A control method of a boost-buck common ground inverter

Abstract

The application discloses a control method of a boost-buck common ground inverter, and belongs to the technical field of inverters. The method comprises the following steps: a control driving unit is used to monitor the output voltage of the inverter in real time, and the output voltage and the size of a reference signal are compared and subtracted to obtain a difference signal of the output voltage and the reference signal voltage, and the difference signal is subjected to voltage processing to obtain a control signal; the control signal and a carrier signal are subjected to size determination, and the on-off of a first switch, a second switch and a third switch is controlled according to the determination result, and then each freewheeling loop or closed loop is turned on to complete the inversion of an input power supply. The application controls the on-off of each control switch through the control driving unit to complete the inversion, solves the problem of low conversion efficiency of a traditional inverter boost-buck conversion, eliminates the common-mode leakage current phenomenon, realizes the boost-buck conversion, and improves the conversion efficiency of the inverter system.

Description

Technical Field

[0001] This invention relates to the field of inverter technology, and in particular to a control method for a buck-boost common-ground inverter. Background Technology

[0002] Non-isolated inverter systems are widely used due to their advantages of simple structure, low cost, low energy loss, and high overall efficiency. However, because of the lack of effective isolation, the common-mode leakage current generated by the photovoltaic panel's capacitance to ground can cause conducted noise and harmonic current interference, even threatening personal safety. Domestic and international experts and scholars have conducted a series of fruitful studies on how to suppress the common-mode leakage current of non-isolated inverters; commonly used methods for suppressing common-mode leakage current include: improving modulation techniques, adding switching devices, adding filters, and improving control methods. However, these methods are susceptible to the effects of parasitic capacitance to ground from the input power supply and changes in circuit parameters.

[0003] Furthermore, photovoltaic modules typically have low output voltages, requiring non-isolated inverters to perform buck-boost conversion to meet user needs within a given voltage range. Traditional methods employ a two-stage conversion approach, cascading a boost converter and an inverter, which reduces system efficiency and increases switching stress.

[0004] Therefore, it is necessary to study inverter topologies and control methods that can fundamentally eliminate common-mode leakage current and achieve high-efficiency buck-boost conversion. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a control method for a buck-boost common-ground inverter, aiming to solve the problem that traditional inverters cannot achieve buck-boost conversion, eliminate common-mode leakage, and improve the inverter's conversion efficiency.

[0006] To achieve the above objectives, the present invention provides a control method for a buck-boost common-ground inverter, comprising a buck-boost common-ground inverter, wherein the buck-boost common-ground inverter includes an input power supply, a filter unit, a control drive unit, diodes, a first switch, a second switch, a third switch, and a load.

[0007] The filtering unit includes a first inductor, a second inductor, a third inductor, a first capacitor, a second capacitor, and a third capacitor. The first terminal of the first inductor is connected to the positive terminal of the input power supply, and its second terminal is connected to the negative terminal of the input power supply via the first switch. The first terminal of the second inductor is connected to the first terminal of the first capacitor via the cathode of the diode, and its second terminal is connected to the second terminal of the second capacitor. The first terminal of the third inductor is connected to the first terminal of the first capacitor via the second switch, and its second terminal is connected to the negative terminal of the input power supply, the second terminal of the first capacitor, the second terminal of the third capacitor, and the second terminal of the load. The first terminal of the second capacitor is connected to the first terminal of the third inductor, and its second terminal is connected to the first terminal of the third capacitor and the first terminal of the load via the third switch. The anode of the diode is connected to the second terminal of the first inductor.

[0008] The control method includes the following steps:

[0009] (1) The control drive unit monitors the output voltage of the buck-boost common ground inverter in real time, and compares the magnitude of the output voltage and the reference signal to obtain the difference signal between the output voltage and the reference signal voltage;

[0010] (2) The difference signal is processed by voltage to obtain a control signal; the control signal is compared with the carrier signal to determine whether the control signal is greater than or less than the carrier signal;

[0011] (3) When the control signal is greater than the carrier signal, the control drive unit controls the second switch to be turned on, and the first switch and the third switch are turned off, so as to turn on the first freewheeling circuit, the second freewheeling circuit and the third freewheeling circuit of the filter unit to complete the inversion of the input power supply;

[0012] (4) When the control signal is less than the carrier signal, the control drive unit adjusts the first switch and the third switch to be turned on, and the second switch to be turned off, so as to turn on the first closed loop, the second closed loop and the third closed loop of the filter unit to complete the inversion of the input power.

