Value determining method for line filter and its components

Abstract: In a line filter including two capacitors (C) connected between a pair of input lines and two inductors (L) inserted between the input side and the load side of the input line and combining them in the sequence of C - L - C - L from the input side, the line filter serves to set a circuit constant as; LT ≧ a (b · CT) -(dF + e) where the total electrostatic capacitance of the capacitors is CT (µF), the total inductance of the inductors is LT (µH), a proportional factor of a frequency is a, a proportional factor of total inductances is b, a proportional factor of a frequency is d, a correction factor of a frequency is e, and a suppressing minimum frequency of a normal mode noise source is F (KHz). Or, in the line filter combines the inductors and the capacitor in the sequence of L - C - L, the line filter served to set the circuit constant as; LT ≦ a (b · CT) The invention offers smaller and cheaper line filters because they have definitive circuit constants.

Description: The present invention relates to a value determining method for a line filter and its components which attenuate noise signals supplied from electronic appliances having built-in high-frequency noise sources (referred to as a noise source appliance) such as switching power source. In general, noise signals spread from a noise source appliance to the external through a power line. When these signals are received by a pseudo-power circuit network (referred to as an LISN), their noise level (referred to as a noise terminal voltage) must be less than or equal to a legally-regulated value. For meeting the requirement, the prior art provides a line filter on the power entry side of the noise source appliance. The line filter can effectively attenuate normal mode and common mode components, but it is considerably bulky because of a built-in coil unless it is much devised. With recent increase of demands for small or lightweight appliances, the line filter has been required to include smaller, more lightweight and cheaper elements whose characteristic values must be as low as possible. In the prior arts, however, engineer's experience and knowledge have significantly determined how to assemble elements for producing a line filter as noise source fundamental frequency components are supplied to the line filter. Comparing an optimal line filter, therefore, has been quite burdensome. It often results in producing a bulky or expensive line filter. The foregoing noises have a common mode noise flowing between a power line and the earth and a normal mode noise flowing between power lines. In both cases, to attenuate the noise level, the line filter is inserted between a power source and a power line. A typical line filter comprises balun coils (balanced-to-unbalanced transformer) respectively providing two set of windings on a ring core, these balun coils being longitudinally connected, capacitors inserted between a power source and a power line on the load side of each coil (referred to as X capacitors), and capacitors inserted between the power line and the earth on the load side of each coil (referred to as Y capacitors). Fig. 16 is a circuit diagram showing the arrangement of a power source circuit employing this type of line filter. In Fig. 16, 1 and 1' denote input terminals, 2 and 2' denote output terminals, G denotes an earth terminal, Rec denotes a diode bridge for rectifying alternate current into direct current, and C F denotes a capacitor for smoothing the rectified voltage. T denotes a transformer consisting of a primary and a secondary coils, Q denotes a switching transistor, D denotes a diode for rectifying an indicative voltaqe on the secondary coil of the transformer T. L 1 , L 2 denote balun coils, C x1 , C x2 denote X capacitors connected between power lines, and C y1 , C y2 denote Y capacitors connected between the power line and the earth. This circuit feeds direct current from the output terminals 3, 3' of the secondary coil to the load. The output terminal 3' is normally connected to the earth for stabilizing operation of an electric circuit. C c means distribution capacitance generated between the primary coil and the secondary coil of the transformer T and serves to pass the common mode noise generated by the transistor Q switching to the sencondary side. The current path of the common mode noise is shown by arrows. As set forth above, the noises can be grouped into common mode components and normal mode ones. The current shown in Fig. 16 has equivalent circuits shown in Figs. 17 and 18, which serve to attenuate these two types of noise component. Fig. 17 is a view showing an equivalent circuit of the circuit shown in Fig. 16, which is dedicated for common mode noises. In Fig. 17, Ec denotes a common mode noise source, R N denotes impedance (generally, R N = 50Ω) of a pseudo-power source