JP5177019B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device that generates a predetermined voltage from a vehicle power supply in a hybrid vehicle or the like.

  Conventionally, there exists a thing of patent document 1 as a switching power supply device. Patent Document 1 introduces a switching power supply apparatus that employs a synchronous rectification method. Synchronous rectification is a switching power supply that uses a transformer to transform the voltage, and instead of rectifying the secondary side with rectification using switching elements instead of diode rectification, high efficiency can be achieved for diode rectification. It has features. Generally, a MOS (Metal Oxide Semiconductor) transistor is used as a switching element for rectification, and a MOS transistor has a parasitic diode called a body diode between its drain and source due to its structure. Rectification can be performed. In Patent Document 1, the current of a switching element for synchronous rectification is detected, a control signal is output when the detected current flows, the control signal is stopped when the detected current stops flowing, and the switching element for synchronous rectification is detected by these control signals Drive. That is, when the switching power supply outputs current, synchronous rectification control is always performed, thereby realizing a low-loss switching power supply.

Japanese Patent Laid-Open No. 7-115766

  By the way, in the above Patent Document 1, when the output current is as large as 100 to 200 A, for example, the current capacity per one switching element for synchronous rectification is exceeded, so a plurality of switching elements are connected in parallel. Also in this configuration, the power loss of the switching element can be reduced by the synchronous rectification control.

  On the other hand, in order to implement synchronous rectification, it is necessary to drive the switching element, resulting in drive loss. For this reason, when the output current is as low as about 0 to 10 A, there is a problem that, compared with diode rectification and synchronous rectification, power loss is larger in synchronous rectification.

  This invention is made | formed in view of such a situation, and it aims at providing the switching power supply device which can reduce a power loss.

The invention according to claim 1 is a switching power supply in which a plurality of secondary rectifier elements combined with diodes are connected in parallel to the secondary winding of the transformer. A current detection means for detecting a load current flowing in a load connected to the side, and a power loss corresponding to a voltage drop of the diode when only the diode combined with the n rectifier elements on the secondary side is driven, The value of the load current in the case where the m is smaller than the first driving loss when only the m smaller than n of the n rectifying elements are driven to perform the synchronous rectification control is set as the first threshold value. The load current value when the first driving loss is smaller than the driving loss at the time of synchronous rectification control of the n rectifying elements is set as a second threshold, and the detected current in the current detecting unit is If less than the first threshold Synchronous rectification control of the rectifying elements is prohibited, and when the first threshold value is exceeded and less than or equal to the second threshold value, only the m rectifying elements are driven to perform synchronous rectification control, and the second threshold value is controlled. And control means for permitting synchronous rectification control of the n rectifying elements when larger than.

  According to this configuration, the second threshold value is such that the first driving loss when the synchronous rectification control is performed by driving only m smaller than n of the n rectifying elements is n rectifying elements. Since the value of the load current is smaller than the drive loss at the time of synchronous rectification control, the first drive loss is greater than the drive loss at the time of synchronous rectification control of n rectifier elements when the second threshold value is exceeded. Becomes larger. Therefore, if the control means permits the synchronous rectification control of the n rectifier elements when the detected current is larger than the second threshold value, the switching power supply device can be driven with the smallest power loss.

  Further, the first threshold value is a load current value when the power loss corresponding to the voltage drop of the diode when only the diode combined with the n rectifying elements is driven is smaller than the first drive loss. It is. That is, when it is less than or equal to the first threshold, the power loss of the switching power supply device is minimized. Therefore, if the control means prohibits the synchronous rectification control of the n rectifier elements when the detected current is equal to or less than the first threshold value, the switching power supply device can be driven with the smallest power loss.

  Further, when the detected current is equal to or smaller than the second threshold, the first driving loss is smaller than the driving loss during the synchronous rectification control of the n rectifying elements, and when the detected current exceeds the first threshold, the first driving is performed. The loss is smaller than the power loss corresponding to the voltage drop of the diode. Therefore, when the control means performs synchronous rectification control by driving only m rectifier elements when the detected current exceeds the first threshold and is equal to or less than the second threshold, the switching power supply is driven with the least power loss. can do. With these controls, the power loss of the switching power supply device can be reduced.

