Bootstrap circuit, power supply device, and vehicle

Abstract

This bootstrap circuit comprises: a first switch configured so as to be applied with a constant voltage at a first end; a capacitor that is connected, at a first end thereof, to a second end of the first switch, and that is configured so as to be applied with a switching voltage at a second end; and a control unit configured to control the first switch on the basis of the switching voltage and a control signal. The switching voltage is a voltage generated at a node connecting first and second switching elements. The second switching element is a switching element provided on a low-potential side of the first switching element and configured to execute switching on the basis of the control signal.

Description

Bootstrap circuit, power supply device, and vehicle

[0001] The invention disclosed herein relates to a bootstrap circuit, a power supply device, and a vehicle.

[0002] 2. Description of the Related Art In power supply devices such as switched capacitor converters and half-bridge DC / DC converters, N-channel MOS field effect transistors with low on-resistance are often used as switching elements in order to maximize efficiency.

[0003] A bootstrap circuit is used to drive an N-channel MOS field effect transistor used as a high-side switching element (see, for example, Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2018-133916

[0005] Although a bootstrap circuit usually has a diode, a switch may be used instead of the diode. By using a switch instead of a diode, the on-resistance of the N-channel MOS field-effect transistor used as the high-side switching element can be further reduced.

[0006] However, using a switch instead of a diode requires timing control of the switch, and it is desirable that this timing control be simple.

[0007] The bootstrap circuit disclosed in this specification includes a first switch configured to have a first terminal to which a constant voltage is applied, a capacitor configured to have a second terminal of the first switch connected to the first terminal and to have a switching voltage applied to the second terminal, and a control unit configured to control the first switch based on the switching voltage and a control signal. The switching voltage is a voltage generated at a connection node between a first switching element and a second switching element. The second switching element is provided on a lower potential side than the first switching element and is configured to switch based on the control signal.

[0008] The power supply device disclosed in this specification includes the bootstrap circuit configured as described above, the first switching element, and the second switching element.

[0009] The vehicle disclosed in this specification has a power supply device having the above-described configuration.

[0010] According to the invention disclosed in this specification, the on-resistance of the switching element driven by the bootstrap voltage can be further reduced, making it easier to control the timing in the bootstrap circuit.

[0011] FIG. 1 is a diagram illustrating a comparative example of a half-bridge DC / DC converter. FIG. 2 is a timing chart illustrating voltages at various components of the half-bridge DC / DC converter illustrated in FIG. 1 when the output current is large. FIG. 3 is a timing chart illustrating voltages at various components of the half-bridge DC / DC converter illustrated in FIG. 1 when the output current is small. FIG. 4 is a diagram illustrating an example of the configuration of a level shifter. FIG. 5 is a diagram illustrating an embodiment of a half-bridge DC / DC converter. FIG. 6 is a timing chart illustrating voltages at various components of the half-bridge DC / DC converter illustrated in FIG. 5 when the output current is large. FIG. 7 is a timing chart illustrating voltages at various components of the half-bridge DC / DC converter illustrated in FIG. 5 when the output current is small. FIG. 8 is an external view of a vehicle. FIG. 9 is a diagram illustrating the configuration of a Dickson-type switched capacitor converter. FIG. 10 is a timing chart illustrating voltages at various components of the switched capacitor converter illustrated in FIG. 9. FIG. 11 is a diagram illustrating a first example of a switched capacitor converter having a topology different from the Dickson-type. FIG. 12 is a diagram illustrating a second example of a switched capacitor converter having a topology different from the Dickson-type. 13 and 14 are diagrams illustrating a third and fourth example of a switched capacitor converter having a topology different from that of the Dickson type.

[0012] In this specification, a MOS (Metal Oxide Semiconductor) field effect transistor refers to a field effect transistor whose gate structure is composed of at least three layers: a layer made of a conductor or a semiconductor such as polysilicon with a low resistance value, an insulating layer, and a P-type, N-type, or intrinsic semiconductor layer. In other words, the gate structure of a MOS field effect transistor is not limited to a three-layer structure of a metal, an oxide, and a semiconductor.

[0013] In this specification, a constant voltage means a voltage that is constant under ideal conditions, but in reality it is a voltage that may fluctuate slightly due to temperature changes and the like.

