Metallic foreign object detection device and method for wireless charging system controlled by SPWM

Abstract

The application discloses a kind of SPWM control's wireless charging system metal foreign matter detection device and method, including wireless power transmitting unit, wireless power receiving unit, metal foreign matter detection unit and control unit, the wireless power transmitting unit includes the SPWM modulation circuit, inverter circuit and transmitting coil connected in sequence, the metal foreign matter detection unit includes detection coil group, detection module connected in sequence, the detection coil group is laid on transmitting coil;The detection module includes parallel resonance circuit, high-pass filter circuit and differential amplifier circuit connected in sequence;The detection coil of the application can improve the detection sensitivity of small metal foreign matter.

Description

Technical Field

[0001] This invention relates to the field of metal foreign object detection in wireless charging systems for electric vehicles, and more particularly to wireless charging systems using SPWM control. Background Technology

[0002] When a wireless charging system for electric vehicles is operating, a strong electromagnetic coupling region exists between the transmitting and receiving coils. Therefore, any metallic foreign object entering or approaching this coupling region will alter the electrical parameters of the coupling mechanism, causing the system to deviate from its normal operating point and affecting its output power and transmission efficiency. Furthermore, due to the eddy current effect of metallic foreign objects, their temperature rises when they enter the electromagnetic coupling region, easily leading to fires and other accidents, posing a significant safety hazard. Summary of the Invention

[0003] Purpose of the invention: In order to detect metallic foreign objects in electromagnetic coupling areas, this invention proposes a metallic foreign object detection device and method for an SPWM-controlled wireless charging system for electric vehicles, including a high-frequency magnetic field generation method, a method for improving the sensitivity of the detection device, and system control logic. This method can improve the detection sensitivity of the detection coil for small metallic foreign objects.

[0004] Technical solution: To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0005] A metal foreign object detection device for a wireless charging system controlled by SPWM includes a wireless power transmitting unit, a wireless power receiving unit, a metal foreign object detection unit, and a control unit. The wireless power transmitting unit includes an SPWM modulation circuit, an inverter circuit, and a transmitting coil connected in sequence. The wireless power receiving unit includes a receiving coil, a resonant capacitor, an AC-DC converter, a DC-DC converter, and a battery load connected in sequence. The metal foreign object detection unit includes a detection coil group and a detection module connected in sequence, with the detection coil group laid on top of the transmitting coil. The detection module includes a parallel resonant circuit, a high-pass filter circuit, and a differential amplifier circuit connected in sequence. The control unit is connected to the SPWM modulation circuit, the DC-DC converter, and the differential amplifier circuit.

[0006] The detection principle is as follows: Metallic foreign objects in a magnetic field will exhibit eddy current and magnetic effects. An SPWM modulation circuit causes the inverter circuit to output a multi-frequency composite current, inducing a multi-frequency magnetic field in space. The high-frequency magnetic field enhances the eddy current effect of the metallic foreign object, leading to an increase in the change in magnetic flux of the detection coil group. The corresponding induced voltage on the detection coil is processed by a detection module consisting of a parallel resonant circuit, a high-pass filter circuit, and a differential amplifier circuit. If a metallic foreign object is detected, the output detection voltage difference control circuit controls the SPWM modulation circuit to switch to metallic foreign object detection mode to confirm its presence. The detection module is designed according to the high-frequency modulation wave, while the wireless power receiving unit is designed according to the low-frequency modulation wave.

[0007] Preferably: the receiving coil is connected in series with a resonant capacitor C. s Resonant capacitor C s The magnitude of the first modulated wave u r0 It is related to the first modulation frequency f0.

[0008] Preferably, the parallel resonant circuit consists of a parallel resonant capacitor and a series sampling resistor, and the resonant frequency of the parallel resonant circuit is related to the second modulation wave u. r1 It is related to the second modulation frequency f1.

[0009] Preferably, the high-pass filter circuit adopts a Butterworth second-order active filter circuit.

[0010] Preferably, the differential amplifier circuit is connected to the control unit via an analog-to-digital converter circuit. The control unit is used to compare the detection voltage with the threshold voltage to determine whether there is foreign object intrusion.