  As described above, according to the present invention, it is possible to provide a switching power supply device that can reduce power loss.

It is a figure which shows the structure of the switching power supply device which concerns on the 1st Embodiment of this invention. It is a voltage waveform diagram for demonstrating operation | movement of the switching power supply device of 1st Embodiment. It is an auxiliary figure for explaining the switching power supply device of a 1st embodiment. It is a figure which shows the relationship between the power loss at the time of characteristic control of the switching power supply device of 1st Embodiment, and load current. It is a figure which shows the relationship between the power loss at the time of the characteristic control of the switching power supply device which concerns on the 2nd Embodiment of this invention, and load current. It is an auxiliary | assistant figure for demonstrating the switching power supply device of 2nd Embodiment.

  Embodiments of the present invention will be described below with reference to the drawings. However, parts corresponding to each other in all the drawings in this specification are denoted by the same reference numerals, and description of the overlapping parts will be omitted as appropriate.

(First embodiment)
FIG. 1 is a diagram showing a configuration of a switching power supply apparatus according to the first embodiment of the present invention.

  A switching power supply device 1 shown in FIG. 1 is used for a hybrid vehicle or the like, and includes a transformer 8 having a primary side coil (primary side winding) 8a and a secondary side coil (secondary side winding) 8b. And four main switching elements (main rectifying elements) 9-1, 9-2, 9-3, 9-4 by MOS transistors in which a diode is connected between the source and drain, and a diode connected between the source and drain The first switching element (rectifying element) 11, the second switching element 12, the third switching element 13, the fourth switching element 14, the coil 15, the capacitor 16, and the current detector ( (Current detection means) 17 and a control unit (control means) 18.

  For the connection of these components, the positive electrode of the battery 20 and the negative electrode of the battery 20 are connected to one end of the primary coil 8a of the transformer 8 via the main switching element 9-1 and the main switching element 9-3, The other end is connected to the positive electrode of the battery 20 via the main switching element 9-2 and the negative electrode of the battery 20 via the main switching element 9-4. The source-drain of the first switching element 11 is connected between one end of the secondary coil 8b of the transformer 8 and the ground, and the second switching element 12 is connected in parallel to the first switching element 11. ing. A source-drain of the third switching element 13 is connected between the other end of the secondary coil 8b and the ground, and a fourth switching element 14 is connected in parallel to the third switching element 13. Yes.

  Further, a load 21 is connected between one end and the other end of the secondary coil 8b via a coil 15, and a capacitor 16 is connected between the coil 15 and the ground. Further, a current detection unit 17 is connected between the coil 15 and the load 21, and a control unit 18 is connected so that a detection current Id from the current detection unit 17 is input. The control unit 18 is further connected to gate terminals of the first to fourth switching elements 11 to 14, and a drive control signal SG is input to each gate terminal. The detection current Id is the same as the output current Iout of the switching power supply device 1.

  In the switching power supply 1 having such a configuration, when the main switching elements 9-1 and 9-4 are turned on, the main switching elements 9-2 and 9-3 are turned off, and a positive current flowing from the battery 20 The energy by Ii is accumulated in the transformer 8, and this accumulated energy is loaded as an output current (load current) Iout through the switching elements 11, 12 or 13, 14 when the main switching element 9 is non-conductive (off). It flows to 21.

  For example, when synchronous rectification control is performed by each of the switching elements 11 to 14 on the secondary side, the main switching elements 9-1 and 9-4 are turned on and the main switching elements 9-2 and 9-9 are turned on at time t1 shown in FIG. -3 is off and current Ii flows from battery 20 to the primary side of transformer 8 on the positive side, current I1 flows to switching elements 11 and 12 on the secondary side of transformer 8. At this time, no current flows through the other switching elements 13 and 14.

  Next, at time t2, when the main switching elements 9-1 and 9-4 are turned off, the main switching elements 9-2 and 9-3 are turned on, and a current Ii flows to the negative side on the primary side. On the secondary side, a current I2 flows through the switching elements 13 and 14. At this time, no current flows through the other switching elements 11 and 12. Thereafter, when the current Ii alternately changes to the + side and the − side on the primary side by sequentially turning on and off the main switching elements 9-1 to 9-4, the current I1 is changed to the secondary side accordingly. I2 flows alternately. As a result, the currents I1 and I2 are combined as the load current Iout and flow to the load 21.

