DC bus voltage monitoring device and electrical system

Abstract

The invention relates to the technology of motors, in particular to a device for monitoring the voltage of a direct current bus in an electrical system and the electrical system comprising the device. According to one aspect of the present application, there is provided an apparatus for monitoring a DC bus voltage in an electrical system, where the electrical system comprises an auxiliary power supply comprising a transformer and a first switching element, a primary winding of the transformer and the first switching element being connected in series between the DC buses, the device comprises a sampling circuit connected with a secondary winding of the transformer; and the controller is connected with the sampling circuit and is configured to determine the voltage of the direct current bus based on a sampling signal acquired by the sampling circuit during the conduction period of the first switch element.

Description

Technical Field

[0001] The present application relates to motor technology, and particularly to a device for monitoring the DC bus voltage in an electrical system and an electrical system including the device. Background Art

[0002] In an electric vehicle, a power source (such as a battery pack) is connected to a motor controller through a DC bus. The motor controller obtains high-voltage direct current from the battery pack through the DC bus. Inside the motor controller, the DC bus is connected to an IGBT module, and the direct current is converted into three-phase alternating current through an inverter to drive the motor. To avoid damage to the vehicle electrical system caused by too high or too low DC bus voltage, it is necessary to monitor the DC bus voltage in real time and take corresponding protection measures (such as reducing the output power or shutting down for protection) when abnormal voltage is detected.

[0003] In the existing DC bus voltage monitoring, a voltage divider resistor is used to divide the bus voltage, and then the voltage signal obtained by voltage division is input to the signal processing side through an isolator. The above method has disadvantages such as high cost and difficulty in miniaturizing the monitoring device. Summary of the Invention

[0004] One object of the present application is to provide a device for monitoring the DC bus voltage in an electrical system and an electrical system including the device, which have advantages such as low implementation cost and being conducive to miniaturization of the device.

[0005] According to one aspect of the present application, there is provided a device for monitoring the DC bus voltage in an electrical system, wherein the electrical system includes an auxiliary power supply, the auxiliary power supply includes a transformer and a first switching element, and the primary winding of the transformer and the first switching element are connected in series between the DC buses.

[0006] The device includes:

[0007] A sampling circuit connected to the secondary winding of the transformer;

[0008] A controller connected to the sampling circuit, configured to determine the voltage of the DC bus based on the sampling signal collected by the sampling circuit during the conduction of the first switching element.

[0009] Optionally, in the above device, the sampling circuit includes:

[0010] An operational amplifier, wherein the non-inverting input terminal of the operational amplifier is connected to one end of the secondary winding, and the inverting input terminal is connected to the output terminal through a first diode with reverse bias.

[0011] Further optionally, in the above device, the sampling circuit further includes a rectifying circuit, and the rectifying circuit includes a forward-biased second diode and a capacitor connected in series between the output terminal of the operational amplifier and the ground.

[0012] Still further optionally, in the above device, the sampling circuit further includes a second switching element connected in parallel with the capacitor and whose on-off state is controlled by the controller, and the controller is configured to periodically make the second switching element in an on state and an off state. And still further optionally, the sampling circuit further includes a third switching element connected between the non-inverting input terminal of the operational amplifier and the ground and whose on-off state is controlled by the controller, and the controller is configured to make the third switching element in an on state and an off state synchronously with the second switching element.

[0013] Optionally, in the above device, the common connection point of the inverting input terminal of the operational amplifier and the first diode is grounded through an adjusting resistor, and the resistance value of the adjusting resistor is set such that the voltage drops of the first diode and the second diode are as close as possible.

[0014] Optionally, in the above device, the controller is configured to obtain the sampling signal collected by the sampling circuit when the first switching element is in an on state and the second switching element and the third switching element are in off states.