[0011] A method for detecting metallic foreign objects in an SPWM-controlled wireless charging system includes the following steps:

[0012] The SPWM modulation circuit modulates the wave u r With triangular carrier u c The comparison controls the switching on and off of the inverter circuit. The SPWM modulation circuit, controlled by the control unit, operates in three states. When only the first modulation wave is used... r0 With triangular carrier u c When comparing signals to control the on / off state of the inverter circuit, the system operates in the energy transfer state, which is the first operating state S1. When the first modulation wave u is used... r0 Second modulation wave u r1 After composite modulation and triangular carrier u c When comparing data to control the inverter's on / off state, the system performs energy transfer and foreign object detection; at this time, the system is in the second operating state S2. In the third operating state S3, when the second modulation wave u is used... r1 With triangular carrier uc When comparing and controlling the switching on and off of the inverter circuit, the eddy current effect of the metal foreign object is enhanced by the high-frequency magnetic field, thereby strengthening the detection capability of small metal foreign objects. At this time, the system is in the third working state S3.

[0013] Preferred method for generating high-frequency magnetic fields:

[0014] The SPWM modulation circuit modulates the first modulation wave with a first modulation frequency of f0. r0 The second modulated wave with a second modulation frequency of f1 r1 Modulation is performed to obtain a composite modulated wave containing a first modulation frequency f0 and a second modulation frequency f1, which controls the inverter circuit to output a PWM voltage wave with frequencies mainly f0 and f1. The PWM voltage wave is directly fed into the transmitting coil, generating a multi-frequency composite magnetic field with frequencies mainly f0 and f1 in space.

[0015] Preferred methods for enhancing sensitivity: Generating a high-frequency magnetic field enhances the eddy current effect of metallic foreign objects in the electromagnetic coupling region of the wireless charging system, thereby increasing the influence of the metallic foreign objects on the magnetic flux in the magnetic field and thus increasing the induced voltage on the detection coil. A parallel resonant circuit utilizes the drop in detection voltage of the detection module in the non-resonant state to improve the sensitivity of the detection circuit to metallic foreign objects. A high-pass filter circuit filters out the influence of low-frequency electrical energy on high-frequency signals, enabling the detection circuit to accurately detect metallic foreign objects.

[0016] Preferred system operating mode:

[0017] a. During normal operation, the control unit controls the SPWM modulation circuit to alternately operate in the first operating state S1 and the second operating state S2. In the second operating state S2, the detection voltage of the detection module is compared with the monitoring voltage. If the voltage does not exceed the threshold voltage, the circuit continues to enter the first operating state S1.

[0018] b. When the detection voltage of the detection module exceeds the monitoring voltage, there may be a metallic foreign object in the electromagnetic coupling mechanism. The control unit controls the SPWM modulation circuit to switch to the third operating state S3, which enhances the eddy current effect of the metallic foreign object through a high-frequency magnetic field, and further compares the detection voltage of the detection module with the detection comparison voltage U. det Size, if the detection voltage of the detection module is higher than the detection comparison voltage U det If a foreign metal object is found to have entered the system, it will stop working.

[0019] If the detection voltage of the detection module is lower than the monitoring voltage, the system switches to the first working state S1 and continues to work.

[0020] Preferably, the inverter circuit is a full-bridge inverter circuit, and the switching transistors in the full-bridge inverter circuit are power MOSFETs. The MOSFETs are controlled using a bipolar SPWM control method.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1) The metal foreign object detection device of the present invention enhances the eddy current effect of metal foreign objects in the magnetic field by generating a high-frequency detection magnetic field, thereby increasing the influence of metal foreign objects on the magnetic flux of the magnetic field, thus enabling the detection coil to sensitively detect small metal foreign objects.

[0023] 2) The high-frequency detection circuit of the present invention shields the change in magnetic flux caused by the change in low-frequency magnetic field, thereby improving the detection accuracy of the detection circuit.

[0024] 3) The parallel resonant circuit of the present invention improves the sensitivity of metal foreign object detection by the rapid drop of the detection voltage in the non-resonant state.

[0025] 4) The system control logic of this invention changes the amplitude and frequency of the modulation wave in the SPWM modulation circuit according to the actual situation, so as to realize real-time monitoring and sensitive detection of metallic foreign objects. Attached Figure Description

[0026] Figure 1 This is the overall circuit schematic diagram of the present invention.

