Vehicle power supply

Abstract

This design suppresses near-short circuits during charging while also reducing unnecessary power loss from the backup power supply. [Solution] The vehicle power supply device 10 includes a main power supply 1, a backup power supply 12, a first voltage detection unit 14 for detecting the voltage of the main power supply 1, a second voltage detection unit 15 for detecting the charging voltage of the backup power supply 12, a boost unit 17 for boosting the voltage to charge the backup power supply 12, a step-down unit 18 for stepping down the voltage to charge the backup power supply 12, and a first control unit 25 for switching control of the boost unit 17 and the step-down unit 18. The first control unit 25 switches to charging the backup power supply 12 with the step-down unit 18 when the second detected voltage by the second voltage detection unit 15 is lower than the first detected voltage by the first voltage detection unit 14, and switches to charging the backup power supply 12 with the boost unit 17 when the second detected voltage by the second voltage detection unit 15 is equal to or greater than the first detected voltage by the first voltage detection unit 14.

Description

Technical Field

[0001] The present invention relates to a vehicle power supply device.

Background Art

[0002] Patent Document 1 discloses a vehicle power supply device including a storage element (backup power supply) that can be charged to a voltage higher than the voltage of a vehicle battery (main power supply). In this power supply device, a booster circuit composed of a DC-DC converter boosts the voltage of the main power supply to charge the backup power supply, and when the voltage of the main power supply is lost or decreased, power is supplied from the backup power supply to the load.

[0003] When charging the backup power supply only by the booster circuit, when the charging voltage of the backup power supply is lower than the voltage of the main power supply, the main power supply and the ground are in a state close to a short circuit, and a large current flows through the circuit, so circuit components may fail. Therefore, in Patent Document 1, a reflux line provided with a diode for blocking the flow of current to the ground side is provided on the ground side of the backup power supply to suppress the state where the main power supply and the ground are close to a short circuit.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When a backup power supply provides power to a load, a voltage drop occurs, so it is desirable to minimize power loss as much as possible. However, in the vehicle power supply device described in Patent Document 1, when power is supplied from the backup power supply to the load, the backup power supply's power is consumed by a short-circuit suppression diode. Therefore, there is room for improvement in suppressing unnecessary power loss from the backup power supply in the vehicle power supply device described in Patent Document 1.

[0006] The present invention relates to a vehicle power supply system equipped with a backup power supply capable of charging to a voltage higher than the voltage of the main power supply, and aims to suppress unnecessary power loss of the backup power supply while suppressing a near-short circuit condition during charging. [Means for solving the problem]

[0007] The present invention provides a vehicle power supply device comprising: a backup power supply capable of being charged to a voltage greater than the voltage of the vehicle's main power supply; a first voltage detection unit for detecting the voltage of the main power supply; a second voltage detection unit for detecting the charging voltage of the backup power supply; a boost unit capable of boosting the voltage from the main power supply to charge the backup power supply; a step-down unit capable of stepping down the voltage from the main power supply to charge the backup power supply; and a first control unit for switching control to charge the backup power supply based on the output voltage from either the boost unit or the step-down unit, wherein the first control unit switches to charging the backup power supply using the step-down unit when the second detected voltage from the second voltage detection unit is lower than the first detected voltage from the first voltage detection unit, and switches to charging the backup power supply using the boost unit when the second detected voltage from the second voltage detection unit is equal to or greater than the first detected voltage from the first voltage detection unit.

[0008] When the second voltage detected by the second voltage detection unit is lower than the first voltage detected by the first voltage detection unit, the first control unit charges the backup power supply using a step-down unit that reduces the voltage from the main power supply. In this way, when the backup power supply is charged with a stepped-down voltage, the main power supply and ground will not be in a state close to a short circuit. Therefore, the backup power supply can be charged normally without damaging circuit components.

[0009] The first control unit charges the backup power supply using a voltage booster unit that increases the voltage from the main power supply when the second voltage detected by the second voltage detection unit is equal to or greater than the first voltage detected by the first voltage detection unit. This allows the output voltage to be increased above the backup power supply's charging voltage, thus preventing the backup power supply from being unable to be charged. Furthermore, when the main power supply voltage is less than or equal to the backup power supply's charging voltage, the main power supply and ground do not become close to a short circuit. Therefore, the backup power supply can be charged normally without damaging circuit components.

