CN112928827B - Control circuit and control method for automatic locking of resonance frequency of non-contact power supply device - Google Patents

Abstract

The application relates to a control circuit and a control method for automatically locking the resonance frequency of a non-contact power supply device, wherein the control circuit comprises a main circuit, a resonant cavity, a current sensor, a zero-crossing comparator, a NAND gate circuit, a differential circuit and a digital signal processor; the output end of the differential circuit is respectively connected with the second input end of the NAND gate circuit and the second input port of the digital signal processor; and the output end of the NAND gate circuit is connected with the capturing port of the digital signal processor. The control method for automatically locking the resonance frequency of the non-contact power supply device is based on real-time detection of the primary transmitting coil current period, controls the inverter output voltage period, calculates the phase difference between the primary transmitting coil current and the inverter output voltage in real time, adjusts the switching frequency in real time by utilizing a digital phase-locked loop algorithm to realize accurate phase locking, realizes soft switching of the inverter, reduces the switching loss of the inverter and the port current of the inverter, and improves the transmission efficiency.

Description

Control circuit and control method for automatic locking of resonant frequency of non-contact power supply device

technical field

The invention relates to the technical field of wireless power supply equipment, in particular to a control circuit and a control method for automatically locking the resonant frequency of a non-contact power supply device.

Background technique

The characteristic of non-contact power supply is that there is no contact between the power supply and the load, and power is supplied through loose coupling. It has the characteristics of no contact, no wear, no spark, waterproof and dustproof, etc. It is used in wireless charging of personal consumer electronics, wireless charging of electric vehicles, Logistics systems and other fields based on automatic guided vehicles have gradually been widely used.

The existing non-contact power supply generally uses the principle of electromagnetic induction, and the primary side power supply applies an excitation voltage to the primary transmitting coil to generate an alternating magnetic field in space. The electric energy induced by the coupling of the secondary receiving coil is adjusted to supply power to the device. According to different application scenarios, the primary transmitting structure has two methods: coil and cable. The former is suitable for the non-running fixed structure of the electric pick-up, and the latter is suitable for the movable structure of the electric pick-up running along a fixed route.

Theoretical analysis and experimental results show that the cable-type transmitting structure needs to cooperate with the resonant cavity circuit, the input square wave voltage is converted into a sinusoidal current with the same frequency, and the power of the transmitting part is proportional to the cable current. Only when the switching frequency is equal to the resonant frequency of the primary-side resonant cavity, that is, when the resonant frequency is locked, the current at the inverter port will be the smallest, and the system can obtain the highest power factor and transmission efficiency. In order to achieve this goal, the traditional solution is to use the analog circuit CD4046 to achieve phase-locking. It is necessary to design a circuit to sample and shape the output voltage of the inverter side of the primary side and the output current of the track side, so as to adjust the voltage-controlled oscillator. Output frequency, so that the output frequency gradually approaches the actual resonance frequency. However, this processing method adopts an analog phase-locking method, and the circuit is relatively complicated, which is not convenient for digital control.

Contents of the invention

Based on this, it is necessary to provide a control circuit and a control method for automatically locking the resonant frequency of a non-contact power supply device for at least one of the above-mentioned problems.

In the first aspect, the present application provides a control circuit for automatically locking the resonant frequency of a non-contact power supply device, including a main circuit, a resonant cavity, a current sensor, a zero-crossing comparator, a NAND gate circuit, a differential circuit and a digital signal processor ;

The main circuit includes a three-phase uncontrolled rectifier bridge and an inverter bridge electrically connected to each other, the resonant cavity is electrically connected to the inverter bridge through a transformer and a current sensor, and the two capacitors of the resonant cavity pass through the primary rail cable connection;

The positive input terminal of the zero-crossing comparator is electrically connected to the current sensor, and the output terminal of the zero-crossing comparator is respectively connected to the first input terminal of the NAND gate circuit and the first input terminal of the digital signal processor. input port connection;

The output end of the differential circuit is respectively connected with the second input end of the NAND gate circuit and the second input port of the digital signal processor; the output end of the NAND gate circuit is connected with the digital signal processor The capture port connection.

