Abstract:
A method of deriving an equivalent circuit model for a passive component includes a first step of providing a given frequency characteristic of a capacitor, a second step of forming one of an RC circuit, an RL circuit, and an RCL circuit using frequency-independent resistances (R), capacitances (C) and/or inductances (L), as the equivalent circuit model representing a circuit capable of performing a simulation in a time domain, a third step of composing an evaluation function for evaluating accuracy of the equivalent circuit model formed in the second step, and a fourth step of determining values of the circuit components by minimizing the evaluation function composed in the third step. The method with a simulator adapted for implementing this method and a computer-readable storage medium containing a recorded program, derives the equivalent circuit model for a capacitor. The model is capable of performing a simulation in the time domain using a common procedure not dependent upon types of passive components.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method for deriving equivalent circuit model of passive components. The invention relates particularly to a method for deriving the equivalent circuit models, a simulator, and a storage medium that are capable of performing simulation in a time domain by a common procedure which is independent from types of the passive components.  
         BACKGROUND OF THE INVENTION  
         [0002]    According to trend toward use of high frequencies and high-speed digitization in data communication devices, it has been important recently to perform highly accurate circuit simulation in a time domain of electronic circuits including passive components.  
           [0003]    Regarding the electronic devices, it is very difficult to estimate electric characteristics of a complicated electronic circuit when the circuit is being designed. For manufacturing the circuit, a lot of trial is repeated in which an actual prototype assembly of the circuit is made and measured in its electric characteristic, and the circuit is designed over again if the assembly does not exhibits desired electric characteristic.  
           [0004]    Circuit simulations for estimating electric characteristics of the electronic circuit are performed with a circuit simulator consisting of a computer and software in order to reduce the trial. As the software, for example, a Simulation Program with Integrated Circuit Emphasis (SPICE) developed by the University of California is known.  
           [0005]    A circuit simulation requires an equivalent circuit model, which specifies electric characteristics of semiconductor devices, such as transistors, FETs, and diodes, and passive components, such as resistors, capacitors, and inductors. It is important to establish a highly accurate equivalent circuit model of the circuit components since accurateness of the circuit simulation depends greatly upon accuracy of the equivalent circuit model.  
           [0006]    For a capacitor, one of the passive components, equivalent circuit models that use comparatively small number of circuit components have been provided. The model includes a three-element model in which first capacitor C 1 , first resistor R 1 , and first inductor L 1  are connected in series as shown in FIG. 6A, and a five-element model in which a series connection of first capacitor C 1  and first resistor R 1  and a series connection of second capacitor C 2  and second resistor R 2  are connected in parallel, and first inductor L 1  is connected in series to the parallel circuit, as shown in FIG. 6B. However, these conventional equivalent circuit models do not have satisfying accuracy. As shown in FIGS. 6C and 6D, the conventional models hardly reproduce an impedance having complex frequency dependence. FIG. 6C shows a real part of the impedance, and FIG. 6D shows a capacitance component of the impedance. Calculated values of a three-element model are represented by solid lines, and calculated values of a five-element model are represented by broken lines. FIGS. 6C and 6D show large differences between the calculated values and actually-measured values given by dotted lines.  
           [0007]    Therefore, for electronic circuits including capacitors, an estimation result of a circuit simulator does not often match with electric characteristics of an actual circuit, and this prevents electronic circuits using the circuit simulator from being designed efficiency.  
         DISCLOSURE OF THE INVENTION  
         [0008]    By a method for deriving an equivalent circuit model, a circuit simulator accurately estimates electric characteristics of an actual passive component.  
           [0009]    The method includes:  
           [0010]    Providing impedance Z(fe) at each of sample frequencies f 1 , . . . ,f N  (where f 1 &lt;f n ) is given as  
             Z ( f   n )= R ( f   n )+ jx ( f   n )  
           [0011]    where  
           [0012]    Z is the impedance of a capacitor,  
           [0013]    R is a real part of Z,  
           [0014]    X is an imaginary part of Z,  
           [0015]    f n  is a value of each sample frequency (n=1, 2, . . . ,N), and  
           [0016]    j is the imaginary unit;  
           [0017]    Forming an equivalent circuit model by adopting any of an RC circuit consisting of a resistance and a capacitance, an RL circuit consisting of a resistance and an inductance, and an RCL circuit consisting of the RC circuit and the RL circuit connected in series;  
           [0018]    Composing an evaluation function Q({right arrow over (P)}) in accordance with formulae:  
           Q        (     P   →     )       =       ∑     n   =   1     N          q        (         R   M          (       f   n     ,     P   →       )       ,       X   M          (       f   n     ,     P   →       )       ,     R        (     f   n     )       ,     X        (     f   n     )         )           ;   and               q        (       R   M     ,     X   M     ,   R   ,   X     )       =         C   R            |       R   M     -   R          |   2         |   R        |   d           +       C   X            |       X   M     -   X          |   2         |   X        |   d           +       C   Z            |       Z   M     -   Z          |   2         |   Z        |   d               ,                         
 
           [0019]    where an impedance of the equivalent circuit model is defined as  
             Z   M ( f   n   , {right arrow over (P)} )= R   M ( f   n   , {right arrow over (P)} )+ jX   M ( f   n   , {right arrow over (P)} ),  
           [0020]    where,  
           [0021]    Z M  is the impedance of the equivalent circuit model,  
           [0022]    R M  is a real part of Z M ,  
           [0023]    X M  is an imaginary part of Z M ,  
           [0024]    f n  is the value of each sample frequency (n=1, 2, . . . ,N),  
           [0025]    j is the imaginary unit,  
           [0026]    {right arrow over (P)}=(P 1 ,P 2 , . . . P K ) is a circuit constant vector including elements being values of R, C and L, and  
           [0027]    C R , C X , and C Z  are positive real numbers or zero; and  
           [0028]    Determining the circuit constant vector {right arrow over (P)} by minimizing the evaluation function Q({right arrow over (P)}).  
