JPH1168497A - Common mode filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a common made filter to effectively reduce common mode noises radiated from a cable of an electronic instrument. SOLUTION: A first transformer 14 is inserted between a transmission line and an AC grounding point 17, and a second transformer 20 is inserted in series with the transmission line. The transformer 14 produces a low impedance for common mode noises, high impedance for a signal, thus the common mode noises are suppressed while no loss is generated for the signal. On the other hand, high impedance is formed for the common mode noise, while low impedance for the signal, thus common mode noises are suppressed and no loss occurs for the signal in the transformer 20. A suppression amount of the common mode noise is given by a ratio between the common mode impedance of the transformer 20 and that of the transformer 14 and a large suppression amount is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a common mode filter, and more particularly to a common mode filter capable of suppressing common mode noise by preventing leakage and intrusion of noise.

[0002]

2. Description of the Related Art A conventional common mode filter of this type is disclosed in, for example,
EC) HENRY W. issued by Publishing Co., Ltd. O
TT's book, supplemented and revised edition, "Practical Noise Reduction Technique" p. 96-1
As shown by reference numeral 06, this is used to prevent noise from leaking from the electronic device and intrusion of noise into the electronic device.

FIG. 17 is a diagram showing an example of a conventional common mode filter of this type.
1 and 22. The winding start 23 of the winding 21 is connected to one input terminal 10, and the winding end is connected to one output terminal 12. Start of winding 24 of winding 22
Is connected to the other input terminal 11, and the winding end is connected to the other output terminal 13.

FIG. 18 is a diagram showing propagation of common mode noise in transmission lines 73 and 74. Common mode noise 6
0 is a pair of transmission paths 73 and 74 of the signal 70 and the ground 17
Occur in between. The impedances 65 and 66 are output impedances on the transmitting side, and the impedance 69 is an input impedance at the receiving end.
A value close to the characteristic impedance of No. 4 is selected. The impedances 67 and 68 are connected to the transmission paths 73 and 74 and the ground 17.
Is the impedance between them.

The common mode currents 171 and 172 are converted from a common mode noise source to output impedances 65 and 6 of the transmitting section.
6 flows into the transmission paths 73 and 74, and the impedance 67 and 68 between the transmission paths 73 and 74 and the ground flows into the loop of the ground 17. Distance between line 73 and ground 17 and line 7
If the distance between 4 and the ground 17 is the same, the impedances 67 and 68 become equal. Output impedance 6
If 5 and 66 are equal, the common mode currents 171 and 17
2 will be equal.

When the common mode currents 171 and 172 are equal, no common mode current flows through the input impedance 69. At this time, the common mode current Ig1 is expressed as follows: Ig1 = Vg / (Z1 + Z3) (1) Since Z1 << Z3, Ig1 = Vg / Z3 (2), and most of the signal current 173 flows through the loop of the output impedance 65, 66 of the transmission section → the transmission paths 73, 74 → the input impedance of the reception end.

The reception voltage VL at both ends of the input impedance 69 is as follows: VL = Z0.Vs / (Z1 + Z2 + Z0) (3)

FIG. 19 is a diagram for explaining the effect of the conventional common mode filter shown in FIG. 17 on common mode noise. The inductance of the first winding 21 is L
1, the inductance of the second winding 22 is L2, and the mutual inductance between the first winding 21 and the second winding 22 is M. The relationship between M and L1, L2 is as follows: M = k√ (L1 · L2) (4), where k is a coupling coefficient and k ≦ 1.

Next, when the common mode current 171 is obtained, Ig1 '= Vg / {Z1 + Z3 + jω (L1 + M)} (5), where ω is the angular frequency (rad / sec). And
Since Z1 << Z3, Ig1 '= Vg / {Z3 + jω (L1 + M)} (6)

The effect of the common mode filter is as follows.
From the equations (2) and (6), the following equation is obtained: Ig1 '/ Ig1 = Z3 / {Z3 + jω (L1 + M)} = 1 / {1 + jω (L1 + M) / Z3} (7) Here, if L1 = L2 = L, k = 1, (7)
The equation is: Ig1 '/ Ig1 = 1 / {1 + jω (2L) / Z3}.
(8) From equation (8), it can be seen that the common mode current can be reduced by increasing the inductance of each winding of the Heisei kenki 20.