In some implementations of the first aspect, the three-phase uncontrolled rectifier bridge includes a capacitor group, a first resistor, and three groups of diode groups connected in parallel with each other, and each group of the diode groups includes two series-connected diodes, two The two diodes are connected to one phase of the three-phase power supply; the second end of the diode group is connected to the first end of the first resistor, and the first resistor is connected in parallel with the first switch; the second end of the diode group is connected in parallel with the first switch; A capacitor group is connected between one end and the second end of the first resistor.

With reference to the first aspect and the above-mentioned implementation manners, in some implementation manners of the first aspect, the capacitor bank includes a first capacitor and a second capacitor connected in series, and the first capacitor and the second capacitor are polar capacitance; the three-phase uncontrolled rectifier bridge also includes a second resistor, a second switch and a third capacitor; the second end of the second resistor is connected to the second end of the first resistor, and the second The first end of the resistor is connected to the first end of the diode group, the second resistor is connected in series with the second switch; the first end of the third capacitor is respectively connected to the first end of the diode group and the connected to the first end of the inverter bridge, and the second end of the third capacitor is respectively connected to the second end of the first resistor and the second end of the inverter bridge.

With reference to the first aspect and the above-mentioned implementation manners, in some implementation manners of the first aspect, the inverter bridge includes a fourth capacitor and a first transformer, and the second output terminal of the inverter bridge is connected to the fourth capacitor The first end of the current sensor is connected to the second end of the fourth capacitor and the first left end of the first transformer respectively, and the first output end of the inverter bridge is connected to the first end of the first transformer The second left end of the connection.

With reference to the first aspect and the above implementation manners, in some implementation manners of the first aspect, the resonant cavity includes a first inductor, a second inductor, a fifth capacitor, and a sixth capacitor, and the first inductor of the first inductor One end is connected to the right first end of the first transformer, the second end of the first inductance is connected to the first end of the second inductance, the second end of the second inductance is connected to the sixth The first end of the capacitor is connected, the first end of the fifth capacitor is connected to the second end of the first inductor, and the second end of the fifth capacitor is connected to the right second end of the transformer; The second end of the sixth capacitor is connected to the second end of the fifth capacitor through the primary track cable.

With reference to the first aspect and the above implementation manners, in some implementation manners of the first aspect, the digital signal processor further includes a processor and a drive unit, the first input port, the second input port and The capture ports are all connected to the processor, and the drive unit is connected to the processor.

In the second aspect, a control method for automatically locking the resonant frequency of a non-contact power supply device adopts the control circuit for automatically locking the resonant frequency of a non-contact power supply device as described in the first aspect of the application of the present invention, including the following steps:

Determine the time reference of the timer in the digital signal processor, collect the current signal of the primary transmitting coil through the current sensor, generate a number of current zero-crossing point information according to the current signal pulse by the zero-crossing comparator, and output Go to the first input port of the digital signal processor to obtain the first counter data at the zero-crossing moment of the falling edge of the square wave corresponding to the primary current;

Based on the time reference, obtain the inverter port voltage signal of the differential circuit, and send it to the second input port of the digital signal processor, and obtain the second zero-crossing moment of the square wave falling edge corresponding to the inverter port voltage. counter data;

determining a phase difference based on the first counter data and the second counter data;

Based on the phase difference, the actual switching period value required to satisfy resonance is determined.

The technical solutions provided in the embodiments of the present invention bring the following beneficial technical effects:

The control method for automatically locking the resonant frequency of the non-contact power supply device provided by the present invention is based on the real-time detection of the current cycle of the primary transmitting coil, controls the inverter output voltage cycle, and calculates the phase difference between the primary transmitting coil current and the inverter output voltage in real time, The digital phase-locked loop algorithm is used to adjust the switching frequency in real time to achieve precise phase-locking, realize the soft switching of the inverter, reduce the switching loss of the inverter and the current of the inverter port, and improve the transmission efficiency.

Additional aspects and advantages of the present application will be presented in subsequent sections, and will be understood in detail from the subsequent description, or learned through specific practice of the present invention.

Description of drawings

Figure 1 is a schematic diagram of the circuit structure of the control of the automatic locking of the resonant frequency of the non-contact power supply device in an embodiment of the application of the present invention;

Fig. 2 is a schematic flowchart of a control method for automatically locking the resonant frequency of a non-contact power supply device in an embodiment of the application of the present invention;

Fig. 3 is a schematic flowchart of a control method for automatically locking the resonant frequency of a non-contact power supply device in an embodiment of the present application.