[0014] 1 is a diagram showing a comparative example of a half-bridge DC / DC converter (i.e., a general configuration to be compared with the embodiments described later). The half-bridge DC / DC converter 100 of this comparative example includes a control unit CNT1, a level shifter LS1, drivers D1 and D2, N-channel MOS field-effect transistors Q1 and Q2, an inductor L1, an output capacitor C0, and a bootstrap circuit 10.

[0015] N-channel MOS field effect transistors Q1 and Q2 are connected in series. N-channel MOS field effect transistor Q1 is a high-side switch provided on the higher potential side of N-channel MOS field effect transistor Q2. N-channel MOS field effect transistor Q2 is a low-side switch provided on the lower potential side of N-channel MOS field effect transistor Q1.

[0016] The control unit CNT1 outputs control signals HD0 and LD. Basically, when one of the control signals HD0 and LD is at a high level, the other is at a low level. The control unit CNT1 provides a dead time during which both the control signals HD0 and LD are at a low level.

[0017] The level shifter LS1 outputs a control signal HD obtained by level-shifting the control signal HD0. The driver D1 amplifies the control signal HD to generate a control signal HG, which is supplied to the gate of the N-channel MOS field-effect transistor Q1. The driver D1 is driven by a voltage between the bootstrap voltage BST and the switching voltage VSW.

[0018] The driver D2 amplifies the control signal LD ​​to generate a control signal LG, which is supplied to the gate of the N-channel MOS field effect transistor Q2. The driver D2 is driven by a voltage between the constant voltage VREG (< the input voltage VIN) and the ground voltage.

[0019] An input voltage VIN is applied to the drain of an N-channel MOS field effect transistor Q1. The source of an N-channel MOS field effect transistor Q2 is connected to the ground potential. Switching of the N-channel MOS field effect transistors Q1 and Q2 generates a switching voltage VSW at the connection node of the N-channel MOS field effect transistors Q1 and Q2.

[0020] The inductor L1 and the output capacitor C0 smooth the switching voltage VSW to generate the output voltage VOUT.

[0021] The bootstrap circuit 10 generates a bootstrap voltage BST that is higher than the input voltage VIN.

[0022] The bootstrap circuit 10 includes an AND gate A1, a level shifter LS2, a driver D3, a P-channel MOS field effect transistor QB, and a capacitor CB.

[0023] A constant voltage VREG is applied to a source of a P-channel MOS field effect transistor QB. A drain of the P-channel MOS field effect transistor QB is connected to a first terminal of a capacitor CB. A switching voltage VSW is applied to a second terminal of the capacitor CB.

[0024] In the bootstrap circuit 10, it is necessary to keep the P-channel MOS field-effect transistor QB off when the switching voltage VSW is greater than 0 V. Therefore, the bootstrap circuit 10 is configured such that an AND gate A1 generates a logical product of a control signal LD ​​and a control signal LG, and the P-channel MOS field-effect transistor QB is controlled based on this logical product. Specifically, a level shifter LS2 level-shifts the output of the AND gate A1 and outputs the result. A driver D3 amplifies and inverts the output of the level shifter LS2 to generate a control signal BG, which is then supplied to the gate of the P-channel MOS field-effect transistor QB. The driver D3 is driven by a voltage between the bootstrap voltage BST and the switching voltage VSW. This allows the N-channel MOS field-effect transistor Q2 to be turned off after the P-channel MOS field-effect transistor QB has been turned off.

[0025] With the bootstrap circuit 10 configured as described above, the P-channel MOS field-effect transistor QB can be kept off when the switching voltage VSW is greater than 0 V, regardless of whether the output current IOUT of the half-bridge DC / DC converter 100 is large (see FIG. 2) or small (see FIG. 3). However, the need for a logical product of the control signal LD ​​and the control signal LG complicates timing control.

[0026] 4 is a diagram showing an example of the configuration of the level shifter LS2. The level shifter LS2 of the example configuration shown in Fig. 4 has an inverter 12, N-channel MOS field effect transistors 13 to 16, P-channel MOS field effect transistors 17 and 18, a first voltage line L11, a second voltage line L12, a third voltage line L13, and a fourth voltage line L14.

[0027] 4, it is necessary to use N-channel MOS field effect transistors 15 and 16 as elements having a withstand voltage of Vin. In other words, since a plurality of elements having a withstand voltage of Vin are required, the level shifter LS2 of the configuration example shown in FIG. 4 has a large area.

