CN104811030A - EMI filter network with impedance mismatching network - Google Patents

Abstract

The invention discloses a design method for an EMI filter network with an impedance mismatching network. The EMI filter network with the impedance mismatching network comprises two parts of the mismatching network and an EMI filter; the EMI filter consists of an inductor L and a capacitor C; the mismatching network comprises an L-type mismatching network and a C-type mismatching network; the L-type mismatching network consists of a resistor RP and an inductor LP which are parallelly connected; the C-type mismatching network consists of a resistor RS and a capacitor CS which are serially connected; the L-type mismatching network is serially connected with the high impedance end of the EMI filter; the C-type mismatching network is parallelly connected with the low impedance end of the EMI filter; and the attenuation of the EMI noise by the EMI filter network is divided to two parts of the insertion loss brought by own EMI filter and the reflection loss after the EMI filter is connected to the mismatching network. High frequency impedance of the noise source and the high frequency impedance of the filter input port after connecting the mismatching network are located in the serious mismatching state, the larger reflection loss is generated to the EMI signal, and the EMI signal from the source end is effectively blocked to be transmitted into other electronic devices.

Description

A kind of electromagnetic interface filter network with impedance mismatching network

Technical field

The invention belongs to electronic technology field, especially relate to a kind of electromagnetic interface filter network design method with impedance mismatching network.

Background technology

Along with the raising of converters operating frequency, the seriousness of the Conducted Electromagnetic Interference problem of power inverter more highlights.In order to prevent the mutual interference between power inverter and other equipment, power inverter, must by relevant electromagnetic compatibility (EMC) standard testing before putting on market.Electromagnetic interface filter is one of important means suppressing conducted noise, and the employing of especially high performance electromagnetic interface filter, has important theory and engineering significance to the interference problem of power electronic equipment.But this kind of electromagnetic interface filter does not often mate by noise source impedance and load impedance to be affected, and causes its working effect not good.Therefore how to eliminate or reduction source, load impedance not mate the impact of electromagnetic interface filter service behaviour be a problem being worth thinking.

Summary of the invention

Goal of the invention: in order to overcome the deficiencies in the prior art, the invention provides a kind of electromagnetic interface filter network design method with impedance mismatching network, utilize noise source and load impedance not to mate produced reflection loss effectively to suppress EMI noise, thus effectively improve the service behaviour of electromagnetic interface filter.

Technical scheme: for achieving the above object, the technical solution used in the present invention is:

A kind of electromagnetic interface filter network with impedance mismatching network, comprise mismatch network and electromagnetic interface filter two parts, electromagnetic interface filter is formed primarily of inductance L and electric capacity C, and mismatch network comprises L-type mismatch network and two kinds, C type mismatch network, and L-type mismatch network is primarily of resistance R _pand inductance L _pformation in parallel, C type mismatch network is primarily of resistance R _swith electric capacity C _sin series;

L-type mismatch network is connected with the high impedance end of electromagnetic interface filter, C type mismatch network is in parallel with the low-impedance end of electromagnetic interface filter, that is: when electromagnetic interface filter be high capacity impedance and low source resistance time, then at the load end series connection L-type mismatch network of electromagnetic interface filter, at the source C type in parallel mismatch network of electromagnetic interface filter; When electromagnetic interface filter be low load impedance and high source impedance time, then at the load end of electromagnetic interface filter C type in parallel mismatch network, at the source series connection L-type mismatch network of electromagnetic interface filter; When electromagnetic interface filter be high capacity impedance and high source impedance time, then respectively at the load end of electromagnetic interface filter and source series connection L-type mismatch network; When electromagnetic interface filter be low load impedance and low source resistance time, then respectively at the load end of electromagnetic interface filter and source C type in parallel mismatch network;

This electromagnetic interface filter network decay to EMI noise be divided into electromagnetic interface filter self with insertion loss and electromagnetic interface filter access the reflection loss two parts after mismatch network.

