JP2014176241A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply capable of reducing conduction loss in a full-bridge circuit 10.SOLUTION: A converter CNV as a switching power supply includes: a transformer 12 having a primary coil 12a and a secondary coil 12b; a full-bridge circuit 10 having a first to a fourth switching elements Q1-Q4; and a full-wave rectifier circuit 14 that rectifies an output current of the secondary coil 12b. On the secondary coil 12b side of the converter CNV, there is formed a booster circuit comprised of an inductor 12c, a first boosting switching element Sla, a second boosting switching element Slb, a first diode Dha and a second diode Dhb.

Description

  The present invention relates to a switching power supply including a transformer having a primary coil and a secondary coil, a switching circuit that applies an AC voltage to the primary coil, and a rectifier circuit that rectifies an output current of the secondary coil.

  As this type of switching power supply, for example, the one shown in Patent Document 1 below is known. Specifically, this power source converts a DC voltage output from a DC power source into an AC voltage by a switching circuit and applies it to the primary coil of the transformer, thereby converting the AC voltage generated in the secondary coil into a full-wave rectifier composed of a diode. Converted to DC voltage by circuit.

JP-A-1-295675

  Here, the switching element included in the switching circuit is turned on / off by, for example, adjusting a time ratio, which is a ratio of an on operation time of the switching element to a specified time. In such a configuration, the inventors of the present invention increase the effective value of the current flowing through the primary coil by increasing the peak value of the current flowing through the primary coil when the on-operation time of the switching element is shortened by decreasing the time ratio. However, they faced the problem of increased conduction loss in the switching circuit. When the conduction loss in the switching circuit increases, there is a concern that the power transmission efficiency in the switching power supply is reduced.

  In particular, when the output voltage range required for the switching power supply is wide, the on-operation time of the switching element tends to be shortened by reducing the time ratio when the output voltage of the switching power supply is low. Therefore, an increase in conduction loss in the switching circuit can be significant.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a switching power supply capable of reducing conduction loss in a switching circuit.

  In order to solve the above-mentioned problem, the invention described in claim 1 includes a transformer (12) having a primary coil (12a) and a secondary coil (12b), a switching element (Q1 to Q1) connected to the primary coil. Q4), a switching circuit (10) for applying an AC voltage to the primary coil by turning on and off the switching element, a rectifier circuit (14) for rectifying the output current of the secondary coil, and the switching power supply Booster circuit (12c, Sla, Slb, Dha, Dhb, 16, Sha, Shb, Sa, Dla, Sb, Dlb, 18) connected to the secondary coil side and boosts the output voltage of the secondary coil And.

  In the above invention, the booster circuit is connected to the secondary coil side of the switching power supply. For this reason, the output voltage of the secondary coil determined by the turn ratio of the primary coil and the secondary coil can be further boosted by the booster circuit. As a result, even when the required voltage of the load connected to the output side of the switching power supply is high, the on-operation time of the switching element can be lengthened, thereby reducing the effective value of the current flowing through the primary coil. be able to. Therefore, the conduction loss in the switching circuit can be reduced, and consequently the power transmission efficiency of the switching power supply can be increased.

The circuit diagram of the converter concerning a 1st embodiment. The figure which shows the operation | movement aspect of the switching circuit concerning the embodiment. The figure which shows the electric current peak value when the time ratio concerning the embodiment is high. The figure which shows the electric current peak value when the time ratio concerning the embodiment is low. FIG. 3 is a circuit diagram showing a boosting operation mode according to the same embodiment. FIG. 3 is a circuit diagram showing a boosting operation mode according to the same embodiment. The flowchart which shows the procedure of the time ratio setting process concerning the embodiment. The figure which shows the pressure | voltage rise permission area | region concerning the embodiment. The figure which shows the setting method of the turns ratio concerning the embodiment. The figure which shows the operation | movement aspects, such as a switching circuit concerning the embodiment. The circuit diagram of the converter concerning a 2nd embodiment. The circuit diagram of the converter concerning a 3rd embodiment. The circuit diagram of the converter concerning other embodiments.

