Filtering method and circuit for UPS (Uninterrupted Power Supply) power frequency voltage and current sampling

Abstract

The invention provides a filtering method and circuit for UPS power frequency voltage and current sampling, and the method comprises the steps: sequentially carrying out the first-stage T-type filtering, operational amplifier isolation amplification and second-stage first-order RC filtering of a voltage sampling signal after isolation and voltage reduction, and forming a second-order voltage sampling filtering channel; sequentially performing first-stage T-type filtering, operational amplifier isolation amplification and second-stage first-order RC filtering on the isolated and converted current sampling signal to form a second-order current sampling filtering channel; wherein the second-order voltage sampling and filtering channel and the second-order current sampling and filtering channel have the same turning frequency fc, and fc is smaller than or equal to 1.7 kHz. The second-order filtering channels with the same low turning frequency are adopted to process the voltage signals and the current signals respectively, the problem that the anti-interference capacity and the signal precision are difficult to consider at the low PWM carrier frequency is effectively solved, PWM switching noise is remarkably restrained, and the sampling precision and the system reliability are improved.

Description

Technical Field

[0001] This application relates to the field of uninterruptible power supply (UPS) technology, and more specifically to a filtering method and circuit for sampling UPS power frequency voltage and current. Background Technology

[0002] In the circuit design of UPS power supply, accurate sampling of AC output voltage and current is the foundation for achieving high-quality control and protection. Figure 1 This paper illustrates a commonly used UPS voltage and current sampling and filtering circuit.

[0003] In this circuit, the UPS phase voltage / line voltage Ua is stepped down by resistor R1, then isolated by voltage transformer T1 to obtain signal Ua1. Ua1 is then filtered by a first-order RC low-pass filter composed of resistor R2 and capacitor C2 to remove high-frequency interference before being sent to the ADC0 port of the microcontroller MCU for analog-to-digital conversion. Similarly, the UPS phase current / line current Ia is isolated by current transformer T11 and converted into voltage signal Uia1 by resistor R11. It is then processed by a first-order RC low-pass filter composed of resistor R12 and capacitor C12 before being sent to the ADC1 port of the MCU.

[0004] The corner frequency fc of the aforementioned first-order RC filter determines its filtering characteristics. For the voltage sampling channel, its corner frequency fc2 = 1 / (2πR2C2); for the current sampling channel, its corner frequency fc12 = 1 / (2πR12C12). To obtain good signal accuracy at the power frequency, such as 50Hz / 60Hz, the filter's corner frequency is usually required to be 15 times or more of the power frequency, typically selected in the range of 1kHz to 2kHz. However, the PWM modulation frequency of a power frequency UPS is usually in the range of 4kHz to 40kHz. According to sampling stability theory, to prevent the PWM carrier frequency from interfering with the sampling accuracy of the power frequency signal, the gain of the filter's amplitude-frequency characteristic at the minimum PWM frequency point should be sufficiently small.

[0005] Figure 2 Showing Figure 1 The Bode plots of the first-order filter circuit shown are presented under different supply voltages, such as Vdd = 5V or 3V, and different corner frequencies fc = 1kHz or 2kHz. Analysis. Figure 2It can be seen that when the PWM frequency is 4kHz, only under the conditions of Vdd=3V and fc=1kHz, the filter's gain curve lies below the 0dB line on the horizontal axis at the 4kHz frequency point, effectively suppressing interference at that frequency. In the other three cases, such as Vdd=5V and fc=1kHz, and Vdd=3V / 5V and fc=2kHz, the gain curve remains above the horizontal axis at 4kHz, meaning that the PWM signal will significantly interfere with the power frequency sampling value. Even if the PWM frequency is increased to 10kHz, the curve for Vdd=5V and fc=2kHz still does not meet the requirements. Considering the accuracy deviation and temperature drift of resistors and capacitors in actual circuits, a PWM frequency of 20kHz or higher is often required to reliably suppress interference.

[0006] Therefore, existing first-order RC sampling filter circuits have a significant drawback: at lower PWM frequencies, especially in the common range of 4kHz to 10kHz, it is difficult to effectively suppress the interference of the PWM carrier frequency on the sampled values ​​while ensuring the accuracy of the power frequency signal. This contradiction limits the design space of the filter circuit parameters and may affect the reliability and performance of medium- and high-power UPS products that use lower PWM frequencies. Summary of the Invention

[0007] In view of this, this application provides a filtering method and circuit for sampling UPS power frequency voltage and current to solve or alleviate the technical problems existing in the prior art, and at least provides a beneficial option.

[0008] The technical solution of this application embodiment is a filtering method for sampling UPS power frequency voltage and current, which includes the following steps: The voltage sampling signal after isolation and step-down is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order voltage sampling filtering channel. The current sampling signal after isolation conversion is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order current sampling filtering channel. The second-order voltage sampling filter channel and the second-order current sampling filter channel both have the same corner frequency fc, and fc≤1.7kHz.