[0013] Furthermore, the input power supply is connected in series with the first switch and the first inductor to form the first closed circuit;

[0014] The input power supply is connected in series with the first inductor, the diode and the first capacitor to form the first freewheeling circuit;

[0015] The second inductor is connected in series with the third switch, the third capacitor and the load in parallel branch, and the first capacitor to form the second closed loop;

[0016] The second inductor is connected in series with the second capacitor and the second switch to form the second freewheeling circuit;

[0017] The third inductor is connected in series with the second capacitor, the third switch, and the parallel branch of the third capacitor and the load to form the third closed loop;

[0018] The third inductor is connected in series with the first capacitor and the second switch to form the third freewheeling circuit.

[0019] Furthermore, the negative terminal of the input power supply is connected to the negative terminal of the load, and the negative terminal of the input power supply and the negative terminal of the load are both grounded.

[0020] Furthermore, the control drive unit has its input terminal connected to the load and its output terminal connected to the first switch, the second switch, and the third switch, respectively, to drive the opening and closing of each switch to connect each circuit and thus complete the DC inverter process.

[0021] Furthermore, the control drive unit includes:

[0022] The sensor system, with its input terminal connected to the load, acquires the output voltage feedback signal of the buck-boost common-ground inverter in real time.

[0023] The DSP, whose input is connected to the output of the sensor system, performs voltage signal processing on the output voltage feedback signal to generate a first switching logic signal and a second switching logic signal.

[0024] The driving circuit has its input terminals connected to the first and second output terminals of the DSP, and its output terminals connected to the first switch, the second switch, and the third switch, respectively. It generates a first driving signal, a second driving signal, and a third driving signal based on the first switch logic signal and the second switch logic signal to drive the opening and closing of each switch accordingly.

[0025] Furthermore, the sensor system includes: an input voltage sensor, whose input terminal is connected to the load and whose output terminal is connected to the input terminal of the DSP, for acquiring the output voltage feedback signal and transmitting it to the DSP.

[0026] Furthermore, the DSP includes:

[0027] The module that generates the reference signal internally produces a constant frequency and voltage sine wave signal;

[0028] The module that generates carrier signals internally produces a fixed-frequency, fixed-voltage triangular wave signal.

[0029] The first analog-to-digital conversion module has its input terminal connected to the output terminal of the sensor system, and performs a first analog-to-digital conversion on the output voltage feedback signal to obtain a first digital signal.

[0030] First comparator: The first input terminal is connected to the output terminal of the module that generates the reference signal, and the second input terminal is connected to the output terminal of the first analog-to-digital converter module. It calculates the difference between the reference signal and the first digital signal to obtain the difference signal.

[0031] Voltage regulator: Its input terminal is connected to the output terminal of the first comparator, and it adjusts the difference signal to obtain a control signal;

[0032] Second comparator: The first input terminal is connected to the output terminal of the voltage regulator, and the second input terminal is connected to the output terminal of the module that generates the carrier signal. It compares the control signal with the carrier signal to obtain the first switch selection signal.

[0033] PWM1: The input terminal is connected to the first output terminal of the second comparator, and outputs the first switch selection signal to obtain the first switch logic signal;

[0034] Inverter: Its input terminal is connected to the second output terminal of the second comparator to obtain the second switch selection signal;

[0035] PWM2: The input terminal is connected to the output terminal of the inverter, and outputs the second switch selection signal to obtain the second switch logic signal.

[0036] Furthermore, the driving circuit includes a first driving circuit, a second driving circuit, and a third driving circuit; the first driving circuit has its input terminal connected to the first output terminal of the DSP and its output terminal connected to the first switch; the second driving circuit has its input terminal connected to the second output terminal of the DSP and its output terminal connected to the second switch; the third driving circuit has its input terminal connected to the first output terminal of the DSP and its output terminal connected to the third switch.

[0037] The beneficial effects of this invention are:

[0038] 1. The inverter provided by this invention is a single-stage converter, which improves the conversion efficiency of the inverter system.

