6%, it is possible to shorten the period during which the input voltage Vin is applied to the primary windings W11, W12, and W13 during startup. Thereby, since the electric current which flows into primary winding W11, W12, W13 at the time of starting can be made small, inrush current can be controlled.

Next, an example of the activation sequence will be described.
FIG. 7 is a diagram for explaining a startup sequence of the DC / DC converter 1 of FIG. In the example of FIG. 7, the duty ratio is 75% at the time t10 when the activation starts. Thereby, the inrush current of the input current Iin can be suppressed as described above.

  At time t10, the phase difference φ is an angle φ0. The angle φ0 is an angle at which the output power output from the second bridge circuit 20 becomes zero. The control circuit 30 increases the phase difference φ from the angle φ0 to 90 ° during startup. Thereby, soft start can be performed, and the rise of the output voltage Vout can be moderated.

  In addition, at the time of starting represents the period from time t10 which started activation to time t13 which starts constant voltage control.

  After time t10, the output voltage Vout gradually increases and reaches the upper limit value V10 at time t11. Since the duty ratio is 75%, the output voltage Vout does not become higher than the upper limit value V10 even if the phase difference φ is 90 ° which is the maximum.

  The control circuit 30 increases the phase difference φ to 90 ° before the output voltage Vout reaches the upper limit value V10. This prevents the output voltage Vout from rising too slowly.

  The control circuit 30 gradually decreases the duty ratio after the output voltage Vout reaches the upper limit value V10 during startup. In the example of FIG. 7, the output voltage Vout reaches the upper limit value V10 at time t11, and then the duty ratio is gradually decreased after waiting until time t12. The duty ratio may be decreased from time t11 without waiting until time t12.

  After time t12, the output voltage Vout rises to the upper limit value V10 or more by reducing the duty ratio.

  The control circuit 30 performs constant voltage control after time t13 when the output voltage Vout reaches a predetermined value V20, and gradually reduces the duty ratio to 50%. The predetermined value V20 is set to a value lower than the target voltage V30 and relatively close to the target voltage V30. As a constant voltage control method, for example, a well-known PID control can be used.

  Thus, after time t13, the output voltage Vout is controlled to the target voltage V30 by controlling the phase difference φ. At time t14, the duty ratio becomes 50%. The duty ratio does not have to be reduced to 50%, but is preferably reduced to at least 66.6% from the viewpoint of power utilization efficiency.

  As described above, according to the present embodiment, the duty ratio at the time of activation of the first and second drive signals G1 to G12 is set larger than the duty ratio at the time of constant voltage control. The period during which the input voltage Vin is applied to the primary windings W11 to W13 can be shortened. Thereby, since the electric current which flows into primary winding W11-W13 at the time of starting can be made small, an inrush current can be controlled.

  Further, such control of the duty ratio can be realized by changing the control of the control circuit 30 without adding a circuit element. Therefore, the inrush current at the time of starting can be suppressed with a simple configuration.

  Note that immediately after the start of activation (immediately after time t10), the control circuit 30 may not output the second drive signals G7 to G12. Even in this case, the inrush current can be suppressed.

  Further, the inductors L1 to L3 may not be provided. In addition to the inductors L1 to L3, an inductor is provided between the node N11 and a connection node between the other end of the first switching element SW1 and one end of the first switching element SW2, and the first switching element SW3 An inductor is provided between the node N12 and a connection node at one end of the other end and the first switching element SW4, and a connection node at the other end of the first switching element SW5 and one end of the first switching element SW6. An inductor may be provided between the node N13. Even with this configuration, the above effects can be obtained.

  Further, the first and second switching elements SW1 to SW12 may be other elements such as an IGBT, and the number thereof is not limited to six.

(Second Embodiment)
In the second embodiment, the activation sequence is different from that of the first embodiment.

  The configuration of the DC / DC converter 1 of the second embodiment is the same as that of the first embodiment, and the function of the control circuit 30 is different from that of the first embodiment. Below, it demonstrates centering around difference.

  FIG. 8 is a diagram for explaining a startup sequence of the DC / DC converter 1 according to the second embodiment. As shown in FIG. 8, the control circuit 30 sets the phase difference φ to 90 ° immediately after activation (time t10). In the example of FIG. 8, the output voltage Vout reaches the upper limit value V10 at time t11a. The operation after time t11a is the same as the operation after time t11 in the first embodiment.