[0010] Regardless of the difference in voltage between the backup power supply's charging voltage and the main power supply's voltage, this design prevents the main power supply and ground from becoming close to a short circuit, thereby suppressing circuit component failures. Therefore, there is no need to provide a return line including a diode on the ground side of the backup power supply. Consequently, when supplying power from the backup power supply to the load, unnecessary power loss in electrical components other than the load can be suppressed. [Effects of the Invention]

[0011] The present invention relates to a vehicle power supply system equipped with a backup power supply capable of charging to a voltage higher than the voltage of the main power supply, which can suppress a near-short circuit state during charging while suppressing unnecessary power loss of the backup power supply. [Brief explanation of the drawing]

[0012] [Figure 1] Circuit diagram of a vehicle power supply device according to an embodiment of the present invention. [Figure 2] A diagram showing the current flow during voltage boosting. [Figure 3]A diagram showing the current flow during voltage reduction. [Figure 4] A flowchart illustrating the control mechanism for switching between step-down and step-up charging. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described below with reference to the drawings.

[0014] Referring to Figure 1, the vehicle power supply device 10 according to an embodiment of the present invention is used in a vehicle and is configured to supply power to a load 2 when the voltage of the battery (main power source) 1 mounted on the vehicle is lost or decreases.

[0015] The vehicle power supply unit 10 includes a backup power supply 12 capable of charging to a voltage greater than the voltage of the battery 1 (e.g., 12V) (e.g., 18V). The vehicle power supply unit 10 also includes a voltage detection unit (first voltage detection unit) 14 for detecting the voltage of the battery 1, a voltage detection unit (second voltage detection unit) 15 for detecting the charging voltage of the backup power supply 12, a transformer circuit 16 consisting of a DC-DC converter, and a control unit (first control unit) 25 for switching.

[0016] The transformer circuit 16 includes a boost circuit (boost unit) 17 that increases the voltage of the battery 1 to charge the backup power supply 12. In this embodiment, the transformer circuit 16 includes a step-down circuit (step-down unit) 18 that steps down the voltage of the battery 1 to charge the backup power supply 12 when the charging voltage of the backup power supply 12 is lower than the voltage of the battery 1.

[0017] The boost circuit 17 and the buck circuit 18 each include a common inductor 19. The boost circuit 17 further includes a field-effect transistor (first field-effect transistor) 20 and a diode (first diode) 21. The buck circuit 18 further includes a field-effect transistor (second field-effect transistor) 22 and a diode (second diode) 23.

[0018] The control unit 25 is configured to switch the transformer circuit 16 to one of the boost circuit 17 and the buck circuit 18 based on the detected voltages Vbatt and Vsc detected by the voltage detection units 14 and 15. The control unit 25 is composed of a single or multiple microcomputers and other electronic devices. Thereby, the control unit 25 charges the backup power supply 12 with the output voltage from one of the boost circuit 17 and the buck circuit 18.

[0019] Specifically, when the detected voltage Vsc detected by the voltage detection unit 15 for the backup power supply 12 is lower than the detected voltage Vbatt detected by the voltage detection unit 14 for the battery 1, the control unit 25 switches the transformer circuit 16 to function as the buck circuit 18. On the other hand, when the detected voltage Vsc detected by the voltage detection unit 15 for the backup power supply 12 is equal to or higher than the detected voltage Vbatt detected by the voltage detection unit 14 for the battery 1, the control unit 25 switches the transformer circuit 16 to function as the boost circuit 17. Thereby, it is possible to prevent the state where the battery 1 and the ground are close to a short circuit. In the following description, the detected voltage Vbatt detected by the voltage detection unit 14 for the battery may be referred to as the voltage Vbatt of the battery 1, and the detected voltage Vsc detected by the voltage detection unit 15 for the backup power supply may be referred to as the voltage Vsc of the backup power supply 12.

[0020] The vehicle power supply device 10 further includes a switching control unit (second control unit) 30, a frequency adjustment unit 31, a current detection unit 32, and an overvoltage protection unit 34. The control unit 30 turns on and off (switches) the field effect transistors (hereinafter referred to as "FETs") 20 and 22 at a predetermined cycle based on signals from the frequency adjustment unit 31, the current detection unit 32, and the overvoltage protection unit 34. The control unit 30 is composed of a logic IC having an arithmetic function and a conversion function.

[0021] Hereinafter, the configuration of the vehicle power supply device 10 will be specifically described.

