CN215871185U - Switching power supply port conduction noise automatic detection cancellation circuit - Google Patents

Abstract

The utility model discloses a switching power supply port conducted noise automatic detection and cancellation circuit, which comprises a common mode noise cancellation circuit and a differential mode noise cancellation circuit, wherein the common mode noise cancellation circuit comprises a noise sampling capacitor and an auto-transformer or a transmission line transformer, a center tap of the auto-transformer or the transmission line transformer is connected with the same potential ground, and two ends of the auto-transformer or two ends of the transmission line transformer are respectively connected with a power line through the noise sampling capacitor; the differential mode noise cancellation circuit consists of a common mode inductor T4 and a differential mode noise separation circuit which are connected in series on a power line, and a high-voltage capacitor is connected between an equipotential ground and a system ground or the ground. The utility model adopts a black box processing mode, does not need to know the internal noise transmission path, directly obtains direct cancellation, has good filtering effect, does not causticize the values of common mode inductance and capacitance compared with the traditional passive filtering, can cancel the noise in both directions inside and outside the power supply, and considers the requirements of safety voltage resistance and leakage current.

Description

Switching power supply port conduction noise automatic detection cancellation circuit

Technical Field

The utility model belongs to the technical field of switching power supplies, and particularly relates to a switching power supply port conduction noise automatic detection cancellation circuit.

Background

The main interference forms present in switching power supplies are conducted interference and near-field radiated interference. The conducted interference is divided into two independent noises: differential mode noise (DM noise) and common mode noise (CM noise). EMC test practices teach that common mode noise is the dominant cause of test problems. It is caused by a voltage jump (high dv/dt) generated during switching operation inside the switching power supply. With the development of switching power supply technology, in order to obtain smaller volume and smaller loss, higher switching frequency and faster switching speed are adopted, resulting in increasingly severe common mode interference. Corresponding standards are set by countries all over the world to limit the electromagnetic interference emission level of electronic and electric products. To meet these criteria, the most efficient approach is to use input and output EMI filters. Conventional EMI filters employ passive components, such as single-stage or multi-stage passive filters composed of a common-mode inductor, a differential-mode inductor, and X, Y capacitors, which are all used with more passive components, such as filter inductors and capacitors. Since the distributed parameters of the passive component, such as the distributed capacitance and lead inductance, etc. (such as the winding distributed capacitance of the difference and common mode inductance, the series equivalent inductance of the filter capacitance, etc.) have great influence on the filtering effect, especially in the high-frequency region, the influence of the distributed parameters is more serious and is difficult to control, thereby greatly attenuating the high-frequency filtering characteristic. The magnetic core, which is the main component of the common-mode and differential-mode filter inductors, has frequency-varying characteristics in electromagnetic parameters, and the performance of the magnetic core itself is limited by the process level and the manufacturing materials, which brings many limitations to the application of the magnetic core. Especially in the common mode noise suppression method, the total amount of capacitance is limited by safety regulations and other factors, and the increasingly high common mode noise can be suppressed only by increasing the common mode inductance. This increases the volume and cost of the common mode filter and makes the filter more and more proportional to the volume and cost of the device. In general, the conventional passive EMI filter has the following disadvantages: the size is large, the manufacturing cost is high, and therefore the requirements of increasingly miniaturization and high density of the switching power supply cannot be met; the attenuation frequency band of the filter is narrow, the insertion loss of the filter is improved by increasing the inductance and the capacitance in a low frequency band, and unnecessary oscillation is possibly caused due to the influence of distribution parameters in a high frequency band to influence the filtering characteristic; the current switching power supply is required to be a green power supply with small volume, light weight, high efficiency, high reliability and high power density, but the reduction of the volume is limited because of a filter which is an important component of the switching power supply. Passive filters are not well suited to the current trend due to volume and weight limitations.

The active filter is still in a search stage, and the active filter has the advantages of small volume, convenience in integration and good dynamic characteristic due to the adoption of an active elimination technology, is widely concerned, but is complex in active filter design, poor in stability and high in cost. Active filters suppress noise by injecting a compensation signal because they use active cancellation techniques, which means that if they are not properly designed or are not working properly, they may increase noise in the circuit. The influence factors affecting the stability and compensation effect of the active filter are many, such as: the internal impedance of a noise source, the amplification factor and the response speed of a compensation circuit, the high-frequency parasitic parameters of a magnetic piece, a control scheme, the structure and the value of a combined passive filter and the like, and because of the influence of the factors, the filter with universality theoretically can not achieve an ideal filtering effect when working in different electronic equipment or working under different operating conditions. The noise source impedance has an effect on the performance of the active filter, which is complex to control. And the active filtering is mostly aimed at the internal switching power supply noise source of the equipment, noise from the outside is not considered, when a feedforward system designed facing internal noise faces external noise, the feedforward system becomes a feedback system, and vice versa, and the control is not good.

