A sampling circuit, active EMI filter and switching power supply

Abstract

The application provides a current sampling circuit, an active EMI filter and a switching power supply, wherein the current sampling circuit comprises a current transformer, a filter device, a current signal to voltage signal network, a first output port and a second output port; the current transformer comprises a primary winding, a first secondary winding and a second secondary winding; the primary winding is connected in series to a first power bus on the grid side, the first secondary winding is connected in series to a second power bus on the grid side, one end of the filter device and one end of the second secondary winding are connected together as the first output port, the other end of the filter device and one end of the current signal to voltage signal network are connected together as the second output port, and the other end of the second secondary winding and the other end of the current signal to voltage signal network are connected together and used for grounding. The application can improve the filtering effect of the cascaded active EMI filter.

Description

Technical Field

[0001] This invention relates to the field of conducted EMI suppression technology for power electronic devices, and particularly to a current sampling circuit, an active EMI filter, and a switching power supply. Background Technology

[0002] Active EMI filters cancel EMI noise by injecting an inverted compensation signal, and are a key technology for miniaturizing filter modules in power electronic systems. Among these, Figure 1 The present invention relates to a cascaded active EMI filter based on current sampling, which is derived from Chinese patent application No. 202411695442.2. The feedback and feedforward paths of the active loop share a current sampling magnetic element, namely a current transformer. Due to the reuse of the current sampling magnetic element, both cost and filtering effect can be taken into account, making it a preferred solution for high-performance applications.

[0003] However, in the aforementioned cascaded active EMI filter scheme, the multiplexing of the current sampling magnetic element means that a single sampling circuit cannot simultaneously match the independent requirements of the two loops for signal phase and amplitude, thus introducing a significant phase deviation in the critical mid-to-low frequency range (150kHz to 1MHz). This deviation directly weakens the noise cancellation accuracy, leading to a decrease in the insertion loss of the cascaded active EMI filter and limiting its full performance.

[0004] Specifically, when the above solution is applied to a high-power low-frequency switching power supply system, a second-order high-pass filter circuit needs to be used in the sampling stage. This addition inevitably leads to the sampling circuit being unable to simultaneously match the independent requirements of the feedforward and feedback paths for signal phase and amplitude, ultimately causing one loop to completely fail. The measured filtering effect of a conventional cascaded active EMI filter solution is as follows: Figure 2 As shown, the horizontal axis represents frequency, and the vertical axis represents the frequency. Curve 1 represents the QP (Quasi-Peak) limit for conducted emissions of Class A devices in the EN55032 standard; curve 2 represents the AV (Average) limit for conducted emissions of Class A devices in the EN55032 standard; curve 3 represents the conducted emissions test results of a conventional cascaded active filter under operating conditions; and curve 4 represents the conducted emissions test results of a conventional cascaded active filter under non-operating conditions. Figure 2It can be seen that when conventional cascaded active filters are applied to high-power low-frequency switching power supply systems, they not only fail to reduce conducted emissions as expected, but also make the conducted emissions worse in some frequency bands. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to provide a current sampling circuit, an active EMI filter, and a switching power supply, which at least partially solves one of the technical problems existing in the prior art.

[0006] As a first aspect of the present invention, the technical solution of the provided current sampling circuit embodiment is as follows:

[0007] A current sampling circuit is applied to an active EMI filter, which is connected between the power grid and a switching power supply. The active EMI filter includes an active filter and a passive filter connected sequentially from the power grid side. The current sampling circuit includes: a current transformer, a filter element, a current-to-voltage signal network, a first output port, and a second output port. The current transformer includes a primary winding, a first secondary winding, and a second secondary winding. The primary winding is connected in series to a first power bus on the power grid side, and the first secondary winding is connected in series to a second power bus on the power grid side. One end of the filter element and one end of the second secondary winding are connected together to form the first output port. The other end of the filter element and one end of the current-to-voltage signal network are connected together to form the second output port. The other end of the second secondary winding and the other end of the current-to-voltage signal network are connected together for grounding.