FIG. 20 is a diagram for explaining the effect of the common mode filter of FIG. 17 on the signal. The reception voltage VL 'is expressed as follows: VL' = Z0.Vs / {Z1 + Z2 + Z0 + j.omega. (L1 + L2-2M)} (9) Here, assuming that L1 = L2 = L and k = 1, the equation (9) becomes as follows: VL '= Z0.Vs / (Z1 + Z2 + Z0) (10), which is equal to the equation (3). This indicates that transformer 20 has no effect on the signal.

In the above description, transformer 20 is an ideal transformer. FIG. 21 shows an equalizing circuit of the transformer 20.
The capacitor 90 is the stray capacitance of the first winding 21, the capacitor 93 is the stray capacitance of the second winding 22, and the capacitor 81
2, 813 represents the stray capacitance between the first winding and the second winding, respectively.

The inductances 803, 804, 808,
Reference numeral 809 denotes a leakage inductance generated because the first winding 21 and the second winding 22 are not completely coupled (k ≠ 1). Inductance 805 indicates the inductance of first winding 21 and second winding 22, and inductances 806 and 807 represent ideal transformers. Resistor 801,
802, 810, 811 represent the resistance of the first winding 21 and the second winding 22, and the resistors 91, 92 represent the loss of the magnetic material.

Next, the stray capacitances 90 and 93 shown in FIG.
Loss 91, 92 of magnetic material, leakage inductance 80
FIG. 12 shows the result of a simulation performed on the effects of 3, 804, 808, and 809 using a simulator. FIG. 22 shows the values of the respective elements at this time. still,
The characteristics of the transformer 20 are values approximating the characteristics of a commercially available transformer.

In FIG. 12, a curve 113 is a simulation result when there are no stray capacitances 90, 93, losses 91, 92 of the magnetic material, and no leakage inductances 803, 804, 808, 809 in FIG. Curve 114 is shown in FIG.
3 is a simulation result. From FIG. 12, 100
It is understood that the characteristic deterioration due to the stray capacitance of the windings 21 and 22 is large above MHz.

FIGS. 14 and 16 show measurement results of common mode noise radiated from a cable in an actual device.
FIG. 14 shows data without a conventional common mode filter. FIG. 16 shows data when a conventional common mode filter is attached. From FIGS. 14 and 16, the effect of the conventional common mode filter is about 5 dB below 100 MHz.

[0017]

The above-mentioned problem of the prior art is that it is difficult to suppress the common mode noise at a frequency of 100 MHz or more. The reason is that the impedance to the common mode filter cannot be increased at a high frequency due to the stray capacitance of the winding of the transformer and the loss of the magnetic material.

An object of the present invention is to provide a common mode filter having a large suppression amount.

[0019]

According to the present invention, a first transmission line is provided between each of a pair of transmission lines and a reference potential point.
And a first transformer comprising a second winding and the first winding.
And a first winding of the second winding is connected to the reference potential point, and a second winding of the second winding is connected to the transmission line.

Further, there is provided a second transformer comprising first and second windings provided in series with each of the pair of transmission lines, and the first and second transformers of the second transformer are provided. Are connected to the input terminals of the pair of transmission paths.

Further, there is provided a second transformer comprising first and second windings provided in series with each of the pair of transmission lines, and the first and second windings of the second transformer are provided. Each of the winding start portions of the second winding is connected to the output terminal side of the pair of transmission paths.

[0022] The invention further comprises a capacitor provided between each of the first and second windings of the first transformer and the reference potential point.

The operation of the present invention will be described. The first transformer inserted between the transmission line and the ground, which is the reference potential point,
Low impedance against common mode noise,
It has a high impedance with respect to the signal, and suppresses the common mode noise while giving almost no loss to the signal.