circuit (referred to as an LISN) network representing equivalent impedance of an AC power line. The other like references denote the like elements shown in Fig. 16. The Y capacitor C y2 recovers most common mode noise emerging through the distribution capacitance C c as i c2 . The Y capacitor C y1 recovers most of the leaked noise as i c1 . The remaining noise flows into the exterior as iRN. To attenuate the noise components of lower harmonics, the Y capacitor C y2 must have larger capacitance. However, it is subject to restrictions on safety standards for leak current since it is connected to the earth. In place of providing the Y capacitor C y2 with larger capacitance, therefore, the balun coil L 2 must have larger inductance. However, the balun coil L 2 passes high-frequency noises, because it is a parasitic element and thus has larger distribution capacitance. In this case, by connecting the balun coil L 1 having smaller distribution capacitance to the input side of the balun coil L 2 , it is possible to suppress the common mode noises of a high frequency. In addition, the present inventors have already developed the method for effectively reducing the common mode components on the assumption of L 1 < L 2 and C y1 < C y2 in the foregoing circuit (as disclosed in the Official Gazettes of Japanese Patent Laid-Open No. 61-157111 and U.S. Patent No. 4,667,173). Fig. 18 is a view showing an equivalent circuit of the circuit shown in Fig. 16, which is dedicated for normal mode noises. In Fig. 18, I denotes switching current, which generates the normal mode noises and whose waveform is, as an example, shown right. C F denotes a smoothing capacitor which generates the switching current I. ESR denotes an equivalent series resistor which is a parasitic element of the capacitor C F . In general, the capacitor C F generates a far larger voltage than ESR, causing the normal mode noise. L e1 and L e2 respectively denote leak inductance of the balun coils L 1 and L 2 , which brings about impedance for suppressing the normal mode noises. R N denotes equivalent impedance of the LISN. The noise current generated in ESR becomes the normal mode noise i N which counter-flows from the line filter to the power source. Most of the noise i N is recovered as i cx1 by the X capacitor C x1 , but is partially leaked from it. The leaked noise counter-flows to the LISN through an AC-power line and is measured as noise. The foregoing conventional line filter includes a capacitor C x2 being connected in parallel with the smoothing capacitor C F (several hundreds µF). Yet, the capacitor C x2 has electrostatic capacitance of 0.1 to 0.5 µF, which is negligible in comparison with the capacitance of the capacitor C F . At this time, the capacitor C x2 has the following impedance in association with switching fundamental waves or lower harmonic noises: |1/ω C x2 |>> ESR 2 + (1/ω C F ) 2 It indicates that the capacitor does not effectively work for attenuating the normal mode noises. This invention is designed to overcome the foregoing shortcoming, and it is an object of this invention to provide a line filter which is excellent in an attenuating effect. The invention provides a line filter as defined in the independent claims. By minimising the inductances this invention can offer a small and inexpensive line filter, capable of filtering normal mode noise. Preferred embodiments of the invention will now be described and compared with conventional filters, with reference to the accompanying schematic drawings, in which:- Fig. 1 is a view showing arrangement of a line filter for attenuating the normal mode noises; Fig. 2 is a view showing an equivalent circuit for obtaining an optimal circuit constant of the line filter; Figs. 3 and 4 are graphs of total capacitance CT against total inductance LT for describing the border on which one of two types of filters should be properly used; Fig. 5 is a view showing an arrangement of a line filter for attenuating common mode noises; Fig. 6 is a view showing an arrangement of combination of the line filter for attenuating normal mode noises with that for attenuating common mode noises; Fig. 7 is a circuit diagram showing a line filter; Fig. 8 is a view showing an arrangement of a circuit to be compared with circuit shown in Fig. 7; Fig. 9 is a view showing an equivalent circuit of the circuit shown in Fig. 8, which works for normal mode noises; Fig. 10 is a view showing a waveform of power current flowing through the circuit shown in Fig. 9; Fig. 11 is a view showing the definition of current and voltage in the circuit shown in Fig. 9; Figs. 12 and 13 are graphs showing the inductances of the choke coils; Fig. 14 is a view showing the relation of a minimal desired value of choke coil L 2e with a choke coil L 2e ; Fig. 15 is a circuit diagram showing the other line filter; Fig. 16 is a view showing one example of power source arrangement