(First embodiment)
Hereinafter, a first embodiment in which a switching power supply according to the present invention is applied to an in-vehicle charger will be described with reference to the drawings.

  As shown in the figure, the converter CNV that is a “switching power supply” is an isolated converter that includes a full-bridge circuit 10 that is a “switching circuit”, a transformer 12, a full-wave rectifier circuit 14, and a smoothing capacitor 16. . Converter CNV has a function of converting a DC voltage output from DC power supply 20 into a voltage across terminals of smoothing capacitor 16.

  The full bridge circuit 10 includes a parallel connection body of a series connection body of the first switching element Q1 and the second switching element Q2 and a series connection body of the third switching element Q3 and the fourth switching element Q4. Yes. In the present embodiment, N-channel MOS field effect transistors are used as the first to fourth switching elements Q1 to Q4. Specifically, the drain of the second switching element Q2 is connected to the source of the first switching element Q1, and the drain of the fourth switching element Q4 is connected to the source of the third switching element Q3. . In the figure, the body diodes of the first to fourth switching elements Q1 to Q4 are indicated by “D1 to D4”. In the present embodiment, the first switching element Q1 and the third switching element Q3 correspond to the “high potential side switching element”, and the second switching element Q2 and the fourth switching element Q4 are “low potential”. Corresponds to the “side switching element”.

  One end of a primary coil 12a constituting the transformer 12 is connected to a connection point between the first switching element Q1 and the second switching element Q2, and a connection point between the third switching element Q3 and the fourth switching element Q4. Is connected to the other end of the primary coil 12a.

  One end of the secondary coil 12b constituting the transformer 12 is connected to a connection point of a series connection body of a first diode Dha and an N-channel MOS field effect transistor (hereinafter referred to as a first boosting switching element Sla). The connection point of the series connection body of the second diode Dhb and the N-channel MOS field effect transistor (hereinafter referred to as second boosting switching element Slb) is connected to the end. Specifically, the drain of the first boost switching element Sla is connected to the anode of the first diode Dha, and the drain of the second boost switching element Slb is connected to the anode of the second diode Dhb. ing. The cathodes of the first diode Dha and the second diode Dhb are connected to each other, and the sources of the first boosting switching element Sla and the second boosting switching element Slb are connected to each other. Here, in the drawing, the body diode of the first boost switching element Sla is indicated by “Dlba”, and the body diode of the second boost switching element Slb is indicated by “Dlbb”.

  In the present embodiment, the first diode Dha and the second diode Dhb correspond to the “boost rectifier element”, and the first diode Dha, the second diode Dhb, the first boost switching element Sla, and The second step-up switching element Slb forms a “rectifier circuit”. Further, the first diode Dha flows current in a direction from the connection point side with the first boosting switching element Sla to the connection point side with the second diode Dhb among both ends of the first diode Dha. Is equivalent to a “first high-potential side element” having a rectifying function that restricts (specifically cuts off) the flow of current in the reverse direction. Furthermore, the second diode Dhb flows current in a direction from the connection point side to the first booster switching element Slb to the connection point side to the first diode Dha among both ends of the second diode Dhb. Is equivalent to a “second high potential side element” having a rectifying function that restricts (specifically cuts off) the flow of current in the reverse direction.

  In addition, the first step-up switching element Sla is connected to the first diode Dha from the connection point side between the first step-up switching element Sla and the second step-up switching element Slb by the body diode Dlba. A rectifying function that allows current flow in the direction toward the connection point to and restricts (specifically cuts off) current flow in the reverse direction is realized. The first boost switching element Sla includes a connection point between the first boost switching element Sla and the secondary coil 12b, and a connection point between the first boost switching element Sla and the second boost switching element Slb. It has an opening and closing function to open and close the electrical path between. Therefore, the first step-up switching element Sla corresponds to the “first low potential side element”.