A typical implementation of the circuit optimization scheme for suppressing common mode interference for the switching power supply in the industry is a common mode inverting cancellation technique. As shown in fig. 1, the reverse phase compensation coil and the compensation capacitor Cg' are added to the original inductance, and the current flowing through the parasitic capacitor Cg is compensated in the reverse direction, so that the current cannot be transmitted to the power port, thereby reducing the common mode interference. This typical common mode anti-phase cancellation technique requires a designer to clearly understand the noise generation and coupling mechanism inside the device and to purposely implement appropriate compensation (e.g., sizing of the compensation capacitor), which may also increase noise; the existing common-mode reverse phase elimination technology and other noise suppression measures derived from the technology are mainly used for compensating and balancing a common-mode noise path inside a power supply, particularly a noise path caused by equivalent common-mode capacitance between a switching device and a heat sink, and a similar mode is difficult to adopt for differential mode noise. Moreover, due to the influence of device parasitic effect, the method can only realize the suppression of common mode noise in the middle and low frequency range.

SUMMERY OF THE UTILITY MODEL

The technical problem to be solved by the utility model is to provide an automatic detection and cancellation circuit for conducted noise of a switch power supply port, which adopts a circuit similar to an LISN to sample noise on a power line, utilizes a small-signal common-mode inductor to obtain a difference common-mode component, and adopts a cancellation technology or directly grounds a common-mode low impedance to remove the noise from the power line.

In order to solve the problems, the technical scheme adopted by the utility model is as follows:

a switching power supply port conducted noise automatic detection and cancellation circuit comprises a common mode noise cancellation circuit and a differential mode noise cancellation circuit, wherein the common mode noise cancellation circuit comprises a noise sampling capacitor and an auto-transformer or a transmission line transformer, a center tap of the auto-transformer or the transmission line transformer is connected with the same potential ground, and two ends of the auto-transformer or two ends of the transmission line transformer are respectively connected with a power line through the noise sampling capacitor; the differential mode noise cancellation circuit comprises a common mode inductor T4 and a differential mode noise separation circuit, wherein the common mode inductor T4 is connected in series with a power line, a primary coil of the common mode inductor is connected in series with the power line, one end of a secondary coil is connected with an equipotential ground, the other end of the secondary coil is connected with the output of the differential mode noise separation circuit, the input of the differential mode noise separation circuit is connected with the power line through a noise sampling capacitor, and a high-voltage capacitor is connected between the equipotential ground and a system ground or the earth.

Further, the power line is an alternating current power line or a direct current power line.

Furthermore, the withstand voltage value of the noise sampling capacitor is half of the working voltage plus a certain allowance, the capacitance value is 0.1 muF, the ESR (equivalent series resistance) is less than 0.5 omega, and the ESL (inductance) is less than muH.

Furthermore, the high-voltage capacitance withstand voltage value is 2kv, the capacitance value is 1000Pf, the parasitic resistance ESR is less than 0.5 omega, and the parasitic inductance ESL is less than muH.

Furthermore, the differential mode noise separation circuit comprises a broadband radio frequency transformer T2 and 2 50 ohm resistors, the dotted end of the primary winding of the broadband radio frequency transformer T2 is connected to the power line through a noise sampling capacitor, the other end of the primary winding is connected to the equipotential ground, the dotted end of the secondary winding of the broadband radio frequency transformer T2 is connected to the power line through the noise sampling capacitor, the other end of the secondary winding is connected to the equipotential ground through 2 series-connected 50 ohm resistors, and the connection node of the 2 series-connected 50 ohm resistors is connected to the common mode inductor T4.

Adopt the produced beneficial effect of above-mentioned technical scheme to lie in:

the utility model adopts the center tap of the autotransformer formed by the small-signal common-mode inductor or the primary center tap of the transmission line transformer without the secondary side to be grounded to eliminate the common mode, has better effect than the common-mode inductor which only depends on a large power line, has small volume and is convenient to realize. Common mode inductance and a differential mode separation circuit connected in series on a power line are adopted to eliminate differential mode. The black box processing mode is adopted, the internal noise transmission path does not need to be known, direct cancellation is directly obtained, the filtering effect is good, compared with the traditional passive filtering, the common mode inductance and capacitance value is not required, both internal and external noise of a power supply can be cancelled, and the requirements of safety voltage resistance and leakage current are met.