  According to the present embodiment, since the phase difference φ is set to the maximum 90 ° immediately after startup, the second bridge circuit 20 can output the maximum output power within a possible output range immediately after startup. For this reason, the increase in the output voltage Vout immediately after time t10 can be made earlier than in the first embodiment.

  Further, since the phase difference φ is not changed until the constant voltage control is started at time t13, the control is easy.

  The DC / DC converter 1 can also be used as a one-way DC / DC converter. When used as a unidirectional DC / DC converter, it is preferable that no reverse current flow from the output terminals T21 and T22 (output side) to the input terminals T11 and T12 (input side). As in the first embodiment, when starting by setting the phase difference φ to an angle φ0 at which the output power becomes zero for soft start, it is set due to variations in elements constituting the DC / DC converter 1 or the like. If an error occurs between the angle φ0 and the angle at which the output power actually becomes zero, a reverse current flows from the output side to the input side, for example, when the load on the output side or the capacitor C1 is charged There is a risk that. On the other hand, according to the present embodiment, since the phase difference is set to 90 °, there is no possibility that a reverse current flows from the output side to the input side even if there are variations in elements.

(Third embodiment)
In the third embodiment, the activation sequence is different from that of the first embodiment.

  The configuration of the DC / DC converter 1 of the third embodiment is the same as that of the first embodiment, and the function of the control circuit 30 is different from that of the first embodiment. Below, it demonstrates centering around difference.

  FIG. 9 is a diagram for explaining a startup sequence of the DC / DC converter 1 according to the third embodiment. As shown in FIG. 9, the control circuit 30 gradually decreases the duty ratio and gradually decreases the phase difference φ after the output voltage Vout reaches the upper limit value V10 during startup. At this time, for example, the amount of decrease in the duty ratio per unit time and the amount of decrease in the phase difference φ per unit time may be set in advance, respectively.

  In the example of FIG. 9, the output voltage Vout reaches the upper limit value V10 at time t11, and after waiting until time t12, the duty ratio and the phase difference φ are gradually reduced until time t13a. The duty ratio and the phase difference φ may be reduced from time t11 without waiting until time t12.

  The operation from time t10 to t12 is the same as the operation from time t10 to t12 in the first embodiment, and the operation after time t13a is the same as the operation after time t13 in the first embodiment.

  According to the present embodiment, the output power can be increased by decreasing the duty ratio, and the output power can be decreased by decreasing the phase difference φ. Therefore, the output voltage Vout between time t12 and time t13a can be reduced. The rise can be moderated.

  Note that the third embodiment may be combined with the second embodiment.

(Fourth embodiment)
In the fourth embodiment, the activation sequence is different from that of the first embodiment.

  The configuration of the DC / DC converter 1 of the fourth embodiment is the same as that of the first embodiment, and the function of the control circuit 30 is different from that of the first embodiment. Below, it demonstrates centering around difference.

  The control circuit 30 performs constant voltage control while gradually decreasing the duty ratio after the output voltage Vout reaches the upper limit value V10 at the time of startup, that is, after time t11 in FIG. 7, for example.

  According to the present embodiment, it is not necessary to control the duty ratio after time t12, and therefore control is easy.

  Note that the fourth embodiment may be combined with the second or third embodiment.

(Fifth embodiment)
The fifth embodiment is different from the first embodiment in that the DC / DC converter 1A is configured with two phases.

  FIG. 10 is a circuit diagram showing a schematic configuration of a two-phase DC / DC converter 1A according to the fifth embodiment. In FIG. 10, the same reference numerals are given to the components common to FIG. 1, and the differences will be mainly described below.

  As shown in FIG. 10, the DC / DC converter 1A includes a primary winding W13, a secondary winding W23, an inductor L3, a first phase, and a first phase configuration from the configuration of the DC / DC converter 1 of FIG. The switching elements SW5 and SW6 and the second switching elements SW11 and SW12 are removed. That is, the transformer T1A is a two-phase transformer, and the first and second bridge circuits 10A and 20A are two-phase full bridge circuits.

