A compensation circuit for reducing leakage current of variable frequency compressor and control method thereof

Abstract

The present invention belongs to the technical field of compressors, and provides a compensation circuit for reducing leakage current of a variable frequency compressor, characterized in that one end of the compensation capacitor of the compensation circuit is grounded, and the other end is connected to AGND and a bus respectively through a switch element, and the switch element is opened and disconnected using software control; the high-frequency common-mode voltage change rate dv / dt in the PWM variable frequency motor drive system is calculated, and when dv / dt is a positive value, the compensation capacitor of the compensation circuit is connected to AGND and disconnected from the bus, and the compensation capacitor absorbs the leakage current of the compressor; when dv / dt is a negative value, the compensation capacitor of the compensation circuit is connected to the bus and disconnected from AGND, generating an external compensation current, and when dv / dt is 0, the compensation capacitor of the compensation circuit is disconnected from AGND and the bus at the same time. The present invention can effectively reduce the leakage current and improve the electrical safety and reliability of the equipment by accurately controlling and compensating the leakage current, without complex components, and saving costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of compressors, and in particular relates to a compensation circuit for reducing leakage current of a variable frequency compressor and a control method thereof. Background Art

[0002] Since its introduction, variable frequency air conditioners have been widely recognized by the market for their high energy efficiency and power saving performance, and are also the mainstream products of major air conditioner manufacturers. However, the use of variable frequency DC compressor PWM control technology and high-speed switching devices introduces more high-frequency common-mode interference into the compressor stator winding, which then leaks to the ground through the coupling capacitor between the stator and the casing, causing the compressor leakage current to increase. The leakage current of variable frequency compressors is generally larger than that of fixed frequency compressors. All countries have corresponding standards for the electrical safety of household appliances, and also have limit requirements for leakage current.

[0003] In the process of developing variable frequency products, three solutions are usually adopted to improve the leakage current exceeding the standard:

[0004] 1) Change the structure of the compressor DC motor and increase the insulation strength between the motor stator and the motor housing;

[0005] 2) Add a passive or active low-pass filter to the three-phase output end of the IPM drive module of the variable frequency controller and then connect the compressor load;

[0006] 3) Use an external leakage current compensation circuit to regularly absorb or compensate for compressor leakage to achieve the purpose of reduction.

[0007] The first solution is the most commonly used solution, which blocks the path of leakage current by strengthening insulation. Related papers and patents have also conducted in-depth research on the second solution, and proposed various effective filter solutions. The third solution of passively reducing leakage current has been less studied in the industry. CN107078657A proposed a leakage current compensation circuit, which uses a high-precision common-mode leakage current transformer to sense the current of the neutral line and the live line, and judges the direction of the leakage current by the difference, triggers the compensation circuit to compensate for the compressor leakage, and obtains the rotor position of the compressor to control the on-off of the compensation circuit to prevent over-compensation. However, this solution has certain disadvantages: high-precision transformers are required and the cost is also high; the on-off of the compensation circuit is based on experimental experience, the compensation is not accurate, and it is difficult to achieve the best effect.

[0008] The present invention proposes a compensation scheme based on a software-controlled compensation circuit, which does not require a mutual inductor as a control trigger and provides more accurate compensation. Summary of the invention

[0009] In view of the above technical problems in the prior art, the purpose of the present invention is to provide a compensation circuit for reducing leakage current of a variable frequency compressor. The compensation circuit adopts software control, proposes a new solution for reducing leakage current of the variable frequency compressor, and reduces compensation costs.

[0010] Another object of the present invention is to provide a control method for the compensation circuit of the variable frequency compressor to reduce the leakage current, and according to the regularity of the leakage current of the variable frequency compressor, a software-controlled leakage current compensation scheme is proposed to accurately compensate the leakage current according to the regularity.

[0011] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical scheme.

[0012] The present invention provides a compensation circuit for reducing leakage current of a variable frequency compressor. One end of a compensation capacitor of the compensation circuit is grounded, and the other end is connected to AGND and a bus respectively through a switch element, and the switch element is opened and disconnected by software control; the high-frequency common-mode voltage change rate dv / dt in a PWM variable frequency motor drive system is calculated, when dv / dt is a positive value, the compensation capacitor of the compensation circuit is connected to AGND and disconnected from the bus, and the compensation capacitor absorbs the leakage current of the compressor; when dv / dt is a negative value, the compensation capacitor of the compensation circuit is connected to the bus and disconnected from AGND, generating an external compensation current, and when dv / dt is 0, the compensation capacitor of the compensation circuit is disconnected from AGND and the bus at the same time.