  In addition, the second booster switching element Sla is connected to the second diode Dhb from the connection point side with the first booster switching element Sla at both ends of the second booster switching element Slb by the body diode Dlbb. A rectifying function that allows current flow in the direction toward the connection point to and restricts (specifically cuts off) current flow in the reverse direction is realized. Further, the second boost switching element Slb includes a connection point between the second boost switching element Slb and the secondary coil 12b, and a connection point between the second boost switching element Slb and the first boost switching element Sla. Open / close function to open / close the electrical path between the two. Therefore, the second step-up switching element Slb corresponds to the “second low potential side element”.

  Here, in the present embodiment, an inductor 12c is described between the secondary coil 12b and the full-wave rectifier circuit 14 in the transformer 12 in the figure. This is an equivalent circuit representation of a leakage inductor generated by increasing the gap between the magnetic core (not shown) of the transformer 12 and the primary coil 12a and the secondary coil 12b. In the figure, the number of turns of the primary coil 12a is indicated by “N1”, and the number of turns of the secondary coil 12b is indicated by “N2”. In the present embodiment, the turn ratio “N1 / N2” of the primary coil 12a and the secondary coil 12b is not necessarily set to a value smaller than 1. Further, in the present embodiment, the inductor 12c corresponds to a “boost inductor”. In addition, the inductor 12c, the first diode Dha, the first diode Dha, the first boosting switching element Sla, and the second boosting switching element Slb constitute a “boost circuit”.

  A load 22 is connected to the output side of the full-wave rectifier circuit 14 via a smoothing capacitor 16. The load 22 includes a storage battery that exchanges power with a rotating machine or the like as an in-vehicle main machine.

  The control device 30 is mainly composed of a microcomputer, and includes an input-side voltage sensor 32 that detects the input voltage Vin of the converter CNV, and an output-side voltage that detects the output voltage Vo (voltage across the smoothing capacitor 16) of the converter CNV. The detection value of the sensor 34 is captured. The control device 30 outputs the operation signals ms1 to ms4 to the first to fourth switching elements Q1 to Q4 based on the detected values and the like, thereby converting the output voltage of the DC power supply 20 and the smoothing capacitor 16. The process applied to is performed. Further, the control device 30 outputs the operation signals gla and glb to the first and second boosting switching elements Sla and Slb, thereby turning on the first and second boosting switching elements Sla and Slb. (Close operation) and off operation (open operation).

  In the present embodiment, as the processing to be applied, as shown in FIG. 2, a set of the first switching element Q1 and the fourth switching element Q4, and a set of the second switching element Q2 and the third switching element Q3. Are complementarily turned on and off based on the first duty ratio Duty1 while giving a dead time. Here, the first duty ratio Duty1 is the ratio (or percentage) of the on-operation time Ton1 to the specified time Tα. Incidentally, in the present embodiment, the control device 30 constitutes a “step-up operation means” and a “primary side operation means”.

  By the way, in-vehicle chargers usually have a wide range of output voltage required for converter CNV. For this reason, the turns ratio of the primary coil 12a and the secondary coil 12b constituting the transformer 12 is normally set so that the converter CNV can output the maximum value of the range that the output voltage can take. When such a setting is made, the first duty ratio Duty1 when the output voltage of the converter CNV is low becomes low, and the peak value of the current flowing through the primary coil 12a can increase. Here, FIGS. 3 and 4 show that the peak current value Ipeak2 when the first duty ratio Duty1 is low is higher than the peak current value Ipeak1 when the first duty ratio Duty1 is high. It was. 3 and 4, (a) shows the transition of the operating state of the first and fourth switching elements Q1, Q4, and (b) shows the transition of the current IL flowing through the secondary coil 12b.

  When the peak value of the current flowing through the primary coil 12a increases, the effective value of the current flowing through the primary coil 12a increases. In the full bridge circuit 10, the first to fourth switching elements Q1 to Q4 and the body diodes D1 to D4. The conduction loss may increase due to the current flowing through.

  In order to solve such a problem, in this embodiment, the converter CNV is provided with an inductor 12c, a first boosting switching element Sla, and a second boosting switching element Slb, and these are arranged on the secondary coil 12b side of the converter CNV. A boost function was provided. Then, the first booster switching element Sla and the second booster switching element Slb are turned on / off based on the second duty ratio Duty2 (detailed later) while setting the first duty ratio Duty1 high. The effective value of the current flowing through the primary coil 12a is reduced, and the conduction loss in the full bridge circuit 10 is reduced.