Drawings

FIG. 1 is a common mode reverse phase cancellation circuit of a prior art switching power supply;

FIG. 2 is a schematic diagram of the common mode isolation circuit of the present invention;

FIG. 3 is a schematic diagram of a differential mode separation circuit of the present invention;

FIG. 4 is a schematic diagram of a transmission line transformer common mode isolation circuit of the present invention;

FIG. 5 is a schematic diagram of a differential mode separation circuit of the transmission line transformer of the present invention;

FIG. 6 is a schematic diagram (not simplified) of the switching power supply port conducted noise automatic detection cancellation circuit of the present invention;

FIG. 7 is a schematic diagram of a differential-common mode noise filter circuit of a conventional switching power supply;

FIG. 8 is a schematic diagram (not simplified) of the common mode noise automatic detection and cancellation circuit of the switching power supply of the present invention;

FIG. 9 is a schematic diagram of the common mode noise automatic detection and cancellation circuit of the switching power supply of the present invention (simplified common mode inductor);

FIG. 10 is a schematic diagram of the automatic detection and cancellation circuit for common mode noise of the switching power supply according to the present invention (simplified common mode separation circuit);

FIG. 11 is a schematic diagram (simplified) of the common mode noise automatic detection and cancellation circuit of the switching power supply of the present invention;

fig. 12 is a schematic diagram of the switching power supply differential-common mode noise cancellation circuit of the present invention.

Detailed Description

The utility model is described in further detail below with reference to the attached drawings:

the utility model relates to an EMI passive filter circuit which is used in a switching power supply and adopts a self-detection automatic cancellation mode, noise on a power line is sampled by adopting a circuit similar to an LISN, a differential common-mode component is obtained by utilizing an autotransformer formed by a transmission line transformer, and a central tap of the autotransformer is adopted to directly discharge low impedance to the ground under the conditions that common-mode capacitance cannot be increased due to the limitation of safety regulations and the volume of common-mode inductance is limited, so that the differential common-mode noise on the power line is further reduced, common-mode noise immunity tests such as the conduction and the conduction immunity of an EMC test, the EFT immunity and the like are facilitated, and the radiation emission of the whole machine caused by the conduction of more than 30Mhz of the power line is reduced. Different from the traditional passive EMI filter based on the principle that the filter is mismatched with the impedance of a noise source, the passive filter for self-detection and automatic cancellation provided by the patent is insensitive to the impedance of the noise source, and can obtain a good filtering effect no matter the impedance is low or high.

In the common mode noise suppression method, the total amount of common mode capacitance cannot be increased due to the limitation of safety regulations and other factors, and the increasingly high common mode noise can be suppressed only by increasing the common mode inductance. However, increasing the common mode inductance is also limited by board area and the like. Active EMI filters, so-called AEFs, are not yet in the reach. What can achieve good filtering effect under the current filtering architecture?

Initial measurements of conducted EMI of current switching power supply products typically show insufficient attenuation of the EMI filter. In order to obtain a proper EMI filter design, the DM and CM noise voltage components of conducted emissions generated by the device under test (EUT) must be studied independently. Processing the DM and CM separately helps to determine and troubleshoot the relevant EMI sources, thereby simplifying the EMI filter design flow. Since EMI filters employ distinct filter elements to suppress DM and CM emissions. In this case, one common diagnostic check method is to separate the conducted noise into a DM noise voltage and a CM noise voltage.

The difference-common mode separating device for conduction noise of the switching power supply is used for intuitively knowing whether the current conduction emission problem of the switching power supply on a certain device is the common mode or the differential mode during EMC test? How large each is? And then a targeted rectification is performed on the circuit to obtain a test pass.

Here, we change the idea that since we know the magnitude of the difference common mode noise component, we can not simplify the circuit of separating the noise samples and implement it on the power board at low cost, and then we can directly obtain good test results if we want to eliminate the common mode or difference mode noise component from the power line?