Reflection loss after electromagnetic interface filter access mismatch network refers to when electromagnetic interface filter port Impedance is mated completely, the current ratio of noise loading is flowed through before equipment access electromagnetic interface filter with after access electromagnetic interface filter, loss on it and electromagnetic interface filter has nothing to do, only relevant with port Impedance mismatch.Analyze known thus, when electromagnetic interface filter parameter is fixed and self insertion loss can not meet noise suppressed demand, obtains larger reflection loss by accessing suitable mismatch network, reaching the object improving electromagnetic interface filter service behaviour.

The design parameter of described electromagnetic interface filter comprises corner frequency f, and corner frequency f comprises common mode corner frequency f _cwith differential mode corner frequency f _d, the selection scheme of the corner frequency f that this case provides is: the EMI noise of test noise source when not accessing electromagnetic interface filter network, the limit of interference that the EMI noise recorded and EMI standard specify is contrasted, (EMI noise recorded adds allowance after deducting the limit of interference that EMI standard specifies at the reduction noise curve realized needed for each frequency range to calculate electromagnetic interface filter, general allowance is 6dB), general attenuation slope after corner frequency is generally 40dB/dec, therefore the straight line of available 40dB/dec slope is to right translation, when straight line and reduction noise curve tangent time, the intersection point of straight line and X-axis is corner frequency f.

In general, described electromagnetic interface filter is single-stage electromagnetic interface filter, and its design is as follows:

$\left\{\begin{array}{c}{f}_c=\frac{1}{2\mathrm{\π}\sqrt{L_{\mathrm{CM}}2C_Y}}\\ f_d=\frac{1}{2\mathrm{\π}\sqrt{2L_{\mathrm{DM}}{C}_X}}\end{array}\right.$

Wherein, f _crepresent common mode corner frequency, L _cMrepresent common mode inductance, C _yrepresent common mode capacitance, f _drepresent differential mode corner frequency, L _dMrepresent differential mode inductance, C _xrepresent differential mode capacitor.

Preferably, the design of described mismatch network meets following constraints:

1. the design of C type mismatch network meets following constraints:

$\sqrt{R_S^2+\frac{1}{{\mathrm{\ω}}_L^2{C}_S^2}}\≤{\mathrm{\ϵ}}_C{\mathrm{\ω}}_{L}L$

Wherein, ω _lrepresent the angular frequency value at electromagnetic interface filter operating frequency range lower limit place, ε _crepresent the mismatch ratio at electromagnetic interface filter access C type mismatch network port place;

2. the design of L-type mismatch network meets following constraints:

$\left|\frac{j{\mathrm{\ω}}_L{R}_P{L}_P}{R_P+j{\mathrm{\ω}}_L{L}_P}\right|\≥\frac{1}{{\mathrm{\ϵ}}_P{\mathrm{\ω}}_{L}C}$

Wherein, ω _lrepresent the angular frequency value (i.e. noise current) at electromagnetic interface filter operating frequency range lower limit place, ε _prepresent the mismatch ratio at electromagnetic interface filter access L-type mismatch network port place;

3. the impedance of C type mismatch network should be large as far as possible and the resistance value of L-type mismatch network should be little as far as possible, that is: C _sresistance value will much larger than R _svalue, L _presistance value then will much smaller than R _p:

$R_S<<\frac{1}{{\mathrm{\&omega;}}_f{C}_S}$

ω _fL _P<<R _P

Wherein, ω _frepresent the angular frequency value of operating current;

4. inoperative at electromagnetic interface filter idle frequency range mismatch network, and play the effect regulating impedance at the frequency range mismatch network of electromagnetic interface filter work, that is: at electromagnetic interface filter operating frequency range lower frequency limit f _ltime, C _sresistance value be less than R _s, L _pimpedance be then greater than R _p:

$\frac{1}{{\mathrm{\&omega;}}_L{C}_S}\&le;R_S$

R _P<ω _LL _P

Wherein, ω _lrepresent the angular frequency value at electromagnetic interface filter operating frequency range lower limit place.