[0013] The software is implemented using a microcontroller, and the microcontroller is connected to the switch element through a network, that is, the switch element is controlled to be opened and disconnected using the microcontroller, and the microcontroller connects two switch elements.

[0014] Usually, the compensation capacitor in the compensation circuit is large, while the coupling capacitor parameters in the motor are very small. It is impossible for the two to be completely consistent, and it is easy for the compensation current to be too large, resulting in increased leakage. It is necessary to select a suitable compensation circuit current limiting resistor Z1 according to the size of the compressor leakage current to limit the compensation current amplitude. Therefore, it is necessary to connect a current limiting resistor to the ground end of the compensation capacitor. The size of the current limiting resistor is determined according to the actual leakage current detected to ensure that the compensation current generated by the compensation circuit offsets the actual leakage current.

[0015] Because the leakage and compensation current amplitudes are very small, the compensation circuit is not limited to using strong electricity as the compensation current generating power source. When using strong current PFC-DC as the compensation current generating power source in the compensation circuit, the switching element uses an IGBT switch tube; when using weak current such as ±15V as the compensation current generating power source of the compensation circuit, the switching element can use a triode. Regardless of whether strong or weak electricity is used as the compensation current generating power source, the control scheme of the compensation circuit is consistent with the following control method.

[0016] The switch element in the present invention uses software to control opening and disconnection according to the regularity of the leakage current of the variable frequency compressor.

[0017] In modern PWM variable frequency power supply systems, two-level, voltage source, AC-DC-AC inverters are usually used. The AC power on the grid side is converted into DC power through a rectifier. It is converted into stable DC power through an intermediate DC link filter, and then converted into AC power with the frequency and amplitude required by the load through an inverter. On the rectifier side, diode uncontrolled rectification is used. On the inverter side, a space vector pulse width modulation signal is used to control the conduction of the switch tube, and then a series of equal amplitude and unequal width voltage signals are output on the inverter output side. With the development of power electronics technology, IGBT is usually used as the switch tube on the inverter side, and the rise time of the switch tube is generally only a few hundred ns. Therefore, the voltage on the output side of the inverter will produce a large dv / dt, which will have a particularly strong impact on the inverter load.

[0018] For the variable frequency compressor circuit, Kirchhoff's voltage theorem can be used to obtain the voltage between the three-phase power supply terminal UVW and the ground:

[0019]

[0020] Adding the three formulas gives:

[0021]

[0022] Among them, U ug , U vg , U wg are the voltages of the inverter output terminal to ground respectively; i a 、i b 、i c are the instantaneous values ​​of the current flowing through the motor winding at the inverter output end; R and L are the equivalent inductances of the motor winding; U Ng It is the voltage between the neutral point of the motor stator winding and the ground, that is, the common mode voltage.

[0023] For a three-phase induction motor, the current flowing through the stator winding at any time is i a 、i b 、i c Since the stator winding is completely symmetrical, at any time: i a +i b +i c =0, so at the inverter output, we can get:

[0024]

[0025] It can be seen that when the motor uses the traditional three-phase symmetrical AC power supply, the neutral point voltage of the stator winding is zero. When the two-level space vector pulse width modulation method is used, the switch tube combination of the upper and lower bridge arms of the inverter IPM is turned on and off, so that the output side of the inverter outputs a series of pulse voltages of equal amplitude and unequal width to be equivalent to AC. At any time, the sum of their amplitudes is not zero, and the upper and lower tubes of the same bridge arm cannot be turned on at the same time. According to its SVPWM modulation principle, there are 8 switch combination states (taking G2, G4, and G6 as an example, turn on 1 and turn off 0), and the corresponding U com The voltage is approximately:

[0026]

[0027] This part of the voltage is the high-frequency common-mode voltage in the system. It does not participate in energy transfer, but has a negative impact on the entire electrical system. Figure 2 The combined capacitance C between the stator winding and the housing is shown in wf , the combined capacitance C between the stator winding and the rotor wr , the coupling capacitance C between the rotor and the casing rf The high-frequency common-mode voltage change rate dv / dt passes through these capacitors, forming a leakage current to the ground.