[0039] 2. The inverter provided by this invention can achieve buck-boost mode switching without mode switching.

[0040] 3. The inverter provided by this invention grounds both the input power supply and the output voltage, which can effectively eliminate the common-mode leakage current of the inverter. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the inverter circuit provided in an embodiment of the present invention.

[0042] Figure 2 This is a circuit diagram of the control drive unit provided in an embodiment of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this application clearer, the patent will be further described below in conjunction with the accompanying drawings. The described embodiments should not be regarded as limitations on the invention. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0044] In the following description, references to "some embodiments" or "one or more embodiments" describe a subset of all possible embodiments. However, it is understood that "some embodiments" or "one or more embodiments" may be the same subset or different subsets of all possible embodiments and may be combined with each other without conflict.

[0045] In the following description, the terms "first, second, third" are used only to distinguish similar objects and do not represent a specific ordering of objects. It is understood that "first, second, third" may be interchanged in a specific order or sequence where permitted, so that the embodiments of the invention described herein can be implemented in an order other than that shown in the illustrations or description.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein is for the purpose of describing embodiments of the invention only and is not intended to limit the invention.

[0047] like Figure 1 The diagram shown illustrates the structure of this step-up / step-down inverter, including: input power supply. U in First Inductor L 1. Second inductor L 2. Third inductor L 3. First switch S1, second switch S2, third switch S3, diode D, first capacitor C 1. Second capacitor C 2. Third capacitor C 3 and control drive unit (attached) Figure 2 (As described in the text).

[0048] Input power U in Its negative terminal is related to the output voltage. u o The negative terminal is connected, and the input power is...U in negative terminal and output voltage u o The negative terminals are grounded together. The input power supply... U in Used to provide electrical energy to the inverter; and input power supply U in With output voltage u o A common ground effectively eliminates the common-mode leakage current of the inverter. In this embodiment, the input power supply... U in It can be used for photovoltaic cells, automotive batteries, fuel cells, etc.

[0049] The filter unit includes a first inductor. L 1. Second inductor L 2. Third inductor L 3. First capacitor C 1. Second capacitor C 2. Third capacitor C 3; First inductor L The first terminal of 1 is connected to the input power supply. U in The positive terminal is connected, and the second terminal is connected to the input power supply through a first switch S1. U in The negative terminal is connected to the second inductor. L The first terminal of 2 is connected to the cathode of diode D and the first capacitor. C The first terminal of 1 is connected, and the second terminal is connected to the second capacitor. C 2. The second end is connected; the third inductor. L The first terminal of 3 is connected to the first capacitor via a second switch S2. C The first end of 1 is connected, and the second end is connected to the input power supply. U in negative terminal, first capacitor C The second and third terminals of capacitor 1 C 3's second end and load Z L The second terminal is connected to the second capacitor. C The first terminal of 2 and the third inductor L The first end of 3 is connected, and the second end is connected to the third capacitor via a third switch S3. C 3 First end and load Z L The first terminal is connected, and the anode of diode D is connected to the first inductor. L The second end of 1 is connected.

[0050] Then the input power supply U in Sequentially connected to the first switch S1 and the first inductorL 1. Connected in series to form the first closed loop;

[0051] The input power supply U in Sequentially connected to the first inductor L 1. The diode D and the first capacitor C 1. Connected in series to form the first freewheeling circuit;

[0052] Then the second inductor L 2. Sequentially connected to the second capacitor C 2 and the second switch S2 are connected in series to form a second freewheeling circuit;

[0053] Second inductor L 2. Sequentially connected to the third switch S3 and the third capacitor C 3 and load Z L Parallel branch, first capacitor C 1. Connected in series to form a second closed loop;

[0054] Then the third inductor L 3. Sequentially connected to the second capacitor C 2. The third switch S3 and the third capacitor C 3 and load Z L The parallel branches are connected in series to form a third closed loop;

[0055] Then the third inductor L 3 sequentially with the first capacitor C Switches 1 and 2 are connected in series to form a third freewheeling circuit.

[0056] The control drive unit has its input terminals connected to the first and second output terminals of the DSP, and its output terminals connected to the first switch S1, the second switch S2, and the third switch S3, respectively. It is used to drive and control the opening and closing of each switch to connect each closed circuit, thereby completing the inversion process of the input power.