employing a conventional line filter; Fig. 17 is a view showing an equivalent circuit of the circuit shown in Fig. 16, which works for common mode noises; and Fig. 18 is a view showing an equivalent circuit of the circuit shown in Fig. 16, which works for common mode noises. Fig. 1 show an arrangement of a line filter for attenuating normal mode noises. L1 and L2 denote inductors inserted between an input side and a load side of a power line, and CX0, CX1 and CX2 denote capacitors connected between a pair of power lines. Fig. 2 shows an equivalent circuit for obtaining an optimal circuit constant of this line filter. CN denotes a capacitor, which is one component of a pseudo-power source circuit network (referred to as LISN), RN denotes a resistor, which is one component of the LISN or internal impedance of a jamming wave level detector Rx, Le denotes an inductance of all components composing the LISN, Re denotes resistance of all components composing the LISN, CX0, CX1 denote capacitors for attenuating normal mode noises, L1e, L2e denote inductors for attenuating normal mode noises, and ESR denotes an equivalent series resistor, which corresponds to the equivalent components of an electrostatic capacitor for smoothing. Further, ISW denotes current produced by switching elements contained in a switching power source, en denotes an equivalent voltage source representing switching noises caused in the ESR, Vn denotes a noise terminal voltage caused in the resistor Rn (to be measured by the jamming wave level detector), I denotes equivalent current produced when the noise terminal voltage Vn is set on the regulated value of the VDE standards, VDE0871, Class B, Vo denotes a voltage caused in a branch path consisting of a capacitor Cn and the resistor Rn, that is, VO = 2RnI + {1/(jω Cn)}· I, IZ denotes current flowing through Le and Re, Ico denotes current flowing through CX0, Ic1 denotes current flowing through CX1, IA denotes current flowing through the inductor Lie, IB denotes current flowing through the inductor L2e, and V2 denotes a voltage caused between A-A'. Moreover, en = V2 is given when the noise terminal voltage Vn keeps the regulated value of VDE0871 and Vn is equal to the regulated value. (1) Normal mode noise source The source is a noise voltage which serves to cause switching current at both ends of input side smoothing capacitor. The noise voltage consists of internal components of the switching current produced on the electrostatic capacitance of the smoothing capacitor and voltage drop components caused by the ESR. If an aluminum electrolytic capacitor is employed, the noise voltage is regarded to mainly consist of the ESR components. (2) Filter circuit A two-stage filter using two Ls is required to adapt the noise terminal voltage to the regulated value of the VDE standards VDE0871, Class B. Herein, the combinations of CX0 - L1 - CX1 - L2 and L1 - CX1 - L2 are considered. (3) LISN equivalent circuit The LISN regulated on the VDE standards VDE0876 is considered. In order to obtain the LISN equivalent constant, it is possible to separate the elements into RN elements, CN elements and all elements except them, RN and CN elements being serially connected to an LISN jamming waves level detector (RX). The elements except RN and CN ones are grouped as equivalent resistors Le and Re. (4) Equivalent circuit used for calculation Fig. 2 shows an equivalent circuit used for calculation for determining an attenuation effect the filter can offer. The source is a power source, but it (the section in the right side of the line A-A') should be replaced with an equivalent voltage source en. (5) Studying noise characteristics The filter element should be selected from the following three cases. 1) CX0 CT/2, CX1 CT/2, CX0 + CX1 = CT 2) CX0 = 0, CX1 = CT 3) CX0 = CT, CX1 = 0 For each case, how to arrange L1, L2, CX0 and CX1 was studied for obtaining the maximum attenuation effect under the condition of LT = L1 + L2 (constant) and when LT = constant. Then, 2) and 3) cases will be compared. Assume that 1(A) normal mode noise current flows through RN for obtaining damping capacity of a filter. The current should have a value so that Vn may be same with the VDE regulated value. At this time, it is necessary to obtain the noise voltage V2 between the load terminals of the filter. (Vn keeps a regulated value of VDE0871, Class B.) V2 = {(RN'I - X 1 I 1 i) - X 2 (I 1 i + IC1i)} + j{(X 1 I 1 r - I/Yn + X 2 (I 1 r + IC1r)} where RN' = 2RN = real number, and i = imaginary number. Further, assuming that IC1 = CX1 current; I 1 = L 1 current; X1, X2 = L1, L2 reactances respectively (= ω L, ωL) Yn, Y0, Y1 = Cn, CX0, CX1 admittances respectively (=ω Cn, ωCX0, ωCX1); X 1 + X 2 = XT (= constant), and Y0 + Y1 = YT (=constant), the formulas representing V2r and V2i are as follows: V2r = RnI - XT (Y0RN' - IZi)I - (XT - X 1 )(XT - Y0){RN' - X 1 (Y0RN' - IZi)} I