[0028] In view of the above considerations, a new embodiment will be proposed below that can further reduce the on-resistance of the switching element driven by the bootstrap voltage and facilitate timing control of the bootstrap circuit.

[0029] <Half-bridge DC / DC converter (embodiment)> Fig. 5 is a diagram showing an embodiment of a half-bridge DC / DC converter. A half-bridge DC / DC converter 200 of this embodiment differs from the half-bridge DC / DC converter 100 described above in that it has a bootstrap circuit 20 instead of the bootstrap circuit 10, but is otherwise basically similar to the half-bridge DC / DC converter 100 described above.

[0030] 6 is a timing chart showing the voltages of the respective parts of the half-bridge DC / DC converter 200 when the output current Iout is large. FIG. 7 is a timing chart showing the voltages of the respective parts of the half-bridge DC / DC converter 200 when the output current Iout is small.

[0031] The bootstrap circuit 20 includes a level shifter 30, a driver D3, a P-channel MOS field effect transistor QB, and a capacitor CB.

[0032] A constant voltage VREG is applied to a source of a P-channel MOS field effect transistor QB. A drain of the P-channel MOS field effect transistor QB is connected to a first terminal of a capacitor CB. A switching voltage VSW is applied to a second terminal of the capacitor CB.

[0033] A control section constituted by the level shifter 30 and the driver D3 controls the P-channel MOS field effect transistor QB based on the switching voltage VSW and the control signal LD.

[0034] The level shifter 30 includes a resistor 31, a P-channel MOS field effect transistor 32, a resistor 33, and an N-channel MOS field effect transistor .

[0035] The control signal LD ​​is applied to a first terminal of a resistor 31. A second terminal of the resistor 31 is connected to the source of a P-channel MOS field effect transistor 32. The drain of the P-channel MOS field effect transistor 32 is connected to a first terminal of a resistor 33, the drain of an N-channel MOS field effect transistor 34, and the input terminal of a driver D3. The switching voltage VSW is applied to the gate of the P-channel MOS field effect transistor 32, the second terminal of the resistor 33, and the source of the N-channel MOS field effect transistor 34.

[0036] The control signal HD is supplied to the gate of an N-channel MOS field effect transistor 34 .

[0037] The driver D3 supplies a control signal BG, which is obtained by amplifying and inverting the output of the level shifter 30, to the gate of the P-channel MOS field effect transistor QB. The driver D3 is driven by a voltage between the bootstrap voltage BST and the switching voltage VSW.

[0038] The bootstrap circuit 20 controls the P-channel MOS field effect transistor QB based on the switching voltage VSW, which facilitates timing control. Specifically, the bootstrap circuit 20 controls the P-channel MOS field effect transistor QB based on the logical product of the control signal LD ​​and the inverted signal of the switching voltage VSW.

[0039] Furthermore, since the bootstrap circuit 20 uses a P-channel MOS field effect transistor QB as a switch instead of a diode, the on-resistance of the N-channel MOS field effect transistor Q1 driven by the bootstrap voltage BST can be further reduced.

[0040] The resistor 33 can pull down the output of the level shifter 30 when the P-channel MOS field effect transistor 32 is off. The N-channel MOS field effect transistor 34 is controlled by a control signal HD. The N-channel MOS field effect transistor 34 can pull down the output of the level shifter 30 when the P-channel MOS field effect transistor 32 is off and the control signal HD is at a high level. Since the bootstrap circuit 20 has a configuration including the N-channel MOS field effect transistor 34, it can more reliably pull down the output of the level shifter 30.

[0041] When the N-channel MOS field effect transistors Q1 and Q2 are Si devices, the bootstrap circuit 20 can turn on the N-channel MOS field effect transistor Q1 at an appropriate timing.

[0042] When the N-channel MOS field effect transistors Q1 and Q2 are GAN devices or SiC devices, that is, when the switching voltage VSW during the dead time is lower by several volts than when the N-channel MOS field effect transistors Q1 and Q2 are Si devices, a configuration may be adopted in which a control signal LG is applied to the first end of the resistor 31 instead of the control signal LD.

[0043] In the level shifter 30, the only element that can withstand a voltage of Vin is the P-channel MOS field effect transistor 32. Therefore, the level shifter 30 can have a smaller area than the level shifter LS2 of the configuration example shown in FIG.