By inserting a second transformer in series with the transmission line in addition to the first transformer, the amount of suppression of the common mode noise is reduced by the common mode impedance of the second transformer and the first transformer. And a larger suppression amount can be obtained.

[0025]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of a first embodiment of the present invention, and the same parts as those in FIG. 17 are denoted by the same reference numerals.
In FIG. 1, the winding start 18 of the first winding 15 of the transformer 14 is connected to the input terminal 10 and the output terminal 12,
The other end of the first winding 15 is connected to the ground 17. The winding start 19 of the second winding 16 is connected to the ground 17, and the other end of the second winding 16 is connected to the input terminal 11 and the output terminal 13.

That is, in the present embodiment, the transformer 14
Are inserted in parallel with a pair of signal transmission lines, and the respective windings 15 and 16 are respectively connected between each transmission line and a ground 17 which is an AC grounding point.

FIG. 2 is a circuit diagram of a second embodiment of the present invention, and the same parts as those in FIG. 1 are denoted by the same reference numerals. In FIG. 2, a second transformer 2 is added to the circuit configuration of FIG.
0 is inserted in series with the signal transmission path and added. Specifically, the first winding 2 of the transformer 20
One winding start 23 is connected to one input terminal 10 and the other end is connected to one output terminal 12. Also,
The winding start 24 of the second winding 22 is connected to the other input terminal 11, and the other end is connected to the other output terminal 13. The winding start 23, 24 of each winding 21, 22 of the second transformer 20 is connected to the output terminal 12, 1,
3 may be connected respectively.

FIG. 3 is a circuit diagram of a third embodiment of the present invention, and the same parts as those in FIG. 1 are denoted by the same reference numerals. In FIG. 3, the circuit configuration of FIG.
0 and 31 are additionally connected between the windings 15 and 16 and the ground 17, respectively.

FIG. 4 is a circuit diagram of a fourth embodiment of the present invention, and the same parts as those in FIG. 2 are denoted by the same reference numerals. In FIG. 4, the circuit configuration of FIG.
0 and 31 are additionally connected between the windings 15 and 16 and the ground 17, respectively.

FIG. 5 is a circuit diagram showing a fifth embodiment of the present invention. The common mode filter circuit 2 shown in FIG.
Are connected in cascade. still,
A plurality of common mode filter circuits shown in FIG. 4 may be connected in cascade.

Next, the operation of the circuit of FIG.
This will be described with reference to FIG. FIG. 6 is an example of a signal transmission line using the common mode filter of FIG. The configuration is the same as that of FIG. 19 described in the related art. If the two output impedances 65 and 66 are equal (Z1 = Z2) and the two lines 73 and 74 are under the same condition with respect to the ground 17, the impedances 67 and 68 are equal (Z3 =
Z4).

If the inductances of the first winding 15 and the second winding 16 of the transformer 14 are equal (L1 = L2), the common mode currents 61 and 62 are equal, and the common mode currents 63 and 64 are also equal. Therefore, the common mode current does not flow through the input impedance 69 of the receiving unit.

The loop voltage and current between the line 73 and the ground 17 in FIG. 6 are as follows: Vg = Z1 ・ I10 + jω (L1−M) (I10−I20) (11) 0 = jω (L1−M) (I20−I10) + Z3.I20 (12) Assuming that the coupling coefficient k = 1, L1 = L2 = M
When this is substituted into the equation (12), I20 = 0, and no common mode current flows through the line 73.

FIG. 7 is a diagram for explaining the effect of the common mode filter of FIG. 1 on the signal. In FIG. 7, the loop voltages and currents of the signal currents 71 and 72 are as follows: Vs = (Z1 + Z2) (I1 + I2) + Z0.multidot.I1 (13) Vs = (Z1 + Z3) (I1 + I2) + j.omega. (L1 + L2 + 2M) I2 ... It is represented by (14).