[0022] The backup power supply 12 is configured by connecting six supercapacitors, which are energy storage elements capable of charging and discharging electrical energy, in series. The positive electrode of the backup power supply 12 is connected to the battery 1 via the transformer circuit 16 and the resistor 33 and is also connected to the load 2. The negative electrode of the backup power supply 12 is connected to the ground.

[0023] The common inductor 19 that constitutes the boost circuit 17 and the buck circuit 18 is a passive element for repeatedly accumulating and releasing electrical energy. One end of the inductor 19 is connected to the battery 1 via the FET 22 and the resistor 33. The other end of the inductor 19 is connected to the backup power supply 12 via the diode 21.

[0024] The FET 20 that constitutes the boost circuit 17 is a switching element for switching the accumulation and release of energy with respect to the inductor 19 when boosting the voltage of the battery 1. The FET 20 is of the N-channel type, the drain is connected between the inductor 19 and the diode 21, the source is connected to the ground, and the gate is connected to the control unit 30 for switching.

[0025] The diode 21 that constitutes the boost circuit 17 is interposed between the inductor 19 and the backup power supply 12, allows the flow of current from the inductor 19 to the backup power supply 12, and blocks the flow of reverse current. That is, the anode of the diode 21 is connected to the inductor 19, and the cathode of the diode 21 is connected to the backup power supply 12.

[0026] The FET 22 that constitutes the buck circuit 18 is a switching element for switching the accumulation and release of energy with respect to the inductor 19 when reducing the voltage of the battery 1. The FET 22 is of the P-channel type, the source is connected to the battery 1 via the resistor 33, the drain is connected to one end of the inductor 19, and the gate is connected to the control unit 30 for switching. The circuit configuration may be changed and an N-channel type FET 22 may be used.

[0027] The diode 23, which constitutes the step-down circuit 18, is connected between the FET 22 and the inductor 19, blocking the flow of current from that connection point to ground. In other words, the cathode of the diode 23 is connected between the FET 22 and the inductor 19, and the anode of the diode 23 is connected to ground.

[0028] The transformer circuit 16 configured in this way functions as a boost circuit 17 that increases the voltage of battery 1 and outputs it by switching FET 20 and turning on FET 22. When FET 20 is on, as shown by the thick solid line in Figure 2, current flows from battery 1 to FET 22, inductor 19, and then to FET 20, causing inductor 19 to store energy. This is because the voltage on the battery 1 side of inductor 19 is higher than the voltage on the backup power supply 12 side. When FET 20 is off, as shown by the thick dashed line in Figure 2, current flows from battery 1 to FET 22, inductor 19, diode 21, and then to backup power supply 12, causing inductor 19 to release (consume) energy and charge backup power supply 12. This is limited to when the voltage on the battery 1 side of inductor 19 is greater than or equal to the voltage on the backup power supply 12 side of inductor 19.

[0029] On the other hand, the transformer circuit 16 functions as a step-down circuit 18 that lowers the voltage of the battery 1 and outputs it by turning off the FET 20 and switching the FET 22. When the FET 22 is ON, as shown by the thick solid line in Figure 3, current flows from the battery 1 in the order of FET 22, inductor 19, diode 21, and backup power supply 12, causing the inductor 19 to store energy and the backup power supply 12 to be charged. This is because the voltage on the battery 1 side of the inductor 19 is higher than the voltage on the backup power supply 12 side. When the FET 22 is OFF, as shown by the thick dashed line in Figure 3, the inductor 19 releases energy, causing current to flow in the order of backup power supply 12 and diode 23, thereby charging the backup power supply 12. This is because the supply from the battery 1 side is cut off.

[0030] Referring to Figure 1, the control unit 25 is configured to switch between the boost circuit 17 and the buck circuit 18, based on the detection results of the voltage detection units 14 and 15, using the boost / buck switching circuit 26. Of the voltage detection unit 14 for the battery, the input terminal is connected between the battery 1 and the resistor 33, and the output terminal is connected to the control unit 25. Of the voltage detection unit 15 for the backup power supply, the input terminal is connected between the backup power supply 12 and the load 2, and the output terminal is connected to the control unit 25.