[0009] A further preferred embodiment: the cutoff frequency fc is set to 1 kHz.

[0010] A further preferred embodiment is that the first-stage T-type filter and the second-stage first-order RC filter are isolated from each other by an operational amplifier.

[0011] Based on the same concept, this application also provides a UPS power frequency voltage and current sampling and filtering circuit, including a voltage sampling branch and a current sampling branch, characterized in that: The voltage sampling branch includes: The first-stage T-type filter unit, the operational amplifier isolation amplifier unit, and the second-stage first-order RC filter unit, connected in sequence, constitute a second-order filter structure. The current sampling branch includes: The first-stage T-type filter unit, the operational amplifier isolation amplifier unit, and the second-stage first-order RC filter unit, connected in sequence, constitute a second-order filter structure. The filtering units at each stage in the voltage sampling branch and the current sampling branch have the same corner frequency fc, and fc≤1.7kHz.

[0012] A further preferred embodiment: the cutoff frequency fc is set to 1 kHz.

[0013] A further preferred embodiment: the first-stage T-type filter unit in the voltage sampling branch is composed of a first resistor (R2), a second resistor (R3), and a first capacitor (C2); The second-stage first-order RC filter unit in the voltage sampling branch consists of a third resistor (R5) and a second capacitor (C5). The first-stage T-type filter unit in the current sampling branch consists of a fourth resistor (R12), a fifth resistor (R13), and a third capacitor (C12). The second-stage first-order RC filter unit in the current sampling branch consists of a sixth resistor (R15) and a fourth capacitor (C15).

[0014] Further optimization: The corner frequency fc2 of the first-stage T-type filter unit satisfies:

[0015] The above formula; The corner frequency fc5 of the second-stage first-order RC filter unit satisfies: The above formula; The corner frequency fc12 of the first-stage T-type filter unit satisfies: The above formula; The corner frequency fc15 of the second-stage first-order RC filter unit satisfies: The above formula.

[0016] A further preferred embodiment includes an AC zero-point boost and isolation circuit, the output of which is connected to the ADC reference terminal of the microcontroller.

[0017] Further preferred: Applicable to UPS systems with an output power greater than or equal to 10kVA.

[0018] The embodiments of this application have the following advantages due to the adoption of the above technical solutions: This application significantly improves the ability to suppress switching noise at lower PWM carrier frequencies by using second-order filter channels with the same cutoff frequency to filter voltage and current signals separately. It solves the problem of difficulty in balancing anti-interference and signal accuracy in the prior art. The method and its circuit structure can more effectively filter out high-frequency components of PWM, ensuring that the sampling signal sent to the microcontroller reflects the true value of the power frequency voltage and current more purely and accurately, thereby improving the control accuracy and operational reliability of the UPS system. It is especially suitable for medium and high power applications of 10kVA and above. At the same time, the voltage and current sampling channels adopt a symmetrical and consistent second-order filter structure, the circuit design is clear, easy to analyze and calculate, and also reduces the complexity of production debugging and subsequent maintenance.

[0019] The above overview is for illustrative purposes only and is not intended to be limiting in any way. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features of this application will become readily apparent from the accompanying drawings and the following detailed description. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a UPS voltage and current sampling and filtering circuit used in existing technology.

[0022] Figure 2 The Bode plot is for a first-order filter circuit used in existing technology.

[0023] Figure 3 The Bode plot is for a second-order filter circuit with the same corner frequency.

[0024] Figure 4 The Bode plot is for a second-order filter circuit with the same corner frequency of 1.7kHz.

[0025] Figure 5 This application discloses a UPS power frequency voltage and current sampling and filtering circuit.

[0026] Figure 6This is a flowchart of a filtering method for sampling power frequency voltage and current in a UPS according to this application. Detailed Implementation

[0028] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of this application. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0029] The embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0030] This application proposes a UPS power frequency voltage and current sampling filtering method and circuit based on a second-order filter structure. The core concept of this method and circuit is to construct second-order filter channels for voltage and current signals, respectively, which are formed by connecting two first-order filter stages with the same low cutoff frequency through operational amplifier isolation.

[0031] Please see Figure 6 The flowchart illustrates an embodiment of the filtering method of this application. The method mainly includes the following steps: The voltage sampling signal after isolation and step-down is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order voltage sampling filtering channel. The current sampling signal after isolation conversion is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order current sampling filtering channel. The second-order voltage sampling filter channel and the second-order current sampling filter channel both have the same corner frequency fc, and fc≤1.7kHz.