When the difference and common mode separation circuit of the conducted noise of the switching power supply is investigated, the following conclusion can be obtained by reading relevant literature data: a good noise separator must have four conditions:

1) the output voltage of the noise separator depends on its input voltage;

2) the input impedance must be equal to that given by the standard, the noise separator must have an input impedance of 50 Ω, and this impedance is not affected by the input voltage, current, or other factors;

3) the CM and DM components of the input signal must pass through the corresponding outputs undistorted, which can be measured by Differential Mode Transmission Ratio (DMTR) and Common Mode Transmission Ratio (CMTR):

4) unwanted mode signals must be suppressed as much as possible. The parameters measuring this index are the differential mode rejection ratio DM rejection ratio (DMRR) and the common mode rejection ratio CM rejection ratio (CMRR).

The simplest noise separator is formed by a resistor, which has the problem that the impedance of the two input ports depends on the voltage applied to it. The noise separator design uses a broadband radio frequency transformer that compensates for the disadvantages of resistive noise separators, where the input impedance is affected by the input voltage. However, the separator using the wideband rf transformer as the main separating device generates a phenomenon of significant mode signal rejection performance degradation due to the stray effect under high frequency conditions, generally with 10-20dB attenuation, and some of them are even more serious, so the performance of the noise separator needs to be further improved.

The power mixer can greatly improve the separation performance of the interference mode signals, especially under the high-frequency condition, but the manufacturing cost is increased a lot, and the power mixer is usually expensive, so that the popularization and the application of the power mixer are influenced, and the power mixer is not to be implemented on a power supply single board.

The autotransformer is used for realizing energy transmission in a directional current transmission mode. A good autotransformer design has a negligible stray impedance compared to the source and load impedances of the circuit. The common mode/differential mode insertion loss (CMIL/DMIL) and the common mode/differential mode rejection ratio (CMRR/DMRR) of the noise separator based on the autotransformer and the common mode choke coil are superior to those of the noise separator based on the common broadband radio frequency transformer.

The noise separator may also be formed by an operational amplifier circuit. The separating circuit formed by the operational amplifier does not have the robustness as good as a noise separator formed by only using a passive device, and the operational amplifier circuit hardly achieves good effect at a high frequency of 30 MHz.

The Wang Shuo noise separator, an improved version of the noise separator, in which the secondary winding of the transformer is removed, thus eliminating the parasitic capacitance between the original secondary windings. Another improvement is the use of a Transmission Line Transformer (Transmission Line Transformer) which has the effect that the range where the differential mode suppression ratio DMRR is lower than-60 dB can be as high as 30 MHZ. The common mode rejection ratio is good, the CMRR interval below-60 dB reaches 15MHz, and the CMRR interval to 30MHz is still below-50 dB.

The transmission line transformer has the characteristics of wide frequency band, high application frequency, small volume, large bearing power and small loss, and is a good radio frequency device.

The transmission line transformer is formed by winding a transmission line (twisted wire, parallel line, coaxial line, etc.) on a magnetic core, wherein the magnetic core is made of ferrite material with high magnetic permeability and low loss, the diameter of the magnetic core can be large or small (determined according to the power), the diameter of the magnetic core is only a few millimeters, and the diameter of the magnetic core is dozens of millimeters.

For a general transformer, its own high frequency characteristics are poor. To improve the low frequency response, the number of primary winding turns is increased (inductance is increased), which in turn results in an increase in distributed capacitance and an increase in high frequency response. The high and low frequency characteristics can be greatly improved by adopting the high magnetic permeability magnetic core, but the magnetic cores have the optimal working frequency band, and when the frequency band is higher than the optimal working frequency band, the loss of the magnetic cores is increased, so that the transmission efficiency is reduced. Due to the influence of distributed capacitance and leakage inductance, even if a common transformer with a high-permeability magnetic core is adopted, the transformer still cannot work in a higher frequency band and transmit broadband signals. Transmission line transformers are often used in the radio frequency range due to their maximum frequency, which can reach hundreds of megahertz and even gigahertz.

