JP2016025774A - Step-down type dc/dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a step-down type DC/DC converter arranged to be able to detect an output current, which never causes overheating nor the decrease in a voltage conversion efficiency, or never raises the part or component cost nor increases a mounting space.SOLUTION: A step-down type DC/DC converter comprises: a first FET 1 connecting between an input-side terminal of a coil 11 and a power supply circuit 4; a second FET 2 connecting between the input-side terminal of the coil 11 and a ground terminal; a backflow preventing device 3 connecting between an output-side terminal of the coil 11 and a load 6; and a smoothing capacitor 10 for smoothing an output voltage. The step-down type DC/DC converter steps down an output voltage of the power supply circuit 4 by PWM controlling the first FET 1 and the second FET 2 respectively and then, makes an output. The smoothing capacitor 10 is connected between the output-side terminal of the coil 11 and the ground terminal. The step-down type DC/DC converter is arranged to detect a difference voltage between paired terminals of the backflow preventing device 3, and to calculate an output current to pass through the backflow preventing device 3 based on the detected difference voltage and a previously detected on-resistance of the backflow preventing device 3.SELECTED DRAWING: Figure 1

Description

  The present invention provides a first FET that connects between the input terminal of the coil and the power supply circuit, a second FET that connects between the input terminal of the coil and the fixed potential terminal, and a backflow prevention that connects between the output terminal of the coil and the load. The present invention relates to a step-down DC / DC converter provided with a power element and a smoothing capacitor for smoothing an output voltage.

  In a vehicle power supply device including a DC / DC converter, a power supply is provided to a vehicle-mounted load within a preset voltage / current range. In supplying power, a mechanism for protecting the circuit after detecting overvoltage, overcurrent, and overheating is necessary so as not to destroy the on-vehicle load and the peripheral circuit.

FIG. 6 is a circuit diagram showing a configuration example of a conventional step-down DC / DC converter.
This step-down DC / DC converter is used in a vehicle power supply device. An N-channel MOSFET is connected to a plus terminal of a high-voltage power supply unit 4 including an alternator (on-vehicle generator, AC generator) and a high-voltage battery. The drain of the (metal oxide semiconductor field effect transistor) 1 is connected, and the source of the FET 1 is connected to the input side terminal of the coil 11. The positive electrode of the smoothing capacitor 9 is connected to the drain of the FET 1, and the negative electrode of the smoothing capacitor 9 is grounded.

  The input side terminal of the coil 11 is also connected to the drain of the N-channel MOSFET 2 and the source of the FET 2 is grounded. The source of the N-channel MOSFET 3 is connected to the output side terminal of the coil 11, the drain of the FET 3 is connected to the positive pole of the smoothing capacitor 10 and one terminal of the shunt resistor 16, and the negative pole of the smoothing capacitor 10 is Grounded. The other terminal of the shunt resistor 16 is connected to each plus terminal of the low voltage power source 5 such as a low voltage battery and the in-vehicle load group 6.

The voltage detector 7 detects the voltage across the shunt resistor 16 (V1-V2), and the detected voltage across the both ends is notified to the control unit 8. The control unit 8 performs PWM control on the FETs 1 and 2 and controls on / off of the FET 3.
In the step-down DC / DC converter having such a configuration, the output voltage of the high-voltage power supply unit 4 is detected by a voltage detector (not shown). The control unit 8 calculates a duty ratio based on the detected output voltage and the set voltage of the low-voltage power supply unit 5, and performs PWM (Pulse Width Modulation) control of the FET 1 based on the calculated duty ratio.

When the PWM control of the FET 1 is performed, the control unit 8 fixes the FET 3 to ON and performs the PWM control with a duty ratio obtained by inverting the FET 2 from the FET 1, whereby the low voltage power source unit 5 A voltage controlled to the set voltage is output.
Based on the voltage across the shunt resistor 16 detected by the voltage detector 7 and the stored resistance value of the shunt resistor 16, the controller 8 outputs current (= voltage across both ends / resistance value) that flows through the shunt resistor 16. Is calculated.