[0027] Figure 2 There are three working state diagrams.

[0028] Figure 3 It is the logic for switching working states.

[0029] Figure 4 It is the non-resonant drop curve of the parallel resistor.

[0030] Figure 5 This is a high-pass filter circuit diagram. Detailed Implementation

[0031] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the invention. After reading this invention, any modifications of the invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0032] A metal foreign object detection device for a wireless charging system controlled by SPWM includes a wireless power transmitting unit, a wireless power receiving unit, a metal foreign object detection unit, and a control unit 8. The wireless power transmitting unit includes an SPWM modulation circuit 1, an inverter circuit 4, and a transmitting coil connected in sequence. The wireless power transmitting unit is laid underground. The inverter circuit receives DC power. The wireless power receiving unit includes a receiving coil, a resonant capacitor, an AC-DC converter, a DC-DC converter, and a battery load connected in sequence.

[0033] The series resonant capacitor C of the receiving coil s Resonant capacitor C s The magnitude of the first modulated wave u r0 Related to the first modulation frequency f0, the resonant capacitance C s The magnitude is determined according to the electrical energy modulation wave u r0 The design is based on frequency f0.

[0034] The wireless power transmitting unit receives AC power at the industrial frequency, which, after being rectified and filtered, is fed into an inverter circuit. The inverter circuit is controlled by an SPWM modulation circuit and outputs a multi-frequency composite PWM voltage wave. The inverter is directly connected to the transmitting coil, which induces a multi-frequency composite magnetic field in space. The wireless power receiving unit is mounted on the chassis of the electric vehicle. In this unit, the receiving coil is coupled to the transmitting coil to transmit electrical energy, and the receiving coil is connected to a resonant capacitor C. s Connected to the power receiving circuit.

[0035] The inverter circuit 4 is a full-bridge inverter circuit, and the switching transistors of the full-bridge inverter circuit are power MOSFETs. The MOSFETs are controlled using a bipolar SPWM control method.

[0036] SPWM modulation circuit 1 modulates the first modulation wave u with a first modulation frequency of f0. r0 The second modulated wave with a second modulation frequency of f1 r1 Modulation is performed to obtain a composite modulated wave containing frequencies f0 and f1, which controls the inverter circuit to output a PWM voltage wave with frequencies mainly f0 and f1.

[0037] The metal foreign object detection unit includes a detection coil group 2 and a detection module 3 connected in sequence. The detection coil group 2 is laid on top of the transmitting coil. The detection coil group is connected to the detection circuit to detect changes in magnetic flux at the upper part of the transmitting coil, at its ports (A1, A2, ..., A...). n and B1, B2, ..., B n An induced voltage is generated at position A; the detection coil A at the corresponding position of the detection coil group n and B n Its induced voltage is U Anand U Bn The detected voltage difference is obtained after processing by the detection circuit as U. on The detection module 3 includes a parallel resonant circuit 5, a high-pass filter circuit 6, and a differential amplifier circuit 7 connected in sequence. The control unit 8 is connected to the SPWM modulation circuit 1, the DC-DC converter, and the differential amplifier circuit 7, respectively. The detection coil group is laid on the surface of the transmitting coil and connected to the detection module 3. The high-pass filter circuit is as follows: Figure 2 As shown, the output is connected to the control circuit, and the high-pass filter circuit adopts a Butterworth second-order active filter circuit. The parallel resonant circuit 5 consists of a parallel resonant capacitor and a series sampling resistor. The resonant frequency of the parallel resonant circuit 5 is related to the second modulation wave u. r1 Related to the second modulation frequency f1, the resonant frequency of the parallel resonant circuit 5 is according to the second modulation wave u. r1 The design is based on the second modulation frequency f1.

[0038] The differential amplifier circuit is followed by an analog-to-digital converter circuit (for simplicity, see attached diagram). Figure 1 The analog-to-digital conversion circuit (not shown) is connected to the control unit 8. The control unit 8 is connected to the SPWM modulation circuit, the detection circuit, and the DC-DC converter. It can generate an SPWM composite modulation wave to control the inverter, control the output voltage of the DC-DC converter, and determine the presence of metallic foreign objects. The control unit 8 is used to compare the detection voltage with the threshold voltage to determine whether foreign object intrusion has occurred.