Detailed ways

In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the associated drawings. Possible embodiments of the invention are shown in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments which have been described herein with reference to the drawings. The embodiments described by referring to the accompanying drawings are exemplary for making the disclosure of the present invention more thorough and comprehensive, and should not be construed as limiting the present invention. Furthermore, if detailed descriptions of known technologies are not technically essential to the illustrated features of the invention, such technical details may be omitted.

Those skilled in the relevant art can understand that, unless otherwise defined, all terms (including technical terms and scientific terms) used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this invention belongs. It should also be understood that terms, such as those defined in commonly used dictionaries, should be understood to have meanings consistent with those in the prior art and will not be used in idealized or overly formal terms unless specifically defined as herein meaning to explain.

Those skilled in the art will understand that unless otherwise stated, the singular forms "a", "an", "said" and "the" used herein may also include plural forms. It should be further understood that the word "comprising" used in the specification of the present application refers to the presence of said features, integers, steps, operations, elements and/or components, but does not exclude the presence or addition of one or more other features, Integers, steps, operations, elements, components, and/or groups thereof. It should be understood that the expression "and/or" used herein includes all or any element and all combinations of one or more associated listed items.

The technical solution of the present invention and how the technical solution solves the above-mentioned technical problems will be described in detail below with specific embodiments.

The first aspect of the application of the present invention provides a control circuit for automatically locking the resonant frequency of a non-contact power supply device, as shown in Figure 1, including a main circuit 10, a resonant cavity 20, a current sensor 30, a zero-crossing comparator 40, and a Gate circuit 50 , differential circuit 60 and digital signal processor 70 .

The main circuit 10 includes a three-phase uncontrolled rectifier bridge 11 and an inverter bridge 12 that are electrically connected to each other. The resonant cavity 20 and the inverter bridge 12 are electrically connected through a transformer and a current sensor 30. The two capacitors of the resonant cavity 20 pass through the primary track cable connect.

The positive input terminal of the zero-crossing comparator 40 is electrically connected to the current sensor 30 , and the output terminal of the zero-crossing comparator 40 is respectively connected to the first input terminal of the NAND circuit 50 and the first input port of the digital signal processor 70 . The output terminal of the differential circuit 60 is respectively connected with the second input terminal of the NAND gate circuit 50 and the second input port of the digital signal processor 70 ; the output terminal of the NAND gate circuit 50 is connected with the capture port of the digital signal processor 70 .

Optionally, in an implementation of the embodiment of the first aspect, as shown in FIG. 1 , the three-phase uncontrolled rectifier bridge 11 includes a capacitor group, a first resistor R1 and three groups of diodes connected in parallel, each The diode group includes two diodes connected in series, and the two diodes are connected to one phase of the three-phase power supply; the second end of the diode group is connected to the first end of the first resistor R1, and the first resistor R1 is connected in parallel with the first switch; A capacitor group is connected between the first end of the diode group and the second end of the first resistor R1. The diode groups can be selected into three groups, such as D1 and D2, D3 and D4, and D5 and D6 in Fig. 1 .

Optionally, in combination with the embodiment of the first aspect and the above-mentioned implementation manners, in other implementation manners of the embodiment of the first aspect, the capacitor group includes a first capacitor C1 and a second capacitor C2 connected in series, and the first capacitor C1 and the second capacitor C2 are polarized capacitors; the three-phase uncontrolled rectifier bridge 11 also includes a second resistor R2, a second switch and a third capacitor C3; the second end of the second resistor R2 is connected to the second end of the first resistor R1 connected, the first end of the second resistor R2 is connected to the first end of the diode group, the second resistor R2 is connected in series with the second switch; the first end of the third capacitor C3 is respectively connected to the first end of the diode group and the inverter bridge 12 The first terminal of the third capacitor C3 is connected to the second terminal of the first resistor R1 and the second terminal of the inverter bridge 12 respectively.

Optionally, in combination with the embodiment of the first aspect and the above implementation manners, in still some implementation manners of the embodiment of the first aspect, the inverter bridge 12 includes a fourth capacitor C4 and a first transformer T1, and the inverter bridge 12 The second output end of the second output terminal is connected with the first end of the fourth capacitor C4, the current sensor 30 is respectively connected with the second end of the fourth capacitor C4 and the left first end of the first transformer T1, and the first output end of the inverter bridge 12 It is connected with the second left terminal of the first transformer T1.