  Patent Document 1 discloses a DC / DC converter connected between a high voltage power storage unit and a low voltage power storage unit, an output current detection unit, a load current detection unit, a low voltage power storage unit voltage detection circuit, and these And a control unit connected to the vehicle is disclosed. When the output current is larger than the predetermined output current, the control unit drives the DC / DC converter so that the low voltage storage unit voltage maintains the predetermined storage unit voltage, while the output current is equal to or lower than the predetermined output current. Repeats stopping and driving of the DC / DC converter in accordance with the amount of discharge charge of the low-voltage power storage unit.

JP 2012-147538 A

  Like the step-down DC / DC converter described above, a current detection method in which a shunt resistor is inserted into an output circuit and a voltage drop due to an output current flowing through the shunt resistor is detected. Therefore, there is a problem that power consumption increases, leading to overheating and a decrease in voltage conversion efficiency. In addition, the current detection method using a Hall element that does not use a shunt resistor has solved the above problems, but it is necessary to add a circuit using a Hall element or a Hall IC, which increases the component cost and mounting. There is a problem of increased space.

  The present invention has been made in view of the above-described circumstances, and can detect an output current without causing overheating and a decrease in voltage conversion efficiency, without causing an increase in component cost and an increase in mounting space. An object of the present invention is to provide a step-down DC / DC converter that can be used.

  A step-down DC / DC converter according to a first aspect of the present invention includes a coil, a first FET that connects between an input side terminal of the coil and a power supply circuit, a second FET that connects between the input side terminal and a fixed potential terminal, A backflow prevention element for connecting between the output side terminal of the coil and the load, and a smoothing capacitor for smoothing the output voltage, and by controlling each of the first FET and the second FET by PWM control, the output voltage of the power supply circuit In the step-down DC / DC converter for stepping down and outputting the voltage, the smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and detects a difference voltage between both terminals of the backflow prevention element. Based on the detected voltage detector, the differential voltage detected by the voltage detector, and the on-resistance of the backflow prevention element detected in advance, the current flows to the backflow prevention element. A calculation means for calculating a force current, characterized in that it comprises a said first 1FET and control unit for the first 2FET respectively PWM control.

In this step-down DC / DC converter, the first FET connects the coil input side terminal and the power supply circuit, and the second FET connects the coil input side terminal and the fixed potential terminal. The backflow prevention element connects between the output terminal of the coil and the load, and the smoothing capacitor smoothes the output voltage. The first FET and the second FET are respectively PWM controlled to step down and output the output voltage of the power supply circuit.
The smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and the voltage detector detects a differential voltage between both terminals of the backflow preventing element. The control unit calculates the output current flowing through the backflow prevention element based on the differential voltage detected by the voltage detector and the on-resistance of the backflow prevention element detected in advance, and the calculation means calculates the first FET and the second FET. Are respectively PWM controlled.

  A step-down DC / DC converter according to a second aspect of the present invention includes a temperature detector that detects a temperature of the backflow prevention element, and a storage unit that stores an on-resistance of the backflow prevention element corresponding to the temperature. Further, the calculation means is configured to calculate the output current based on an on-resistance stored in the storage means corresponding to the temperature detected by the temperature detector. .

  In this step-down DC / DC converter, the temperature detector detects the temperature of the backflow prevention element, and the storage means stores the on-resistance of the backflow prevention element corresponding to the temperature. The calculation means calculates the output current based on the on-resistance stored in the storage means corresponding to the temperature detected by the temperature detector.

  A step-down DC / DC converter according to a third aspect of the present invention includes a coil, a first FET that connects the input side terminal of the coil and a power supply circuit, a second FET that connects the input side terminal and the fixed potential terminal, A backflow prevention element for connecting between the output side terminal of the coil and the load, and a smoothing capacitor for smoothing the output voltage, and by controlling each of the first FET and the second FET by PWM control, the output voltage of the power supply circuit In the step-down DC / DC converter for stepping down and outputting the current, the smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and has current detection means for detecting a conduction current, Two IPDs (Intelligent Power Devices) connected in reverse and connected in parallel to the backflow prevention elements, current values detected by the current detection means, Based on the previously detected said backflow prevention element and the on-resistance of the IPD, characterized in that it comprises a calculating means for calculating an output current flowing in the reverse flow preventing device.