V2i = XT (1 + IZr + Y0/Yn)I + (XT - X 1 )(YT - Y0){1/Yn - X 1 (1 + IXr + Y0/Yn)}I - I/Yn where IZr and IZi denote a real number and an imaginary number of a formula in which z represents current flowing through a branch paths of Le and Re, as shown below; IZr = (ReRn - XeXn) / (Re 2 + Xe 2 ) IZi = (RnXe - Re/Yn) / (Re 2 + Xe 2 ) V2 is equal to an allowable value of the source noise voltage en. Assuming that; en = V2r 2 + V2i 2 K = en/Vn = V2r 2 + V2i 2 / (RN·I) and the damping capacity IL of a filter IL = 20 logk, optimising the line filter for normal mode noises results in selecting circuit elements so that IL is maximized under the conditions of CT = constant and LT = constant. For making the conditions less complicated, it is necessary to substitute the conditions of the formulas 1) to 3) for the formulas 4) and 5). (i) Y0 1/2 YT, Y1 1/2 YT (the case 1)) For representing K using X 1 , substitute the formula 7) for the formulas 4) and 5) and then the result for the formula 6), resulting in providing the following formulas; K 2 RN 2 = A0X 1 4 + A1X 1 3 + A2X 1 2 + A3X 1 + A4 A0 = Mr 2 + Mi 2 A1 = -2 (MrNr + MiNi) A2 = Nr 2 + Ni 2 - 2MrPr + 2MiPi A3 = 2(NrPr + NiPi) A4 = Pr 2 + Pi 2 Mr = (YT/2)(YT Rn/2) - IXi Nr = YT Rn/2 + (YT/2)XT (YT Rn/2 - IZi) Pr = RN(1 - YT XT) + XT IZi Mi = (YT/2)(1 + IZr + YT/2 Yn) Ni = (YT/2) {1/Yn + XT(1 + IZr + YT/2 Yn)} Pi = XT (1 + IZr + YT/YN) - 1/Yn Then, substitute f(X 1 ) for the left side of the formula 8), resulting in the following formula; f(X 1 ) = A6X 1 4 + A1X 1 3 + A2X 1 2 + A3X 1 + A4 Herein, it is possible to obtain X 1 maximizing f(X1). This value is X 1 which brings about a maximum value of IL in Y0 = Y1 = 1/2YT. (ii) Y0 = 0, Y1 = YT (case 2)) Y0 = 0, Y1 = YT For representing K using A4, substitute the formula 10) for the formulas 4) and 5) and then the result for the formula 6), resulting inproviding the following formula; f(X 1 ) = A0X 1 4 + A1X 1 3 + A2X 1 2 + A3X 1 + A4 where, A0 = Mr 2 + Mi 2 A1 = 2(MrNr + MiNi) A2 = Nr 2 + Ni 2 + 2MrPr + 2MiPi A3 = 2(NrPr - NiPi) A4 = Pr 2 + Pi 2 Mr = YTIZi Nr = YT(RN - XYIZi) Pr = RN(1 - YTXT) + XTIZi Mi = YT(1 + IZr) Ni = YT{1/Yn + Xt(1 + IZn)} Pi = XT(1 + IZr + YT/Yn) = 1/Yn Like the (i) case, it is possible to obtain a value of X 1 which maximizes f(X 1 ). The X 1 is a value provided when Il maximizes under Y0 = 0 and Y1 = YT. Under the same condition of YT and XT, comparing a maximum value of f(X 1 ) obtained from the formula 9) with that obtained from the formula 11) and picking up the circuit arrangement having a larger value, under F = 50 KHz, two areas appear, that is, (i) area where the C - L - C - L combination has a larger value of damping capacity IL and (ii) area where the L - C - L combination has a larger value. The border extends on the line where both combinations have the same IL value. Likewise, under F = 100 KHz, the border extends as shown in Fig. 4. By providing a minimum frequency F and a certain CT and LT, therefore, it is possible to define filter arrangement having higher damping capacity with reference to the borders as shown in Figs. 3 and 4. Once the filter arrangement is defined, it is also possible to obtain CX0 and CX1 from CT. Since the X 1 maximizing f(X 1 ) can be obtained with reference to the formulas 9) and 11), the following formulas compute L 1 and L 2 . L 1 = X 1 /ω, L 2 = (XT - X 1 )/ω Figs. 3 and 4 show graphs plotting an approximation formula computing out a border in the form of; LT0 = a (b · CT) -(dF + e) where a denotes a proportional factor of total inductance (= 2.39 x 10 6 ), b denotes a proportional factor of total electrostatic capacitance (= 141), d denotes a proportional factor of a frequency (= 0.0132), e denotes a correction factor (= -1.055), and LT : µH, CT : µF, F : KHz. By providing CT, LT and F, therefore, it is possible to select the combination of CX0 - L 1 - CX1 - L 2 if LT ≧ LT0 or the combination of L 1 - CX - L 2 if LT < LT0. Reference has been heretofore directed to the line filter arrangement for attenuating normal mode noises. The line filter normally has a capability of attenuating common mode components, because the noise source generally includes common mode components. The common mode choke coil for attenuating the common mode noises, included in the line filter arrangement as shown in Fig. 1, may serve to attenuate the normal mode noise components if it employs leak inductance. Fig. 5 shows line filter arrangement for normal mode noise components, which employs the common choke coil. In Fig. 5, L 1 and L 2 denote inductors employing common mode choke coils, the leak inductance of which serves to suppress the normal mode noises. CX0, CX1 and CX2 denote capacitors for suppressing the normal mode noises. Fig. 6 shows the filter arrangement for attenuating the normal mode noise components as well as the common mode noise components in cooperation with a Y capacitor. In Fig. 6, CX1 