[0008] Furthermore, when the active EMI filter is working, the second secondary winding can acquire a first current signal characterizing the magnitude of the noise current on the power bus. The first current signal is divided into two branches at one end of the second secondary winding. The first branch current flows through the second secondary winding, and the second branch current flows through the filter device and the current signal to voltage signal network. The voltage at the first output port is the first current signal multiplied by the parallel impedance of the first branch and the second branch; The voltage at the second output port is the second branch current multiplied by the impedance of the current-to-voltage signal network; The inductance value of the second secondary winding is chosen such that the parallel impedance of the first branch and the second branch tends to be determined by the inductance of the second secondary winding, so that the first output port has the transmission characteristics of a first-order high-pass filter. The inductance values ​​of the second secondary winding, the impedance values ​​of the filter device, and the impedance values ​​of the current signal to voltage signal network make the second output port exhibit the transmission characteristics of a second-order high-pass filter.

[0009] Optionally, the inductance of the second secondary winding is greater than or equal to 10uH.

[0010] Optionally, the filtering device includes a high-pass filter device.

[0011] Optionally, the high-pass filtering device is a high-pass capacitor.

[0012] Optionally, the capacitance value of the high-pass capacitor is greater than or equal to 1nF and less than or equal to 1uF.

[0013] Optionally, the current-to-voltage signal network includes resistors.

[0014] Optionally, the resistance value of the resistor is greater than or equal to 1Ω and less than or equal to 1kΩ.

[0015] As a second aspect of the present invention, the technical solution of the provided active EMI filter embodiment is as follows: An active EMI filter is connected between a power grid and a switching power supply, comprising an active filter and a passive filter connected sequentially from the power grid side, wherein: the active filter includes a feedforward loop, a current sampling circuit as described in any of the first aspects above, and a feedback loop; The input terminal of the feedforward loop is connected to the first output terminal of the current sampling circuit, and the output terminal is connected to the power bus between the grid side and the current transformer to provide active insertion loss. The input terminal of the feedback loop is connected to the second output terminal of the current sampling circuit, and the output terminal is connected to the power bus between the current transformer and the passive filter to provide active insertion loss. The passive filter is used to provide passive insertion loss and to provide a high-impedance condition for the output of the feedback loop.

[0016] As a first aspect of the present invention, the technical solution of the provided switching power supply embodiment is as follows: A switching power supply, comprising the active EMI filter described in the second aspect above.

[0017] The advantages of this invention compared to the prior art are as follows: The current sampling circuit provided in this embodiment of the invention includes two output ports. The voltage signals acquired by the two output ports are different and are respectively output to the input terminal of the feedforward loop and the input terminal of the feedback loop to adapt to the phase deviation requirements of different loops for the input signals. This solves the phase deviation problem caused by the multiplexing of current sampling magnetic components in existing cascaded active EMI filters, ensuring that the phase deviation of each loop is within a reasonable range, thereby improving the filtering effect of the cascaded active EMI filter. Attached Figure Description

[0018] Figure 1 An active EMI filter disclosed in patent application 202411695442.2; Figure 2 The image shows the actual filtering effect of a conventional cascaded active EMI filter scheme. Figure 3 This is a topology block diagram of a dual-output noise current sampling circuit and a cascaded active EMI filter according to the present invention. Figure 4 Voltage-current gain curve of a cascaded EMI active filter using the current sampling circuit of the present invention; Figure 5 Insertion loss diagram of a cascaded EMI active filter using the current sampling circuit of the present invention; Figure 6 The diagram shows the filtering effect of a cascaded EMI active filter using the current sampling circuit of this invention. Detailed Implementation

[0019] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this application.

[0020] Furthermore, unless otherwise specified, the various embodiments and features described in this application may be combined with each other.

[0021] As a specific embodiment of the present invention, a current sampling circuit is provided. Please refer to [link / reference]. Figure 3This is applied to active EMI filters, which are connected between the power grid and the switching power supply. The active EMI filter includes an active filter and a passive filter connected sequentially from the power grid side. The current sampling circuit includes: a current transformer TX1, a filter element, a current-to-voltage signal network, a first output port Terminal1, and a second output port Terminal2. The current transformer includes a primary winding P1, a first secondary winding S1, and a second secondary winding S2. The primary winding P1 is connected in series to the first power bus on the power grid side, and the first secondary winding S1 is connected in series to the second power bus on the power grid side. One end of the filter element and one end of the second secondary winding S2 are connected together to form the first output port. The other end of the filter element and one end of the current-to-voltage signal network are connected together to form the second output port Terminal1. The other end of the second secondary winding S2 and the other end of the current-to-voltage signal network are connected together for grounding.