[0044] <Application Example> Fig. 8 is an external view of a vehicle X. The vehicle X in this configuration example is equipped with various electronic devices X11 to X18 that operate by receiving a supply of voltage output from a battery (not shown). Note that the installation positions of the electronic devices X11 to X18 in this figure may differ from the actual positions for convenience of illustration.

[0045] The electronic device X11 is an engine control unit that performs engine-related controls (injection control, electronic throttle control, idling control, oxygen sensor heater control, auto-cruise control, etc.).

[0046] The electronic device X12 is a lamp control unit that controls the turning on and off of a high intensity discharged lamp (HID) or a daytime running lamp (DRL).

[0047] The electronic device X13 is a transmission control unit that performs control related to the transmission.

[0048] The electronic device X14 is a braking unit that performs control related to the movement of the vehicle X (ABS (anti-lock brake system) control, EPS (electric power steering) control, electronic suspension control, etc.).

[0049] The electronic device X15 is a security control unit that controls the driving of door locks, security alarms, and the like.

[0050] The electronic devices X16 are electronic devices that are installed in the vehicle X at the factory as standard equipment or manufacturer options, such as wipers, power door mirrors, power windows, dampers (shock absorbers), power sunroofs, and power seats.

[0051] The electronic device X17 is an electronic device that is optionally installed in the vehicle X as a user option, such as an in-vehicle A / V (audio / visual) device, a car navigation system, and an ETC (electronic toll collection system).

[0052] The electronic device X18 is an electronic device equipped with a high-voltage motor, such as an in-vehicle blower, an oil pump, a water pump, or a battery cooling fan.

[0053] The half-bridge DC / DC converter 200 described above can be incorporated into any of the electronic devices X11 to X18. Furthermore, the use of the half-bridge DC / DC converter 200 described above is not limited to a power supply mounted on the vehicle X, and may also be a power supply mounted on industrial equipment, for example.

[0054] <Others> In addition to the above-described embodiments, various modifications can be made to the configuration of the invention without departing from the spirit of the invention. The above-described embodiments are illustrative in all respects and should be considered not to be limiting. The technical scope of the present invention is defined by the claims, not by the description of the above-described embodiments, and should be understood to include all modifications that fall within the meaning and scope of the claims.

[0055] The bootstrap circuit 20 can also be mounted on power supply devices other than the half-bridge DC / DC converter 200. For example, the bootstrap circuit 20 can be mounted on a Dickson-type switched capacitor converter shown in Fig. 9. Fig. 10 is a timing chart showing voltages at various parts of the switched capacitor converter shown in Fig. 9.

[0056] The switched capacitor converter shown in FIG. 9 includes switching elements M1 to M8, capacitors C1 to C3, an output capacitor Cout, and a control unit CNT2.

[0057] A first terminal of the switching element M1 is connected to the positive electrode of a DC voltage source VS1, and a negative electrode of the DC voltage source VS1 is connected to the ground potential. The DC voltage source VS1 supplies an input voltage Vin to the first terminal of the switching element M1.

[0058] The second terminal of the switching element M1 is connected to the first terminal of the switching element M2 and the first terminal of the capacitor C3. The second terminal of the switching element M2 is connected to the first terminal of the switching element M3 and the first terminal of the capacitor C2. The second terminal of the switching element M3 is connected to the first terminal of the switching element M4 and the first terminal of the capacitor C1.

[0059] The second terminal of the switching element M4 is connected to a first terminal of the switching element M7, a first terminal of the load LD1, a first terminal of the switching element M6, and a first terminal of the output capacitor Cout. The second terminal of the switching element M7 is connected to a first terminal of the switching element M8, a second terminal of the capacitor C1, and a second terminal of the capacitor C3. The second terminal of the switching element M6 is connected to a first terminal of the switching element M5 and a second terminal of the capacitor C2. The second terminal of the switching element M8, the second terminal of the load LD1, the second terminal of the switching element M5, and the second terminal of the output capacitor Cout are connected to ground potential.

[0060] The control unit CNT2 controls the switching elements M1, M3, M5, and M7 by a first control signal Φ1, and controls the switching elements M2, M4, M6, and M8 by a second control signal Φ2.

[0061] The control unit CNT2 controls the on / off of the switching elements M1, M3, M5, and M7 and the switching elements M2, M4, M6, and M8 in a complementary manner.