[0031] The step-up / step-down switching circuit 26 includes a pair of transistors 27 and 28. Of the transistor 27 for FET 20 (first switching element), the collector (one end) is connected between FET 20 and the control unit 30, the emitter (other end) is connected to ground, and the base is connected to the control unit 25. Of the transistor 28 for FET 22 (second switching element), the collector (one end) is connected between FET 22 and the control unit 30, the emitter (other end) is connected to ground, and the base is connected to the control unit 25. Other switching elements besides transistors 27 and 28 may be used as long as they can switch between conduction and disconnection states.

[0032] The control unit 25 switches the transformer circuit 16 to function either the boost circuit 17 or the buck circuit 18 by controlling the on / off state of transistors 27 and 28. This supplies the output voltage from either the boost circuit 17 or the buck circuit 18 to the backup power supply 12, thereby charging the backup power supply 12.

[0033] Specifically, when the detected voltage (second detected voltage) Vsc by the voltage detection unit 15 for the backup power supply is lower than the detected voltage (first detected voltage) Vbatt by the voltage detection unit 14 for the battery (Vsc < Vbatt), the control unit 25 turns on the transistor 27 and turns off the transistor 28. As a result, due to the conduction of the transistor 27, the gate of the FET 20 is shorted to the ground to maintain the off state. In this state, by switching the FET 22, the backup power supply 12 is charged by the step-down circuit 18.

[0034] On the other hand, when the detected voltage (second detected voltage) Vsc by the voltage detection unit 15 for the backup power supply is greater than or equal to the detected voltage (first detected voltage) Vbatt by the voltage detection unit 14 for the battery (Vsc ≧ Vbatt), the control unit 25 turns off the transistor 27 and turns on the transistor 28. As a result, due to the conduction of the transistor 28, the gate of the FET 22 is shorted to the ground to maintain the on state. In this state, by switching the FET 20, the backup power supply 12 is charged by the boost circuit 17.

[0035] More specifically, when the voltage Vsc of the backup power supply 12 is lower than the voltage Vbatt (e.g., 12V) of the battery 1, the control unit 25 steps down the voltage of the battery 1 to an output voltage (e.g., Vsc + 0.3V) higher than the charging voltage of the backup power supply 12 by switching to the step-down circuit 18 and charges the backup power supply 12. On the other hand, when the voltage Vsc of the backup power supply 12 is greater than or equal to the voltage Vbatt (e.g., 12V) of the battery 1, the control unit 25 steps up the voltage of the battery 1 to an output voltage (e.g., Vsc + 0.3V) higher than the charging voltage of the backup power supply 12 by switching to the boost circuit 17 and charges the backup power supply 12.

[0036] In this case, if only the boost circuit 17 is installed without the step-down circuit 18, when the voltage Vsc of the backup power supply 12 is lower than the voltage Vbatt of the battery 1, even if the FET 20 is switched, current continues to flow through the inductor 19 and electrical energy continues to accumulate (see Figure 2). As a result, the battery 1 and ground become close to a short circuit, and a large current flows through the circuit, which can cause circuit components to fail. In contrast, in this embodiment, when the voltage Vsc of the backup power supply 12 is lower than the voltage Vbatt of the battery 1, the step-down circuit 18 steps down the voltage of the battery 1 and supplies it to the backup power supply 12. As a result, the inductor 19 repeatedly accumulates and releases electrical energy, the battery 1 and ground do not become close to a short circuit, a large current does not flow through the circuit, and circuit components do not fail.

[0037] The switching control unit 30 is connected to the gates of FETs 20 and 22, respectively. A frequency adjustment unit 31 and a current detection unit 32 are further connected to the control unit 30. The overvoltage protection unit 34 is connected to the connection line between the current detection unit 32 and the control unit 30.

[0038] The frequency adjustment unit 31 is configured to supply the control unit 30 with a clock frequency for switching the FETs 20 and 22 on and off. The control unit 30, including the frequency adjustment unit 31, may be composed of one or more microcomputers.

[0039] The current detection unit 32 is configured to switch the operating state of the control unit 30 based on the current value output from the battery 1. Specifically, a resistor 33 for current detection is interposed between the battery 1 and the FET 22. The current detection unit 32 is connected between the battery 1 and the resistor 33, and between the resistor 33 and the FET 22. When the detected current is greater than or equal to the set current (e.g., 3A), the current detection unit 32 enters a Hi state, turning off the control unit 30's control of the FETs 20 and 22. This stops the charging of the backup power supply 12. On the other hand, when the detected current is less than the set current, the current detection unit 32 enters a Lo state, turning on the control unit 30's control of the FETs 20 and 22. This enables constant current control.