For transmission line transformers, the line-to-line capacitance at any point is large and evenly distributed across the line because the coil is formed by two wires wound together in close proximity. Since the wire is wound around a high permeability core, the inductance of each small segment of the wire is large and evenly distributed throughout the wire. Therefore, the transmission line can be regarded as a coupling chain consisting of a plurality of inductors and capacitors, and the transmission line transformer utilizes the coupling between the inductors and the capacitors to complete the energy transmission. Therefore, in the transmission line transformer, the distributed capacitance between the two lines does not affect the transmission of high frequency energy, but is a necessary condition for electromagnetic energy conversion. Because the electromagnetic wave is mainly transmitted in the medium between the wires, the influence of the loss of the magnetic core on the signal transmission can be greatly reduced, so the highest working frequency of the transmission line transformer can be greatly improved, and the transmission of high-frequency and broadband signals by the transmission line transformer becomes possible. The preferred conducted noise differential-common mode separation circuit currently recognized in the industry is shown in fig. 2 and 3. Where VCM is (VL + VN)/2, VDM is (VL-VN)/2, T1, T2 use wideband RF transformers to achieve acceptable separation results over the frequency range covered by EMI, where the characteristic impedances (ZO) of T1 and T2 are 50 Ω and 100 Ω, respectively.

To reduce the number of devices, a radio frequency transformer or a transmission line transformer with a characteristic impedance of 50 ohms or 100 ohms may be uniformly used to obtain VCM and VDM, as shown in fig. 4 and 5.

The common mode voltage does not cause transformer action because the transformer needs a differential voltage for its operation, so that current is generated in the winding and a magnetic flux flows in the core. Ideally, the common mode choke presents zero impedance to the differential mode current, and for the common mode current, it is equivalent to connect a large inductor with a large impedance in series, so as to realize the suppression of the common mode current.

It is known that voltage is actually potential difference, all voltage measurements are potential difference tests of a test point relative to a certain reference point, and conducted emission tests or tests of conducted sensitivity, electric fast pulse group and the like are all based on a laboratory ground reference plane, namely the ground, as a potential zero base point. To perform the subtraction of VDM ═ 2 (VL-VN)/as shown in fig. 3, i.e. to subtract VN from VL, we connect VL and VN to the two terminals of the common mode inductance, which are the same name, and strip the differential mode signal from the total signal at the other end of the common mode inductance. VN is grounded at the other end of the common mode inductance coil, namely, the VN is in equipotential connection with a grounding reference point, so that VN exists in VL, VN also exists in the grounding reference, the VN and the grounding reference are synchronous, and then the grounding reference point is used as a reference, and VN is reduced from VL by looking at VL.

Therefore, the inventor simplifies the noise sampling and separating circuit and implements the circuit on the power panel, and then eliminates the common mode noise component from the power line, and proposes the circuit shown in fig. 6, where T3, T4, and T5 are common mode inductors (to carry the operating current) for the common power line, and T1 and T2 are broadband small-signal common mode inductors, where T1 preferably eliminates the transmission line autotransformer on the secondary side. Because the differential mode noise is equal in size and opposite in direction on the L line and the N line, the differential mode noise can be eliminated from the L line. The common mode inductance functions as common mode rejection, and noise cancellation is achieved by equipotential connection to ground. Moreover, in test practice, the fact that two small inductance value common mode inductors are connected in series is found that the filtering effect is better than that of one large inductor, because the parasitic capacitance of the large inductor is large, the filtering effect is influenced.

It must be emphasized here that in the face of internal noise sources from the switching power supply, the sequence is sampling in order, VCM and VDM are acquired, then common mode cancellation is performed first, then differential mode cancellation is performed, common mode is performed first, then differential mode is performed, and the sequence cannot be reversed. This is to make the original common mode detected from the noise sources of the L line and the N line of the cancelled VCM and the original L line or N line together undergo common mode inductance attenuation and then are respectively cancelled.

After the noises VL and VN on the power supply line L & N are sampled by C1 and C2, the common mode VCM and the differential mode VDM are acquired by a circuit composed of small-signal common mode inductors (TI, T2), the common mode VCM is removed from the L line and the N line by the power supply line common mode inductors, and the differential mode VDM is subtracted from the L line.

From the conduction test related principles, it can be derived: VL is VCM + VDM, VN is VCM-VDM;

by adopting the small-signal common-mode inductor which accords with the corresponding characteristic impedance, particularly by adopting a transmission line transformer, the VCM and the VDM can obtain more accurate results in a required frequency band. But the power line common mode inductance we employ may not perform well at high frequencies. After the processing, the difference common mode noise on the L line and the N line should be well filtered, and the filtering effect depends on the high-frequency performance of the power line common mode inductor. However, the conducted emission test is not unable to detect noise, but the noise detected is only detected if the specified corresponding limit for CLASS B or CLASS a is met. The conducted sensitivity CS, the electrical fast pulse burst EFT and other EMS electromagnetic sensitivity tests also require that the system can meet the specified performance criterion A or B under the noise interference of a certain specification. Therefore, it is not possible nor necessary to completely eliminate the noise.