In this step-down DC / DC converter, the first FET connects the coil input side terminal and the power supply circuit, and the second FET connects the coil input side terminal and the fixed potential terminal. The backflow prevention element connects between the output terminal of the coil and the load, and the smoothing capacitor smoothes the output voltage. The first FET and the second FET are respectively PWM controlled to step down and output the output voltage of the power supply circuit.
The smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and has current detection means for detecting a flowing current, and is connected in reverse series, and two IPDs are connected in parallel to the element for preventing backflow. It is connected to the. The calculation means calculates an output current flowing through the backflow prevention element based on the current value detected by the current detection means and the on-resistances of the backflow prevention element and the IPD detected in advance.

  In the step-down DC / DC converter according to a fourth aspect of the present invention, the backflow prevention element includes a plurality of FETs connected in parallel, and the calculation is performed when the plurality of FETs are fixed on. When the output current calculated by the means is smaller than the first current value, the calculation means detects the current value detected by the current detection means and the detection in advance with only one of the plurality of FETs fixed to ON. Based on the on-resistances of the FET and the IPD, the output current flowing through the FET is recalculated. When only one of the plurality of FETs is fixed on, the calculation means calculates When the output current is larger than the second current value (> first current value), the plurality of FETs are fixed to be on.

  In this step-down DC / DC converter, the backflow prevention element is composed of a plurality of FETs connected in parallel. If the output current calculated by the calculation means is smaller than the first current value when the plurality of FETs are fixed to ON, the calculation means performs the current in a state in which only one of the plurality of FETs is fixed to ON. Based on the current value detected by the detection means and the on-resistances of the FET and IPD detected in advance, the output current flowing through the FET is recalculated. If only one of the plurality of FETs is fixed on, and the output current calculated by the calculating means is larger than the second current value (> first current value), the plurality of FETs are fixed on.

  A step-down DC / DC converter according to a fifth aspect of the present invention is a coil, a first FET that connects between the input side terminal of the coil and the power supply circuit, a second FET that connects between the input side terminal and the fixed potential terminal, A backflow prevention element for connecting between the output side terminal of the coil and the load, and a smoothing capacitor for smoothing the output voltage, and by controlling each of the first FET and the second FET by PWM control, the output voltage of the power supply circuit In the step-down DC / DC converter for stepping down and outputting the current, the smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and the backflow preventing element is a current detection means for detecting a conduction current. Each of which has two IPDs (Intelligent Power Devices) connected in reverse series, and the current detection means detects the output current. It characterized that you have.

In this step-down DC / DC converter, the first FET connects the coil input side terminal and the power supply circuit, and the second FET connects the coil input side terminal and the fixed potential terminal. The backflow prevention element connects between the output terminal of the coil and the load, and the smoothing capacitor smoothes the output voltage. The first FET and the second FET are respectively PWM controlled to step down and output the output voltage of the power supply circuit.
The smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil. The backflow prevention element is composed of two IPDs each having current detection means for detecting a conduction current and connected in reverse series, and the current detection means of the IPD detects the output current.

  A step-down DC / DC converter according to a sixth aspect of the present invention includes a coil, a first FET that connects the input side terminal of the coil and the power supply circuit, a second FET that connects the input side terminal and the fixed potential terminal, A backflow prevention element for connecting between the output side terminal of the coil and the load, and a smoothing capacitor for smoothing the output voltage, and by controlling each of the first FET and the second FET by PWM control, the output voltage of the power supply circuit In the step-down DC / DC converter for stepping down the voltage, the smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and the backflow prevention element is connected to two IPDs connected in reverse series. (Intelligent Power Device), a voltage detector for detecting a difference voltage between both ends of the two IPDs, a difference voltage detected by the voltage detector, and a prediction voltage. Based on the on-resistance of said detected IPD, characterized in that it comprises a calculating means for calculating an output current Tsuryu to the IPD.

  In this step-down DC / DC converter, the first FET connects the coil input side terminal and the power supply circuit, and the second FET connects the coil input side terminal and the fixed potential terminal. The backflow prevention element connects between the output terminal of the coil and the load, and the smoothing capacitor smoothes the output voltage. The first FET and the second FET are respectively PWM controlled to step down and output the output voltage of the power supply circuit. The smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and the backflow prevention element is composed of two IPDs connected in reverse series. The voltage detector detects a differential voltage across the two IPDs, and the calculation means outputs an output current that flows through the IPD based on the differential voltage detected by the voltage detector and each on-resistance of the IPD detected in advance. Is calculated.