[0015] According to another aspect of the present application, there is provided an electrical system, including:

[0016] A DC power supply;

[0017] A motor controller;

[0018] A DC bus connecting the DC power supply and the motor controller;

[0019] An auxiliary power supply, including a transformer and a first switching element, and the primary winding of the transformer and the first switching element are connected in series between the DC buses;

[0020] A DC bus voltage monitoring device, including:

[0021] A sampling circuit connected to the secondary winding of the transformer;

[0022] A controller connected to the sampling circuit, and the controller is configured to determine the voltage of the DC bus based on the sampling signal collected by the sampling circuit during the conduction of the first switching element.

[0023] Optionally, in the above electrical system, the sampling circuit includes:

[0024] An operational amplifier, wherein the non-inverting input terminal of the operational amplifier is connected to one end of the secondary winding, and the inverting input terminal is connected to the output terminal through a first diode with reverse bias.

[0025] Optionally, in the above electrical system, the sampling circuit further includes a rectifying circuit, and the rectifying circuit includes a second diode with forward bias and a capacitor connected in series between the output terminal of the operational amplifier and the ground.

[0026] Further optionally, in the above electrical system, the sampling circuit further includes a second switching element connected in parallel with the capacitor and whose on-off state is controlled by the controller, and the controller is configured to periodically make the second switching element in the on state and the off state. Still further optionally, the sampling circuit further includes a third switching element connected between the non-inverting input terminal of the operational amplifier and the ground and whose on-off state is controlled by the controller, and the controller is configured to make the third switching element in the on state and the off state synchronously with the second switching element.

[0027] Optionally, in the above electrical system, the common connection point of the output terminal of the operational amplifier and the first diode is grounded through an adjusting resistor, and the resistance value of the adjusting resistor is set such that the voltage drops of the first diode and the second diode are as close as possible.

[0028] Optionally, in the above electrical system, the controller is configured to obtain the sampling signal collected by the sampling circuit when the first switching element is in the on state and the second switching element and the third switching element are in the off state.

[0029] Optionally, in the above electrical system, the auxiliary power supply further includes a secondary rectifying circuit connected to the secondary winding of the transformer.

[0030] In some embodiments of the present application, the isolation characteristic and voltage transformation characteristic of the transformer in the auxiliary power supply are utilized to realize the functions of high-voltage isolation and step-down sampling. Since the auxiliary power supply including the transformer is usually an essential component in many electrical systems, this multiplexing method eliminates additional voltage-dividing resistors and isolation devices, which is beneficial to cost reduction and device miniaturization. In addition, in the sampling circuit, by using the second switching element and the third switching element to provide a charge discharge channel for the capacitor in the rectifying circuit, the voltage signal obtained by the controller can quickly respond to the change of the DC bus voltage, so as to better monitor the change of the DC bus voltage. Description of the Drawings

[0031] The above and / or other aspects and advantages of the present application will become clearer and easier to understand through the following description of various aspects in conjunction with the accompanying drawings, where the same or similar units are denoted by the same reference numerals. The accompanying drawings include:

[0032] Figure 1 A schematic block diagram of an electrical system according to an embodiment of the present application.

[0033] Figure 2 A circuit schematic diagram of a device for monitoring the DC bus voltage in an electrical system according to another embodiment of the present application.

[0034] Figure 3 A schematic diagram showing the waveforms of the voltage of the primary winding and the voltage of the corresponding terminal on the secondary winding.

[0035] Figure 4 A schematic diagram showing the waveform of the voltage on capacitor C1 changing with the increase of the DC bus voltage during the rapid rise stage of the DC bus voltage.

[0036] Figure 5 A schematic diagram showing the waveform that the voltage on capacitor C1 cannot quickly respond to the decrease of the DC bus voltage during the rapid drop stage of the DC bus voltage.

[0037] Figure 6 A timing diagram of the sampling control signal and the on-off control signal. Detailed implementation manners

[0038] The present application will be described more comprehensively below with reference to the accompanying drawings which illustrate schematic embodiments of the present application. However, the present application can be implemented in different forms and should not be construed as limited to the embodiments given herein. The above-described embodiments are intended to make the disclosure herein complete and to convey the scope of protection of the present application more fully to those skilled in the art.