[0028] By building a drag system model to simulate the motor system, Figure 2 The motor coupling capacitor parameter model is established as shown in Figure 1 As shown in B, the simulation results are as follows Figure 3 As shown, I leakage is the compressor leakage current, U com is the common-mode voltage of the stator winding. It can be concluded that when the common-mode voltage dv / dt is positive, that is, on the rising edge, the compressor coil will have a positive leakage current to the ground. When dv / dt is negative, that is, on the falling edge, the compressor coil will have a negative leakage current to the ground, which has a certain phase regularity.

[0029] Therefore, the compensation circuit can output a compensation current with a phase opposite to the leakage current to offset the overall external leakage current. One end of the compensation capacitor of the compensation circuit of the present invention is grounded, and the other end is connected to AGND and the busbar through a switch element respectively. When dv / dt is a positive value, the compensation capacitor of the compensation circuit is connected to AGND, and the compensation capacitor absorbs the leakage current of the compressor; when dv / dt is a negative value, the compensation capacitor of the compensation circuit is connected to the busbar to generate an external compensation current.

[0030] The present invention also provides a control method for a compensation circuit for reducing leakage current of the above variable frequency compressor, the method comprising the following steps:

[0031] 1) Continuously collect the voltage of the UVW output relative to AGND of the i+1 group of inverter IPMs, where i=0,1,2,…;

[0032] 2) Calculate the common mode voltage U com , determine the current switch state according to the value;

[0033] 3) If U comi comi+1 , then open the switch element connected to AGND and disconnect the switch element connected to the bus; if U comi >U comi+1 , then the switch element connected to the bus is turned on and the switch element connected to AGND is turned off; if there is no level change, the switch element connected to AGND and the bus is kept disconnected, where U comi , U comi+1 The common-mode voltage calculated for the i-th group or the i+1-th group.

[0034] The calculation formula of the common mode voltage is:

[0035]

[0036] Among them, U ug , U vg , U wg are the voltage of the inverter output terminal to ground, U Ng It is the voltage between the neutral point of the motor stator winding and the ground, that is, the common mode voltage.

[0037] In a further technical solution, in order to ensure the stability of control, a checking step is added after step 2), that is, the common mode voltage U calculated in step 2) is determined. com Is it close to 0, Udc / 3, 2Udc / 3, Udc within the error range? If so, set U com Equal to the theoretical value of the corresponding switch state, stored in the temporary register to obtain U comi , U comi+1 Otherwise, repeat steps 1) and 2). The theoretical value of the corresponding switch state is 0, Udc / 3, 2Udc / 3, Udc and the calculated common mode voltage U com The closest value.

[0038] In the above method, if U comi comi+1 , then dv / dt>0; if U comi >U comi+1 , then dv / dt<0; the level does not change, that is, U comi =U comi+1 ​​and dv / dt=0. According to the rule obtained from the above simulation, when the common mode voltage dv / dt is positive, that is, at the rising edge, the compressor coil will have a positive leakage current to the ground, and when dv / dt is negative, that is, at the falling edge, the compressor coil will have a negative leakage current to the ground. Then, if U comi comi+1 , that is, the rising edge, the compressor coil will have a positive leakage current to the ground. At this time, open the switch element connected to AGND, disconnect the switch element connected to the bus, and the compensation circuit will absorb the leakage current. comi >U comi+1 , that is, when it is the falling edge, the compressor coil will have a negative leakage current to the ground. At this time, opening the switch element connected to the bus and disconnecting the switch element connected to AGND will generate an external compensation current. comi =U comi+1 , that is, no level change occurs, the compressor does not need to absorb or compensate current, and only needs to keep the switching element disconnected from AGND and the bus.

[0039] The present invention also provides another control method for a compensation circuit for reducing leakage current of the above-mentioned variable frequency compressor, the method comprising the following steps:

[0040] 1) Obtain the driving signal levels of the three switch tubes of the inverter controlled by the SVPWM algorithm, with 1 representing a high level and 0 representing a low level;

[0041] 2) Add the three switch states together;

[0042] 3) Check the table based on the previous state and current state to confirm the current state and predict the next state;

[0043] 4) Output the compensation circuit switch element control signal when the next set of inverter switch signals is given. If the sum of the three switch states of the next state is greater than the sum of the three switch states of the current state, control the switch element connected to AGND to open and disconnect the switch element connected to the bus; if the sum of the three switch states of the next state is less than the sum of the three switch states of the current state, control the switch element connected to the bus to open and disconnect the switch element connected to AGND; if the sum of the three switch states of the next state is the same as the sum of the three switch states of the current state, disconnect the switch element between AGND and the bus.