[0039] Control logic:

[0040] A wireless power transmission system controlled by a composite modulation wave is employed, selecting the appropriate modulation wave signal frequency according to different operating conditions; when only the first modulation wave u is used... r0 As modulating wave and carrier u c The inverter is driven by a comparison, and the system operates in the energy transfer state, which is state S1; when the first modulation wave u is used... r0 Second modulation wave u r1 As modulating wave and carrier u c The inverter is driven by a comparison, during which the system performs energy transfer and foreign object detection, and operates in state S2; when the second modulation wave u is used... r1 As modulating wave and carrier u c The inverter is driven by comparison, and the eddy current effect of the metal foreign object is enhanced by the high-frequency magnetic field, thereby strengthening the detection capability of small metal foreign objects. At this time, the system is working in the S3 state.

[0041] The system operates in states S1 and S2 during normal operation. In state S1, the system achieves efficient energy transfer. It periodically switches to state S2 to monitor for foreign metal intrusion. When the foreign metal detection circuit outputs voltage U... onHigher than the monitoring voltage U mon At this time, there may be metallic foreign objects in the electromagnetic coupling mechanism; after switching to the S3 working state through the control circuit, if the output voltage U of the detection circuit is... on Greater than the detection voltage U det At this point, a foreign metal object has intruded.

[0042] A method for detecting metallic foreign objects in an SPWM-controlled wireless charging system, such as... Figure 3 , 4 As shown, it includes the following steps:

[0043] The SPWM modulation circuit 1 modulates the wave u r With triangular carrier u c The comparison is used to control the switching on and off of the inverter circuit 4. The SPWM modulation circuit 1, controlled by the control unit 8, operates in three states. When only the first modulation wave u is used... r0 With triangular carrier u c When comparing and controlling the switching on and off of inverter circuit 4, the system operates in energy transfer state, which is the first operating state S1. When the first modulation wave u is used... r0 Second modulation wave u r1 After composite modulation and triangular carrier u c When comparing data to control the inverter's on / off state, the system performs energy transfer and foreign object detection; at this time, the system is in the second operating state S2. In the third operating state S3, when the second modulation wave u is used... r1 With triangular carrier u c When comparing and controlling the switching on and off of inverter circuit 4, the eddy current effect of metal foreign objects is enhanced by the high-frequency magnetic field, thereby strengthening the detection capability of small metal foreign objects. At this time, the system is in the third working state S3.

[0044] Methods for generating high-frequency magnetic fields:

[0045] SPWM modulation circuit 1 modulates the first modulation wave u with a first modulation frequency of f0. r0 The second modulated wave with a second modulation frequency of f1 r1 Modulation is performed to obtain a composite modulated wave containing a first modulation frequency f0 and a second modulation frequency f1, which controls the inverter circuit 4 to output a PWM voltage wave with frequencies mainly f0 and f1. The PWM voltage wave is directly fed into the transmitting coil, generating a multi-frequency composite magnetic field with frequencies mainly f0 and f1 in space.

[0046] Sensitivity enhancement methods: By generating a high-frequency magnetic field, the eddy current effect of metallic foreign objects in the electromagnetic coupling region of the wireless charging system is enhanced, thereby increasing the influence of the metallic foreign objects on the magnetic flux in the magnetic field and thus increasing the induced voltage on the detection coil. A parallel resonant circuit utilizes the drop in detection voltage of detection module 3 in the non-resonant state to improve the sensitivity of the detection circuit to metallic foreign objects. A high-pass filter circuit filters out the influence of low-frequency electrical energy on high-frequency signals, enabling the detection circuit to accurately detect metallic foreign objects.

[0047] The system operates as follows:

[0048] a. During normal operation, the control unit 8 controls the SPWM modulation circuit 1 to alternately operate in the first operating state S1 and the second operating state S2. In the second operating state S2, the detection voltage of the detection module 3 is compared with the monitoring voltage U. mon If the voltage does not exceed the threshold voltage, the system continues to enter the first operating state S1.