and CX2 denote capacitors for suppressing the common mode noises. L 1 and L 2 denote inductors which respectively suppress the common mode noises using the common mode inductance and the normal mode noises using the leak inductance. Further, G denotes an earth terminal which is part of a common noise path. CX0, CX1 and CX2 denote capacitors for suppressing the normal mode noises. CX2 is an additive capacitor for attenuating the normal mode noises. The foregoing description has been directed to a single line filter. It is also possible to dispersively locate L 1 , L 2 , CX0, CX1, CX2, CY1, and CY2 in a power supply or an electronic device. As set forth above, the line filter of this embodiment comprises at least two normal mode inductors and at least one normal mode capacitor. Assuming that the total electrostatic capacitance is CT (µF), total inductance of the two inductors is LT (µH), a proportional factor of total inductance is a, a proportional factor of total capacitance is b, a proportional factor of a frequency is d, a correction factor of frequency is e, and a suppressed minimum frequency of a normal mode noise source is F (KHz), it is possible to select the combination of CX0 - L 1 - CX1 - L 2 if LT ≧ LT0 or the combination of L 1 - CX1 - L 2 if LT < LT0, where LT denotes total normal mode noise inductance and LT0 is given by the formula; LT0 = a (b x CT) -(dF + e) The foregoing description results in offering definitive element constants. Next, a value determining method for line filter components will be described. Fig. 7 shows one embodiment of a line filter used for this method. In Fig. 7, L 1 denotes a balun coil inserted on a power line on the input side, L 2 denotes a balun coil inserted on the power line on the load side, and C x1 denotes an X capacitor inserted on the power line on the input side of the balun coil L 2 . C y1 denotes a Y capacitor connected between the earth and power line on the load side of the balun coil L 1 , C y2 denotes power line on the load side of the balun coil L 2 , 1 and 1' denote input terminals to be connected to power lines, 2 and 2' denote output terminals to be connected to load lines, and G denotes an earth terminal connected to the earth of an AC-powered source. In this circuit, the balun coil L 2 has a larger inductance than the balun coil L 1 , and the Y capacitor C y2 has larger electrostatic capacitance than the Y capacitor C y1 . As shown in Fig. 7, the circuit of this embodiment provides the X capacitor for suppressing the normal mode noises on a middle spot between the balun coils L 1 and L 2 . Fig. 8 shows the arrangement of a circuit to be compared with the circuit shown in Fig. 7. In Fig. 8, an X capacitor C x0 is inserted between the power lines on the input side of the balun coil L 1 , and an X capacitor C x1 is inserted between the power lines on the input side of the balun coil L 2 . The other elements are common to those in the circuit shown in Fig. 7. Then, reference will be directed to the process of forming the circuit shown in Fig. 1 with reference to Fig. 8. Fig. 9 shows an equivalent circuit of the circuit shown in Fig. 7, which works for attenuating the normal mode noises. In Fig. 9, I denotes switching current which generates the normal mode noises. C F denotes a smoothing capacitor, which is a source of the switching current I. R F denotes an equivalent series resistor which is a parasitic element of the capacitor C F , and C x0 and C x1 denote X capacitors connected between the power lines 1 and 1'. L 1e and L 2e denote leak inductances of the balun coils L 1 and L 2 , which provide impedance working as an obstacle against the normal mode noises. R N denotes equivalent impedance of the LISN. C N denotes a high-pass filter capacitor which inhibits commercial frequency components and allows measuring noises to pass. Le and Re compose the remaining part of the LISN, which respectively correspond to a reactance element and a resistance element for correcting low-pass impedance. 3 and 3' denote terminals for picking up the normal mode noises and connecting to a disturbance waves level detector, V N denote a voltage value of the normal mode noise components, and en denotes a voltage value of a source noise. This circuit has a filter composed of L 1e , L 2e , C x0 , and C x1 . It serves to attenuate the noises generated in the equivalent series resistor R F which is a parasitic element of the capacitor C F . In the circuit shown in Fig. 16, the X capacitor c x1 is inserted between the power lines on the load side of the balun coil L 1 , and the X capacitor C x2 is inserted between the power lines on the load side of the balun coil L 2 . In the circuit shown in Fig. 2, the X capacitor C x0 is inserted between the power lines of the balun coil L 1 , and the X capacitor C x1 is inserted between the power lines of the balun coil L 2 . The line filter employing the