Also in the present embodiment, the control circuit 30A determines the duty ratios of the first and second drive signals G1 to G4 and G7 to G10 at the time of startup and the first and second drive signals G1 to G4 at the time of constant voltage control. , G7 to G10 are set larger than the duty ratio. Specifically, the control circuit 30A sets the duty ratio at start-up to be greater than 1/2 (50%), and sets the duty ratio at constant voltage control to be 1/2 or less.
The activation sequence is the same as that in the first embodiment.

Thereby, according to the present embodiment, as in the first embodiment, the period during which the input voltage Vin is applied to the primary windings W11 and W12 during startup can be shortened. Thereby, since the electric current which flows into primary winding W11, W12 at the time of starting can be made small, inrush current can be controlled.
Also, other effects of the first embodiment can be obtained.

  Also in the fifth embodiment, the activation sequence of the second to fourth embodiments may be used.

  The aspect of the present invention is not limited to the individual embodiments described above, and includes various modifications that can be conceived by those skilled in the art, and the effects of the present invention are not limited to the contents described above. That is, various additions, modifications, and partial deletions can be made without departing from the concept and spirit of the present invention derived from the contents defined in the claims and equivalents thereof.

1, 1A DC / DC converter T1, T1A Transformers W11, W12, W13 Primary windings W21, W22, W23 Secondary windings 10, 10A First bridge circuits SW1-SW6 First switching elements 20, 20A Second Bridge circuits SW7 to SW12 Second switching elements 30, 30A Control circuits L1 to L3 Inductors

Claims (11)

A transformer having a primary winding and a secondary winding;
A first bridge circuit that converts an input DC input voltage into a first AC voltage and supplies the first AC voltage to the primary winding by a switching operation of the plurality of first switching elements;
A second bridge circuit that converts a second AC voltage supplied from the secondary winding of the transformer into a DC output voltage and outputs the DC voltage by a switching operation of a plurality of second switching elements;
A first driving signal for switching each first switching element on or off and a second driving signal for switching each second switching element on or off are output, and during constant voltage control, A control circuit that controls a phase difference between the first drive signal and the second drive signal so that an output voltage approaches a target voltage,
The control circuit sets a duty ratio of the first and second drive signals at start-up to be larger than a duty ratio of the first and second drive signals at the constant voltage control. DC / DC converter. The first and second bridge circuits are three-phase full bridge circuits,
2. The DC / DC circuit according to claim 1, wherein the control circuit sets the duty ratio at the time of startup to be greater than 2/3, and sets the duty ratio at the time of the constant voltage control to 2/3 or less. DC converter. The first and second bridge circuits are two-phase full bridge circuits,
2. The DC / DC circuit according to claim 1, wherein the control circuit sets the duty ratio at the time of startup to be larger than ½, and sets the duty ratio at the time of the constant voltage control to ½ or less. DC converter.   4. The control circuit according to claim 1, wherein the control circuit sets the start-up duty ratio to be smaller than a value at which the output power output from the second bridge circuit becomes zero. 5. The DC / DC converter described in 1.   The control circuit, after the output voltage reaches an upper limit value at the start-up, gradually reduces the duty ratio, and performs the constant voltage control after the output voltage reaches a predetermined value. The DC / DC converter according to any one of claims 1 to 4, wherein the DC / DC converter is characterized.   6. The DC according to claim 5, wherein the control circuit increases the phase difference from the angle at which the output power output from the second bridge circuit becomes zero to 90 ° at the start-up. / DC converter.   The DC / DC converter according to claim 6, wherein the control circuit increases the phase difference to 90 ° before the output voltage reaches the upper limit value.   The DC / DC converter according to claim 5, wherein the control circuit sets the phase difference to 90 ° immediately after startup.   9. The control circuit according to claim 5, wherein the control circuit gradually decreases the duty ratio and gradually decreases the phase difference after the output voltage reaches the upper limit value at the start-up. DC / DC converter in any one.   5. The control circuit according to claim 1, wherein the control circuit gradually decreases a duty ratio and performs the constant voltage control after the output voltage reaches an upper limit value at the start-up. The DC / DC converter described. The duty ratio of the first drive signal is a high period / cycle of the first drive signal for switching the first switching element on the high side,
11. The duty ratio of the second drive signal is a high period / cycle of the second drive signal for switching the second switching element on the low side. A DC / DC converter according to claim 1.

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