Beneficial effect: the electromagnetic interface filter network design method with impedance mismatching network provided by the invention, impedance mismatching network is adopted to improve filter service behaviour, there is following advantage: after selecting suitable mismatch network and filter element, operating current can be zero-decrement by the electromagnetic interface filter after improvement in the least, and now filter only plays the effect of signal transmission; Noise source high-frequency resistance and the filter input end mouth high-frequency resistance after accessing mismatch network will be in severe mismatch state, and produce larger reflection loss to EMI signal, the EMI signal effectively blocked from source imports in other electronic equipments; Simultaneously, load end high-frequency resistance and the filter output mouth high-frequency resistance after accessing mismatch network also will be in severe mismatch state, produce larger reflection loss to EMI signal, the EMI signal effectively blocked from other power electronic equipment imports in converter.

Accompanying drawing explanation

Fig. 1 is FL-network structured flowchart;

Fig. 2 is FL-network topology under low source resistance, high capacity impedance;

Fig. 3 is FL-network topology under high source impedance, low load impedance;

Fig. 4 is FL-network topology under low source resistance, low load impedance;

Fig. 5 is FL-network topology under high source impedance, high capacity impedance;

Fig. 6 is filter electrical schematic diagram when not accessing mismatch network;

Fig. 7 is circuit topology after filter access.

Embodiment

Below in conjunction with accompanying drawing, the present invention is further described.

Be illustrated in figure 1 a kind of electromagnetic interface filter network with impedance mismatching network, on the original insertion loss basis of electromagnetic interface filter, increase the mismatch of port of wave filter impedance to provide larger reflection loss by termination mismatch network, reach the object improving filter service behaviour.Comprise mismatch network and electromagnetic interface filter two parts, electromagnetic interface filter is formed primarily of inductance L and electric capacity C, and mismatch network comprises L-type mismatch network and two kinds, C type mismatch network, and L-type mismatch network is primarily of resistance R _pand inductance L _pformation in parallel, C type mismatch network is primarily of resistance R _swith electric capacity C _sin series; L-type mismatch network is connected with the high impedance end of electromagnetic interface filter, and C type mismatch network is in parallel with the low-impedance end of electromagnetic interface filter; This electromagnetic interface filter network decay to EMI noise be divided into electromagnetic interface filter self with insertion loss and electromagnetic interface filter access the reflection loss two parts after mismatch network.

The design parameter of described electromagnetic interface filter comprises corner frequency f, and corner frequency f comprises common mode corner frequency f _cwith differential mode corner frequency f _d, the selection scheme of corner frequency f is: the EMI noise of test noise source when not accessing electromagnetic interface filter network, the limit of interference that the EMI noise recorded and EMI standard specify is contrasted, (EMI noise recorded adds allowance after deducting the limit of interference that EMI standard specifies at the reduction noise curve realized needed for each frequency range to calculate electromagnetic interface filter, general allowance is 6dB), general attenuation slope after corner frequency is generally 40dB/dec, therefore the straight line of available 40dB/dec slope is to right translation, when straight line and reduction noise curve tangent time, the intersection point of straight line and X-axis is corner frequency f.Because electromagnetic interface filter adopts single-stage electromagnetic interface filter as shown in Figure 6 usually, its design is as follows:

$\left\{\begin{array}{c}{f}_c=\frac{1}{2\mathrm{\&pi;}\sqrt{L_{\mathrm{CM}}2C_Y}}\\ f_d=\frac{1}{2\mathrm{\&pi;}\sqrt{2L_{\mathrm{DM}}{C}_X}}\end{array}\right.---\left(1\right)$

Wherein, f _crepresent common mode corner frequency, L _cMrepresent common mode inductance, C _yrepresent common mode capacitance, f _drepresent differential mode corner frequency, L _dMrepresent differential mode inductance, C _xrepresent differential mode capacitor.

Due to common mode capacitance C _yfor ground capacity, limit by leakage current, its value should more than 5nF; And differential mode inductance L _dMgenerally can be provided by the leakage inductance of common mode choke, its span is generally between (10-20) μ H; Therefore first should determine common mode capacitance C when designing _ywith differential mode inductance L _dM, then convolution (1) calculates common mode inductance L respectively _cMwith differential mode capacitor C _x.Complete common mode capacitance C _y, differential mode inductance L _dM, common mode inductance L _cMwith differential mode capacitor C _xdesign after namely complete the design of electromagnetic interface filter.