The noise is mainly analyzed from the internal noise source of the power supply, and it can be seen that the output is controlled by the input, and is a feed-forward system, and the use of passive devices makes the system have no delay, and can process the noise in real time.

In the scheme, the common-mode inductor is used for addition and subtraction, noise on the power line is directly cancelled, and the common-mode inductor has a direct advantage that the common-mode inductor has no special requirement on inductance of the common-mode inductor and does not need to increase the inductance and capacitance to improve the filtering effect, so that the common-mode inductor is small in size and weight.

Compared with the traditional mode that the L line and the N line share one power supply common mode inductor, the mode that the L line and the VCM and the N line and the VCM share one power supply common mode inductor has a better effect because common mode noise is separately stripped, the VCM and a reference ground are in equipotential connection at the other end of the common mode inductor, and therefore a test taking the reference ground point as a reference is carried out, the VCM cannot be tested on the L line and the N line, and synchronous cancellation is achieved. Here, the ground cancellation is mainly relied on, and the traditional method is not purely relying on the large impedance of the common-mode inductor to suppress the common mode.

In EMC testing practice, we note that the common mode problem is usually large and the differential mode noise can be solved in the conventional way (differential mode inductance and differential mode capacitance). As shown in FIG. 7, CX1/CX2/LDM1/LDM2 is differential mode filtering, and LCM/CY1/CY2 is common mode filtering. In view of this, we first solve the common mode noise, and the circuit shown in fig. 6 is subtracted as shown in fig. 8.

In practice, the common mode inductor with three wires wound in parallel has mature application. We can also use a three-wire parallel-wound power line common mode inductor, i.e. the circuit shown in fig. 9.

Still further, considering that the purpose is not to make an accurate mathematical calculation but to eliminate the common mode noise as much as possible, if the center tap of the autotransformer T1 is not grounded directly, the resolved common mode noise is grounded with low impedance, i.e. equipotential with earth, and the circuit evolves to the circuit shown in fig. 10. On the basis of longitudinally blocking the common mode noise by adopting a T3 power line common mode inductor, the center tap of the autotransformer is transversely and directly grounded, and the decomposed common mode noise is grounded with low impedance, namely is connected with the ground at the same potential. T3 is power line common mode inductance, T1 is transmission line transformer without secondary side, and the connection impedance of the center tap to the ground determines the filtering effect.

It is verified that the power line common mode inductor T3 can be eliminated, and the common mode filter circuit is simplified to the circuit shown in fig. 11, thereby saving one power line common mode inductor. The finally obtained common mode noise cancellation circuit comprises noise sampling capacitors C1 and C2 and an autotransformer T1 or a transmission line transformer, wherein a center tap of the autotransformer T1 or the transmission line transformer is connected with the same potential ground, and two ends of the autotransformer T1 or two ends of the transmission line transformer are respectively connected with an L line and an N line of a power line through the noise sampling capacitors C1 and C2. In the case of a dc switching power supply, both ends of the autotransformer T1 or both ends of the transmission line transformer are connected to a P (positive pole) N (negative pole) line of the power supply line via noise sampling capacitors C1 and C2, respectively.

While the VCM is grounded, a differential mode cancellation circuit can be added, as shown in fig. 12, the common mode VCM on the center tap of the autotransformer is grounded and then the differential mode VDM is cancelled. The final cancellation circuit is as follows: the common mode noise cancellation circuit consists of a transmission line transformer T1 and noise sampling capacitors C1 and C2, wherein the center tap of the transmission line transformer T1 is connected with the equipotential ground, and two ends of the transmission line transformer T1 are respectively connected with an L line and an N line of a power line through the noise sampling capacitors C1 and C2; the differential mode noise cancellation circuit comprises a common mode inductor T4 and a differential mode noise separation circuit, wherein the common mode inductor T4 is connected in series on a power line, a primary coil of the common mode inductor T4 is connected in series in an L line of the power line, one end of a secondary coil is connected with an equipotential ground, the other end of the secondary coil is connected with the output of the differential mode noise separation circuit, the input of the differential mode noise separation circuit is connected with the L line and the N line of the power line through noise sampling capacitors C1 and C2, and a high-voltage capacitor is connected between the equipotential ground and a system ground or the ground.