[0062] The switching voltage VSW1 switches between the value of Vin and the value of Vin x 3 / 4. The switching voltage VSW1 is generated at the connection node between the switching element M1 and the switching element M2.

[0063] The switching voltage VSW2 switches between a value of Vin×3 / 4 and a value of Vin / 2. The switching voltage VSW2 is generated at the connection node between the switching element M2 and the switching element M3.

[0064] The switching voltage VSW3 switches between the value of Vin / 2 and the value of Vin / 4. The switching voltage VSW3 is generated at the connection node between the switching element M3 and the switching element M4.

[0065] The switching voltage VSW6 switches between the value of Vin / 4 and 0 (ground potential). The switching voltage VSW6 is generated at the connection node between the switching element M5 and the switching element M6.

[0066] The switching voltage VSW7 switches between the value of Vin / 4 and 0 (ground potential). The switching voltage VSW7 is generated at the connection node between the switching element M7 and the switching element M8.

[0067] The output voltage Vout has a value of Vin / 4. The output voltage Vout is generated at the connection node between the switching element M4, the switching element M6, and the switching element M7. The output voltage Vout is supplied to the load LD1.

[0068] When the switching elements M1 to M8 are N-channel MOS field effect transistors, six bootstrap circuits 20 for driving the switching elements M1 to M4, M6, and M7, respectively, may be mounted on the switched capacitor converter shown in FIG.

[0069] It should be noted that the positions of the DC voltage source VS1 and the load LD1 may be interchanged in the switched capacitor converter shown in Fig. 9. When the positions of the DC voltage source VS1 and the load LD1 are interchanged, the voltage supplied to the load LD1 from the switched capacitor converter shown in Fig. 9 (the output voltage of the switched capacitor converter shown in Fig. 9) becomes larger than the voltage supplied from the DC voltage source VS1 to the switched capacitor converter shown in Fig. 9 (the input voltage of the switched capacitor converter shown in Fig. 9).

[0070] The bootstrap circuit 20 can also be mounted on a switched capacitor converter with a topology different from the Dickson type. Examples of switched capacitor converters with a topology different from the Dickson type include the switched capacitor converters shown in FIGS. 11 to 14.

[0071] The bootstrap circuit (20) described above has a first switch (QB) configured to have a first terminal to which a constant voltage is applied, a capacitor (CB) configured to have a second terminal of the first switch connected to its first terminal and to have a switching voltage applied to its second terminal, and a control unit (30, D3) configured to control the first switch based on the switching voltage and a first control signal, wherein the switching voltage is a voltage generated at a connection node between a first switching element (Q1) and a second switching element (Q2), and the second switching element is provided on a lower potential side than the first switching element and is configured to switch based on the first control signal (first configuration).

[0072] The bootstrap circuit of the first configuration can further reduce the on-resistance of the switching element (first switching element) driven by the bootstrap voltage, making timing control easier.

[0073] In the bootstrap circuit of the first configuration, the control section may have a level shifter (30) configured to level-shift the first control signal (second configuration).

[0074] The bootstrap circuit of the second configuration can use a level shifter with a small circuit area.

[0075] In the bootstrap circuit of the second configuration, the level shifter may have an N-channel MOS field effect transistor (32), and the N-channel MOS field effect transistor may be configured so that the switching voltage is supplied to a gate thereof (third configuration).

[0076] In the bootstrap circuit of the third configuration, the high-voltage elements can be made up of only N-channel MOS field-effect transistors.

[0077] In the bootstrap circuit of the third configuration, the N-channel MOS field effect transistor may be configured so that the first control signal is supplied to its drain (fourth configuration).

[0078] In the bootstrap circuit of the fourth configuration, the high-voltage elements can be made up of only N-channel MOS field-effect transistors.

[0079] In the bootstrap circuit of the third or fourth configuration, the level shifter may have a configuration (fifth configuration) including a resistor (33) provided between the source and drain of the N-channel MOS field effect transistor.

[0080] The bootstrap circuit of the fifth configuration can pull down the output of the level shifter when the N-channel MOS field effect transistor is off.

[0081] In the bootstrap circuits of the third to fifth configurations, the first switching element may be a switching element configured to switch based on a second control signal, and the level shifter may have a second switch (34) provided between the source and drain of the N-channel MOS field effect transistor, and the second switch may be configured to switch based on the second control signal (sixth configuration).