[0022] Please continue reading Figure 3 When the active EMI filter is working, the second secondary winding S2 group can acquire a first current signal that characterizes the magnitude of the noise current on the power bus. The first current signal is split into two branches at one end of the second secondary winding. The current in the first branch flows through the second secondary winding S2, and the current in the second branch flows through the filter device and the current signal to voltage signal network. The voltage at the first output port Terminal1 is the first current signal multiplied by the parallel impedance of the first branch and the second branch. The voltage at the second output port Terminal2 is the current in the second branch multiplied by the impedance of the current signal to voltage signal network.

[0023] As a specific embodiment of the present invention, a current sampling circuit is provided: the inductance value of the second secondary winding S2 is chosen such that the parallel impedance of the first branch and the second branch tends to be determined by the inductance of the second secondary winding S2, thereby the first output port Terminal1 has the transmission characteristics of a first-order high-pass filter, i.e., low attenuation and low offset; the inductance value of the second secondary winding S2, the impedance value of the filtering device, and the impedance value of the current signal to voltage signal network make the second output port Terminal2 exhibit the transmission characteristics of a second-order high-pass filter, i.e., high attenuation and high offset. Since the voltage signals obtained by the two output ports are different, they are respectively output to the input end of the feedforward loop and the input end of the feedback loop to adapt to the phase deviation requirements of different loops for the input signals, thereby solving the phase deviation problem caused by the multiplexing of current sampling magnetic components in existing cascaded active EMI filters, so that the phase deviation of each loop is within a reasonable range, thereby improving the filtering effect of the cascaded active EMI filter.

[0024] As a specific embodiment of the present invention, a current sampling circuit is provided, wherein the inductance of the second secondary winding S2 is greater than or equal to 10uH, thereby enabling the parallel impedance of the first branch and the second branch to be determined by the inductance of the second secondary winding S2, so that the first output port Terminal1 has the transmission characteristics of a first-order high-pass filter.

[0025] As a specific embodiment of the present invention, a current sampling circuit is provided, wherein the filtering device includes a high-pass filter device.

[0026] As a specific embodiment of the present invention, a current sampling circuit is provided, wherein the high-pass filter device is a high-pass capacitor C1.

[0027] As a specific embodiment of the present invention, a current sampling circuit is provided, wherein the capacitance value of the high-pass capacitor is greater than or equal to 1nF and less than or equal to 1uF, thereby setting the cutoff frequency of the current sampling circuit in a reasonable range, thereby ensuring that the current sampling circuit provides sufficient attenuation in the low-frequency band and reducing the impact on the phase of the sampling signal in the high-frequency band.

[0028] As a specific embodiment of the present invention, a current sampling circuit is provided, wherein the current signal to voltage signal network includes resistor 1.

[0029] As a specific embodiment of the present invention, a current sampling circuit is provided, wherein the resistance value of resistor R1 is greater than or equal to 1Ω and less than or equal to 1kΩ. By setting the resistance value of resistor R1 within this range, the transmission characteristics of the current sampling circuit are reasonably corrected.

[0030] As a specific embodiment of the present invention, an active EMI filter is provided. Please refer to [link / reference]. Figure 3 It is connected between the power grid and the switching power supply, including an active filter and a passive filter connected in sequence from the power grid side. The active filter includes a feedforward loop, any of the above-mentioned current sampling circuits, and a feedback loop. The input of the feedforward loop is connected to the first output of the current sampling circuit, and the output is connected to the power bus between the grid side and the current transformer to provide active insertion loss. The input of the feedback loop is connected to the second output of the current sampling circuit, and the output is connected to the power bus between the current transformer and the passive filter to provide active insertion loss. Passive filters are used to provide passive insertion loss and to provide a high-impedance condition at the output of the feedback loop.

[0031] It should be noted that the present invention does not limit the specific circuits of the feedforward loop and the feedback loop. Those skilled in the art can select them as needed. For example, the circuit may include a voltage amplifier circuit, a power amplifier circuit, and an output impedance circuit in sequence. The voltage amplifier circuit is used to amplify the voltage signal output from the output terminal of the current sampling circuit and output it to the power amplifier circuit. The power amplifier circuit is used to output the amplified voltage signal to the output impedance circuit and provide additional current to improve the current output capability. The output impedance circuit is used to convert the amplified voltage signal into a current signal to cancel the common-mode current in the power bus.

[0032] Furthermore, the present invention does not limit the specific circuit of the passive filter, and those skilled in the art can make selections as needed, such as including one or more common-mode inductors in series, and one or more safety Y capacitors in parallel after the common-mode inductors.