[0057] In this embodiment, the capacitor C 1. Capacitor C 2. Capacitor C 3 is a non-polarized capacitor.

[0058] The second switch S2 and the third switch S3 are both metal-oxide-semiconductor field-effect transistors (MOS) and / or insulated-gate bipolar transistors (IGBTs), and the first switch S1 is a MOS transistor.

[0059] like Figure 2As shown, the control drive unit also includes: a sensor system 1, a digital signal processor (DSP) 2, and a drive circuit 3.

[0060] Sensor system 1, with its input terminal connected to the load Z L Connect and collect load. Z L Real-time output voltage feedback signal u of ;

[0061] DSP 2, whose input terminal is connected to the output terminal of the sensor system 1, provides feedback signal for the output voltage. u of The voltage signal is processed in three stages: a first voltage signal processing stage, a second voltage signal processing stage, and a third voltage signal processing stage, and the door opening / closing signals are generated accordingly.

[0062] The input terminal of the driving circuit 3 is connected to the output terminal of DSP2, and the output terminal is connected to the first switch S1, the second switch S2 and the third switch S3 respectively. Based on the first switch logic signal O1 and the second switch logic signal O2 respectively, the first driving signal and the second driving signal are generated to drive the opening and closing of the switches S1-S3 respectively.

[0063] Sensor system 1 includes:

[0064] Output voltage sensor 101, input terminal connected to load Z L The output terminal is connected to the input terminal of DSP 2 for acquiring the output voltage feedback signal. u of And transmit it to the DSP 2;

[0065] Digital Signal Processor (DSP) 2 includes:

[0066] The module that generates the reference signal internally produces a constant frequency and voltage sine wave signal;

[0067] The module that generates carrier signals internally produces a fixed-frequency, fixed-voltage triangular wave signal.

[0068] The first analog-to-digital converter module AD1 has its input terminal connected to the output terminal of the output voltage sensor 101, and provides feedback signal to the output voltage sensor. u of Perform the first analog-to-digital conversion to obtain the first digital signal. u of ;

[0069] First comparator 201: Its first input terminal is connected to the output terminal of the module generating the reference signal, and its second input terminal is connected to the output terminal of the first analog-to-digital converter module AD1, converting the reference signal and the first digital signal. u of Perform subtraction to obtain the difference signal;

[0070] Voltage regulator 203: Its input terminal is connected to the output terminal of the first comparator 201, and it adjusts the difference signal to obtain a control signal;

[0071] Second comparator 202: The first input terminal is connected to the output terminal of voltage regulator 203, and the second input terminal is connected to the output terminal of the module that generates carrier signal. It compares the control signal with the carrier signal to obtain the first switch selection signal.

[0072] PWM1: The input terminal is connected to the first output terminal of the second comparator 202, outputs the first switch selection signal, and outputs the first switch mode reference signal;

[0073] Inverter 204: Its input terminal is connected to the second output terminal of the second comparator 202, and it outputs a second switch selection signal;

[0074] PWM2: The input terminal is connected to the output terminal of the inverter 204, and outputs the second switch selection signal to obtain the second switch mode reference signal.

[0075] The driving circuit 3 includes: a first driving circuit 301, a second driving circuit 302, and a third driving circuit 303. The first driving circuit 301 has its input terminal connected to the first output terminal of DSP2 and its output terminal connected to the first switch S1. The second driving circuit 302 has its input terminal connected to the second output terminal of DSP2 and its output terminal connected to the second switch S2. The third driving circuit 303 has its input terminal connected to the first output terminal of DSP2 and its output terminal connected to the third switch S3.

[0076] The voltage regulator 203 employs either PI control or proportional-resonant control. The first drive circuit 301, the second drive circuit 302, and the third drive circuit 303 can be driven through optocoupler or transformer isolation.

[0077] This invention provides a control method for a buck-boost common-ground inverter, which is based on... Figure 1 The buck-boost common-ground inverter shown includes the following steps:

[0078] S101, The control drive unit monitors the output voltage feedback signal of the load. u of and the output voltage feedback signalu of The difference between the output voltage and the reference signal voltage is obtained by comparing the magnitude of the reference signal and the reference signal.