[0039] In this specification, terms such as "comprising" and "including" mean that in addition to the units and steps directly and explicitly recited in the specification and claims, the technical solutions of the present application do not exclude the situation of having other units and steps not directly or explicitly recited.

[0040] In this specification, "connection" refers to an electrical connection for transmitting electric power or a communication connection for transmitting signals between two or more hardware entities, which includes the situation where two hardware entities are directly connected and also includes the situation where two hardware entities are connected through other hardware entities.

[0041] Many electrical systems usually include multiple power sources, such as a main power source and an auxiliary power source. Among them, the main power source is responsible for the basic power supply requirements of the core functions of the device (such as motor drive and energy storage, etc.), and the auxiliary power source supplies power to specific subsystems (such as control systems and sensors, etc.) to implement precise control functions or specific operation functions. Examples of these electrical systems include, but are not limited to, electric vehicles, industrial equipment (such as motor drives, photovoltaic inverters, energy storage systems), urban rail vehicles, medical equipment (such as X-ray machines, CT scanners), and communication base stations and data centers, etc.

[0042] Taking an electric vehicle as an example, the DC bus transmits high-voltage electrical energy to the motor controller, and the motor controller converts direct current into alternating current through an inverter to drive the motor. On the other hand, the auxiliary power source draws power from the DC bus and supplies electrical energy to the low-voltage system and the drive circuit of the motor controller. In a typical auxiliary power source, the primary winding of the transformer is connected between the DC buses, and the secondary winding is connected to the rectifier circuit to supply electrical energy to the low-voltage system. In some embodiments of the present application, the isolation characteristics and voltage transformation characteristics of the transformer in the auxiliary power source are utilized to implement the functions of high-voltage isolation and step-down sampling. Specifically, the voltage signal at the secondary winding can be collected and the voltage of the DC bus can be monitored based on this voltage signal. Since the auxiliary power source containing the transformer is usually an essential component in an electric vehicle, the above method eliminates additional voltage-dividing resistors and isolation devices, which is beneficial for cost reduction and device miniaturization.

[0043] The following describes multiple embodiments of the present application with the aid of the accompanying drawings.

[0044] Figure 1 It is a schematic block diagram of an electrical system (such as a vehicle electrical system) according to an embodiment of the present application. Refer to Figure 1 , the illustrated electrical system 10 includes a DC power source (such as a battery pack and a supercapacitor, etc.) 110, a motor controller 120, DC buses HVDC+ and HVDC-, an auxiliary power source 130, and a DC bus voltage monitoring device 140. The DC power source 110 can be, for example, the power battery of an electric vehicle, as Figure 1 shown, which is connected to the motor controller 120 via the DC buses HVDC+ and HVDC-. In addition, the auxiliary power source 130 includes a transformer (not shown), and the primary winding of the transformer is connected to the DC bus to draw power from the DC bus. Refer to Figure 1 , the DC bus voltage monitoring device 140 is connected to the auxiliary power source 130, and it can, for example, collect a voltage sampling signal from the secondary winding of the transformer of the auxiliary power source 130 and determine the voltage of the DC bus based on this voltage sampling signal. The structure and working principle of the DC bus voltage monitoring device 140 will be further described below.