[0032] In this embodiment, it is understood more specifically as follows: Step S10: Isolate and step down the input voltage signal Ua to obtain a first intermediate signal, such as Ua1. Isolate and convert the input current signal Ia to obtain a second intermediate signal, such as Uia1. Step S20: The first intermediate signal is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order voltage sampling filter channel. Step S30: The second intermediate signal is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order current sampling filter channel.

[0033] The second-order voltage sampling filter channel and the second-order current sampling filter channel are configured to have the same corner frequency fc, and fc ≤ 1.7kHz.

[0034] In a preferred embodiment, the cutoff frequency fc is set to 1kHz. This value satisfies the sampling accuracy requirement of more than 15 times that of the power frequency signal, taking into account fluctuations up to 65Hz, and also lays a theoretical foundation for providing sufficient attenuation margin at the 4kHz frequency point. See below for details. Figure 3 , Figure 4 Analysis.

[0035] In steps S20 and S30, the first-stage filter and the second-stage filter are isolated from each other by an operational amplifier, which ensures that the characteristics of the two-stage filters are independent and easy to design and analyze.

[0036] To more intuitively understand the effects of the technical solution in this application, please refer to... Figure 3 and Figure 4 Please provide an explanation. Figure 3 The Bode plot is shown when two second-order filter circuits with the same corner frequency are used, at fc=1kHz. As shown in the figure, regardless of whether the MCU power supply voltage Vdd is 5V or 3V, its amplitude-frequency response curve corresponds to... Figure 3 In the diagram, ① and ② are both stably located below the horizontal axis at the 4kHz frequency point, with a safety margin. This indicates that the proposed solution can effectively suppress interference at this common PWM frequency. Figure 4 This further demonstrates that even if the actual cutoff frequency fc rises to 1.7kHz due to component parameter deviations, under the condition of Vdd=5V, its curve can still be located near the lower horizontal axis at 4kHz, indicating that the system still possesses basic anti-interference capabilities. This proves the rationality and robustness of setting the upper limit of fc to 1.7kHz in this application.

[0037] Please see Figure 5 It demonstrates a specific circuit embodiment for implementing the above method: a UPS power frequency voltage and current sampling and filtering circuit, including a voltage sampling branch and a current sampling branch, characterized in that: The voltage sampling branch includes: The first-stage T-type filter unit, the operational amplifier isolation amplifier unit, and the second-stage first-order RC filter unit, connected in sequence, constitute a second-order filter structure. The current sampling branch includes: The first-stage T-type filter unit, the operational amplifier isolation amplifier unit, and the second-stage first-order RC filter unit, connected in sequence, constitute a second-order filter structure. The filtering units at each stage in the voltage sampling branch and the current sampling branch have the same corner frequency fc, and fc≤1.7kHz.

[0038] In this embodiment, the corner frequency fc is set to 1kHz.

[0039] In this embodiment, the first-stage T-type filter unit in the voltage sampling branch is composed of a first resistor R2, a second resistor R3, and a first capacitor C2; The second-stage first-order RC filter unit in the voltage sampling branch consists of a third resistor R5 and a second capacitor C5. The first-stage T-type filter unit in the current sampling branch consists of a fourth resistor R12, a fifth resistor R13, and a third capacitor C12. The second-stage first-order RC filter unit in the current sampling branch consists of a sixth resistor R15 and a fourth capacitor C15.

[0040] Specifically, in combination Figure 5 visible: The first-stage T-type filter unit in the voltage sampling branch consists of a first resistor R2, a second resistor R3, and a first capacitor C2. The current flows in at the connection point of R2 and R3, and one end of C2 is grounded, forming a T-type structure.

[0041] The operational amplifier isolation amplifier unit in the voltage sampling branch is based on operational amplifier N1:2. Its inverting input is connected to the output of the aforementioned T-type filter unit, i.e., the connection point of R3 and R4. The non-inverting input is connected to the reference ground or bias circuit via a resistor. The feedback resistor R4 is connected between the output and the inverting input to form a proportional amplifier.

[0042] The second-stage first-order RC filter unit in the voltage sampling branch consists of a third resistor R5 and a second capacitor C5, connected in series between the output of op-amp N1:2 and the ADC1 port of the MCU.

[0043] The current sampling branch has a structure symmetrical to the voltage branch: its first-stage T-type filter unit consists of the fourth resistor R12, the fifth resistor R13 and the third capacitor C12; the operational amplifier isolation amplifier unit is based on N1:3 and is equipped with feedback resistor R14; the second-stage first-order RC filter unit consists of the sixth resistor R15 and the fourth capacitor C15, and is output to the ADC2 port of the MCU.

[0044] In this embodiment, The corner frequency fc2 of the first-stage T-type filter unit satisfies:

[0045] The above formula; The corner frequency fc5 of the second-stage first-order RC filter unit satisfies: The above formula; The corner frequency fc12 of the first-stage T-type filter unit satisfies: The above formula; The corner frequency fc15 of the second-stage first-order RC filter unit satisfies: The above formula.