The differential mode noise separation circuit comprises a broadband radio frequency transformer T2 and 2 50-ohm resistors, the dotted end of a primary winding of the broadband radio frequency transformer T2 is connected with a power line through a noise sampling capacitor, the other end of the primary winding is connected with an equipotential ground, the dotted end of a secondary winding of the broadband radio frequency transformer T2 is connected with the power line through the noise sampling capacitor, the other end of the secondary winding is connected with the equipotential ground through 2 series-connected 50-ohm resistors, and the connecting node of the 2 series-connected 50-ohm resistors is connected with a common mode inductor T4.

The common mode inductor is not directly connected with the ground but connected with the ground through the high-voltage capacitor, the insulation strength of the enameled wire is limited when the common mode inductor is wound on the magnetic core by the enameled wire, the L & N line has a voltage-resistant requirement on the ground in safety regulations, and the insulation strength of the enameled wire is not required when the common mode inductor is connected with the ground through the high-voltage capacitor. Meanwhile, two 0.1uF sampling capacitors are connected in series with the autotransformer and then connected in parallel between the L line and the N line, and half of the working voltage is considered to be added with a certain margin for voltage withstand value of the capacitors, and 160V is taken here. C1 ═ C2 ═ 0.1 μ F, ± -10%, 160V, parasitic resistance ESR < 0.5 Ω, and parasitic inductance ESL < 1 μ H are recommended.

For grounding equipment, the L & N line has the requirement of isolating 1500Vac from the ground PGND, so the circuit is not suitable for being directly connected with the ground, but is grounded through a 2kV 1000p high-voltage capacitor, and a high-voltage capacitor with the parasitic resistance ESR less than 0.5 omega and the parasitic inductance ESL less than 1 muH is recommended to be selected.

Thus, the equivalent Y capacitance of the L line or the N line to the earth PGND is equal to 160V 0.1 muF string 2kV 1000pF, the equivalent capacitance value is less than 0.1 muF, the withstand voltage is 2160V, and the withstand voltage and leakage current limitation of the L & N line to the earth are not influenced. C1 and C2 are connected in series with the coil and then connected in parallel between the L line and the N line, the total withstand voltage is 320V, and the maximum voltage of 220V usually does not exceed 264V.

For an electric appliance powered by a 220V/50Hz alternating current power grid, the leakage current is regulated to be not more than 10mA in many countries; there are different requirements for leakage current for other electric products such as handheld and mobile devices. Therefore, if the power supply product needs to be connected with a Y capacitor, the capacity is not very large, so that the capacity of the Y capacitor is basically not more than 0.1 muF.

In a floating ground device, there is no ground PGND, only systematically, there is a parasitic capacitance between the system ground and ground. Any power supply will typically have some common mode capacitance connected to ground. These capacitors may be discrete capacitors, or may be interlayer capacitors of the PCB, or a combination of both types of capacitors. The power supply is not really floating and is connected to ground via parasitic capacitances. The requirement for conducting or conducted immunity testing is that the EUT be placed on a wooden stand 40 cm from the reference ground plane for consistency testing.

We know that conducted noise measurements are based on a ground reference plane, i.e., the ground reference plane PGND, which is a small radio frequency potential difference between PGND and ground of our detection cancellation circuit. This is a poor place in the united states. Partial improvement can be achieved by controlling the parasitic parameters of the devices such as inductance and capacitance in the circuit.

The utility model can be used for AC switching power supply equipment and DC switching power supply equipment, and the equipment can be connected with the ground or not. The needle is butted with an isolated direct current port (direct current switch power supply) on the earth equipment, so that the equipotential points are equal

Connected to the ground through a high-voltage capacitor

Aiming at the direct current port on the earth equipment, the equipotential point is

Connected to system ground through high-voltage capacitor

Aiming at the situation that the AC power port on the earth equipment is not connected, the equipotential point is equal

And is connected to the secondary ground of the system through a high-voltage capacitor. When the needle is connected with an alternating current power port on the earth equipment, the needle is at an equal potential point

Connected to the ground through a high-voltage capacitor

The technical scheme aims at the condition that the noise source is inside the switching power supply.

It should be noted that, when the noise source is from outside the switching power supply, it is better to adopt a mode of eliminating the common mode noise by grounding the center tap, because there is no directivity. Since the external noise (such as conduction sensitivity test CS and electrical fast burst EFT in EMS test) is common mode noise, VDM is almost zero, the common mode inductor T4 does not work, and the supply current is straight through. The common mode noise rejection circuit with center tap grounded is still effective for external common mode noise.