[0045] Figure 2The circuit schematic diagram of a device for monitoring the DC bus voltage in an electrical system according to another embodiment of the present application. The shown device can be used to implement Figure 1 the DC bus voltage monitoring device 140 in

[0046] For illustrative purposes only, Figure 2 in Figure 2 the auxiliary power supply 130 that provides the voltage sampling signal adopts a flyback power supply topology. In particular, the auxiliary power supply 130 includes a transformer T1, a first rectifier circuit 131, a switching element Q3 ( Figure 2 where a metal-oxide-semiconductor field effect transistor (MOS transistor) is used as a specific implementation method in ), and a power manager 132. The primary winding of the transformer T1 is connected in series with the switching element Q3 between the DC bus HVDC+ and HVDC-. The secondary winding of the transformer T1 is connected to the first rectifier circuit 131. The power manager 132 applies a pulse width modulation (PWM) signal to the control terminal of the switching element Q3, so that the high-voltage circuit where the primary winding is located periodically switches between the conducting state and the open state, thereby reflecting the primary voltage to the secondary winding according to the turns ratio relationship. It should be noted that other power supply topologies are also suitable for the various embodiments of the present application as long as they have the functions of high-voltage and low-voltage conversion and isolation. The first rectifier circuit 131 includes a diode D1 and a capacitor C2 connected in series across the two ends of the secondary winding to rectify the output of the secondary winding, thereby generating a stable voltage V R .

[0047] In Figure 2 the shown embodiment, the device 20 includes a sampling circuit 210 and a controller 220. The sampling circuit 210 includes voltage-dividing resistors R1 and R2, an operational amplifier U1, and a second rectifier circuit 211. Referring to Figure 2 , the voltage-dividing resistors R1 and R2 are connected in series between the secondary winding and the ground. The non-inverting input terminal of the operational amplifier U1 is connected to the common connection point of the voltage-dividing resistors R1 and R2, so it can be connected to one end of the secondary winding of the transformer T1 through the voltage-dividing resistor R1. In addition, the negative and positive electrodes of the diode D3 are respectively connected to the inverting input terminal and the output terminal of the operational amplifier U1, that is, the inverting input terminal of the operational amplifier U1 is connected to the output terminal through the reversely biased diode D3.

[0048] Continuing to refer to Figure 2The second rectifier circuit 211 includes a diode D2 and a capacitor C1, wherein the anode of the diode D2 is connected to the output terminal of the operational amplifier U1, and the cathode is grounded via the capacitor C1, that is, the forward-biased diode D2 and the capacitor C1 are connected in series between the output terminal of the operational amplifier U1 and the ground. Optionally, the common point of the inverting input terminal of the operational amplifier U1 and the diode D3 is grounded via an adjustment resistor R3, and the voltage drops of the diodes D2 and D3 can be made as close as possible by adjusting the resistance value of the adjustment resistor R3.

[0049] Figure 2 The device 20 shown also includes a switching element Q2 ( Figure 2 The MOS tube is used as a specific implementation method), the gate of which is connected to the controller 220, the source (or drain) is connected to the positive electrode of the diode D2 through the resistor R4, and the drain (or source) is connected to the ground. The device 20 further includes a switch element Q1 ( Figure 2 A MOS tube is used as a specific implementation method), the gate of which is connected to the controller 220, the source (or drain) is connected to the common point of the resistors R1 and R2, and the drain (or source) is connected to the ground.

[0050] like Figure 2 As shown, the port P1 (e.g., analog input port) of the controller 220 is connected to the anode of the diode D2 to receive the voltage sampling signal output by the sampling circuit 210. In addition, the port P2 of the controller 220 is connected to the gates of the switching elements Q1 and Q2, which periodically turn on and off the switching elements by applying a PWM signal. Optionally, the application of the PWM signal causes the switching elements Q1 and Q2 to be in an on state and an off state synchronously. It will be further recognized from the following description that the switching elements Q1 and Q2 are used to discharge the charge stored in the capacitor C1.

[0051] It should be pointed out that although Figure 2 The switching elements Q1-Q3 in the figure are shown in the form of MOS tubes, but other types of switching elements are also available, such as but not limited to bipolar junction transistors (BJT) and junction field effect transistors (JFET).

[0052] exist Figure 2 In the example, the controller 220 can be various types of controllers, including but not limited to a microcontroller, a field programmable gate array, and a programmable logic controller.