From equation (13), I2 = {Vs- (Z1 + Z2 + Z0) I1} / (Z1 + Z2) (15) is obtained. Therefore, when the equation (15) is substituted into the equation (14), the following equation is obtained: I1 = jω (L1 + L2 + M) Vs / {Z0 (Z1 + Z2) + jω (L1 + L2 + M) (Z1 + Z2 + Z0)} (16) .

With respect to the inductances L1 and L2 of the transformer 14, if ω (L1 + L2 + M) >> Z0 (17), the expression (16) is as follows: I1 = Vs / (Z1 + Z2 + Z0) (18) The voltage VL at both ends of the impedance 69 is as follows: VL = Z0.Vs / (Z1 + Z2 + Z0) (19), which is equal to the equation (3).

As described above, by inserting the common mode filter of FIG. 1 into the transmission lines 73 and 74 as shown in FIG. 6, the common mode current flowing through the transmission lines 73 and 74 can be reduced to zero, and the signal Has no effect. However, in practice, since the degree of coupling between the first winding 15 and the second winding 16 of the transformer 14 is not perfect, the leakage inductances 803, 804, 808, 809 and the winding resistances 801 and 801 shown in FIG.
Since there are 802, 810, 811, they do not become zero,
The impedance of the transformer 20 with respect to the common mode noise is the leakage inductance 803, 804, 808, 80
9 and the winding resistance 801, 802, 810, 811.

The common mode filter of FIG. 2 improves the characteristics of the common mode filter of FIG. 1, and the amount of common mode noise suppression is reduced when the impedances 67 and 68 are much larger than the common mode impedance of the transformer 14. Is the voltage dividing ratio of the common mode impedance of the transformer 20 and the common mode impedance of the transformer 14.

The capacitors 30 and 40 of FIGS. 3 and 4
Is to prevent low frequency noise such as commercial AC from entering the transmission line from the ground.

In the cascade connection configuration of FIG. 5, a larger suppression amount than the common mode filter shown in FIG. 2 of the basic configuration can be obtained.

[0042]

Next, embodiments of the present invention will be described with reference to the drawings. In the following embodiment, the parameters of the transformer 14 and the transformer 20 correspond to the leakage inductance 80 shown in FIG. 21 which is a main factor for deteriorating the suppression amount of the common mode noise.
Only 3,804,808,809 and stray capacitances 90,93 are used.

FIG. 8 shows an embodiment of the common mode filter shown in FIG. 1, FIG. 9 shows an embodiment of the common mode filter shown in FIG. 2, and FIG. 10 shows an embodiment of the common mode filter shown in FIG. . FIG. 11 shows the values of the respective elements.

FIG. 12 shows the simulation results of the transmission characteristics of the common mode noise of the embodiments of FIGS. 8, 9 and 10. Curve 110 is the common mode filter of FIG. 8, and curve 111 is the common mode filter of FIG. A curve 112 is a simulation result of each of the common mode filters of FIG.

The common mode filter according to the present invention has a regulation band of 30 MHz to 1 GHz, which is a regulation band of the Voluntary Control Council for Interference by Information Technology Equipment (VCCI) for regulating electromagnetic interference.
It can be seen that the suppression amount is larger at z than in the conventional common mode filter 6 of the curve 114. In particular, it can be seen that the suppression amount of the common mode filter of FIG. 2 of the curve 111 and the common mode filter of FIG.

FIG. 13 shows the result of simulation of the signal transmission characteristics of the common mode filter. Curve 120
1, curve 121 is the simulation result of the common mode filter of FIG. 2, curve 122 is the simulation result of the common mode filter of FIG. 4, and curve 123 is the simulation result of the conventional common mode filter of FIG. In both cases, the loss increases in the low band and the high band, but the characteristics are flat over a wide band.

The filter shown in FIG.
The reason why the loss is large near z is that the inductance and mutual inductance of each winding of the transformer 14 and the capacitors 30 and 31 resonate.