  Incidentally, in the present embodiment, the specified time as the denominator of the second time ratio Duty2 and the specified time as the denominator of the first time ratio Duty1 are set to the same value “Tα”. Therefore, in the present embodiment, the second duty ratio Duty2 is the ratio (or percentage) of the ON operation time Ton2 of the first boosting switching element Sla or the second boosting switching element Slb with respect to the specified time Tα. expressed.

  Next, the boosting function will be described with reference to FIGS.

  As shown in FIG. 5, when the first and fourth switching elements Q1 and Q4 are turned on and the second and third switching elements Q2 and Q3 are turned off, the first boosting switching element Sla is fixed to be turned off, and the second step-up switching element Slb is turned on and off based on the second duty ratio Duty2. Specifically, as shown in FIG. 5A, when the second boost switching element Slb is turned on, the secondary coil 12b, the second boost switching element Slb, and the first boost switching element Sla. Current flows in a closed circuit including the body diode Dlba and the inductor 12c, and magnetic energy is stored in the inductor 12c. Thereafter, as shown in FIG. 5B, when the second step-up switching element Slb is switched to the OFF operation, the magnetic energy accumulated in the inductor 12c is released, whereby the secondary coil 12b, the second coil 12b, Current flows through a closed circuit including the diode Dhb, the smoothing capacitor 16, the body diode Dlba, and the inductor 12c.

  After that, as shown in FIG. 6, when the first and fourth switching elements Q1, Q4 are switched to the off operation and the second and third switching elements Q2, Q3 are switched to the on operation, The step-up switching element Slb is fixed to be turned off, and the first step-up switching element Sla is turned on / off based on the second time ratio. Specifically, as shown in FIG. 6A, when the first boosting switching element Sla is turned on, the secondary coil 12b, the inductor 12c, the first boosting switching element Sla, and the second boosting switching element. A current flows through a closed circuit including the body diode Dlbb of the switching element Slb, and magnetic energy is accumulated in the inductor 12c. Thereafter, as shown in FIG. 6B, when the first step-up switching element Sla is switched to the OFF operation, the magnetic energy accumulated in the inductor 12c is released, whereby the secondary coil 12b and the inductor 12c. A current flows through a closed circuit including the first diode Dha, the smoothing capacitor 16, and the body diode Dlbb. By repeating the on / off operation described above, the step-up function on the secondary coil 12b side can be operated.

  Subsequently, a method for setting the first duty ratio Duty1 and the second duty ratio Duty2 will be described with reference to FIG. Here, FIG. 7 is a flowchart showing a procedure for setting the duty ratios Duty1 and Duty2. This process is repeatedly executed by the control device 30 at a predetermined cycle, for example.

  In this series of processing, first, in step S10, the input voltage Vin detected by the input side voltage sensor 32 and the output voltage Vo detected by the output side voltage sensor 34 are acquired.

  In the subsequent step S12, the total duty ratio DUTY is calculated based on the detected input voltage Vin and output voltage Vo. Here, the total duty ratio DUTY is set to a higher value as the value obtained by subtracting the input voltage Vin from the output voltage Vo is larger, for example.

  In the subsequent step S14, it is determined whether or not the total duty ratio DUTY exceeds the upper limit value (hereinafter, first upper limit value D1max) of the first duty ratio Duty1. In this embodiment, the process of this step constitutes “permission means”.

  If a negative determination is made in step S14, the operation of the boosting function on the secondary coil 12b side is prohibited, and the process proceeds to step S16. In step S16, the first duty ratio Duty1 is set to the total duty ratio DUTY, and the second duty ratio Duty2 is set to “0”. That is, the first and second boosting switching elements Sla and Slb are turned off without operating the boosting function on the secondary coil 12b side.