[0044] ​According to the SVPWM seven-segment drive signal rule, usually, no matter which of the six sectors it is in, the sum of the conduction states of the drive signal (taking G2, G4, G6 as an example, turn on 1, turn off 0) will always change according to the rule of (0, 1, 2, 3, 3, 2, 1, 0, 0, 1...). By obtaining the switch state at the previous moment and the switch state at this moment, it is possible to predict the next state change U com In case of rising or falling edge, the compensation circuit switch tube driving signal is output on time at the next state change to perform compensation.

[0045] From the above content, we can know that according to the SVPWM modulation principle, there are 8 switch combination states (taking G2, G4, G6 as an example, turn on 1 and turn off 0), and the corresponding U com The voltage is approximately:

[0046]

[0047] It can be seen that when the sum of the switch states gradually increases, U com The voltage also gradually increases, that is, dv / dt is positive; when the sum of the switching states gradually decreases, U com The voltage also gradually decreases, that is, dv / dt is negative; when the sum of the switch states remains unchanged, U com The voltage does not change, that is, dv / dt is 0. Then, when the next set of inverter switching signals is given, the compensation circuit switch element control signal is output. If the sum of the three switch states of the next state is greater than the sum of the three switch states of the current state, the switch element connected to AGND is controlled to be opened, and the switch element connected to the bus is disconnected; if the sum of the three switch states of the next state is less than the sum of the three switch states of the current state, the switch element connected to the bus is controlled to be opened, and the switch element connected to AGND is disconnected; if the sum of the three switch states of the next state is the same as the sum of the three switch states of the current state, the switch element between AGND and the bus is disconnected.

[0048] The present invention has the following beneficial effects: 1. By accurately controlling and compensating the leakage current, the leakage current can be effectively reduced, and the electrical safety and reliability of the equipment can be improved; 2. There are no complicated components, which saves costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 A schematic diagram of a PWM variable frequency power supply system and a leakage current compensation circuit in an embodiment of the present invention;

[0050] Figure 2 A schematic diagram of a coupling capacitor of a variable frequency compressor in an embodiment of the present invention;

[0051] Figure 3A schematic diagram of the phase law of leakage current in an embodiment of the present invention;

[0052] Figure 4 This is a flow chart of the control scheme in Example 1 of the present invention;

[0053] Figure 5 This is a flow chart of the control scheme in Example 2 of the present invention;

[0054] Among them, 1 is the stator, 2 is the permanent magnet, 3 is the rotor, and 4 is the housing. DETAILED DESCRIPTION

[0055] The technical solution of the present invention is described below in conjunction with specific embodiments. However, the following embodiments are only a few of the technical solutions of the present invention, not all of them, and the following embodiments should not be understood as limiting the technical solutions of the present invention.

[0056] Example 1

[0057] refer to Figure 1 , this embodiment is a commonly used PWM modulated variable frequency motor drive system A (PFC takes Boost as an example, but not limited to) to set a compensation circuit. In the drive system A, first, the AC power on the grid side is converted into DC power through a rectifier; then, it is converted into stable DC power through an intermediate DC link filter, and then the DC power is converted into AC power with the frequency and amplitude required by the load through an inverter. On the rectifier side, diode uncontrolled rectification is used. On the inverter side, a space vector pulse width modulation signal is used to control the conduction of the switch tube, and then a series of equal-amplitude and unequal-width voltage signals are output on the output side of the inverter.

[0058] This embodiment is designed Figure 1 The compensation circuit shown in C has one end of the compensation capacitor of the compensation circuit grounded, and the other end connected to AGND through a switch tube G9, and connected to the bus through a switch tube G8, and the switch element is opened and disconnected using software control; the high-frequency common-mode voltage change rate dv / dt in the PWM variable frequency motor drive system is calculated, when dv / dt is a positive value, the compensation capacitor of the compensation circuit is connected to AGND and disconnected from the bus, and the compensation capacitor absorbs the leakage current of the compressor; when dv / dt is a negative value, the compensation capacitor of the compensation circuit is connected to the bus and disconnected from AGND, generating an external compensation current, and when dv / dt is 0, the compensation capacitor of the compensation circuit is disconnected from AGND and the bus at the same time.