FIGS. 14 and 15 show the results of measuring the common mode noise radiated from the cable of the apparatus. The measurement was performed by the 3 m method of VCCI. Line 131 is VCCI
The class B limits of

FIG. 14 shows the radiation noise when the common mode filter is not used, and FIG. 15 shows the radiation noise when the common mode filter of FIG. 2 is used. When the common mode filter shown in FIG. 2 was used, data at a level near the limit of the measurement equipment was obtained at 300 MHz or less.

[0050]

An advantage of the present invention is that common mode noise can be greatly suppressed. The reason is that a transformer with high impedance for common mode noise is inserted in the transmission line in series, and a transformer with low impedance for common mode noise is inserted between the transmission line and AC ground point to suppress common mode noise. To do that.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a third embodiment of the present invention.

FIG. 4 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 5 is a circuit diagram showing a fifth embodiment of the present invention.

FIG. 6 is a diagram illustrating the operation of the first exemplary embodiment of the present invention.

FIG. 7 is a diagram illustrating an operation of the first exemplary embodiment of the present invention.

FIG. 8 is a configuration diagram illustrating an example of the first exemplary embodiment of the present invention.

FIG. 9 is a configuration diagram illustrating an example of the second exemplary embodiment of the present invention.

FIG. 10 is a configuration diagram showing an example of the fourth embodiment of the present invention.

FIG. 11 is a diagram showing element values in a simulation of an example of the present invention.

FIG. 12 shows simulation data of a common mode filter of the present invention and a conventional common mode filter.

FIG. 13 shows simulation data of a common mode filter of the present invention and a conventional common mode filter.

FIG. 14 is a measured value of noise radiated from a device when a common mode filter is not used.

FIG. 15 shows measured values of noise radiated from the device when the common mode filter of the present invention is used.

FIG. 16 shows a measured value of noise radiated from a device when a conventional common mode filter is used.

FIG. 17 is a diagram illustrating an example of a conventional common mode filter.

FIG. 18 is an explanatory diagram of a path through which common mode noise and a signal pass.

FIG. 19 is a diagram illustrating the operation of a conventional common mode filter.

FIG. 20 is a diagram illustrating the operation of a conventional common mode filter.

FIG. 21 is a diagram showing an equivalent circuit of a transformer.

FIG. 22 is a diagram showing element values in a simulation of a conventional common mode filter.

FIG. 23 is a diagram showing a specific example of a conventional common mode filter.

[Explanation of symbols]

10,11 Input terminal 12,13 Output terminal 14,20 Transformer 15,16,21,22 Transformer winding 17 Ground 18,19,23,24 Winding winding start 30,31,80,81,90 , 93 capacitor 60 common mode noise source 61, 62, 63, 64, 171, 172 common mode noise current 65, 66 output impedance 67, 68 impedance between transmission line and ground 69 receiving side input impedance 70 signal source 71, 72, 173 Signal current 73, 74 Transmission line 81, 82, 91, 92 Resistor

Claims (6)

[Claims] A first transformer comprising first and second windings provided between each of a pair of transmission lines and a reference potential point, wherein the first and second windings are provided. A common mode filter, wherein one winding start of the wire is connected to the reference potential point, and the other winding start is connected to the transmission line. 2. The apparatus according to claim 2, further comprising a second transformer comprising first and second windings provided in series with each of said pair of transmission lines, wherein said first and second windings of said second transformer are provided. 2. The common mode filter according to claim 1, wherein each winding start of the two windings is connected to an input terminal side of the pair of transmission paths. 3. 3. A second transformer comprising first and second windings provided in series with each of the pair of transmission lines, wherein the first and second transformers of the second transformer are provided. 2. The common mode filter according to claim 1, wherein each winding start of the two windings is connected to an output terminal side of the pair of transmission paths. 3. 4. The apparatus according to claim 1, further comprising a capacitor provided between each of said first and second windings of said first transformer and said reference potential point. Common mode filter as described. 5. A common mode filter comprising a plurality of cascade-connected common mode filters according to claim 1. 6. The common mode filter according to claim 1, wherein the reference potential point is an AC ground point.