At this time, the power loss between the switching elements 11 and 12 and the switching elements 13 and 14, that is, the power loss Pr of the switching power supply device 1 is expressed by the following equation (1).
Pr = 2 × Pd + (R / 2) × Iout ² (1)

  However, Pd is a driving loss for one switching element, and R is an on-resistance value for one switching element. When the power loss Pr of the equation (1) is represented in a graph, a curve L1 is obtained in the relationship between the power loss Pr and the load current (output current) Iout shown in FIG. However, in FIG. 3, r2 of the power loss is a drive loss for two switching elements.

Further, when synchronous rectification control is not performed in the switching power supply device 1 and control is performed only with the diodes of the switching elements 11 to 14 on the secondary side, the power loss of each diode, that is, the power loss Pr of the switching power supply device 1 is It is represented by (2).
Pr = Vf × Iout (2)

  Vf is the voltage drop of the diode. If the power loss Pr of Expression (2) is represented in a graph, a curve L2 shown in FIG. 3 is obtained.

  Therefore, in the present embodiment, as shown in FIG. 4, the curve L1 of the relationship between the power loss Pr and the load current Iout when the synchronous rectification control of the secondary side switching elements 11 to 14 is performed, and the synchronous rectification control The threshold value is an intersection point Ith1 with the curve L2 when the control is performed only by the diode without performing the above. In other words, when only the diode combined with the secondary-side switching elements 11 to 14 is driven, the power loss corresponding to the voltage drop of the diode is driven during the synchronous rectification control of the secondary-side switching elements 11 to 14. The value of the load current Iout when it becomes smaller than the loss (power loss) is defined as a threshold value Ith1.

  This threshold value Ith1 is set in the control unit 18, and the control unit 18 prohibits the synchronous rectification control of the switching elements 11 to 14 by the drive control signal SG when the detection current Id in the current detection unit 17 is equal to or less than the threshold value Ith1. To do. In this case, the relationship between the power loss Pr and the load current Iout is as indicated by a curve L3.

  That is, when the load current Iout (detection current Id) is less than or equal to the threshold value Ith1, the synchronous rectification control is prohibited, so that the current flows through the diode, so that the load current Iout flows similarly to the curve L2. On the other hand, when the load current Iout is larger than the threshold value Ith1, the synchronous rectification control is permitted and the current flows through the switching elements 11 to 14, whereby the load current Iout flows similarly to the curve L1.

  Thus, in the switching power supply device 1 of the first embodiment, the battery 20 is connected to the primary side coil 8a of the transformer 8 through the main switching elements 9-1 to 9-4 in which the diodes are combined. The secondary side coil 8b of the transformer 8 includes a plurality of secondary side switching elements 11 to 14 combined with diodes connected in parallel, and the main switching elements 9-1 and 9-4 are turned on to perform main switching. When the elements 9-2 and 9-3 are off, energy due to the current from the battery 20 is stored in the transformer 8, and this stored energy is stored in the main switching element 9 when the main switching elements 9-1 and 9-4 are off. -2 and 9-3 flow as a load current Iout to the load 21 via the secondary-side switching elements 11 to 14 that are synchronously rectified when ON.

  In this configuration, the current detection unit 17 that detects the load current Iout flowing through the load 21 and the power loss corresponding to the voltage drop of the diode when only the diode combined with the secondary side switching elements 11 to 14 is driven. When the value of the load current Iout when the driving loss of the secondary side switching elements 11 to 14 is smaller than the driving loss at the time of the synchronous rectification control is set to the threshold value Ith1, and The control part 18 which prohibits the synchronous rectification control of the secondary side switching elements 11-14, and permits the synchronous rectification control of the secondary side switching elements 11-14 when larger than the threshold value Ith1 was provided.

  Therefore, the power loss corresponding to the voltage drop of the diode when only the diode combined with the secondary side switching elements 11 to 14 is driven is smaller than the driving loss of the switching elements 11 to 14 during the synchronous rectification control. The synchronous rectification control is prohibited. Due to this prohibition, only the secondary-side diode is driven with a power loss smaller than the driving loss of the switching elements 11 to 14 at the time of synchronous rectification control, so that the switching power supply device 1 is driven. Can be reduced.