[0053] The working principle of the device 20 is described below.

[0054] In the auxiliary power supply 130, a PWM signal is applied to the gate of the switch element Q3 to turn the switch element Q3 on and off periodically, thereby changing the voltage of the primary winding of the transformer T1 according to the turns ratio relationship N.p :N s is reflected to the secondary winding. In Figure 2 the example shown, the voltage signal sampled at the same-name terminal on the secondary winding when the switching element Q3 is in the on state is used to determine the DC bus voltage.

[0055] Figure 3 The waveform diagrams of the voltage V0 of the primary winding and the voltage V1 of the same-name terminal on the secondary winding are shown, where the vertical axis represents the voltage magnitude and the horizontal axis represents the time t.

[0056] In Figure 3 , V HVDC is the voltage difference between HVDC+ and HVDC- of the DC bus, V R is the output voltage of the first rectifier circuit 131, V or1 is the voltage reflected from the secondary winding of the transformer T1 to the primary winding, V or2 is the voltage reflected from the primary winding of the transformer T1 to the secondary winding. V or1 and V or2 can be determined by the following formulas (1) and (2) respectively:

[0057]

[0058] When the switching element Q3 is in the on state, the voltage V1 of the same-name terminal on the secondary winding is equal to the sum of the voltage reflected from the primary winding to the secondary winding and the output voltage of the first rectifier circuit 131, that is:

[0059]

[0060] Therefore, the DC bus voltage V HVDC .

[0061] Refer to Figure 2 In, the voltage V1 at the secondary winding terminal is divided by the voltage dividing resistors R1 and R2 in the sampling circuit 210 to form an input voltage V2 at the non-inverting input terminal of the operational amplifier U1. The operational amplifier U1 acts as a follower and outputs a voltage V3 at its output terminal. The voltages V2 and V3 can be determined by the following formulas (4) and (5) respectively:

[0062]

[0063] V3 - V D3 = V2 (5)

[0064] where R1 and R2 are the resistances of the voltage dividing resistors R1 and R2 respectively, and V D3 is the voltage drop of the diode D3.

[0065] To make the voltage signal collected on the P1 port of the controller 220 relatively stable, in Figure 2 In the illustrated example, the sampling circuit 210 further includes a second rectifying circuit 211 to rectify the output voltage of the operational amplifier U1. As Figure 2 shown, the second rectifying circuit 211 is composed of a diode D2 and a capacitor C1, and the output voltage V4 thereof is sent to the P1 port of the controller 220. The voltage V4 can be determined by the following formula (6):

[0066] V4 + V D2 = V3 (6)

[0067] wherein, V D2 is the voltage drop of the diode D2.

[0068] In summary, the output voltage V4 of the second rectifying circuit 211 can be expressed as:

[0069]

[0070] Thus, the controller 220 can determine the DC bus voltage V HVDC according to the following formula (8) by using the voltage V4 input to its port P1:

[0071]

[0072] In Figure 2 the illustrated circuit, the voltage drop V D3 of the diode D3 depends on the current flowing through the diode D3. Therefore, by selecting a suitable resistance value for the regulating resistor R3, the voltage drops V D2 and V D3 of the diodes D2 and D3 can be made as close as possible. In this case, the DC bus voltage V HVDC can be determined according to the following formula (9):

[0073]

[0074] In the above formula, since the voltage drops V D2 and V D3 of the diodes D2 and D3 do not need to be considered, the calculation of the DC bus voltage can be simplified.