  On the other hand, when an affirmative determination is made in step S14, the operation of the step-up function on the secondary coil 12b side is permitted, and the process proceeds to step S18. In step S18, the first duty ratio Duty1 is set to the first upper limit value D1max, and the second duty ratio Duty2 is set to a value obtained by subtracting the first upper limit value D1max from the total duty ratio DUTY. FIG. 8 shows how the first duty ratio Duty1 and the second duty ratio Duty2 are set according to the present embodiment. Specifically, FIG. 8 shows how the time ratios Duty1 and Duty2 are set when the input voltage Vin is constant. In the figure, the horizontal axis indicates the output voltage Vo of the converter CNV, and the vertical axis indicates the output power of the converter CNV. Furthermore, in the figure, the upper and lower limit values of the output voltage Vo are indicated by “Vo-max” and “Vo-min”, and the upper and lower limit values of the output power are indicated by “Po-max” and “Po-min”. . Incidentally, in the present embodiment, the upper limit value “Vo-max” of the output voltage Vo corresponds to the “maximum required value of the output voltage of the switching power supply”.

  In addition, when the process of step S16, S18 is completed, this series of processes is once complete | finished.

  Here, in the present embodiment, the turn ratio “N1 / N2” of the primary coil 12a and the secondary coil 12b is, as shown in FIG. 9, when the first duty ratio Duty1 is the first upper limit value D1max. Even so, the output voltage of the converter CNV is set to be less than the upper limit value Vo-max. FIG. 9 shows an example in which such setting is realized by setting the turn ratio “N1 / N2” to “Ra”.

  FIG. 10 shows an example of the operation mode of the boost function of the secondary coil 12b. Specifically, FIG. 10A shows the transition of the operating state of the first and fourth switching elements Q1 and Q4, and FIG. 10B shows the operating state of the second and third switching elements Q2 and Q3. FIG. 10C shows the transition of the operating state of the first boosting switching element Sla, and FIG. 10D shows the transition of the operating state of the second boosting switching element Slb. . In FIG. 10, the dead time between the set of the first and fourth switching elements Q1 and Q4 and the set of the second and third switching elements Q2 and Q3 is omitted.

  As shown in the figure, in this embodiment, the output timing of the on operation signal glb to the second boosting switching element Slb and the output timing of the on operation signals ms1 and ms4 to the first and fourth switching elements Q1 and Q4. Are synchronized with the on-operation timing (time t1, t5) of the second step-up switching element Slb and the on-operation timing of the first and fourth switching elements Q1, Q4. In addition, the output timing of the on operation signal gla to the first boosting switching element Sla and the output timing of the on operation signals ms2 and ms3 to the second and third switching elements Q2 and Q3 are synchronized, so that the first The ON operation timing (time t3, t7) of the step-up switching element Sla and the ON operation timing of the second and third switching elements Q2, Q3 are synchronized.

  According to the embodiment described in detail above, the following effects can be obtained.

  (1) A step-up circuit having an inductor 12c, a first step-up switching element Sla, a second step-up switching element Slb, a first diode Dha, and a second diode Dhb on the secondary coil 12b side of the converter CNV Prepared. Therefore, the output voltage of the secondary coil 12b determined by the winding ratio “N1 / N2” of the primary coil 12a and the secondary coil 12b can be further boosted by the booster circuit. Thereby, even when the required voltage of the load 22 is high, the ON operation time of the first to fourth switching elements Q1 to Q4 can be lengthened, so that the effective value of the current flowing through the primary coil 12a is Can be reduced. Therefore, the conduction loss in the full bridge circuit 10 can be reduced, and as a result, the power transmission efficiency of the converter CNV can be increased.

  In particular, in this embodiment, when it is determined that the first duty ratio Duty1 exceeds the first upper limit value D1max, the operation of the boost function on the secondary coil 12b side is permitted, and the first duty ratio Duty1. Is the first upper limit value D1max, the turn ratio “N1 / N2” of the primary coil 12a and the secondary coil 12b is set so that the output voltage of the converter CNV is less than the upper limit value Vo-max. The setting greatly contributes to reducing the conduction loss by avoiding the decrease in the first duty ratio Duty1.