  According to the step-down DC / DC converter according to the present invention, step-down DC capable of detecting an output current without causing overheating and a decrease in voltage conversion efficiency, without causing an increase in component cost and an increase in mounting space. / DC converter can be realized.

1 is a circuit diagram showing a schematic configuration of an embodiment of a step-down DC / DC converter according to the present invention. 1 is a circuit diagram showing a schematic configuration of an embodiment of a step-down DC / DC converter according to the present invention. It is a circuit diagram which shows the structure around the element for backflow prevention of embodiment of the pressure | voltage fall type DC / DC converter which concerns on this invention. It is a flowchart which shows operation | movement of embodiment of the pressure | voltage fall type DC / DC converter which concerns on this invention. 1 is a circuit diagram showing a schematic configuration of an embodiment of a step-down DC / DC converter according to the present invention. It is a circuit diagram which shows the schematic structural example of the conventional step-down DC / DC converter.

Hereinafter, the present invention will be described with reference to the drawings illustrating embodiments thereof.
(Embodiment 1)
FIG. 1 is a circuit diagram showing a schematic configuration of a step-down DC / DC converter according to a first embodiment of the present invention.
This step-down DC / DC converter is used for a power supply device for a vehicle. A source is connected to a coil 11 and an input side terminal of the coil 11, and an alternator (on-vehicle generator, AC generator) and a high-voltage battery. The N-channel MOSFET (metal oxide semiconductor field effect transistor) 1 whose drain is connected to the plus terminal of the high-voltage power supply unit 4 configured by the above, and the plus electrode is connected to the drain of the FET (first FET) 1 and minus And a smoothing capacitor 9 having a pole grounded (connected to a ground terminal (fixed potential terminal)).

  In this step-down DC / DC converter, the drain is connected to the input side terminal of the coil 11, the N-channel MOSFET (second FET) 2 whose source is grounded, and the source is connected to the output side terminal of the coil 11. And an N channel type MOSFET (backflow prevention element) 3 and a smoothing capacitor 10 having a positive electrode connected to the source of the FET 3. The drain of the FET 3 is connected to each plus terminal of the low-voltage power source 5 such as a low-voltage battery and the in-vehicle load group 6, and the minus pole of the smoothing capacitor 10 is grounded.

  This step-down DC / DC converter also includes a voltage detector 7 for detecting the voltage across the FET 3 (source-drain voltage) (V1-V2), a temperature detector 3d for detecting the temperature of the FET 3, and voltage detection. And a control unit 8 to which the voltage value detected by the device 7 and the temperature detected by the temperature detector 3d are provided. The control unit 8 has a built-in memory (storage means) 8a that stores the on-resistance of the FET 3 corresponding to the temperature, performs PWM control on the FETs 1 and 2, and controls on / off of the FET 3. A body diode (parasitic diode) is formed in each of the FETs 1, 2, and 3.

In the step-down DC / DC converter having such a configuration, the output voltage of the high-voltage power supply unit 4 is detected by a voltage detector (not shown). The control unit 8 calculates a duty ratio based on the detected output voltage and the set voltage of the low-voltage power supply unit 5, and performs PWM (Pulse Width Modulation) control of the FET 1 based on the calculated duty ratio.
When the PWM control of the FET 1 is performed, the control unit 8 fixes the FET 3 to ON, and performs the PWM control with a duty ratio obtained by inverting the FET 2 with respect to the FET 1. A controlled voltage is output.

The control unit 8 also flows through the FET 3 based on the voltage across the FET 3 detected by the voltage detector 7, the temperature detected by the temperature detector 3d, and the ON resistance stored in the memory 8a and corresponding to the detected temperature. Output current (= voltage across both ends / resistance value) is calculated.
Conventionally, since the smoothing capacitor 10 is connected to the output side of the FET 3 (FIG. 6), the voltage across the FET 3 is not smoothed and has ripples. In the first embodiment, the smoothing capacitor 10 Since it is smooth, a stable value can be detected.
The control unit 8 determines overcurrent, overheating, and the like based on the calculated output current, and uses it to prevent a failure of the step-down DC / DC converter. Even when a diode is used in place of the FET 3, the same operation and effect can be obtained.