As shown in Figure 7, the reflection loss expression formula of electromagnetic interface filter is:

$A=201g\left|\frac{2\sqrt{z_S{z}_L}}{z_S+z_L}\right|=-101g(1-{\left|\mathrm{\&Gamma;}\right|}^2)---\left(2\right)$

Wherein, Z _srepresent noise source impedance, Z _lrepresent noise loading impedance, represent reflection coefficient.

Known according to formula (2), noise source impedance Z _swith noise loading impedance Z _ldifference larger, the reflection loss of electromagnetic interface filter is larger; Otherwise, when noise source impedance Z _swith noise loading impedance Z _lwhen mating completely, the reflection loss of electromagnetic interface filter levels off to 0, and namely noise signal nondestructively can pass through electromagnetic interface filter.Therefore connected with the high impedance end of electromagnetic interface filter by L-type mismatch network, the mode that C type mismatch network is in parallel with the low-impedance end of electromagnetic interface filter, the mismatch at the impedance of electromagnetic interface filter noise source and load impedance two ends can be aggravated, and then obtain larger reflection loss, reach the object improving electromagnetic interface filter service behaviour.

The insertion loss expression formula of electromagnetic interface filter is:

$\mathrm{IL}=201\mathrm{og}\left|\frac{T_{11}{T}_{12}+T_{21}{Z}_S{S}_L+T_{22}{Z}_S}{Z_S+Z_L}\right|---\left(3\right)$

Wherein, T is the transmission matrix of electromagnetic interface filter, T ₁₁, T ₁₂, T ₂₁, T ₂₂for the element of T.

Therefore while the loss of acquisition maximum reflection, obtain maximum insertion loss in order to ensureing, suitable mismatch network and filter construction can be selected according to different noise source, loading condition, that is: when electromagnetic interface filter be high capacity impedance and low source resistance time, then at the load end series connection L-type mismatch network of electromagnetic interface filter, at the source C type in parallel mismatch network of electromagnetic interface filter, as shown in Figure 2; When electromagnetic interface filter be low load impedance and high source impedance time, then at the load end of electromagnetic interface filter C type in parallel mismatch network, at the source series connection L-type mismatch network of electromagnetic interface filter, as shown in Figure 3; When electromagnetic interface filter be high capacity impedance and high source impedance time, then respectively at the load end of electromagnetic interface filter and source series connection L-type mismatch network, as shown in Figure 4; When electromagnetic interface filter be low load impedance and low source resistance time, then respectively at the load end of electromagnetic interface filter and source C type in parallel mismatch network, as shown in Figure 5.

For the load impedance of electromagnetic interface filter be high impedance, source impedance for Low ESR, for suppressing EMI noise better, electromagnetic interface filter structure should be chosen as L-C type; Meanwhile, in order to increase the reflection loss of electromagnetic interface filter network, should at its load end series connection access L-type mismatch network, source parallel connection access C type mismatch network (as shown in Figure 2 structure).As shown in Figure 2, after access mismatch network, the source impedance Z' of electromagnetic interface filter _scan be considered C type mismatch network impedance and former noise source impedance Z _sparallel connection; The load impedance Z' of electromagnetic interface filter _lcan be considered L-type mismatch network impedance and former noise loading impedance Z _lseries connection, that is:

Z' _S＝Z _S||Z _C(4)

Z' _L＝Z _L+Z _P

Wherein, Z _crepresent C type mismatch network impedance, Z _prepresent L-type mismatch network impedance.

When after the design completing electromagnetic interface filter, can design mismatch network.On the basis not affecting converter normal working performance, for increasing the reflection loss of electromagnetic interface filter network, following 4 constraints are proposed to the value of mismatch network counter element:

1., for the EMI noise that interference source produces, on the basis of electromagnetic interface filter self insertion loss, by access C type mismatch network to provide larger reflection loss, guarantee that noise is effectively suppressed, now Z' _scertain mismatch ratio should be met, that is: with the minimum input impedance of entering viewed from noise source

|Z' _S|≤ε _C|Z _1min| (5)