  Further, in the present embodiment, in order to realize the boosting function, the fact that some elements of the full-wave rectifier circuit 14 are diverted and that the leakage inductor 12c of the transformer 12 is used as the inductor, This greatly contributes to avoiding an increase in cost.

  (2) In addition to the first diode Dha and the second diode Dhb, the rectifying function of the full-wave rectifier circuit 14 includes the body diode Dlba and the second boost switching element Slb of the first boost switching element Sla. Realized by the body diode Dlbb. Thereby, an increase in the size and cost of converter CNV can be avoided.

(Second Embodiment)
Hereinafter, the second embodiment will be described with reference to the drawings with a focus on differences from the first embodiment.

  FIG. 11 shows a circuit configuration of the converter CNV according to the present embodiment. In FIG. 11, the same members as those shown in FIG. 1 are denoted by the same reference numerals for convenience.

  As shown in the figure, in this embodiment, the full-wave rectifier circuit 14 performs synchronous rectification. Specifically, instead of the first diode Dha, the first high-potential side switching element Sha is provided in the converter CNV, and instead of the second diode Dhb, the second high-potential side switching element Shb is included in the converter CNV. Is provided. Here, in this embodiment, N channel MOS field effect transistors are used as the switching elements Sha and Shb. In the figure, the body diodes of the first and second high potential side switching elements Sha and Shb are indicated by “Dhba” and “Dhbb”.

  The source of the first high potential side switching element Sha is connected to the drain of the first boosting switching element Sla, and the source of the second high potential side switching element Shb is connected to the second boosting switching element Slb. The drain is connected. The drains of the first and second high potential side switching elements Sha, Shb are connected to each other. In the present embodiment, the first high potential side switching element Sha corresponds to a “first high potential side element” having a rectifying function, and the second high potential side switching element Shb has a rectifying function. Corresponds to “2 high potential side element”.

  The control device 30 outputs the operation signals gha and ghb to the first and second high potential side switching elements Sha and Shb, thereby turning on and off the first and second high potential side switching elements Sha and Shb. To do.

  In such a configuration, when the first and fourth switching elements Q1 and Q4 are turned on and the second and third switching elements Q2 and Q3 are turned off, the second boost switching element Slb is turned off. When operated, the first high potential side switching element Sha is turned off and the second high potential side switching element Shb is turned on. On the other hand, in a situation where the first and fourth switching elements Q1 and Q4 are turned off and the second and third switching elements Q2 and Q3 are turned on, the first boost switching element Sla is turned off. In this case, the first high potential side switching element Sha is turned on and the second high potential side switching element Shb is turned off. Thereby, synchronous rectification can be performed with respect to an element on the high potential side among the elements constituting the full-wave rectifier circuit 14.

  According to this embodiment described above, in addition to the effect obtained in the first embodiment, it is possible to obtain an effect that the power conversion efficiency in the converter CNV can be further increased.

(Third embodiment)
Hereinafter, the third embodiment will be described with reference to the drawings with a focus on differences from the first embodiment.

  FIG. 12 shows a circuit configuration of the converter CNV according to the present embodiment. In FIG. 12, the same members as those shown in FIG. 1 are given the same reference numerals for the sake of convenience.

  As shown in the figure, in this embodiment, instead of the first boost switching element Sla, the converter CNV includes a parallel connection body of the first low-potential side diode Dla and the first switching element Sa. ing. Further, instead of the second step-up switching element Slb, the converter CNV includes a parallel connection body of the second low-potential side diode Dlb and the second switching element Sb for switching. Here, the first and second low potential side diodes Dla, Dlb and the first and second switching elements Sa, Sb are separate members. The controller 30 outputs the operation signals ga and gb to the first and second switching elements Sa and Sb, thereby turning on and off the first and second switching elements Sa and Sb. To do. In the present embodiment, the parallel connection body of the first low potential side diode Dla and the first switching element Sa for switching is equivalent to the “first low potential side element”, and the second low potential side diode Dlb. The parallel connection body of the second switching element Sb for opening and closing corresponds to the “second low-potential side element”.

  Also according to the present embodiment described above, it is possible to obtain an effect according to the effect obtained in the first embodiment.