[0046] During the design process, appropriate resistor and capacitor values ​​are selected so that fc2, fc5, fc12, and fc15 are all equal to the set fc, such as 1kHz.

[0047] Furthermore, in this embodiment, such as Figure 5 As shown, the circuit embodiment of this application also includes an AC zero-point boosting and isolation circuit, the output of which is connected to the ADC0 terminal of the microcontroller. This circuit mainly consists of voltage divider resistors R31 and R32 and a voltage follower, where the voltage follower can be constructed from operational amplifiers N1:1. R31 and R32 divide the power supply voltage to generate a DC bias reference voltage. After sampling this reference voltage with high input impedance and driving it with low output impedance, the voltage follower outputs it to an ADC terminal of the MCU, as shown in ADC0. This design effectively reduces the impact of voltage divider network accuracy and load changes on the reference voltage, providing a stable and accurate AC signal zero-point reference for the analog-to-digital conversion of voltage signal Ua3 and current signal Uia3, thereby improving the calculation accuracy of the entire sampling system.

[0048] In this embodiment, it is applied to UPS systems with an output power greater than or equal to 10kVA. In such medium- to high-power applications, the switching losses of power devices, heat dissipation, and electromagnetic environment are more complex, placing higher demands on the anti-interference capability and reliability of the sampling circuit. This solution, through its excellent anti-PWM interference characteristics and stable second-order filtering structure, can significantly improve the operational stability and control accuracy of such high-performance UPS systems.

[0049] In summary, this application effectively solves the technical contradictions pointed out in the background art by constructing symmetrical second-order filter channels with the same low corner frequency, and supplementing them with load isolation, precise frequency design and stable zero-point reference, thus achieving significant beneficial effects.

[0050] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various variations or substitutions within the technical scope disclosed in this application, and these should all be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A filtering method for sampling UPS power frequency voltage and current, characterized in that, Includes the following steps: The voltage sampling signal after isolation and step-down is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order voltage sampling filtering channel. The current sampling signal after isolation conversion is sequentially subjected to first-stage T-type filtering, operational amplifier isolation amplification, and second-stage first-order RC filtering to form a second-order current sampling filtering channel. The second-order voltage sampling filter channel and the second-order current sampling filter channel both have the same corner frequency fc, and fc≤1.7kHz.

2. The filtering method according to claim 1, characterized in that, The cutoff frequency fc is set to 1 kHz.

3. The filtering method according to claim 1, characterized in that, The first-stage T-type filter and the second-stage first-order RC filter are isolated from each other by an operational amplifier.

4. A UPS power frequency voltage and current sampling and filtering circuit, comprising a voltage sampling branch and a current sampling branch, characterized in that: The voltage sampling branch includes: The first-stage T-type filter unit, the operational amplifier isolation amplifier unit, and the second-stage first-order RC filter unit, connected in sequence, constitute a second-order filter structure. The current sampling branch includes: The first-stage T-type filter unit, the operational amplifier isolation amplifier unit, and the second-stage first-order RC filter unit, connected in sequence, constitute a second-order filter structure. The filtering units at each stage in the voltage sampling branch and the current sampling branch have the same corner frequency fc, and fc≤1.7kHz.

5. The sampling filter circuit according to claim 4, characterized in that, The cutoff frequency fc is set to 1 kHz.

6. The sampling filter circuit according to claim 4, characterized in that, The first-stage T-type filter unit in the voltage sampling branch consists of a first resistor (R2), a second resistor (R3), and a first capacitor (C2); The second-stage first-order RC filter unit in the voltage sampling branch consists of a third resistor (R5) and a second capacitor (C5). The first-stage T-type filter unit in the current sampling branch consists of a fourth resistor (R12), a fifth resistor (R13), and a third capacitor (C12). The second-stage first-order RC filter unit in the current sampling branch consists of a sixth resistor (R15) and a fourth capacitor (C15).

7. The sampling filter circuit according to claim 6, characterized in that, The corner frequency fc2 of the first-stage T-type filter unit satisfies: ； The above formula; The corner frequency fc5 of the second-stage first-order RC filter unit satisfies: ； The above formula; The corner frequency fc12 of the first-stage T-type filter unit satisfies: ； The above formula; The corner frequency fc15 of the second-stage first-order RC filter unit satisfies: ； The above formula.

8. The sampling filter circuit according to claim 4, characterized in that, It also includes an AC zero-point boost and isolation circuit, the output of which is connected to the ADC reference terminal of the microcontroller.

9. The sampling filter circuit according to any one of claims 4 to 8, characterized in that, It is applicable to UPS systems with an output power greater than or equal to 10kVA.

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