(Second Embodiment)
FIG. 5 is a diagram showing the relationship between the power loss Pr and the load current Iout of the switching power supply according to the second embodiment of the present invention, and FIG. 6 is a diagram for explaining the switching power supply according to the second embodiment. It is an auxiliary figure.

  The difference between the switching power supply device of the second embodiment and the first embodiment is that the prohibition and permission of the synchronous rectification control of the switching elements 11 to 14 on the secondary side by the control unit 18 are indicated by a curve L5 shown in FIG. It is to be done along.

First, in FIG. 6, a curve L4 indicates a power loss Pr and a load current when synchronous rectification control is performed by driving only one element (for example, 11 or 13) among the switching elements 11, 12, 13, and 14. This shows the relationship with Iout. However, r1 of the power loss Pr shown in FIG. 6 is a drive loss for one switching element, and the power loss Pr during the synchronous rectification control in this case is expressed by the following equation (3).
Pr = Pd + R × Iout ² (3)

  In the present embodiment, the intersection Ith11 between the curve L2 and the curve L4 when the secondary-side switching elements 11 to 14 are controlled by only the diode without performing the synchronous rectification control is set as the first threshold value, and the secondary-side switching element. The intersection Ith12 between the curve L1 and the curve L4 of the relationship between the power loss Pr and the load current Iout when the synchronous rectification control of 11 to 14 is performed is set as the second threshold value.

  In other words, when only the diode combined with the switching elements 11 to 14 on the secondary side is driven, the power loss corresponding to the voltage drop of the diode is 1 element 11 of the switching elements 11, 12 or 13, 14. Alternatively, the value of the load current Iout when the drive loss (power loss) is smaller than when driving only 13 and performing synchronous rectification control is set as the first threshold value Ith11. Further, among the switching elements 11, 12, 13, 14 on the secondary side, the drive loss when the synchronous rectification control is performed by driving only one element 11, 13 is the synchronization of all the switching elements 11-14. The value of the load current Iout when it becomes smaller than the drive loss during the rectification control is set as the second threshold value Ith12.

  As a result, the load current Iout along the curve L2 has the smallest power loss Pr below the first threshold Ith11, and the load current Iout along the curve L4 exceeds the first threshold Ith11 and below the second threshold Ith12. Has the smallest power loss Pr, and when the second threshold value Ith12 is exceeded, the load current Iout along the curve L1 has the smallest power loss Pr.

  Therefore, the first and second threshold values Ith11 and Ith12 are set in the control unit 18, and the control unit 18 uses the drive control signal SG to detect each of the cases where the detection current Id in the current detection unit 17 is equal to or less than the first threshold value Ith11. Synchronous rectification control of the switching elements 11 to 14 is prohibited, and when the first threshold value Ith11 is exceeded and the second threshold value Ith12 is less than or equal to one, only one switching element 11 or 13 is driven to perform synchronous rectification control. When the threshold value Ith12 is exceeded, all the switching elements 11, 12 or 13, 14 are synchronously rectified.

  By this control, the load current Iout flows along the curve L5 shown in FIG. That is, when the detection current Id (load current Iout) is equal to or less than the first threshold value Ith11, the synchronous rectification control is prohibited, so that the load current Iout flows similarly to the curve L2 when the current flows through the diode. In addition, when the first threshold value Ith11 is exceeded and the second threshold value Ith12 is less than or equal to one, only one switching element 11 or 13 is driven and synchronous rectification control is performed, so that the load current Iout flows similarly to the curve L4, Furthermore, when larger than the second threshold value Ith12, the synchronous rectification control is permitted and the current flows through all the switching elements 11, 12, or 13, 14, so that the load current Iout flows similarly to the curve L1.

  However, in the above, the curve L4 performs synchronous rectification control by driving only one switching element 11 or 13 when two switching elements 11, 12 or 13, 14 are connected in parallel on the secondary side. However, when n switching elements are connected in parallel on the secondary side, even when m switching elements smaller than n are driven to perform synchronous rectification control, a curve is obtained. L4 has a relationship corresponding to the curve L5 as described above.