(Embodiment 2)
FIG. 2 is a circuit diagram showing a schematic configuration of the step-down DC / DC converter according to the second embodiment of the present invention.
This step-down DC / DC converter includes two IPDs 12 and 13 connected in anti-series and connected in parallel to the FET 3. The IPDs 12 and 13 are power supply control ICs (Integrated Circuits) having a current detection function, which detect the current flowing through the built-in branch circuit and calculate the current flowing through the IPD main circuit by proportional calculation.
The IPDs 12 and 13 give the calculated conduction current value to the control unit 8, and the control unit 8 performs on / off control of the IPDs 12 and 13 simultaneously with the FET 3.

The IPDs 12 and 13 may be relatively inexpensive and have an on-resistance that is relatively larger than that of the FET 3. However, if the on-resistance is too large, the detected current value becomes small and causes an error. is there.
The FET 3 has been connected so that an output current flows in a direction opposite to the normal direction so as not to flow backward through a body diode when it is off.

  On the other hand, in the general IPD, the switch is an FET having a body diode, but does not detect a reverse current (minus current). Therefore, the IPD 12 is connected in accordance with the direction of the output current for current detection, and the IPD 13 is connected in reverse series to the IPD 12 in order to prevent backflow through the body diode when off. The other configuration is the same as the configuration of the step-down DC / DC converter according to the first embodiment (FIG. 1) described above, and a description thereof will be omitted. However, the temperature detector 3d and the memory 8a shown in FIG.

In the step-down DC / DC converter having such a configuration, the control unit 8 fixes the FET 3 and the IPDs 12 and 13 to ON when the FET 1 is PWM-controlled.
The control unit 8 also calculates an output current I flowing through the FET 3 based on the current value Is detected by the current sensor of the IPD 12 and the on-resistances of the IPDs 12 and 13 and the FET 3 detected and stored in advance.

Here, if the on-resistances of the IPDs 12 and 13 and the FET 3 are R2, R2 and R1, the output current I flowing through the FET 3 is
I = Is × (2R2 / R1)
Is calculated by
Other operations are the same as those of the step-down DC / DC converter according to the first embodiment described above, and thus description thereof is omitted.

(Embodiment 3)
The configuration of Embodiment 3 of the step-down DC / DC converter according to the present invention is the same as the configuration of the step-down DC / DC converter shown in FIG. Further, for example, three FETs 3a, 3b, and 3c connected in parallel are replaced. The FETs 3a, 3b, and 3c are on / off controlled by the control unit 8. Other configurations are the same as the configuration of the step-down DC / DC converter according to the second embodiment (FIG. 2) described above, and a description thereof will be omitted.

An example of the operation of the step-down DC / DC converter having such a configuration will be described below with reference to the flowchart of FIG.
As shown in FIG. 3B, when the plurality of FETs 3a, 3b, 3c are on (S1), the control unit 8 reads the current value Is detected by the current sensor of the IPD 12 (S3). Next, based on the read current value Is and the on-resistances of the IPDs 12 and 13 and the FETs 3a, 3b, and 3c that have been detected and stored in advance, the (total) output current I that flows through the FETs 3a, 3b, and 3c is calculated. (S5).

Here, assuming that the on-resistances of the IPDs 12, 13, and FETs 3a, 3b, 3c are R2, R2, R3, R3, R3, the (total) output current I flowing through the FETs 3a, 3b, 3c is:
I = Is * (2R2 / (R3 / 3)) = Is * (6R2 / R3)
Is calculated by

As a result, the (synthetic) on-resistance of the backflow prevention elements FET3a, 3b, and 3c becomes 比 べ compared to the case of one backflow prevention element FET, and the backflow prevention elements FET3a, 3b, and 3c flow. Loss and heat generation due to current can be reduced.
Here, the on-resistances of the FETs 3a, 3b, and 3c are equal to R3, but they may be different from each other as Ra, Rb, and Rc. In this case, R3 / 3 in the above formula is Ra × Rb × Rc / (Rb × Rc + Ra × Rc + Ra × Rb).