arrangement shown in Fig. 8 has a large attenuating effect for the normal mode noises. The present inventors detail this fact in the aforementioned Japanese Patent Specification filed on December 16, 1987. The present invention discloses how to select the values of C x0 , C x1 , L 1e , and L 2e for making the line filter arrangement shown in Fig. 8 smaller and more lightweight. At first, assuming that the waveform of I is 50 KHz (T = 20 µS), in Fig. 9, C x0 + C x1 = 0.47 µF (constant) is established in the values of; Le 100 µH Re 10 Ω RN = 50 CN = 0.1 µ or less CF = 27 µF or less RF = 0.2 Ω en = 0.163 V (50 KHz, Ton = 3.75 µS) Vn = 2.118 mV (50 KHz) and L 1e = 0.2 mH is given, the process for obtaining a value of L 2e will be described. In Fig. 9, because of R F » 1/ C F , : switching frequency, the voltage en of the source noise can be represented by; en = R F a(n) 2 + b(n) 2 a(n) = I 2 sin /nπ - (I 2 - I ) x (1 - cos)/nπ b(n) = I 1 /nπ - I 2 cos /nπ + (I 2 - I 1 ) sin /nπ = 2πnδ, δ = 0.5 Ei min/E I 1 and I 2 respectively denote current values in the passage of switching current at the times of t = 0 and t - Ton. Ei min denotes a minimum input voltage where pulse width of switching current is Ton = 1/2T. This is a minimum voltage which allows the pulse width to be controlled. Assuming that an input voltage Ei = 280 V and Ei min = 105 V, the voltage en of the source noise is; e(1) = 0.163 V for a fundamental wave (n = 1) shown in Fig. 10, or e(2) = 0.1365 V, e(3) = 0.0983 V for a second or third high frequency. Assuming that C x0 , C x1 ≧ 0.1 µF L 1e , L 2e ≧ 0.05 mH the values of filter components may be computed in relation to the fundamental wave only. At the B level of VDE0871, the regulated value is; Vn(1) = 2.118 mV (50 KHz, 55.6 dB) Vn(2) = 1.20 mV (100 KHz, 61.6 dB) where by giving en (1), Vn(1), C x0 + C x1 = CT (= constant), and L 1e , L 2e is computed as follows. Then, reference will be directed to several factors such as a current value or a voltage value in the circuit shown in Fig. 9, which factors are defined as shown in Fig. 11. At first, flowing 1A through R N , Vn = 1(A) x 1/2R N = 1/2 R N is given. Then, both terminals of C x0 : VC 0 = R N - jXn current of Xe and Re: iz = Vco/(Re + jXe) = izr - jizi izr = (RNRe - XnXe)/(Re 2 + Xe 2 ) izi = (RNXe + ReXn)/(Re 2 + Xe 2 ) current of C x0 : ico = jω C x0 V co = XnY co + jRnY co Y co = ω C x0 current of L 1e : i Li = 1 + iz + i co i Li = i Lir + ij L1i i Lir = 1 + izr + XnY co i L1i = RNY co - izi voltage of L 1e : V L1 = jω L 1e · I Li ; V 1 = ω L 1e ∴ V L1 = - X Li i L1 i + jX L1 i L1 r voltage of C x1 : V c1 = V co + V L1 ∴ V c1 = RN - X L1 i L1 i + j(X L1 i L1 r - X n ) current of C x1 : ic 1 = j C x1 V c1 ; Y c1 = C x1 ∴ i c1 = - Y c1 V c1 i + jY c1 iV c1 r current of L 2 : i L2 = i L1 + i L1 + i c1 ∴ i L2 = i L2 r + ji L2 i i L2 r = i L1 r + ic 1 r i L2 i = i Li i + ic 1 i voltage of L 2 : V L2 = jω L2 ei L2 ; ω L2e = X L2 V L2 = - X L2 i L2 i + jX L2 i L2 r source voltage: et = V c1 + V L2 ∴ et = etr + jeti etr = V c1 r - X L2 i L2 i eti = V c1 i + X L2 i L2 r By dividing both sides of all foregoing formulas with Rn, these formulas are transformed into the relation formulas of various amounts per 2Vn because of the given relation of RN = 2 Vn. Thus, assuming that (et/2Vn) 2 = (en/wVn) 2 = r and representing an unknown X L2 for obtaining L 2e with x, x is given by the following formula; x = (ag - bf±√D)/(f 2 + g 2 ) where a = V c1 r, b = V c1 i, f = i L2 r, g = i L2 i, D = r(f 2 + g 2 ) - (ag + bg) 2 Thus, by giving the source noise voltage en, the regulated values Vn, C x0 , C x1 and L 1e , L 2e is given by; L 2e = x/ω Assuming that L 1e is 0.2 mH if C x0 + C x1 = CT = 0.47 µF, therefore, L 2e can be computed out. At the same time, the source noise fundamental wave voltage of R F is 0.163 V, and Vn is given by; Vn = 0.00118 V in the state of the B level of the noise voltage regulated value VDE0871 at the measuring terminal R N and at 50 KHz. The damping capacity of the filter corresponds to (0.163/0.00118), where it is necessary to give L 2e a value shown in Fig. 12. Further, in Fig. 12, Ca denotes a value of C x0 (= 0 µF) minimizing L 2e , and Cb denotes a value of C x0 (= 0.32 µF) locally minimizing L 2e . The smaller-sized line filter requires as small a value of L 2e as possible. Hence, assuming that; C x0 = Ca = 0 µF, and C x1 = 0.47 µF = C x0 = 0.47 µF, L 2e has a value given by; L 2e = 0.3 mH In Fig. 12, C x01 and C x02 respectively denote a local maximum and a maximum value, which points respectively denote series resonance points between L 1e //L 2e (serial synthetic value of inductance) and Cx0 and between L 1e and C x1 . Fig. 14 shows a characteristic curve of L 2e depicted as a result of changing L 1e in the range of 0.04 mH to 0.6 mH. The minimum value of L 2e rests in case of; C x0 = 0 µF, C x1 = 0.47 µF Further, assuming that C x0 + C x1 = 0.68 µF is given, L 