[0049] b. The detection voltage of detection module 3 exceeds the monitoring voltage U. mon At this time, there may be metallic foreign objects in the electromagnetic coupling mechanism. The control unit 8 controls the SPWM modulation circuit 1 to switch to the third working state S3, which enhances the eddy current effect of the metallic foreign objects through a high-frequency magnetic field, and further compares the detection voltage U of the detection module 3. on And the detection comparison voltage U det Size, if the detection voltage of detection module 3 is higher than the detection comparison voltage U det If a foreign metal object is found to have entered the system, it will stop working.

[0050] If the detection voltage of the detection module 3 is lower than the monitoring voltage, the system switches to the first working state S1 and continues to work.

[0051] like Figure 4 As shown, the system operates in states S1 and S2 during normal operation. In state S1, the system achieves efficient energy transfer; it periodically switches to state S2 to monitor for metal foreign object intrusion. When the metal foreign object detection circuit outputs voltage U... on Higher than the monitoring voltage U mon At this time, there may be metallic foreign objects in the electromagnetic coupling mechanism; after switching to the S3 working state through the control circuit, if the output voltage U of the detection circuit is... on Greater than the detection comparison voltage U det At this point, a foreign metal object has intruded.

[0052] The following is a specific embodiment of the non-resonant drop curve of the parallel resistor of the present invention:

[0053] Transmitting coil voltage U OC =24V, operating frequency f=100kHz, equivalent inductance L of transmitting coilP =136uH, equivalent resistance R P =0.76Ω; Equivalent inductance L of the detection coil D = 8.04uH, resonant capacitor C D =374nF, sampling resistor R of the detection circuit S =100Ω, internal resistance R of the detection coil D =0.065Ω; Mutual inductance between the detection coil and the transmitting coil M = 13.2uH.

[0054] Based on the above parameters, the non-resonant voltage drop curve of the parallel resistor is calculated. Point A corresponds to the induced voltage of the detection coil when there is no foreign object, and point B corresponds to the induced voltage of the detection coil when there is a foreign object. At a frequency of 100kHz, point A is in a fully resonant state, and point B is in a partially resonant state. It can be seen that after the intrusion of a metallic foreign object, the voltage U across the sampling resistor is adjusted through the parameter design of the parallel resistor. SAn There is a significant drop, which increases the voltage difference between the detection coils.

[0055] It is easy to see that the SPWM-controlled metal foreign object detection unit and detection method proposed in this paper improve the detection sensitivity of the detection unit for small metal foreign objects and ensure the safe operation of the electric vehicle wireless charging system.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A detection method for a metal foreign object detection device in an SPWM-controlled wireless charging system, characterized in that: The system includes a wireless power transmitting unit, a wireless power receiving unit, a metal foreign object detection unit, and a control unit (8). The wireless power transmitting unit includes an SPWM modulation circuit (1), an inverter circuit (4), and a transmitting coil connected in sequence. The wireless power receiving unit includes a receiving coil, a resonant capacitor, an AC-DC converter, a DC-DC converter, and a battery load connected in sequence. The metal foreign object detection unit includes a detection coil group (2) and a detection module (3) connected in sequence. The detection coil group (2) is laid on top of the transmitting coil. The detection module (3) includes a parallel resonant circuit (5), a high-pass filter circuit (6), and a differential amplifier circuit (7) connected in sequence. The control unit (8) is connected to the SPWM modulation circuit (1), the DC-DC converter, and the differential amplifier circuit (7) respectively. The system includes the following steps: The SPWM modulation circuit (1) modulates the wave u r With triangular carrier u c The comparison is used to control the switching on and off of the inverter circuit (4); the SPWM modulation circuit (1) is controlled by the control unit (8) and operates in three working states; when only the first modulation wave is used... u r0 With triangular carrier u c When comparing the switching on and off of the inverter circuit (4), the system operates in the energy transfer state, at which time the system is in the first operating state. S 1. When using the first modulation wave u r0 Second modulation wave u r1 Composite modulation and triangular carrier u c When comparing parameters to control the inverter's on / off state, the system performs energy transfer and foreign object detection; at this time, the system is in its second operating state. S 2nd step; Third working state S 3. When using the second modulation wave u r1 With triangular carrier u c When comparing the switching on and off of the inverter circuit (4), the eddy current effect of the metal foreign object is enhanced by the high-frequency magnetic field, thereby strengthening the detection capability of small metal foreign objects. At this time, the system is in the third working state. S 3 times; High-frequency magnetic field generation method: SPWM modulation circuit (1) modulates the first modulation frequency to be f 0's first modulation wave u r0 The second modulation frequency is f The second modulation wave of 1 u r1 Modulation is performed to obtain a signal containing the first modulation frequency. f 0 and second modulation frequency f The composite modulation wave of 1 controls the output frequency of the inverter circuit (4) to be mainly 1. f 0 and f A PWM voltage wave of 1; the PWM voltage wave is directly fed into the transmitting coil, generating a frequency in space that is mainly [frequency value missing]. f 0 and f 1. Multi-frequency composite magnetic field; by generating a high-frequency detection magnetic field, the eddy current effect of metallic foreign objects in the magnetic field is enhanced, thereby increasing the influence of metallic foreign objects on the magnetic flux of the magnetic field. The high-frequency detection circuit shields the change in magnetic flux caused by changes in low-frequency magnetic field. The parallel resonant circuit (5) improves the sensitivity of metal foreign object detection by the rapid drop of the detection voltage in the non-resonant state.