The utility model regards the whole switching power supply device as a 'black box', does not consider the internal noise generation and coupling mechanism, performs online self-detection, directly detects the common mode noise on the power line, and grounds the common mode noise with low impedance, thereby realizing the suppression of the noise on the power line. Moreover, the scheme provided by the utility model has good inhibition effect on noise sources from an internal power supply and external noise sources (such as conduction immunity CS, EFT and the like applied by EMS test and common mode interference from the outside to a power line), and the adoption of the transmission line transformer ensures that the circuit of the utility model has good high-frequency characteristics, which is particularly important for automobile electronics, and different from ITE equipment, the power conduction emission of the automobile electronics covers 150kHz to 108MHz and is wider than 150k to 30MHz of ITE, which undoubtedly puts higher requirements on the power conduction emission, but the utility model can cover higher frequency bands, and can solve the problems of power port conduction emission and radiation emission caused by the conduction of a power supply 30MH in the above frequency bands.

The passive filter is bidirectional as the traditional passive filter, the impedance of a noise source can be within the external and the internal, the passive device has zero response time, and the delay and the phase are not required to be compensated like the active filter. The common mode is eliminated by adopting the center tap of the autotransformer formed by the small-signal common mode inductor or the grounding of the primary center tap of the transmission line transformer without the secondary side, and the common mode inductor has better effect and small volume than the common mode inductor which only depends on a large power line and is convenient to realize.

The utility model has the following characteristics:

the black box processing mode is adopted, the internal noise propagation path does not need to be known, direct cancellation is directly obtained, and the filtering effect is good;

active filtering is not used, but the cancellation effect only achieved by the active filtering is achieved;

compared with the traditional passive filtering, the values of the common-mode inductor and the capacitor are not required;

both external and internal directions can be filtered;

independent of noise source impedance;

can be processed together in both differential and common modes;

passive devices, no delay response;

the requirements of safety voltage resistance and leakage current are met;

and a transmission line transformer is adopted, so that the frequency band is wide.

Claims (5)

1. A switching power supply port conducted noise automatic detection cancellation circuit is characterized in that: the common mode noise cancellation circuit comprises a noise sampling capacitor and an autotransformer or a transmission line transformer, wherein a center tap of the autotransformer or the transmission line transformer is connected with the same potential ground, and two ends of the autotransformer or two ends of the transmission line transformer are respectively connected with a power line through the noise sampling capacitor; the differential mode noise cancellation circuit comprises a common mode inductor T4 and a differential mode noise separation circuit, wherein the common mode inductor T4 is connected in series with a power line, a primary coil of the common mode inductor is connected in series with the power line, one end of a secondary coil is connected with an equipotential ground, the other end of the secondary coil is connected with the output of the differential mode noise separation circuit, the input of the differential mode noise separation circuit is connected with the power line through a noise sampling capacitor, and a high-voltage capacitor is connected between the equipotential ground and a system ground or the earth.

2. The automatic detection and cancellation circuit for conduction noise of the switching power supply port according to claim 1, wherein: the power line is an alternating current power line or a direct current power line.

3. The automatic detection and cancellation circuit for conduction noise of the switching power supply port according to claim 1 or 2, wherein: the withstand voltage value of the noise sampling capacitor is half of the working voltage plus a certain allowance, the capacitance value is 0.1 muF, the ESR of the parasitic resistance is less than 0.5 omega, and the ESL of the parasitic inductance is less than muH.

4. The automatic detection and cancellation circuit for conduction noise of the switching power supply port according to claim 3, wherein: the withstand voltage value of the high-voltage capacitor is 2kv, the capacitance value is 1000Pf, the ESR of the parasitic resistance is less than 0.5 omega, and the ESL of the parasitic inductance is less than muH.

5. The automatic detection and cancellation circuit for conduction noise of the switching power supply port according to claim 4, wherein: the differential mode noise separation circuit comprises a broadband radio frequency transformer T2 and 2 50-ohm resistors, the dotted end of a primary winding of the broadband radio frequency transformer T2 is connected with a power line through a noise sampling capacitor, the other end of the primary winding is connected with an equipotential ground, the dotted end of a secondary winding of the broadband radio frequency transformer T2 is connected with the power line through the noise sampling capacitor, the other end of the secondary winding is connected with the equipotential ground through 2 series-connected 50-ohm resistors, and the connecting node of the 2 series-connected 50-ohm resistors is connected with a common mode inductor T4.

Images