[0040] The overvoltage protection unit 34 is configured to switch the operating state of the control unit 30 based on the charge state of the backup power supply 12. Specifically, the overvoltage protection unit 34 is connected to the switching control unit 25 and also between the switching control unit 30 and the current detection unit 32. The switching control unit 25 has a function to determine whether the backup power supply 12 is 100% charged (fully charged) based on the detection result of the voltage detection unit 15 for the backup power supply. When it determines that the power supply is fully charged, it outputs a signal to the overvoltage protection unit 34, and when it determines that the power supply is not fully charged, it does not output a signal to the overvoltage protection unit 34. If the overvoltage protection unit 34 does not receive a signal from the switching control unit 25, it does not output a signal to the switching control unit 30 and allows the control unit 30 to switch the FETs 20 and 22 for charging. On the other hand, if the overvoltage protection unit 34 receives a signal from the switching control unit 25, it outputs a signal to the switching control unit 30, causing the control unit 30 to switch the FETs 20 and 22 so that the charging of the backup power supply 12 is stopped. For example, by keeping FET22 in the OFF state, charging of the backup power supply 12 can be stopped.

[0041] Next, with reference to Figure 4, the charging process (switching control) by the switching control unit 25 will be explained.

[0042] In the charging process, the switching control unit 25 first determines in step S1 whether the backup power supply 12 is less than fully charged based on the detection result of the voltage detection unit 15 for the backup power supply. If it determines that the backup power supply 12 is fully charged, it proceeds to step S2 and outputs a signal to the overvoltage protection unit 34, thereby stopping the charging of the backup power supply 12 via the switching control unit 30, and returns to step S1. On the other hand, if it determines that the backup power supply 12 is not fully charged, it proceeds to step S3.

[0043] In step S3, the switching control unit 25 determines, based on the detection results of the voltage detection units 14 and 15, whether the voltage Vsc of the backup power supply 12 is lower than the voltage Vbatt of the battery 1. If the voltage Vsc of the backup power supply 12 is lower than the voltage Vbatt of the battery 1, the process proceeds to step S4, where the transformer circuit 16 is switched to step-down charging (step-down circuit 18) via the step-up / step-down switching circuit 26, and the process returns to step S1. On the other hand, if the voltage Vsc of the backup power supply 12 is equal to or greater than the voltage Vbatt of the battery 1, the process proceeds to step S5, where the transformer circuit 16 is switched to step-up charging (step-up circuit 17) via the step-up / step-down switching circuit 26, and the process returns to step S1.

[0044] The vehicle power supply unit 10 configured as described above has the following features.

[0045] The switching control unit 25 charges the backup power supply 12 using a step-down circuit 18 that steps down the voltage from the battery 1 when the voltage Vsc detected by the backup power supply voltage detection unit 15 is lower than the voltage Vbatt detected by the battery voltage detection unit 14. When the backup power supply 12 is charged using this stepped-down voltage, the battery 1 and ground will not be in a near-short circuit state. Therefore, the backup power supply 12 can be charged normally without damaging any circuit components.

[0046] The switching control unit 25 charges the backup power supply 12 using a boost circuit 17 that increases the voltage from battery 1 when the voltage Vsc detected by the backup power supply voltage detection unit 15 is equal to or greater than the voltage Vbatt detected by the battery voltage detection unit 14. This allows the output voltage to be increased above the charging voltage of the backup power supply 12, thus preventing the problem of the backup power supply 12 not being able to be charged. Also, when the voltage of battery 1 is less than or equal to the charging voltage of the backup power supply 12, battery 1 and ground do not become close to a short circuit. Therefore, the backup power supply 12 can be charged normally without damaging circuit components.

[0047] Regardless of the difference in voltage between the backup power supply 12's charging voltage Vsc and the battery 1's voltage Vbatt, this design prevents the battery 1 from becoming nearly short-circuited to ground, thereby suppressing circuit component failures. Therefore, there is no need to provide a return line including a diode on the ground side of the backup power supply 12. Consequently, when supplying power from the backup power supply 12 to the load 2, unnecessary power loss in electrical components other than the load 2 can be suppressed.