(Other embodiments)
Each of the above embodiments may be modified as follows.

  In the first embodiment, when the first and fourth switching elements Q1 and Q4 are turned on, and when the second and third switching elements Q2 and Q3 are turned off, the first booster Although the switching element Sla is fixed to be turned off, the present invention is not limited to this, and it may be fixed to be turned on. Thereby, it is possible to avoid a current from flowing through the body diode of the first boost switching element Sla, and to reduce conduction loss. Further, when the second and third switching elements Q2 and Q3 are turned on, and when the first and fourth switching elements Q1 and Q4 are turned off, the second step-up switching element Slb is fixed to be turned on. May be. Such an operation method can be similarly applied to FIG. 11 of the second embodiment.

  In the first embodiment, as the “boost switching element” and the “boost rectifier element”, the first boost switching element Sla and the first diode Dha, and the second boost switching element Slb and Although both of the two diodes Dhb are used, the present invention is not limited to this. For example, only one of the first boost switching element Sla and the first diode Dha, and the second boost switching element Slb and the second diode Dhb may be used.

  In the first embodiment, the operation of the step-up function on the secondary coil 12b side is permitted only when it is determined that the first duty ratio Duty1 exceeds the first upper limit value D1max, but the present invention is not limited to this. Before the first duty ratio Duty1 exceeds the first upper limit value D1max, the operation of the step-up function on the secondary coil 12b side may be permitted. Even in this case, the power conversion efficiency of the converter CNV can be increased.

  The “boost inductor” is not limited to the leakage inductor of the transformer 12 but may be an inductor as a passive element. Here, FIG. 13 illustrates a configuration in which the boosting inductor is changed to the inductor 18 as a passive element based on the converter CNV shown in FIG. 11 of the second embodiment.

  The operation method of the first to fourth switching elements Q1 to Q4 is not limited to the one based on the hard switching process using the time ratio. For example, for the first switching element Q1 (second switching element Q2) and the fourth switching element Q4 (third switching element Q3), the timing of switching to the ON state and the OFF state while fixing the duty ratio It may be based on phase shift processing that adjusts the flow rate while shifting the timing of switching to. Even in this case, if there is a possibility that the effective value of the current flowing through the primary coil 12a may increase due to the shortened ON operation time of the first to fourth switching elements Q1 to Q4, the application of the present invention. Is effective.

  The method for setting the ON operation timing of the first boosting switching element Sla is not limited to the one exemplified in the first embodiment. For example, a sensor for detecting the current flowing through the secondary coil 12b is provided, and the flow direction of the current flowing through the secondary coil 12b is determined based on the detection value of the sensor, and the ON operation timing of the first boost switching element Sla is determined. It may be set. The same applies to the second step-up switching element Slb.

  The “boost circuit” is not limited to one configured by diverting part of elements included in the transformer 12 and the full-wave rectifier circuit 14. For example, in FIG. 1 of the first embodiment, a booster circuit (boost chopper circuit) provided between the smoothing capacitor 16 and the load 22 may be used.

  In the first embodiment, the “first high potential side element” and the “second high potential side element” are not limited to diodes, and may be thyristors, for example.

  The “switching circuit” is not limited to a full bridge circuit, and may be, for example, a half bridge circuit including a series connection body of a pair of switching elements.

  The charger on which the converter CNV is mounted is not limited to the one mounted on the vehicle, and may be installed on a house, for example.

  DESCRIPTION OF SYMBOLS 10 ... Full bridge circuit, 12 ... Transformer, 12a ... Primary coil, 12b ... Secondary coil, 12c ... Inductor, 14 ... Full-wave rectifier circuit, Sla ... First boost switching element, Slb ... Second boost Switching element, Dha ... first diode, Dhb ... second diode, Q1-Q4 ... first to fourth switching elements.