2e has a minimum value in case of; C x0 = 0 µF, C x1 = 0.68 µF Then, increasing the value as shown below; C x0 + C x1 = 1 µF L 2e has a minimal value on the following point as shown in Fig. 13; In case of C x0 + C x1 = 1 µF, under the state of L 1e ≦ 0.2 mH, L 2e has a minimal value when; C x0 = Ca = 0 µF g, h and i on Fig. 14 indicate characteristic curves of L 1e depicted in relation to the foregoing minimal values of L 2e . Each curve determines its required L 2e if L 1e is given in the state of; C x0 + C x1 = constant These curves respectively coincide at points D, E and F with straight lines a, b and c which satisfy the condition of; L 1e + L 2e = constant These points correspond to points minimizing the sum of L 1e + L 2e The characteristic curves g, h, i are depicted under the state of C x0 + C x1 = constant. Hence, each curve shows an optimal sum of the X capacitors. The point of contact D, E or F indicates a minimal point of the choke coil. Hence, it shows optimal inductance. The point of contact D, E or F means the point optimizing both the X capacitor and the choke coil. The k portion on the i curve means a section having a minimal value of L 2e nearby; The portion satisfying the condition; L 1e ≦ 0.26 mH is a point where L 2e has a minimal value on the point of; C x0 = 0, C x1 = 10 µF These minimal points D, E, F satisfy the condition of; C x0 = 0, C x1 = max, L 1e < L 2e Lastly, reference will be directed to why the optimal point rests on the point of C x0 = 0. If C x0 > 0, the circuit state approches a series resonance state caused by C x0 and L e // L 1e so that the noise voltage slightly rises between the terminal 1' and 1. It means that lowering the noise voltage to the regulated value requires a larger value of L 2e . If a sum of C x0 + C x1 rises to about 1 uF, the L - C step type attenuating effect resulting from equally-divided 1 uF located on C x0 is more excellent than the attenuating effect resulting from 1 pF focused on C x1 . As shown in Fig. 13, therefore L 2e has a minimal value nearby; Fig. 15 shows the embodiment of the arrangement employing the foregoing phenomenon. This embodiment corresponds to the K section in Fig. 14 under the state of; C x0 = C x1 , L 1 < L 2 If the size of a choke coil is proportional to the energy amount E; E = 1/2 LI 2 (L: inductance, I: supply current) condensed in the choke coil, the sum of two choke coils become minimal and most lightweight in the state of; E 1 + E 2 = minimal or L 1e = L 2e = minimal The points D, E and F in Fig. 14 correspond to the points for the minimal and most lightweight choke coil, meeting the foregoing conditions. Further, the curve shown in Figs. 12 and 13 are based on the computation in the state of C x0 + C x1 . Hence, by meeting the conditions of; C x0 = 0, C x1 = max and C x0 = C x1 = 1/2 (max) C x0 and C x1 are optimized. The point of the smallest and most lightweight X capacitor rests on the curve g, h or i shown in Fig. 14. The circuit shown in Fig. 9 employs leak inductance of a balun type common mode choke coil as L 1e and L 2e , but it may employ an independent choke coil. Considering that the balun type common mode choke coil has small normal mode inductance, the independent choke coil may be more advantageous. If a shield is applied on a switching converter and transformer to attenuate the electrostatic capacitance between the primary coil and the secondary coil, the normal mode noises are more dominant. In this case, the circuits shown in Figs. 7 and 15 may employ a choke coil in place of a balun coil. Though the foregoing description does not distinctively separate the inductance of the balun coil into the self-inductance L and leak inductance Le, the balun coil itself serves to attenuate the common mode noises using the self-inductance. On the other hand, the leak inductance Le serves to suppress the normal mode noises. The foregoing description employs Le for removing the normal mode noises and L for removing the common mode noises. On order to determine the values of the choke coils L 1e and L 2e , this embodiment has the process of plotting a straight line having a minimal value of L 2e as shown below; L 1e + L 2e = constant and selecting a point of constant on the line or its near point. Further, in order to select the values of the X capacitors C x0 andc C x1 , under the condition of C x0 + C x1 = constant, it is possible to obtain a minimal value of L 2e by giving a value of L 1e or vice versa and then select a value of C x0 or C x1 corresponding to the minimal value or its near value. It results in offering a line filter which is small and lightweight and has a high effect of attenuating the normal mode noises. Further, the line filter of this invention offers a higher effect of attenuating the noises in a high-pass frequency, because the C y2 located on the conventional line filter works for the X capacitor for normal noises.