2. The detection method of the SPWM-controlled wireless charging system metal foreign object detection device according to claim 1, characterized in that: The series resonant capacitor of the receiving coil C s resonant capacitor C s The magnitude and the first modulated wave u r0 and the first modulation frequency f 0 related.

3. The detection method of the SPWM-controlled wireless charging system metal foreign object detection device according to claim 2, characterized in that: The resonant frequency of the parallel resonant circuit (5) is related to the second modulation wave. u r1 Second modulation frequency f 1. Related.

4. The detection method of the SPWM-controlled wireless charging system metal foreign object detection device according to claim 3, characterized in that: The high-pass filter circuit adopts a Butterworth second-order active filter circuit.

5. The detection method of the SPWM-controlled wireless charging system metal foreign object detection device according to claim 4, characterized in that: The differential amplifier circuit is connected to the control unit (8) via an analog-to-digital converter circuit. The control unit (8) is used to compare the detection voltage with the threshold voltage to determine whether there is foreign object intrusion.

6. The detection method of the SPWM-controlled wireless charging system metal foreign object detection device according to claim 5, characterized in that, Sensitivity enhancement methods: By generating a high-frequency magnetic field, the eddy current effect of the metal foreign object in the electromagnetic coupling area of ​​the wireless charging system is enhanced, thereby increasing the influence of the metal foreign object on the magnetic flux in the magnetic field and thus increasing the induced voltage on the detection coil; the parallel resonant circuit enhances the sensitivity of the detection circuit to the metal foreign object by utilizing the drop in the detection voltage of the detection module (3) in the non-resonant state; the high-pass filter circuit achieves accurate detection of the metal foreign object by filtering out the influence of low-frequency power on the high-frequency signal.

7. The detection method of the SPWM-controlled wireless charging system metal foreign object detection device according to claim 6, characterized in that, The system operates as follows: a. During normal operation, the control unit (8) controls the SPWM modulation circuit (1) to alternately operate in the first operating state. S 1 and second working states S 2nd step; in the second working state S 2. The detection voltage of the detection module (3) is compared with the monitoring voltage. If the voltage does not exceed the threshold voltage, the system continues to enter the first working state. S 1; b. When the detection voltage of the detection module (3) exceeds the monitoring voltage, there may be a metal foreign object in the electromagnetic coupling mechanism. The control unit (8) controls the SPWM modulation circuit (1) to switch to the third working state. S 3. By enhancing the eddy current effect of the metallic foreign object through a high-frequency magnetic field, the detection voltage of the detection module (3) and the detection comparison voltage are further compared. U det Size, if the detection voltage of the detection module (3) is higher than the detection comparison voltage U det If a foreign metal object is found to have entered the system, it will stop working. If the detection voltage of the detection module (3) is lower than the monitoring voltage, the system switches to the first working state. S 1. Continue working.

8. The detection method of the SPWM-controlled wireless charging system metal foreign object detection device according to claim 7, characterized in that: The inverter circuit (4) is a full-bridge inverter circuit, and the switching transistor of the full-bridge inverter circuit is a power MOSFET switching transistor; the control mode of the MOSFET switching transistor adopts the bipolar SPWM control method.

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