[0082] The bootstrap circuit of the sixth configuration can pull down the output of the level shifter when the N-channel MOS field effect transistor is off.

[0083] In the bootstrap circuits of the first to sixth configurations, the first switching element and the second switching element may each be a Si device, and the first control signal may be an input signal of a driver configured to drive the second switching element (seventh configuration).

[0084] The bootstrap circuit of the seventh configuration can turn on the first switch at a timing suitable for the first switching element and the second switching element.

[0085] In the bootstrap circuits of the first to sixth configurations, the first switching element and the second switching element may each be a GAN device or a SiC device, and the first control signal may be an output signal of a driver configured to drive the second switching element (eighth configuration).

[0086] The bootstrap circuit of the eighth configuration can turn on the first switch at a timing suitable for the first switching element and the second switching element.

[0087] In the bootstrap circuits of the first to eighth configurations, the first switch may be turned off when the switching voltage is greater than a predetermined value (ninth configuration).

[0088] The bootstrap circuit of the ninth configuration can prevent the switching voltage from reaching an abnormal value.

[0089] The power supply device (200) described above has a configuration (tenth configuration) that includes a bootstrap circuit of any one of the first to ninth configurations, the first switching element, and the second switching element.

[0090] The power supply device having the tenth configuration can further reduce the on-resistance of the switching element (first switching element) driven by the bootstrap voltage, making it easier to control the timing of the bootstrap circuit.

[0091] The vehicle (X) described above has a configuration (eleventh configuration) that includes the power supply device of the tenth configuration.

[0092] In the vehicle having the eleventh configuration, the on-resistance of the switching element (first switching element) driven by the bootstrap voltage can be further reduced, making it easier to control the timing of the bootstrap circuit.

[0093] 10, 20 Bootstrap circuit 12 Inverter 13 to 16, 34, Q1, Q2 N-channel MOS field effect transistor 17, 18, 32, QB P-channel MOS field effect transistor 30, LS1, LS2 Level shifter 31, 33 Resistor 100, 200 Half-bridge DC / DC converter A1 AND gate C0, Cout Output capacitor C1 to C3, CB Capacitor CNT1, CNT2 Control unit D1 to D3 Driver L1 Inductor L11 to L14 First to fourth voltage lines LD1 Load M1 to M8 Switching element X Vehicle X11 to X18 Electronic device

Claims

1. A first switch configured to apply a constant voltage to a first terminal; a capacitor having a first terminal connected to the second terminal of the first switch and configured to receive a switching voltage at a second terminal; a controller configured to control the first switch based on the switching voltage and a first control signal; having the switching voltage is a voltage generated at a connection node between a first switching element and a second switching element, The second switching element is provided on a lower potential side than the first switching element and is configured to switch based on the first control signal.

2. The bootstrap circuit of claim 1 , wherein the control unit comprises a level shifter configured to level shift the first control signal.

3. the level shifter has a P-channel MOS field effect transistor; 3. The bootstrap circuit according to claim 2, wherein the P-channel MOS field effect transistor is configured so that the switching voltage is supplied to a gate of the P-channel MOS field effect transistor.

4. 4. The bootstrap circuit according to claim 3, wherein the P-channel MOS field effect transistor is configured so that the first control signal is supplied to a drain of the P-channel MOS field effect transistor.

5. 4. The bootstrap circuit according to claim 3, wherein the level shifter has a resistor provided between the gate and drain of the P-channel MOS field effect transistor.

6. the first switching element is a switching element configured to switch based on a second control signal; the level shifter has a second switch provided between a gate and a drain of the P-channel MOS field effect transistor, The bootstrap circuit of claim 3 , wherein the second switch is configured to switch based on the second control signal.

7. the first switching element and the second switching element are each a Si device; 2. The bootstrap circuit of claim 1, wherein the first control signal is an input signal of a driver configured to drive the second switching element.

8. the first switching element and the second switching element are each a GAN device or a SiC device; 2. The bootstrap circuit of claim 1, wherein the first control signal is an output signal of a driver configured to drive the second switching element.

9. 2. The bootstrap circuit of claim 1, wherein the first switch is off when the switching voltage is greater than a predetermined value.

10. A bootstrap circuit according to any one of claims 1 to 9; The first switching element; the second switching element.

11. A vehicle comprising the power supply device according to claim 10.