Claims: A line filter to be used in an electronic apparatus having an AC input and a switching power source unit, comprising: a first coil with a first inductance (L1) disposed in a line at an input side of said line filter; a second coil with a second inductance (L2) disposed in the line at a load side of said line filter, the second coil connected in series with the first coil; a third coil with the first inductance (L1) disposed on another line at the input side of said line filter; a fourth coil with the second inductance (L2) disposed on the other line at the load side of said line filter, the fourth coil connected in series with the third coil; and a first X capacitor of a first capacitance Cx1 disposed between a first node connecting said first coil with said second coil and a second node connecting said third coil with said fourth coil; characterized by a second X capacitor of a second capacitance Cx0 disposed between the lines at the input side of said first coil and said third coil; wherein said first and third coils in combination exhibit a normal mode leak inductance of L1e, and said second and fourth coils in combination exhibit a normal mode leak inductance of L2e, L1e and L2e are such as to minimise their sum L1e+L2e for a given value of the sum Cx1+Cxo of the first and second capacitances; and wherein, for that given value of Cx1+Cxo, the second capacitance Cxo is such as to minimise (Ca, Figs. 12, 13), or locally to minimise (Cb, Figs. 12, 13), the second normal mode leak inductance L2e, on the basis of a relationship (Fig. 12, 13) between the second normal mode leak inductance L2e and the second capacitance Cx0; said relationship (Fig. 12, 13) having been determined on the basis of an analysis of a pseudo-power source circuit (Le, Re) connected to the input side, in an equivalent circuit of the line filter. A line filter according to Claim 1, in which L1e=L2e. A line filter according to Claim 1 or 2, for suppressing a frequency of at least F kilohertz, in which: where a circuit constant Lt0=ab.Ct -(dF+e) is greater than the total inductance Lt=L1+L2 measured in millihenries, the first X capacitor is the only capacitor between the said lines on the input side of the second and fourth coils; and where the circuit constant Lt0 is less than the total inductance Lt, a second X capacitor (Cx0) is disposed between the lines on the input side of said first and third coils (L1), the capacitance of said second X capacitor being approximately equal to that of the first X capacitor; where: Ct is the sum of the capacitance of said first X capacitor and of any said second capacitor measured in microfarads; a is a constant of proportionality equal to 2. 39 X 10E6; b is a constant of proportionality equal to 141; d is a frequency proportionality factor equal to 0. 0132; and e is a frequency correction factor equal to -1.055. A line filter according to Claim 3, comprising first Y capacitors (Cy1) disposed between ground and the first and second nodes on the load side of said first X capacitor. A line filter according to Claim 4, comprising second Y capacitors (Cy2) disposed between the lines and ground on the load side of said second and fourth coils, the total capacitance of said second Y capacitors being larger than that of said first Y capacitors. A line filter according to any preceding claim, wherein said first and said third coils are coils of a first balun. A line filter according to any preceding claim, wherein said second and said fourth coils are coils of a second balun.

Inventors: KASAMA NORIO, MATSUYA HIDEJI, OKOCHI SADAO, KASAMA, NORIO, MATSUYA, HIDEJI, OKOCHI, SADAO, KASAMA, NORIO C/O PATENT DIVISION, MATSUYA, HIDEJI C/O PATENT DIVISION, OKOCHI, SADAO C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H03H7
IPC codes: H03H7