[0079] S102. Perform voltage processing on the difference signal to obtain a control signal; compare the control signal with the carrier signal to determine whether the control signal is greater than or less than the carrier signal;

[0080] S103. When the control signal is greater than the carrier signal, the control drive unit controls the second switch S2 to be turned on, and the first switch S1 and the third switch S3 to be turned off, so as to turn on the first freewheeling circuit, the second freewheeling circuit and the third freewheeling circuit of the filter unit to complete the inversion of the input power supply.

[0081] The control drive unit controls the second switch S2 to be turned on, and the first switch S1 and the third switch S3 to be turned off. Then the first freewheeling circuit, the second freewheeling circuit and the third freewheeling circuit are turned on; the first closed circuit, the second closed circuit and the third closed circuit are turned off, thereby completing the inversion of the input power.

[0082] S104. When the control signal is less than the carrier signal, the control drive unit adjusts the first switch S1 and the third switch S3 to be turned on, and the second switch S2 to be turned off, so as to turn on the first closed loop, the second closed loop and the third closed loop of the filter unit to complete the inversion of the input power.

[0083] The control drive unit controls the second switch S2 to open, and the first switch S1 and the third switch S3 to open, so that the first freewheeling circuit, the second freewheeling circuit and the third freewheeling circuit are disconnected; the first closed circuit, the second closed circuit and the third closed circuit are opened, thereby completing the inversion of the input power.

[0084] Working principle of the invention:

[0085] The sensor system monitors the output voltage in real time. It performs a first voltage signal processing, subtracting the output signal from the reference signal to obtain a difference signal. A second voltage signal processing is performed on the difference signal to obtain a control signal. A third voltage signal processing is performed on the control signal and the carrier signal. When the control signal is greater than the carrier signal, the control drive unit regulates the inverter's second switch to conduct the first, second, and third freewheeling circuits, causing the inverter to output a corresponding voltage signal to complete the input power inversion process. When the control signal is less than the carrier signal, the control drive unit regulates the inverter's first and third switches to conduct the first, second, and third closed loops, causing the inverter to output a corresponding voltage signal to complete the input power inversion process.

[0086] In summary, the control method for a buck-boost common-ground inverter of the present invention solves the problem of low buck-boost conversion efficiency of traditional inverters, eliminates common-mode leakage current, realizes buck-boost conversion, and improves the conversion efficiency of the inverter system.

[0087] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Any modifications, equivalent substitutions, and improvements made within the spirit and scope of this application are included within the scope of protection of this application.

Claims

1. A control method for a buck-boost common-ground inverter, characterized in that: The system includes a buck-boost common-ground inverter, which comprises an input power supply, a filter unit, a control drive unit, diodes, a first switch, a second switch, a third switch, and a load. The filtering unit includes a first inductor, a second inductor, a third inductor, a first capacitor, a second capacitor, and a third capacitor. The first terminal of the first inductor is connected to the positive terminal of the input power supply, and its second terminal is connected to the negative terminal of the input power supply via the first switch. The first terminal of the second inductor is connected to the first terminal of the first capacitor via the cathode of the diode, and its second terminal is connected to the second terminal of the second capacitor. The first terminal of the third inductor is connected to the first terminal of the first capacitor via the second switch, and its second terminal is connected to the negative terminal of the input power supply, the second terminal of the first capacitor, the second terminal of the third capacitor, and the second terminal of the load. The first terminal of the second capacitor is connected to the first terminal of the third inductor, and its second terminal is connected to the first terminal of the third capacitor and the first terminal of the load via the third switch. The anode of the diode is connected to the second terminal of the first inductor. The control method includes the following steps: (1) The control drive unit monitors the output voltage of the buck-boost common ground inverter in real time, and compares the magnitude of the output voltage and the reference signal to obtain the difference signal between the output voltage and the reference signal voltage; (2) The difference signal is processed by voltage to obtain a control signal; the control signal is compared with the carrier signal to determine whether the control signal is greater than or less than the carrier signal; (3) When the control signal is greater than the carrier signal, the control drive unit controls the second switch to be turned on, and the first switch and the third switch are turned off, so as to turn on the first freewheeling circuit, the second freewheeling circuit and the third freewheeling circuit of the filter unit to complete the inversion of the input power supply; (4) When the control signal is less than the carrier signal, the control drive unit adjusts the first switch and the third switch to be turned on, and the second switch to be turned off, so as to turn on the first closed loop, the second closed loop and the third closed loop of the filter unit to complete the inversion of the input power.