Optionally, in combination with the embodiment of the first aspect and the above implementation manners, in some implementation manners of the embodiment of the first aspect, the resonant cavity 20 includes a first inductor L1, a second inductor L2, a fifth capacitor C5 and The sixth capacitor C6, the first end of the first inductor L1 is connected to the first right end of the first transformer T1, the second end of the first inductor L1 is connected to the first end of the second inductor L2, and the second end of the second inductor L2 The two ends are connected to the first end of the sixth capacitor C6, the first end of the fifth capacitor C5 is connected to the second end of the first inductor L1, and the second end of the fifth capacitor C5 is connected to the right second end of the first transformer T1. Connection; the second end of the sixth capacitor C6 is connected to the second end of the fifth capacitor through the primary track cable.

Optionally, in some implementations of the embodiment of the first aspect, the digital signal processor 70 further includes a processor and a drive unit, and the first input port, the second input port and the capture port are all connected to the processor, The drive unit is connected with the processor.

From a hardware point of view, the processor 1001 can be a CPU (Central Processing Unit, central processing unit), a general-purpose processor, a DSP (Digital Signal Processor, a data signal processor), an ASIC (Application Specific Integrated Circuit, an application specific integrated circuit), an FPGA (Field -Programmable GateArray, Field Programmable Gate Array) or other programmable logic devices, transistor logic devices, hardware components or any combination thereof. It can implement or execute the various illustrative logical blocks, modules and circuits described in connection with the present disclosure. The processor 1001 may also be a combination that implements computing functions, for example, a combination of one or more microprocessors, a combination of DSP and a microprocessor, and the like.

Based on the same technical idea, the embodiment of the second aspect of the application of the present invention provides a control method for automatic locking of the resonant frequency of a non-contact power supply device, using the automatic locking of the resonant frequency of the non-contact power supply device as described in the first aspect of the application of the present invention The control circuit, as shown in Figure 2, includes the following steps:

S100: Determine the time reference of the timer in the digital signal processor 70, collect the current signal of the primary transmitting coil through the current sensor 30, generate a number of current zero-crossing point information according to the current signal pulse by the zero-crossing comparator, and output it to the digital signal processing The first input port of the device 70 is used to obtain the first counter data at the time when the falling edge of the square wave corresponding to the primary side current crosses zero.

S200: Based on the time reference, obtain the inverter port voltage signal of the differential circuit 60, and send it to the second input port of the digital signal processor 70, and obtain the second counter data at the time when the falling edge of the square wave corresponding to the inverter port voltage crosses zero. .

S300: Determine a phase difference according to the first counter data and the second counter data.

S400: According to the phase difference, determine an actual switching cycle value required to satisfy the resonance.

The control method for automatically locking the resonant frequency of the non-contact power supply device provided by the present invention is based on the real-time detection of the current cycle of the primary transmitting coil, controls the inverter output voltage cycle, and calculates the phase difference between the primary transmitting coil current and the inverter output voltage in real time, The digital phase-locked loop algorithm is used to adjust the switching frequency in real time to achieve precise phase-locking, realize the soft switching of the inverter, reduce the switching loss of the inverter and the current of the inverter port, and improve the transmission efficiency.

As shown in Figure 3, the device is initialized, especially the digital signal processor 70 is initialized, the timer T is set as the timing time base, and the first input port and the second input port are set to the falling edge capture mode , set the capture port as rising and falling capture mode. After that, the timer T is started.

The current sensor 30 collects the current signal of the primary transmitting coil, and outputs the current sampling signal to the zero-crossing comparator 40, and the zero-crossing comparator 40 generates current zero-crossing point information in a pulsed manner, and sends it to the first input of the digital signal processor 70 port.

The differential circuit 60 collects the terminal voltage signal of the full-bridge inverter circuit composed of IGBT (Insulated Gate Bipolar Transistor, Insulated Gate Bipolar Transistor), and sends it to the second input port of the digital signal processor 70 after being processed by the adjustment circuit.