[0048] Since the boost circuit 17 and the buck circuit 18 share a common inductor 19, the transformer circuit 16 can be constructed with a simple configuration. In addition, a switching control unit 30 is connected to FET 20 and FET 22, respectively. Therefore, the backup power supply 12 can be reliably charged by the boost circuit 17 including FET 20 or the buck circuit 18 including FET 22.

[0049] The circuit includes a transistor 27 with its collector connected between FET 20 and the control unit 30 and its emitter connected to ground, and a transistor 28 with its collector connected between FET 22 and the control unit 30 and its emitter connected to ground. The control unit 25 also controls the on / off state of transistors 27 and 28 when charging the backup power supply 12. This ensures that the backup power supply 12 is reliably charged by the output voltage from either the boost circuit 17 or the buck circuit 18.

[0050] The system includes an overvoltage protection unit 34 that stops charging the backup power supply 12 by the boost circuit 17 and the buck circuit 18 when the backup power supply 12 is fully charged. This prevents degradation of the backup power supply 12 due to overcharging.

[0051] Furthermore, the present invention is not limited to the configuration of the above-described embodiment, and various modifications are possible.

[0052] For example, the transformer circuit 16 can be modified as needed, as long as it includes a boost circuit (boost section) 17 and a buck circuit (buck section) 18.

[0053] The switching control unit 25 may be configured to switch between the boost circuit (boost unit) 17 and the buck circuit (buck unit) 18 by means other than the boost / buck switching circuit 26. [Explanation of Symbols]

[0054] 1. Battery (main power source) 2 loads 10. Vehicle power supply unit 12 Backup power supply 14. Voltage detection unit (first voltage detection unit) 15 Voltage detection unit (second voltage detection unit) 16. Transformer Circuit 17. Boost Circuit (Boost Section) 18 Step-down circuit (step-down section) 19 Inductors 20 FET (Field-Effect Transistor) 21 Diode (First Diode) 22 FET (Field-Effect Transistor) 23 Diode (2nd Diode) 25 Control Unit (First Control Unit) 26. Step-up / Step-down Converter 27. Transistor (First Switching Element) 28. Transistor (Second Switching Element) 30 Control Unit (Second Control Unit) 31 Frequency adjustment section 32 Current detection unit 33 Resistors 34 Overvoltage protection section

Claims

1. A backup power supply capable of charging to a voltage higher than the vehicle's main power supply voltage, A first voltage detection unit for detecting the voltage of the main power supply, A second voltage detection unit for detecting the charging voltage of the backup power supply, A boost unit capable of boosting the voltage from the main power supply to charge the backup power supply, A step-down unit capable of reducing the voltage from the main power supply to charge the backup power supply, A first control unit that controls switching to charge the backup power supply using the output voltage from either the boost unit or the buck unit. Equipped with, The first control unit is, When the second voltage detected by the second voltage detection unit is lower than the first voltage detected by the first voltage detection unit, the step-down unit switches to charging the backup power supply. When the second voltage detected by the second voltage detection unit is equal to or greater than the first voltage detected by the first voltage detection unit, the boost unit switches to charging the backup power supply. Vehicle power supply unit.

2. The boost unit and the buck unit each have a common inductor having one end connected to the main power supply and the other end connected to the backup power supply. The aforementioned boosting unit is A first diode has its anode connected to the inductor and its cathode connected to the backup power supply, A first field-effect transistor is provided, with one end connected between the inductor and the first diode and the other end connected to ground. It further possesses, The aforementioned step-down unit is A second field-effect transistor, one end of which is connected to the main power supply side and the other end of which is connected to the one end of the inductor, A second diode is connected between the second field-effect transistor and the inductor, with its cathode connected to ground and its anode connected to ground. It further possesses, The vehicle power supply device according to claim 1, further comprising a second control unit for switching the first field-effect transistor and the second field-effect transistor, respectively.

3. A first switching element, with one end connected between the first field-effect transistor and the second control unit and the other end connected to ground, A second switching element, with one end connected to the second field-effect transistor and the second control unit and the other end connected to ground, It has, The vehicle power supply device according to claim 2, wherein the first control unit supplies the output voltage from one of the boost unit and the buck unit to the backup power supply by on / off control of the first switching element and the second switching element.

4. The vehicle power supply device according to any one of claims 1 to 3, further comprising an overvoltage protection unit for stopping the charging of the backup power supply by the boost unit and the buck unit when the backup power supply is fully charged.

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