Next, the control unit 8 determines whether or not the calculated output current I of the FETs 3a, 3b, and 3c is smaller than the predetermined current value I1 (S7). (S19), the current value Is detected by the current sensor of the IPD 12 is read again (S3).
If the calculated output current I of the FETs 3a, 3b, 3c is smaller than the predetermined current value I1 (S7), the control unit 8 selects one FET from the FETs 3a, 3b, 3c, as shown in FIG. 3A. Only the selected FET is turned on (S9).

  Next, the control unit 8 reads the current value Is detected by the current sensor of the IPD 12 (S11), reads the current value Is, and the on-resistances of the IPDs 12 and 13 that are detected and stored in advance and the selected FET. Based on the above, the output current I flowing through the selected FET is calculated (S13).

Here, since the on-resistances of the IPDs 12 and 13 and the selected FET are R2, R2, and R3 as described above, the output current I flowing through the selected FET is
I = Is × (2R2 / R3)
Is calculated by

As a result, when the output current I is the same, the current value Is detected by the current sensor of the IPD 12 is three times that in the case where the backflow prevention elements FET are three in parallel, so that current detection with higher accuracy is possible.
In this case, each of the on-resistances of the FETs 3a, 3b, and 3c can be selected as R3, and any of them can be selected. However, when the on-resistances of the FETs 3a, 3b, and 3c are different from each other, they are determined in advance. The FET should be selected.

  Next, the control unit 8 determines the calculated output current of the selected FET as I (S15), and then determines whether or not the output current I is larger than the predetermined current value I2 (S17), and determines the predetermined current value I2 If not, the current value Is detected by the current sensor of the IPD 12 is read (S11). The predetermined current value I2 is sufficiently larger than the predetermined current value I1.

If the output current I is larger than the predetermined current value I2 (S17), the controller 8 turns on the plurality of FETs 3a, 3b, 3c (S21), and then reads the current value Is detected by the current sensor of the IPD 12 ( S3).
As shown in FIG. 3A, the control unit 8 reads the current value Is detected by the current sensor of the IPD 12 when the plurality of FETs 3a, 3b, and 3c are not on and only one FET is on (S1). (S11).
Other operations are the same as those of the step-down DC / DC converter according to the second embodiment described above, and thus the description thereof is omitted.

(Embodiment 4)
FIG. 5 is a circuit diagram showing a schematic configuration of a step-down DC / DC converter according to a fourth embodiment of the present invention.
This step-down DC / DC converter includes two IPDs 14 and 15 connected in reverse series as a backflow preventing element. The IPDs 14 and 15 are power supply control ICs having a current detection function. The IPDs 14 and 15 detect the conduction current of the built-in branch circuit and perform proportional calculation to the conduction current of the IPD main circuit (the output current of the step-down DC / DC converter). ) Is calculated.

The IPDs 14 and 15 give the calculated conduction current value to the control unit 8, and the control unit 8 controls the IPDs 14 and 15 on / off simultaneously.
Conventionally, the backflow preventing element is connected so that an output current flows through the FET in the opposite direction from that of the FET so as not to flow back through the body diode when the FET is off.

  On the other hand, in the general IPD, the switch is an FET having a body diode, but does not detect a reverse current (minus current). For this reason, the IPD 14 is connected in accordance with the direction of the output current for current detection, and the IPD 15 is connected in reverse series to the IPD 14 in order to prevent backflow through the body diode when it is off. The other configuration is the same as the configuration of the step-down DC / DC converter according to the first embodiment (FIG. 1) described above, and a description thereof will be omitted. However, the temperature detector 3d and the memory 8a shown in FIG.

In the step-down DC / DC converter having such a configuration, the control unit 8 fixes the IPDs 14 and 15 to ON when the FET 1 is PWM-controlled.
The control unit 8 is also given the current value Is detected by the current sensor of the IPD 14 as the output current I.

As in the first embodiment, the voltage detector 7 is provided for detecting the voltage (V1-V2) across the backflow prevention elements IPD14, 15, and the control unit 8 detects the voltage across the voltage detected by the voltage detector 7. It is also possible to calculate the output current (= voltage between both ends / (R2 + R2)) flowing through the IPDs 14 and 15 based on the respective on-resistances R2 and R2 detected in advance of the IPDs 14 and 15.
Other operations are the same as those of the step-down DC / DC converter according to the first embodiment described above, and thus description thereof is omitted.