Claims (7)

A transformer (12) having a primary coil (12a) and a secondary coil (12b);
A switching circuit (10) connected to the primary coil and having switching elements (Q1 to Q4), and applying an AC voltage to the primary coil by an on / off operation of the switching element;
A rectifier circuit (14) for rectifying the output current of the secondary coil;
A booster circuit (12c, Sla, Slb, Dha, Dhb, 16, Sha, Shb, Sa, Dla, Sb, Dlb) that is connected to the secondary coil side of the switching power supply and boosts the output voltage of the secondary coil. 18)
A switching power supply comprising: The switching circuit is
A series connection body of switching elements (Q1, Q3) on the high potential side and switching elements (Q2, Q4) on the low potential side;
An alternating voltage is applied to the primary coil by an on / off operation of the high-potential side switching element and the low-potential side switching element;
The booster circuit includes:
Boosting switching elements (Sla, Slb, Sa, Sb);
A step-up inductor (12c, 18);
A boosting rectifier element (Dha,) having one end connected to a connection point of the boosting switching element and the boosting inductor, and rectifying a current output from the boosting inductor when the boosting switching element is turned off. Dhb, Sha, Shb)
A step-up operation means (30) for turning on and off the step-up switching element to step up the output voltage of the secondary coil;
With
An AC voltage is applied to the primary coil by performing an on / off operation on each of the high potential side switching element and the low potential side switching element based on a time ratio that is a ratio of an on operation time of the switching element to a specified time. Primary operating means (30) to apply;
Permission means for permitting boosting of the output voltage of the secondary coil by the boosting circuit, provided that the duty ratio used by the primary side operating means is an upper limit value;
The switching power supply according to claim 1, further comprising:   Even if the turn ratio of the primary coil and the secondary coil is the case where the duty ratio used by the primary side operation means is the upper limit value, the output voltage of the switching power supply is less than the maximum required value. The switching power supply according to claim 2, wherein the switching power supply is set to be   4. The switching power supply according to claim 2, wherein the boosting inductor is a leakage inductor (12c) of the transformer. The rectifier circuit includes a series connection body of a first high potential side element (Dha, Sha) and a first low potential side element (Sla, Dla, Sa), and a second high potential side element (Dhb, Shb). And a parallel connection body with a series connection body of the second low potential side elements (Slb, Dlb, Sb),
A connection point of the first high potential side element and the first low potential side element is connected to one end of both ends of the secondary coil, and the second high potential side element and the other end are connected to the other end. A connection point of the second low potential side element is connected;
The first high potential side element is connected to the connection point side with the second high potential side element from the connection point side with the first low potential side element of both ends of the first high potential side element. Has a rectifying function that allows the flow of current in the direction toward and restricts the flow of current in the reverse direction,
The second high potential side element is connected to a connection point side with the first high potential side element from a connection point side with the second low potential side element among both ends of the second high potential side element. Has a rectifying function that allows the flow of current in the direction toward and restricts the flow of current in the reverse direction,
The first low potential side element is connected to the connection point side with the first high potential side element from the connection point side with the second low potential side element among both ends of the first low potential side element. A rectifying function that allows current to flow in the direction toward and restricts current flow in the reverse direction, and a connection point between the first low potential side element and the secondary coil and the first low potential side element; An open / close function to open and close an electrical path between the connection point of the second low potential side element;
The second low potential side element is connected to the connection point side with the second high potential side element from the connection point side with the first low potential side element among both ends of the second low potential side element. A rectifying function that allows current to flow in the direction toward and restricts current flow in the reverse direction, and a connection point between the second low-potential side element and the secondary coil, the second low-potential side element, and Having an opening / closing function of opening / closing an electrical path between the first low potential side element and a connection point;
The step-up switching element and the step-up rectifying element include the first low potential side element, the first high potential side element, the second low potential side element, and the second high potential side element. The switching power supply according to any one of claims 2 to 4, wherein the switching power supply is at least one of them. Each of the first low potential side element (Sla) and the second low potential side element (Slb) is a field effect transistor,
6. The rectifying function of each of the first low potential side element and the second low potential side element is realized by a body diode (Dlba, Dlbb) of the field effect transistor. Switching power supply.   Each of the first low potential side element and the second low potential side element is a parallel connection body of a diode (Dla, Dlb) and a switching element for switching (Sa, Sb). 5. The switching power supply according to 5.

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