[0075] Continue to refer to Figure 2, during the rapid rise stage of the DC bus voltage, when the switching element Q3 is in the conducting state, the output voltage of the operational amplifier U1 charges the capacitor C1 through the diode D2, causing the voltage of the capacitor C1 to continuously increase. When the switching element Q3 is in the off state, due to the reverse cutoff effect of the diode D2, the charge on the capacitor C1 cannot be quickly released, so the voltage on the capacitor C1 remains relatively stable. When the switching element Q3 repeatedly alternates between the conducting state and the off state, the voltage on the capacitor C1 will increase correspondingly as the DC bus voltage increases. Figure 4 Figure 2 shows a waveform schematic diagram of the voltage on the capacitor C1 changing with the increase of the DC bus voltage during the rapid rise stage of the DC bus voltage, where the vertical axis represents the voltage magnitude and the horizontal axis represents the time t.

[0076] On the other hand, during the rapid drop stage of the DC bus voltage, when the switching element Q3 is in the off state, if there is no discharge path available for the capacitor C1, due to the reverse cutoff effect of the diode D2, the voltage on the capacitor C1 still remains at a relatively high level and cannot quickly respond to the change of the DC bus voltage. Figure 5 Figure 7 shows a waveform schematic diagram of the voltage on the capacitor C1 being unable to quickly respond to the decrease of the DC bus voltage during the rapid drop stage of the DC bus voltage, where the vertical axis represents the voltage magnitude and the horizontal axis represents the time t.

[0077] To avoid a situation similar to Figure 5 shown, as Figure 2 shown, the switching element Q2 is connected in parallel across the two ends of the capacitor C1. Therefore, when the switching element Q2 is in the conducting state, a discharge path for the charge of the capacitor C1 is provided. In some specific embodiments, the controller 220 controls the on / off state of the switching element Q2 by applying a PWM signal to the gate of the switching element Q2, so that the charge stored in the capacitor C1 is periodically discharged. Continuing to refer to Figure 2, in addition to the switching element Q2, a switching element Q1 is also connected between the non-inverting input terminal of the operational amplifier U1 and the ground. When the switching element Q2 is in the on state, although the charge on the capacitor C1 can be discharged through the path formed by the resistor R4 and the switching element Q2, at the same time, since the voltage V2 at the non-inverting input terminal of the operational amplifier U1 may be greater than the voltage at the inverting input terminal, the operational amplifier U1 will continue to charge the capacitor C1 through the diode D2, thus hindering the rapid discharge of the charge on the capacitor C1. In some other specific embodiments, the controller 220 controls the on / off states of the switching elements Q1 and Q2 by simultaneously applying PWM signals to the gates of the switching elements Q1 and Q2, so that the switching elements Q1 and Q2 are simultaneously in the on state. At this time, the charge on the capacitor C1 will be quickly released through the path formed by the resistor R4 and the switching element Q2, and the conduction of the switching element Q1 also ensures that the voltage at the non-inverting input terminal of the operational amplifier U1 is lower than the voltage at the inverting input terminal to prevent the operational amplifier U1 from charging the capacitor C1.

[0078] Figure 6 is a timing diagram of the sampling control signal and the on / off control signal. As Figure 6 shown, the on / off control signal (which can be, for example, the PWM signal output by the controller 220 through the port P2) for controlling the on / off states of the switching elements Q1 and Q2 alternately assumes a high level state and a low level state, where the durations of the high level state and the low level state are t1 and (t2 + t3), respectively. When the on / off control signal is in the high level state, the switching elements Q1 and Q2 are in the on state, so the voltage on the capacitor C1 is quickly discharged. On the other hand, when the on / off control signal is in the low level state, the switching elements Q1 and Q2 are in the off state, and at this time, the sampling control signal can be enabled throughout the entire duration (for example, Figure 6 the duration (t2 + t3) shown) or in a partial duration (for example, Figure 6 the duration t3 shown) to receive the voltage signal V4 output by the sampling circuit 210 through the port P1, for example.