Start the internal timer T of the digital signal processor 70 as a time reference, and capture the first counter data Ti_f(n) of the T counter at the moment when the falling edge of the square wave corresponding to the primary side current passes through 0 o'clock. The digital signal processor 70 captures the second counter data Tv_f(n) of the T counter when the falling edge of the square wave corresponding to the inverter port voltage passes through 0 o'clock. Define the sign function Sign(dT) to define the voltage and current phase relationship variables:

dT=Ti_f(n)-Tv_f(n)………Formula (1)

When Sign(dT)>0, the current on the phase leads the voltage; when Sign(dT)<0, the current on the phase lags the voltage. If |dT|<1us, it is considered to be in a phase-locked state, and there is no need to adjust the switching frequency.

The square wave corresponding to the primary current drops and the square wave corresponding to the primary voltage passes through an AND circuit to obtain a waveform, and the high level width of the waveform indicates the phase difference. After the waveform is processed by the conditioning circuit, it is sent to the first capture port of the digital signal processor 70. The capture mode is set to be the rising edge and the falling edge of the square wave signal. The phase difference is defined as dP(n), and the phase difference is PW. Then there is the following formula (3):

dP(n)=Sign(dT)*PW………Formula (3)

The digital phase-locked loop controller is implemented by a program. In order to improve the phase-locked precision, the controller can specifically adopt a discrete proportional-integral (PI) controller. The given value of the controller is 0, the feedback value is the phase difference with direction, and its output is used as the switching period correction value of the switching tube drive signal. The switching period correction value is defined as dTP(n), and there is the following relationship group (4):

dTP(n)=dTP(n-1)+(Kp+Ki)*dP(n)-Kp*dP(n-1)

dTP(n-1)=dTP(n)

dP(n-1)=dP(n)... Relational formula group (4)

Among them, dTP(n) is the output of the current pulse beat of the digital phase-locked loop controller; dTP(n-1) is the output of a pulse beat on the digital phase-locked loop controller; dP(n) is the current pulse beat of the digital phase-locked loop Phase difference; dP(n-1) is the phase difference of a pulse beat on the digital phase-locked loop. Kp is the proportional coefficient of the digital proportional-integral controller, and Ki is the integral coefficient of the digital proportional-integral controller.

The initial setting value that defines the switching period is TP0. Then the switching cycle obtained after processing by the digital phase-locked controller is:

TP=TP0+dTP(n)……Formula (5)

The switching tubes that make up the inverter bridge send out driving signals at the updated frequency through T1A/B and T2A/B according to the switching cycle after the above automatic adjustment, and sending out high-frequency signals can ensure that the voltage at the inverter port is in phase with the voltage, thus Resonance is achieved, the port current has the minimum power factor and the highest efficiency.

Those skilled in the art can understand that the various operations, methods, and steps, measures, and schemes in the processes that have been discussed in this application can be replaced, changed, combined, or deleted. Furthermore, the various operations, methods, and other steps, measures, and schemes in the processes that have been discussed in this application may also be replaced, changed, rearranged, decomposed, combined, or deleted. Further, steps, measures, and schemes in the prior art that have operations, methods, and processes disclosed in the present application may also be alternated, changed, rearranged, decomposed, combined, or deleted.

The terms "first" and "second" are used for descriptive purposes only, and cannot be interpreted as indicating or implying relative importance or implicitly specifying the quantity of indicated technical features. Thus, a feature defined as "first" and "second" may explicitly or implicitly include one or more of these features. In the description of the present application, unless otherwise specified, "plurality" means two or more.

In the description of this application, it should be noted that unless otherwise specified and limited, the terms "installation", "connection", and "connection" should be understood in a broad sense, for example, it can be a fixed connection or a detachable connection. Connected, or integrally connected; it can be directly connected, or indirectly connected through an intermediary, and it can be the internal communication of two elements. Those of ordinary skill in the art can understand the specific meanings of the above terms in this application in specific situations.

It should be understood that although the various steps in the flow chart of the accompanying drawings are displayed sequentially according to the arrows, these steps are not necessarily executed sequentially in the order indicated by the arrows. Unless otherwise specified herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some of the steps in the flowcharts of the accompanying drawings may include multiple sub-steps or multiple stages, and these sub-steps or stages may not necessarily be executed at the same time, but may be executed at different times, and the order of execution is also It is not necessarily performed sequentially, but may be performed alternately or alternately with at least a part of other steps or sub-steps or stages of other steps.