[0059] The software is implemented using a microcontroller, and the microcontroller is connected to the switch element through a network, that is, the switch element is controlled to open and disconnect by the microcontroller. The microcontroller connects two switch elements, such as the microcontroller directly sends an electrical signal after calculation, which is electrically transmitted to the switch element to control its opening and disconnection, or the microcontroller sends a radio signal (it can also be an infrared, Bluetooth, wireless network, etc. signal) after calculation, and the signal receiving circuit connected to the switch element receives the signal and converts it into a driving signal to drive the switch element to open and disconnect.

[0060] The microcontroller may be a control mainboard of an electrical appliance (such as an air conditioner, a refrigerator, etc.) using the variable frequency compressor.

[0061] In the actual compensation circuit, the compensation capacitor C4 is relatively large, while the coupling capacitor parameters in the motor are very small. It is impossible for the two to be completely consistent. It is easy for the compensation current to be too large, resulting in increased leakage. It is necessary to select a suitable compensation circuit current limiting resistor Z1 according to the size of the compressor leakage current to limit the compensation current amplitude.

[0062] Because the leakage and compensation current amplitudes are very small, the compensation circuit is not limited to using strong electricity as the compensation current generating power source. When using strong current PFC-DC as the compensation current generating power source in the compensation circuit, the switching element uses an IGBT switch tube; when using weak current such as ±15V as the compensation current generating power source of the compensation circuit, the switching element can use a triode. Regardless of whether strong or weak electricity is used as the compensation current generating power source, the control scheme of the compensation circuit is consistent with the following control method.

[0063] The control flow of the compensation circuit of this embodiment by the software is as follows: Figure 4 As shown, the specific process is as follows:

[0064] 1) The software chip detects and samples the voltage U of the inverter IPM output UVW relative to AGND ug , U vg , U wg , continuous sampling i+1 groups, where i=0,1,2,…;

[0065] 2) Calculate the common mode voltage U com , determine the current switch state according to the value, where the calculation formula for the common-mode voltage is:

[0066]

[0067] U Ng It is the voltage between the neutral point of the motor stator winding and the ground, that is, the common mode voltage;

[0068] 3) Determine the common mode voltage U calculated in step 2) comIs it close to 0, Udc / 3, 2Udc / 3, Udc within the error range? If so, set U com Equal to the theoretical value of the corresponding switch state, stored in the temporary register to obtain U comi , U comi+1 , otherwise, repeat steps 1) and 2);

[0069] The theoretical value of the corresponding switch state is 0, Udc / 3, 2Udc / 3, Udc and the calculated common mode voltage U com The closest value;

[0070] 4) Perform rising or falling edge detection: If U comi comi+1 , then the G9 switch is turned on and the G8 switch is turned off; if U comi >U comi+1 , then the G8 switch tube is turned on and the G9 switch tube is turned off; if there is no level change, the G9 and G8 switch tubes remain turned off;

[0071] 5) Clear the temporary register.

[0072] Example 2

[0073] This embodiment is exactly the same as the PWM modulated variable frequency motor drive system A and the compensation circuit C of the embodiment 1, but the software control method is different.

[0074] According to the SVPWM seven-segment drive signal rule, usually, no matter which of the six sectors it is in, the sum of the conduction states of the drive signal (taking G2, G4, G6 as an example, turn on 1, turn off 0) will always change according to the rule of (0, 1, 2, 3, 3, 2, 1, 0, 0, 1...). By obtaining the switch state at the previous moment and the switch state at this moment, it is possible to predict the next state change U com In the case of the rising edge or falling edge of the compensation circuit, the compensation circuit switch tube driving signal is output on time at the next state change to perform compensation. The control flow of the compensation circuit of this embodiment by the software is as follows: Figure 5 As shown, the specific process is as follows:

[0075] 1) Obtain the driving signal level of the inverter switch tubes G2, G4, and G6 controlled by the SVPWM algorithm, with 1 representing a high level and 0 representing a low level;

[0076] 2) Add the three switch states together;