Wherein, Z _1minrepresent input impedance Z ₁minimum value; ε _crepresent the mismatch ratio at electromagnetic interface filter access C type mismatch network port place, be the ratio of port Impedance after access C type mismatch network and input impedance, its span is generally (0,1).Work as ε _cvalue more levels off to 0, represents that port mismatch is larger, also larger to the reflection loss of noise, but now operating current degree of susceptibility is also larger; Work as ε _cvalue level off to 1 time, represent that port mismatch is less, match condition better, namely noise imports in filter completely by loss-free, now produce high requirement by the insertion loss of filter self, this situation is also considered to " worst case " of filter work usually, and mismatch ratio ε _cfollowing relationship is there is with reflection coefficient Γ:

$\left|\mathrm{\&Gamma;}\right|=\frac{1-{\mathrm{\&epsiv;}}_C}{1+{\mathrm{\&epsiv;}}_C}---\left(6\right)$

As shown in Figure 2, when load impedance short circuit, input impedance Z ₁to obtain minimum value, its expression formula is:

$Z_1=\mathrm{j\&omega;L}+\frac{1}{\mathrm{j\&omega;C}}\left|\right|\left(R_P\right|\left|\mathrm{j\&omega;}{L}_p\right)---\left(7\right)$

Wherein, ω represents the angular frequency of electromagnetic interface filter operating frequency, because electromagnetic interface filter is operated in high band, therefore when ω is larger, and Z ₁can abbreviation be:

$Z_1\&ap;\frac{1}{R_p{\mathrm{\&omega;}}^2{C}^2}+j(\mathrm{\&omega;L}-\frac{1}{R_p\mathrm{\&omega;C}})---\left(8\right)$

Because the noise brought due to filter own resonance will be avoided in electromagnetic interface filter design process to amplify, so the resistance value of inductance will much larger than 1/ (R _pω C) value, therefore can be obtained by formula (8), at filter operating frequency lower limit place, | Z ₁| obtain minimum value and can be similar to and think that its minimum value is ω _ll ω _ll, substituting into formula (5) can obtain:

|Z _S|·|Z _C|≤ε _Cω _LL (9)

Generally, after C type mismatch network and source impedance parallel connection, the modulus value of impedance should be less than C type mismatch network direct impedance modulus value, therefore formula (9) can obtain by abbreviation:

$\sqrt{R_S^2+\frac{1}{{\mathrm{\&omega;}}_L^2{C}_S^2}}\&le;{\mathrm{\&epsiv;}}_C{\mathrm{\&omega;}}_{L}L---\left(10\right)$

ω _lrepresent the angular frequency value at electromagnetic interface filter operating frequency range lower limit place.

In like manner can obtain, because C type mismatch network is always connected with the inductance end of filter, therefore when noise source, load impedance are other situations, the value of C type mismatch network all should meet expression formula (10).

2. the EMI noise for preventing other equipment from producing imports source into, and access L-type mismatch network, to provide larger reflection loss, alleviates the operating pressure of filter, now Z' _lcertain mismatch ratio should be met, that is: with the maximum output impedance of entering viewed from load end

|Z' _S|≥|Z _2max|/ε _P(11)

Wherein, Z _2minrepresent output impedance Z ₂minimum value; ε _prepresent the mismatch ratio at electromagnetic interface filter access L-type mismatch network port place, be the ratio of port Impedance after access L-type mismatch network and output impedance, its span is generally (0,1).

Can be obtained by Fig. 2, when source impedance is opened a way, output impedance Z ₂maximum, its expression formula is as follows:

$Z_2=\frac{1}{\mathrm{j\&omega;C}}\left|\right|(\mathrm{j\&omega;L}+R_S+\frac{1}{\mathrm{j\&omega;}{C}_S})---\left(12\right)$

In like manner can obtain, when ω is larger, | Z ₂| at filter operating frequency lower limit, place obtains maximum, and can be similar to and think that its maximum is 1/ (ω _lc), obtain in substitution formula (12):

$|Z_P+Z_L|\&GreaterEqual;\frac{1}{{\mathrm{\&epsiv;}}_P{\mathrm{\&omega;}}_{\mathrm{LC}}}---\left(13\right)$