2. The control method for a buck-boost common-ground inverter as described in claim 1, characterized in that: The input power supply is connected in series with the first switch and the first inductor to form the first closed circuit; The input power supply is connected in series with the first inductor, the diode and the first capacitor to form the first freewheeling circuit; The second inductor is connected in series with the third switch, the third capacitor and the load in parallel branch, and the first capacitor to form the second closed loop; The second inductor is connected in series with the second capacitor and the second switch to form the second freewheeling circuit; The third inductor is connected in series with the second capacitor, the third switch, and the parallel branch of the third capacitor and the load to form the third closed loop; The third inductor is connected in series with the first capacitor and the second switch to form the third freewheeling circuit.

3. The control method for a buck-boost common-ground inverter as described in claim 1, characterized in that: The negative terminal of the input power supply is connected to the negative terminal of the load, and the negative terminals of the input power supply and the load are both grounded.

4. The control method for a buck-boost common-ground inverter as described in claim 1, characterized in that: The control drive unit has its input terminal connected to the load and its output terminal connected to the first switch, the second switch and the third switch respectively. It is used to drive the opening and closing of each switch to connect each circuit and thus complete the DC inverter process.

5. The control method for a buck-boost common-ground inverter as described in claim 1, characterized in that, The control drive unit includes: The sensor system, with its input terminal connected to the load, acquires the output voltage feedback signal of the buck-boost common-ground inverter in real time. The DSP, whose input is connected to the output of the sensor system, performs voltage signal processing on the output voltage feedback signal to generate a first switching logic signal and a second switching logic signal. The driving circuit has its input terminals connected to the first and second output terminals of the DSP, and its output terminals connected to the first switch, the second switch, and the third switch, respectively. It generates a first driving signal, a second driving signal, and a third driving signal based on the first switch logic signal and the second switch logic signal to drive the opening and closing of each switch accordingly.

6. The control method for a buck-boost common-ground inverter as described in claim 5, characterized in that, The sensor system includes an input voltage sensor, whose input terminal is connected to the load and whose output terminal is connected to the input terminal of the DSP, for acquiring the output voltage feedback signal and transmitting it to the DSP.

7. The control method for a buck-boost common-ground inverter as described in claim 5, characterized in that: The DSP includes: The module that generates the reference signal internally produces a constant frequency and voltage sine wave signal; The module that generates carrier signals internally produces a fixed-frequency, fixed-voltage triangular wave signal. The first analog-to-digital conversion module has its input terminal connected to the output terminal of the sensor system, and performs a first analog-to-digital conversion on the output voltage feedback signal to obtain a first digital signal. First comparator: The first input terminal is connected to the output terminal of the module that generates the reference signal, and the second input terminal is connected to the output terminal of the first analog-to-digital converter module. It calculates the difference between the reference signal and the first digital signal to obtain the difference signal. Voltage regulator: Its input terminal is connected to the output terminal of the first comparator, and it adjusts the difference signal to obtain a control signal; Second comparator: The first input terminal is connected to the output terminal of the voltage regulator, and the second input terminal is connected to the output terminal of the module that generates the carrier signal. It compares the control signal with the carrier signal to obtain the first switch selection signal. PWM1: The input terminal is connected to the first output terminal of the second comparator, and outputs the first switch selection signal to obtain the first switch logic signal; Inverter: Its input terminal is connected to the second output terminal of the second comparator to obtain the second switch selection signal; PWM2: The input terminal is connected to the output terminal of the inverter, and outputs the second switch selection signal to obtain the second switch logic signal.

8. The control method for a buck-boost common-ground inverter as described in claim 5, characterized in that: The driving circuit includes a first driving circuit, a second driving circuit, and a third driving circuit; the first driving circuit has its input terminal connected to the first output terminal of the DSP, and its output terminal connected to the first switch. The second driving circuit has its input terminal connected to the second output terminal of the DSP, and its output terminal connected to the second switch; The third driving circuit has its input terminal connected to the first output terminal of the DSP, and its output terminal connected to the third switch.

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