[0077] 3) Check the table based on the previous state and the current state to confirm the current state position and predict whether the next state is a rising edge or a falling edge;

[0078] ​4) When the next set of inverter switching signals is given, the compensation circuit switch element control signal is output. If the sum of the three switch states of the next state is greater than the sum of the three switch states of the current state, that is, it is a rising edge, then the G9 switch tube is turned on and the G8 switch tube is turned off; if the sum of the three switch states of the next state is less than the sum of the three switch states of the current state, that is, it is a falling edge, then the G8 switch tube is turned on and the G9 switch tube is turned off; if the sum of the three switch states of the next state is the same as the sum of the three switch states of the current state, then the switch elements of AGND and the bus are disconnected, that is, no compensation is performed.

Claims

1. A compensation circuit for reducing leakage current of a variable frequency compressor, characterized in that: One end of the compensation capacitor of the compensation circuit is grounded, and the other end is connected to AGND and the bus through a switch element respectively, and the switch element is opened and disconnected using software control; calculate the high-frequency common-mode voltage change rate in the PWM variable frequency motor drive system d v / d t, when d v / d When t is positive, the compensation capacitor of the compensation circuit is connected to AGND and disconnected from the busbar, and the compensation capacitor absorbs the compressor leakage current; when d v / dt When it is negative, the compensation capacitor of the compensation circuit is connected to the bus and disconnected from AGND, generating an external compensation current. d v / d When t is 0, the compensation capacitor of the compensation circuit is disconnected from AGND and the bus at the same time; the calculation of the high-frequency common-mode voltage change rate in the PWM variable-frequency motor drive system d v / d Follow these steps: 1) Obtain the driving signal levels of the three switch tubes of the inverter controlled by the SVPWM algorithm, with 1 representing a high level and 0 representing a low level; 2) Add the three switch states together; 3) Check the table based on the previous state and current state to confirm the current state, and predict the next state and the next state d v / d t; if the change rule of the sum of the inverter switch states is: 0, 1, 2, 3, 3, 2, 1, 0, 0, 1…, then if the sum of the three switch states of the next state is greater than the sum of the three switch states of the current state, then d v / d t is a positive value; if the sum of the three switch states of the next state is less than the sum of the three switch states of the current state, then d v / dt is a negative value; if the sum of the three switch states of the next state is the same as the sum of the three switch states of the current state, then d v / d t is 0.

2. The compensation circuit according to claim 1, characterized in that: The ground terminal of the compensation capacitor is connected to a current limiting resistor.

3. The compensation circuit according to claim 2, characterized in that: The size of the current limiting resistor is determined according to the size of the leakage current actually detected.

4. The compensation circuit according to claim 1, characterized in that: When a strong current PFC-DC is used as the power source for generating the compensation current in the compensation circuit, an IGBT switch tube is used as the switching element; when a weak current is used as the power source for generating the compensation current in the compensation circuit, a triode is used as the switching element.

5. A control method for a compensation circuit for reducing leakage current of a variable frequency compressor according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: 1) Obtain the driving signal levels of the three switch tubes of the inverter controlled by the SVPWM algorithm, with 1 representing a high level and 0 representing a low level; 2) Add the three switch states together; 3) Check the table based on the previous state and current state to confirm the current state, and predict the next state and the next state d v / d The size of t; 4) Output the compensation circuit switch element control signal when the next set of inverter switching signals is given. d v / d When t is positive, the switch element connected to AGND is controlled to open and the switch element connected to the bus is disconnected; when d v / dt When it is negative, the switch element connected to the bus is controlled to open and the switch element connected to AGND is disconnected. d v / d When t is 0, the switch element between AGND and the bus is disconnected.

6. The control method according to claim 5, characterized in that: If the change rule of the sum of the switch states of the inverter is: 0, 1, 2, 3, 3, 2, 1, 0, 0, 1..., then in step 4), if the sum of the three switch states of the next state is greater than the sum of the three switch states of the current state, the switch element connected to AGND is controlled to be opened, and the switch element connected to the bus is disconnected; if the sum of the three switch states of the next state is less than the sum of the three switch states of the current state, the switch element connected to the bus is controlled to be opened, and the switch element connected to AGND is disconnected; if the sum of the three switch states of the next state is the same as the sum of the three switch states of the current state, the switch element between AGND and the bus is disconnected.

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