Because of Z _lfor high impedance, therefore work as | Z _p| when meeting output impedance mismatch ratio, | Z _p+ Z _l| necessarily meet formula (13), namely formula (13) can be reduced to

$\left|\frac{j{\mathrm{\&omega;}}_L{R}_P{L}_P}{R_P+j{\mathrm{\&omega;}}_L{L}_P}\right|\&GreaterEqual;\frac{1}{{\mathrm{\&epsiv;}}_P{\mathrm{\&omega;}}_{L}C}---\left(14\right)$

In like manner can obtain, because L-type mismatch network is always connected with the capacitance terminal of filter, therefore when noise source, load impedance are other situations, the value of L-type mismatch network all should meet expression formula (14).

No matter 3. which kind of situation noise source, load impedance are, for enabling useful signal zero-decrement by electromagnetic interface filter network, the impedance of the C type mismatch network of access should be large as far as possible and the resistance value of L-type mismatch network should be little as far as possible, that is: C _sresistance value will much larger than R _svalue, L _presistance value then will much smaller than R _p:

$R_S<<\frac{1}{{\mathrm{\&omega;}}_f{C}_S}---\left(15\right)$

ω _fL _P<<R _P

No matter 4. which kind of situation noise source, load impedance are, when regulating port Impedance, Optimal action only accesses resistance, so both can reach the object regulating impedance, also new resonance can not be produced because of access perception or capacitive element, but in engineering, can not use such mismatch network, because it is too large to working signal loss.Therefore inoperative at electromagnetic interface filter idle frequency range mismatch network, and play the effect regulating impedance at the frequency range mismatch network of electromagnetic interface filter work, that is: at electromagnetic interface filter operating frequency range lower frequency limit f _ltime, C _sresistance value be less than R _s, L _pimpedance be then greater than R _p:

$\frac{1}{{\mathrm{\&omega;}}_L{C}_S}\&le;R_S---\left(16\right)$

R _P<ω _LL _P

Wherein, ω _lrepresent the angular frequency value at electromagnetic interface filter operating frequency range lower limit place.

Based on four constraintss above, can show that the design procedure of electromagnetic interface filter network is as follows:

Step one: determine source, load impedance character, select suitable filter construction.When designing filter, or the insertion loss of noise attentuation demand being provided to determine the value of filter element in full rate section.

Step 2: determine ε _cand ε _psize, its value can select desired value according to noise jamming situation in (0,1) scope, if disturbed condition is serious, then by value get a little bit smaller, as 1/10th, to obtain larger reflection loss; Otherwise, then get greatly a bit.

Step 3: determine mismatch network counter element value.Because element value constraints in mismatch network is all provide with inequality, therefore meeting random selection element value in inequality claimed range, but the factor such as economy and volume can be considered, C _swith L _pvalue should be a little bit smaller as far as possible.

Step 4: if the ε chosen _cand ε _psize improper and cause the mismatch network element value calculated not realize, then should reselect mismatch ratio, and repeat step 3.

The above is only the preferred embodiment of the present invention; be noted that for those skilled in the art; under the premise without departing from the principles of the invention, can also make some improvements and modifications, these improvements and modifications also should be considered as protection scope of the present invention.

Claims (4)

1. the electromagnetic interface filter network with impedance mismatching network, it is characterized in that: comprise mismatch network and electromagnetic interface filter two parts, electromagnetic interface filter is formed primarily of inductance L and electric capacity C, and mismatch network comprises L-type mismatch network and two kinds, C type mismatch network, and L-type mismatch network is primarily of resistance R _pand inductance L _pformation in parallel, C type mismatch network is primarily of resistance R _swith electric capacity C _sin series;

L-type mismatch network is connected with the high impedance end of electromagnetic interface filter, C type mismatch network is in parallel with the low-impedance end of electromagnetic interface filter, that is: when electromagnetic interface filter be high capacity impedance and low source resistance time, then at the load end series connection L-type mismatch network of electromagnetic interface filter, at the source C type in parallel mismatch network of electromagnetic interface filter; When electromagnetic interface filter be low load impedance and high source impedance time, then at the load end of electromagnetic interface filter C type in parallel mismatch network, at the source series connection L-type mismatch network of electromagnetic interface filter; When electromagnetic interface filter be high capacity impedance and high source impedance time, then respectively at the load end of electromagnetic interface filter and source series connection L-type mismatch network; When electromagnetic interface filter be low load impedance and low source resistance time, then respectively at the load end of electromagnetic interface filter and source C type in parallel mismatch network;

This electromagnetic interface filter network decay to EMI noise be divided into electromagnetic interface filter self with insertion loss and electromagnetic interface filter access the reflection loss two parts after mismatch network.