[0033] As a specific embodiment of the present invention, a switching power supply is provided, including any of the above-mentioned active EMI filters.

[0034] The following combination Figure 3 A detailed analysis of the working principle of the embodiments of the present invention is provided below: Current transformer TX1 acquires the raw noise current signal on the power bus, which can be expressed in the frequency domain as a function of frequency. The current signal is shunted by the secondary winding branch of the current transformer TX1 (i.e., the first branch mentioned above) and the series branch of the high-pass capacitor C1 and voltage conversion resistor R1 (referred to as the RC series branch, i.e., the second branch mentioned above). The impedances of the two branches are respectively... , Where f is the operating frequency of the circuit, This refers to the inductance of the second secondary winding. Let C1 be the capacitance value. Let R1 be the resistance value.

[0035] For the first output port Terminal1, the output voltage signal is: The product of the parallel impedances of the two branches, and when the inductance of the second secondary winding is small, its actual impedance is basically determined by the secondary inductance of TX1. Therefore, the output port Terminal1 has the transmission characteristics of a first-order high-pass filter, namely low attenuation and low offset.

[0036] For the second output port Terminal2, the output voltage signal is the current obtained from the RC series branch. The product of the impedance and the resistor R1 exhibits the transmission characteristics of a classic second-order high-pass filter, namely high attenuation and high offset.

[0037] The signal of the first output port Terminal1 As the input signal of the feedforward loop; the signal of the second output port Terminal2. As the input signal to the feedback loop; ultimately, the insertion loss of the feedforward loop is... The function is described as follows: The insertion loss of the feedback loop is... The function is described as follows: Total insertion loss is , and passive filter insertion loss The function is described as follows: .

[0038] Input current - output voltage gain of the first output port Terminal1 It is a function of frequency, described as:

[0039] in: The current transformer turns ratio is given by f, where f is the operating frequency of the circuit. This refers to the inductance of the second secondary winding. Let C1 be the capacitance value. Let R1 be the resistance value.

[0040] Input current - output voltage gain of the second signal output port Terminal2 It is a function of frequency, described as:

[0041] in: The current transformer turns ratio is given by f, where f is the operating frequency of the circuit. This refers to the inductance of the second secondary winding. Let C1 be the capacitance value. Let R1 be the resistance value.

[0042] right Taking the first derivative, we know that in the stopband of this current sampling circuit, the sampled signal is attenuated with a slope of 40dB / Dec, and its low-frequency phase shift is at most 90°; for Taking the first derivative, we can see that in the stopband of this current sampling circuit, the sampling signal is attenuated with a slope of 40dB / Dec, and its low-frequency phase shift can reach up to 180°. and Transmission characteristics such as Figure 4As shown, the horizontal axis represents frequency, and the vertical axis, from top to bottom, shows the following curves: the first curve represents the phase gain of the second output port; the second curve represents the phase gain of the first output port; the third curve represents the amplitude gain of the first output port; and the fourth curve represents the amplitude gain of the second output port.

[0043] For the feedforward loop, since it requires relatively small loop gain (only unity current gain is needed), but has strict phase deviation requirements, the voltage signal output from the first output port Terminal1 is used as the input signal. For the feedback loop, since it requires relatively large loop gain, but has less stringent phase deviation requirements, the voltage signal output from the second output port Terminal2 is used as the input signal. The component parameters are set as follows: The feedforward loop achieves unity current gain, the feedback loop gain is 10dB, and the passive filter is a 5mH (measured at 10kHz) nanometer amorphous common-mode inductor; the final insertion loss is as follows: Figure 5 As shown, the final filtering effect is as follows: Figure 6 As shown. Figure 5 The horizontal axis represents frequency, and the vertical axis, from top to bottom, shows the following curves: the first curve represents the insertion loss when the feedforward loop operates alone; the second curve represents the insertion loss when the feedback loop operates alone; and the third curve represents the insertion loss when both the feedforward and feedback loops operate together. Figure 6 The horizontal axis represents frequency, and the vertical axis represents the following curves: Curve 1 shows the QP (Quasi-Peak) limit for conducted emissions of Class A devices in the EN55032 standard; Curve 2 shows the AV (Average) limit for conducted emissions of Class A devices in the EN55032 standard; Curve 3 shows the conducted emissions test results when the cascaded active EMI filter using the current sampling circuit of this invention is not operating; Curve 4 shows the conducted emissions test results when the cascaded active EMI filter using the current sampling circuit of this invention is operating. Figure 4 It can be seen that the cascaded active EMI filter using the current sampling circuit of this invention exhibits good filtering performance in both the low-frequency and mid-frequency ranges when applied to high-power low-frequency switching power supply systems. Clearly, both filter loops of the cascaded active EMI filter using the current sampling circuit of this invention can achieve considerable insertion loss, with the final total insertion loss reaching over 80dB.