  As described above, in the switching power supply according to the second embodiment, the control unit 18 drives the power of the voltage drop of the diode when only the diode combined with the n switching elements 11 to 14 on the secondary side is driven. The value of the load current Iout when the loss is smaller than the first driving loss when only the m smaller than n of the n switching elements 11 to 14 is driven to perform the synchronous rectification control is the first value. The threshold value Ith11 is set to 1, the load current Iout when the first drive loss is smaller than the drive loss during the synchronous rectification control of the n switching elements 11 to 14 is set to the second threshold value Ith12, and current detection is performed. When the detection current Id in the unit 17 is equal to or less than the first threshold value Ith11, the synchronous rectification control of the n switching elements 11 to 14 is prohibited, exceeds the first threshold value Ith11, and the second By driving the switching elements of the m only performs synchronous rectification control when threshold Ith12 below, so as to allow the synchronous rectification control of the n switching elements is greater than the second threshold value Ith12.

  In this case, the second threshold value Ith12 is such that the first driving loss when the synchronous rectification control is performed by driving only m smaller than n of the n switching elements 11 to 14 is n switching. Since the value of the load current is smaller than the driving loss at the time of synchronous rectification control of the elements 11 to 14, when the second threshold value Ith12 is exceeded, the driving loss at the time of synchronous rectification control of the n switching elements 11 to 14 As a result, the first driving loss becomes larger. Therefore, when the control unit 18 permits the synchronous rectification control of the n switching elements 11 to 14 when the detected current Id is larger than the second threshold value Ith12, the switching power supply device can be driven with the smallest power loss. it can.

  Further, the first threshold value Ith11 is obtained when the power loss corresponding to the voltage drop of the diode when driving only the diode combined with the n switching elements 11 to 14 is smaller than the first drive loss. This is the load current value. That is, when the first threshold value Ith11 or less, the power loss of the switching power supply device is the smallest. Therefore, when the control unit 18 prohibits the synchronous rectification control of the n switching elements 11 to 14 when the detected current Id is equal to or less than the first threshold value Ith11, the switching power supply device can be driven with the smallest power loss. .

  Further, when the detection current Id is equal to or smaller than the second threshold value Ith12, the first driving loss is smaller than the driving loss at the time of the synchronous rectification control of the n switching elements 11 to 14, and exceeds the first threshold value Ith11. The first drive loss is smaller than the power loss corresponding to the voltage drop of the diode. Accordingly, when the control unit 18 performs synchronous rectification control by driving only m switching elements when the detection current Id exceeds the first threshold value Ith11 and is equal to or less than the second threshold value Ith12, switching is performed with the least power loss. The power supply device can be driven. With these controls, the power loss of the switching power supply device can be reduced.

DESCRIPTION OF SYMBOLS 1 Switching power supply device 8 Transformer 8a Primary side coil 8b Secondary side coil 9-1 to 9-4 Main switching element 11, 12, 13, 14 Switching element 15 Coil 16 Capacitor 17 Current detection part 18 Control part 20 Battery 21 Load Ii Primary side current I1, I2 Secondary side current Id Detection current Iout Load current (output current)
SG drive control signal

Claims (1)

In a switching power supply apparatus in which n secondary rectifier elements combined with diodes are connected in parallel to the secondary winding of the transformer,
Current detection means for detecting a load current flowing in a load connected to the secondary side of the transformer;
When only the diode combined with the n rectifying elements on the secondary side is driven, the power loss corresponding to the voltage drop of the diode is driven only to m smaller than n of the n rectifying elements. The value of the load current when the first drive loss is smaller than the first drive loss when the synchronous rectification control is performed is set as a first threshold, and the first drive loss is the synchronous rectification control of the n rectifier elements. The value of the load current when the driving loss is smaller than the driving loss is set as a second threshold value, and the synchronous rectification of the n rectifying elements is performed when the detected current of the current detection unit is equal to or less than the first threshold value. Control is prohibited, and when the first threshold value is exceeded and less than or equal to the second threshold value, only the m rectifying elements are driven to perform synchronous rectification control, and when larger than the second threshold value, the n Control means for allowing synchronous rectification control of individual rectifier elements Switching power supply unit, characterized in that it comprises a.

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