2. the electromagnetic interface filter network with impedance mismatching network according to claim 1, it is characterized in that: the design parameter of described electromagnetic interface filter comprises corner frequency f, corner frequency f comprises common mode corner frequency f _cwith differential mode corner frequency f _d; The selection scheme of corner frequency f is: the EMI noise of test noise source when not accessing electromagnetic interface filter network, the limit of interference that the EMI noise recorded and EMI standard specify is contrasted, calculate the reduction noise curve that electromagnetic interface filter realizes needed for each frequency range, with the straight line of 40dB/dec slope to right translation, when straight line and reduction noise curve tangent time, the intersection point of straight line and X-axis is corner frequency f.

3. the electromagnetic interface filter network with impedance mismatching network according to claim 1 and 2, is characterized in that: described electromagnetic interface filter is single-stage electromagnetic interface filter, and its design is as follows:

$\left\{\begin{array}{c}{f}_c=\frac{1}{2\mathrm{\&pi;}\sqrt{L_{\mathrm{CM}}2C_Y}}\\ f_d=\frac{1}{2\mathrm{\&pi;}\sqrt{2L_{\mathrm{DM}}{C}_X}}\end{array}\right.$

Wherein, f _crepresent common mode corner frequency, L _cMrepresent common mode inductance, C _yrepresent common mode capacitance, f _drepresent differential mode corner frequency, L _dMrepresent differential mode inductance, C _xrepresent differential mode capacitor.

4. the electromagnetic interface filter network with impedance mismatching network according to claim 1 and 2, is characterized in that: the design of described mismatch network meets following constraints:

1. the design of C type mismatch network meets following constraints:

$\sqrt{R_S^2+\frac{1}{{\mathrm{\&omega;}}_L^2{C}_S^2}}\&le;{\mathrm{\&epsiv;}}_C{\mathrm{\&omega;}}_{C}L$

Wherein, ω _lrepresent the angular frequency value at electromagnetic interface filter operating frequency range lower limit place, ε _crepresent the mismatch ratio at electromagnetic interface filter access C type mismatch network port place;

2. the design of L-type mismatch network meets following constraints:

$\left|\frac{j{\mathrm{\&omega;}}_L{R}_P{L}_P}{R_P+j{\mathrm{\&omega;}}_L{L}_P}\right|\&GreaterEqual;\frac{1}{{\mathrm{\&epsiv;}}_P{\mathrm{\&omega;}}_{L}C}$

Wherein, ω _lrepresent the angular frequency value at electromagnetic interface filter operating frequency range lower limit place, ε _prepresent the mismatch ratio at electromagnetic interface filter access L-type mismatch network port place;

3. the impedance of C type mismatch network should be large as far as possible and the resistance value of L-type mismatch network should be little as far as possible, that is: C _sresistance value will much larger than R _svalue, L _presistance value then will much smaller than R _p:

$R_S<<\frac{1}{{\mathrm{\&omega;}}_f{C}_S}$

ω _fL _P<<R _P

Wherein, ω _frepresent the angular frequency value of operating current;

4. inoperative at electromagnetic interface filter idle frequency range mismatch network, and play the effect regulating impedance at the frequency range mismatch network of electromagnetic interface filter work, that is: at electromagnetic interface filter operating frequency range lower frequency limit f _ltime, C _sresistance value be less than R _s, L _pimpedance be then greater than R _p:

$\frac{1}{{\mathrm{\&omega;}}_L{C}_S}\&le;R_S$

R _P<ω _LL _P

Wherein, ω _lrepresent the angular frequency value at electromagnetic interface filter operating frequency range lower limit place.