[0079] In some specific embodiments, the durations of the on / off control signal in the high level state and the low level state (such as Figure 6 the durations t1 and (t2 + t3) in Figure 6 and the duration of the sampling control signal (such as the duration t3 in

[0080] It should be noted that, through research, the inventors of the present application have found that setting the switching frequencies of switching elements Q1 and Q2 (the number of times the switching elements switch between the on state and the off state per unit time) higher than that of switching element Q3 is beneficial for discharging the charge on capacitor C1. When making a compromise between the charge discharge rate and the implementation cost, it is particularly advantageous to set the switching frequencies of switching elements Q1 and Q2 to several times (such as 2 to 10 times) that of switching element Q3.

[0081] Although only some specific embodiments of the present application have been described, those of ordinary skill in the art should understand that the present application can be implemented in many other forms without departing from its gist and scope. Therefore, the examples and embodiments shown are regarded as illustrative rather than restrictive, and the present application may cover various modifications and substitutions without departing from the spirit and scope of the present application as defined by the appended claims.

[0082] The embodiments and examples provided herein are for the purpose of best illustrating the embodiments in accordance with the present technology and its specific applications, and thereby enabling those skilled in the art to implement and use the present application. However, those skilled in the art will know that the above description and examples are provided only for the convenience of illustration and exemplification. The described presentation is not intended to cover all aspects of the present application or to limit the present application to the precise forms disclosed.

Claims

1. A device for monitoring a DC bus voltage in an electrical system, wherein: The electrical system includes an auxiliary power supply, the auxiliary power supply includes a transformer and a first switching element, the primary winding of the transformer and the first switching element are connected in series between the DC busbars, The device comprises: a sampling circuit connected to the secondary winding of the transformer; A controller connected to the sampling circuit is configured to determine the voltage of the DC bus based on a sampling signal collected by the sampling circuit during the conduction period of the first switching element.

2. The device according to claim 1, wherein: The sampling circuit comprises: An operational amplifier, wherein a non-inverting input terminal of the operational amplifier is connected to one end of the secondary winding, and an inverting input terminal is connected to an output terminal via a reverse biased first diode.

3. The device as claimed in claim 2, wherein: The sampling circuit further includes a rectifier circuit including a forward-biased second diode and a capacitor connected in series between an output terminal of the operational amplifier and ground. 4 . The device of claim 3 , wherein the sampling circuit further comprises a second switch element connected in parallel with the capacitor and whose on / off state is controlled by the controller, and the controller is configured to periodically turn the second switch element into an on state and an off state.

5. The device as claimed in claim 4, wherein the sampling circuit further comprises a third switch element connected between the non-inverting input terminal of the operational amplifier and ground and whose on / off state is controlled by the controller, and the controller is configured to make the third switch element in an on state and an off state synchronously with the second switch element.

6. The device according to claim 3, wherein: A common point of the inverting input terminal of the operational amplifier and the first diode is grounded via an adjusting resistor, and the resistance of the adjusting resistor is set to make the voltage drops of the first diode and the second diode as close as possible.

7. The device according to claim 5, wherein: The controller is configured to obtain a sampling signal collected by the sampling circuit when the first switching element is in an on state and the second switching element and the third switching element are in an off state.

8. An electrical system comprising: DC power supply; Motor controller; A DC bus connecting the DC power supply and the motor controller; An auxiliary power supply, comprising a transformer and a first switching element, wherein the primary winding of the transformer and the first switching element are connected in series between the DC bus; The DC bus voltage monitoring device comprises: a sampling circuit connected to the secondary winding of the transformer; A controller connected to the sampling circuit is configured to determine the voltage of the DC bus based on a sampling signal collected by the sampling circuit during the conduction period of the first switching element.

9. The electrical system of claim 8, wherein: The sampling circuit comprises: An operational amplifier, wherein a non-inverting input terminal of the operational amplifier is connected to one end of the secondary winding, and an inverting input terminal is connected to an output terminal via a reverse biased first diode.

10. The electrical system of claim 9, wherein: The sampling circuit further includes a rectifier circuit including a forward-biased second diode and a capacitor connected in series between an output terminal of the operational amplifier and ground.

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