The above description is only part of the implementation of the present application, and it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present application, some improvements and modifications can also be made, and these improvements and modifications are also It should be regarded as the protection scope of this application.

Claims (7)

1. The control circuit is characterized by comprising a main circuit, a resonant cavity, a current sensor, a zero-crossing comparator, a NAND gate circuit, a differential circuit and a digital signal processor;

the main circuit comprises a three-phase uncontrolled rectifier bridge and an inverter bridge which are electrically connected with each other, the resonant cavity is electrically connected with the inverter bridge through a transformer and a current sensor, and two capacitors of the resonant cavity are connected through a primary side track cable;

the positive input end of the zero-crossing comparator is electrically connected with the current sensor, and the output end of the zero-crossing comparator is respectively connected with the first input end of the NAND gate circuit and the first input port of the digital signal processor;

the output end of the differential circuit is respectively connected with the second input end of the NAND gate circuit and the second input port of the digital signal processor; and the output end of the NAND gate circuit is connected with the capturing port of the digital signal processor.

2. The control circuit of claim 1, wherein the three-phase uncontrolled rectifier bridge comprises a capacitor bank, a first resistor, and three sets of diodes connected in parallel with each other, each set of diodes comprising two diodes connected in series, the two diodes being connected to one of the three-phase power sources; the second end of the diode group is connected with the first end of a first resistor, and the first resistor is connected with a first switch in parallel; a capacitor group is connected between the first end of the diode group and the second end of the first resistor.

3. The control circuit of claim 2, wherein the capacitor bank includes a first capacitor and a second capacitor in series, the first capacitor and the second capacitor being polar capacitors; the three-phase uncontrolled rectifier bridge further comprises a second resistor, a second switch and a third capacitor; the second end of the second resistor is connected with the second end of the first resistor, the first end of the second resistor is connected with the first end of the diode group, and the second resistor is connected with the second switch in series; the first end of the third capacitor is connected with the first end of the diode group and the first end of the inverter bridge respectively, and the second end of the third capacitor is connected with the second end of the first resistor and the second end of the inverter bridge respectively.

4. The control circuit of claim 1, wherein the inverter bridge comprises a fourth capacitor and a first transformer, wherein the second output terminal of the inverter bridge is connected to the first terminal of the fourth capacitor, wherein the current sensor is connected to the second terminal of the fourth capacitor and the left first terminal of the first transformer, respectively, and wherein the first output terminal of the inverter bridge is connected to the left second terminal of the first transformer.

5. The control circuit of claim 4, wherein the resonant cavity comprises a first inductor, a second inductor, a fifth capacitor, and a sixth capacitor, a first end of the first inductor being connected to a right first end of the first transformer, a second end of the first inductor being connected to a first end of the second inductor, a second end of the second inductor being connected to a first end of the sixth capacitor, a first end of the fifth capacitor being connected to a second end of the first inductor, a second end of the fifth capacitor being connected to a right second end of the transformer; and the second end of the sixth capacitor is connected with the second end of the fifth capacitor through the primary side track cable.

6. The control circuit of claim 1, wherein the digital signal processor further comprises a processor and a drive unit, the first input port, the second input port, and the capture port are all coupled to the processor, and the drive unit is coupled to the processor.

7. A control method of automatic locking of a resonance frequency of a non-contact power supply device, characterized by adopting the control circuit of automatic locking of a resonance frequency of a non-contact power supply device according to any one of claims 1 to 6, comprising the steps of:

determining a time reference of a timer in the digital signal processor, collecting a current signal of a primary transmitting coil through the current sensor, generating a plurality of current zero crossing point information by the zero crossing comparator according to the current signal pulse, outputting the current zero crossing point information to a first input port of the digital signal processor, and obtaining first counter data of square wave falling edge zero crossing point moments corresponding to primary side current;

acquiring an inversion port voltage signal of the differential circuit based on the time reference, and transmitting the inversion port voltage signal to a second input port of the digital signal processor to acquire second counter data of square wave falling edge zero crossing point time corresponding to the inversion port voltage;

determining a phase difference from the first counter data and the second counter data;

from the phase difference, the actual switching period value required to meet resonance is determined.