1 FET (first FET)
2 FET (second FET)
3, 3a, 3b, 3c FET (Backflow prevention element)
3d Temperature detector 4 High voltage power supply unit 5 Low voltage power supply unit 6 In-vehicle load group 7 Voltage detector 8 Control unit 8a Memory (storage means)
9, 10 Smoothing capacitor 11 Coil 12, 13, 14, 15 IPD

Claims (6)

A coil, a first FET that connects the input side terminal of the coil and the power supply circuit, a second FET that connects the input side terminal and the fixed potential terminal, and a backflow prevention that connects the output side terminal of the coil and the load In a step-down DC / DC converter that includes a power device and a smoothing capacitor for smoothing the output voltage, and performs PWM control on the first FET and the second FET, respectively, to step down and output the output voltage of the power supply circuit ,
The smoothing capacitor is connected between an output side terminal and a fixed potential terminal of the coil, and detects a voltage difference between both terminals of the backflow prevention element, and a difference detected by the voltage detector. Control for calculating the output current flowing through the backflow prevention element based on the voltage and the on-resistance of the backflow prevention element detected in advance, and controlling the PWM control of the first FET and the second FET, respectively And a step-down DC / DC converter.   A temperature detector for detecting the temperature of the backflow prevention element; and storage means for storing the on-resistance of the backflow prevention element corresponding to the temperature. The calculation means includes: 2. The step-down DC / DC converter according to claim 1, wherein the output current is calculated based on an on-resistance stored in the storage unit corresponding to the detected temperature. A coil, a first FET that connects the input side terminal of the coil and the power supply circuit, a second FET that connects the input side terminal and the fixed potential terminal, and a backflow prevention that connects the output side terminal of the coil and the load In a step-down DC / DC converter that includes a power device and a smoothing capacitor for smoothing the output voltage, and performs PWM control on the first FET and the second FET, respectively, to step down and output the output voltage of the power supply circuit ,
The smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and has current detection means for detecting a conduction current, and is connected in reverse series and in parallel to the backflow prevention element. Based on the two IPDs (Intelligent Power Devices), the current value detected by the current detection means, and the on-resistance of the backflow prevention element and the IPD detected in advance, the current flows to the backflow prevention element. A step-down DC / DC converter comprising a calculating means for calculating an output current.   The backflow prevention element is constituted by a plurality of FETs connected in parallel, and when the plurality of FETs are fixed to ON, the output current calculated by the calculation means is smaller than the first current value. When only one of the plurality of FETs is fixed on, the calculation means is based on the current value detected by the current detection means and the on-resistances of the FET and IPD detected in advance. When the output current flowing through the FET is recalculated and only one of the plurality of FETs is fixed to ON, the output current calculated by the calculating means is a second current value (> first current value). 4. The step-down DC / DC converter according to claim 3, wherein the plurality of FETs are fixed to ON when the value is larger. A coil, a first FET that connects the input side terminal of the coil and the power supply circuit, a second FET that connects the input side terminal and the fixed potential terminal, and a backflow prevention that connects the output side terminal of the coil and the load In a step-down DC / DC converter that includes a power device and a smoothing capacitor for smoothing the output voltage, and performs PWM control on the first FET and the second FET, respectively, to step down and output the output voltage of the power supply circuit ,
The smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and the backflow prevention element has two IPDs connected in reverse series each having current detection means for detecting a conduction current. A step-down DC / DC converter characterized in that the current detecting means is configured to detect an output current. A coil, a first FET that connects the input side terminal of the coil and the power supply circuit, a second FET that connects the input side terminal and the fixed potential terminal, and a backflow prevention that connects the output side terminal of the coil and the load In a step-down DC / DC converter that includes a power device and a smoothing capacitor for smoothing the output voltage, and performs PWM control on the first FET and the second FET, respectively, to step down and output the output voltage of the power supply circuit ,
The smoothing capacitor is connected between the output side terminal and the fixed potential terminal of the coil, and the backflow prevention element is composed of two IPDs (Intelligent Power Devices) connected in reverse series. A voltage detector for detecting a voltage difference between both ends of the IPD, and a calculation for calculating an output current flowing through the IPD based on the difference voltage detected by the voltage detector and each on-resistance of the IPD detected in advance. And a step-down DC / DC converter.

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