[0044] In summary, in existing cascaded active EMI filter solutions, the multiplexing of current sampling magnetic components means that a single sampling circuit cannot simultaneously match the independent requirements of the two loops for signal phase and amplitude. This introduces a significant phase deviation in the critical mid-to-low frequency range (150kHz to 1MHz), which directly weakens the noise cancellation accuracy, leading to a decrease in the insertion loss of the dual-loop active EMI filter and limiting its full performance. The dual-output noise current sampling circuit provided by this invention can ensure that the phase deviation of each filtering loop is within a reasonable range, thereby improving the filtering effect of the dual-loop active EMI filter.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A current sampling circuit applied to an active EMI filter, the active EMI filter being connected between the power grid and a switching power supply, comprising an active filter and a passive filter connected sequentially from the power grid side, characterized in that, The current sampling circuit includes: a current transformer, a filter element, a current signal to voltage signal network, a first output port, and a second output port; the current transformer includes a primary winding, a first secondary winding, and a second secondary winding; the primary winding is connected in series to the first power bus on the grid side, the first secondary winding is connected in series to the second power bus on the grid side, one end of the filter element and one end of the second secondary winding are connected together to form the first output port, the other end of the filter element and one end of the current signal to voltage signal network are connected together to form the second output port, and the other end of the second secondary winding and the other end of the current signal to voltage signal network are connected together for grounding.

2. The current sampling circuit according to claim 1, characterized in that: When the active EMI filter is working, the second secondary winding can acquire a first current signal that characterizes the magnitude of the noise current on the power bus. The first current signal is divided into two branches at one end of the second secondary winding. The first branch current flows through the second secondary winding, and the second branch current flows through the filter device and the current signal to voltage signal network. The voltage at the first output port is the first current signal multiplied by the parallel impedance of the first branch and the second branch; The voltage at the second output port is the second branch current multiplied by the impedance of the current-to-voltage signal network; The inductance value of the second secondary winding is chosen such that the parallel impedance of the first branch and the second branch tends to be determined by the inductance of the second secondary winding, so that the first output port has the transmission characteristics of a first-order high-pass filter. The inductance values ​​of the second secondary winding, the impedance values ​​of the filter device, and the impedance values ​​of the current signal to voltage signal network make the second output port exhibit the transmission characteristics of a second-order high-pass filter.

3. The current sampling circuit according to claim 2, characterized in that: The inductance of the second secondary winding is greater than or equal to 10uH.

4. The current sampling circuit according to claim 1, characterized in that: The filtering device includes a high-pass filter device.

5. The current sampling circuit according to claim 1, characterized in that: The high-pass filter device is a high-pass capacitor.

6. The current sampling circuit according to claim 5, characterized in that: The capacitance value of the high-pass capacitor is greater than or equal to 1nF and less than or equal to 1uF.

7. The current sampling circuit according to claim 1, characterized in that: The current-to-voltage signal network includes resistors.

8. The current sampling circuit according to claim 7, characterized in that: The resistance value is greater than or equal to 1Ω and less than or equal to 1kΩ.

9. An active EMI filter connected between a power grid and a switching power supply, comprising an active filter and a passive filter connected sequentially from the power grid side, characterized in that: The active filter includes a feedforward loop, a current sampling circuit as described in any one of claims 1 to 8, and a feedback loop; The input terminal of the feedforward loop is connected to the first output terminal of the current sampling circuit, and the output terminal is connected to the power bus between the grid side and the current transformer to provide active insertion loss. The input terminal of the feedback loop is connected to the second output terminal of the current sampling circuit, and the output terminal is connected to the power bus between the current transformer and the passive filter to provide active insertion loss. The passive filter is used to provide passive insertion loss and to provide a high-impedance condition for the output of the feedback loop.

10. A switching power supply, characterized in that: Includes the active EMI filter as described in claim 9.

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