           [0029]    This method of deriving equivalent circuit model is applicable commonly to capacitors regardless of their kinds, and is also applicable in general to other passive components, such as resistors and inductors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a flowchart showing a method for deriving an equivalent circuit model of a capacitor according to exemplary embodiments of the present invention.  
         [0031]    [0031]FIG. 2A is a diagram of an RC ladder circuit which is an equivalent circuit model of a capacitor according to the embodiments.  
         [0032]    [0032]FIG. 2B is a diagram of an RL ladder circuit which is an equivalent circuit model of a capacitor according to the embodiments.  
         [0033]    [0033]FIG. 2C is a diagram of an RCL ladder circuit which is an equivalent circuit model of a capacitor according to the exemplary embodiments.  
         [0034]    [0034]FIG. 3A through FIG. 3E are diagrams of RC circuits which are other equivalent circuit models of a capacitor.  
         [0035]    [0035]FIG. 3L through FIG. 3P are diagrams of RL circuits which are still other equivalent circuit models of a capacitor.  
         [0036]    [0036]FIG. 4 is a flowchart showing a method of determining circuit constant vectors of the ladder circuits shown in FIG. 2A through FIG. 2C.  
         [0037]    [0037]FIG. 5A shows an equivalent circuit model of a solid tantalum electrolytic capacitor according to the embodiments.  
         [0038]    [0038]FIG. 5B is a graphical representation showing a real part of a reproduced impedance of the solid tantalum electrolytic capacitor according to the embodiments.  
         [0039]    [0039]FIG. 5C is a graphical representation showing a reproduced capacitance of the solid tantalum electrolytic capacitor according to the embodiments.  
         [0040]    [0040]FIG. 6A shows a three-element model representing an equivalent circuit model of a solid tantalum electrolytic in a conventional method.  
         [0041]    [0041]FIG. 6B shows a five-element model in a conventional method.  
         [0042]    [0042]FIG. 6C is a graphical representation showing a real part of a reproduced impedance of the solid tantalum electrolytic capacitor by the conventional method.  
         [0043]    [0043]FIG. 6D is a graphical representation showing a reproduced capacitance of the solid tantalum electrolytic capacitor by the conventional method. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0044]    (Exemplary Embodiment 1)  
         [0045]    [0045]FIG. 1 is a flowchart of processes in a method for deriving an equivalent circuit model of a capacitor according to an exemplary embodiment. Impedances as sampled values for frequencies shown by equation 1, which will be described later, are provided. (Step  11 ). Using frequency-independent resistance (R), capacitance (C), and inductance (L), one of an RC circuit consisting of the resistor and the capacitor, an RL circuit consisting of the resistor and the inductor, and an RCL circuit consisting of the RC circuit and the RL circuit connected in series is formed as an equivalent circuit model representing a circuit enabling a simulation in a time domain (step  12 ). An evaluation function defined by equation 3, described later, is set with equations 1 and 2 (step  13 ). A circuit constant vector is determined by minimizing equation 3 (step  14 ).  
         [0046]    [0046]FIG. 2A through FIG. 2C show circuit diagrams of the equivalent circuit models of a capacitor according to the embodiment. FIG. 2A illustrates a single-stage RC ladder circuit having first resistance R C ( 1 ) and first capacitance C( 1 ) connected in series, a two-stage RC ladder circuit having a series circuit consisting of second resistance R C ( 2 ) and second capacitance C( 2 ) connected in parallel with the first capacitance C( 1 ), and an N C -stage RC ladder circuit (“N C ” is a natural number) formed similarly. FIG. 2B illustrates a single-stage RL ladder circuit having first resistance R L ( 1 ) and first inductance L( 1 ) connected in series, a two-stage RL ladder circuit having a series circuit consisting of second resistance R L ( 2 ) and second inductance L( 2 ) connected in parallel with first inductance L( 1 ), and an NL-stage RL ladder circuit (“N L ” is a natural number) formed similarly. FIG. 2C shows an (N C +N L )-stage RCL ladder circuit composed of the N C -stage RC ladder circuit and the N L -stage RL ladder circuit that are connected in series.  
         [0047]    Besides those circuit diagrams shown in FIG. 2A through FIG. 2C, other diagrams are useful as long as constructing an equivalent circuit model of a capacitor, such as RC circuits shown in FIG. 3A through FIG. 3E, RL circuits shown in the FIG. 3L through FIG. 3P, and RCL circuits formed by connecting in series any of the RC circuits with any of the RL circuits (not shown in these figures). If using the diagrams, however, it is necessary to determine values of circuit components by another method suitable for each of the diagrams.  
         [0048]    [0048]FIG. 4 is a flowchart showing a method of determining the circuit constant vector of any of the ladder circuits shown in FIG. 2A through FIG. 2C according to this exemplary embodiment. This corresponds to Step  14  of FIG. 1. In accordance with equations 6 through 9 discussed below, values of circuit components of the RC circuit in FIG. 2A through FIG. 2C are distributed at an equal ratio (step  41 ). A ratio for the equal-ratio distribution of the circuit component values is determined by minimizing equation 4 described later (step  42 ). A circuit constant vector is calculated by minimizing equation 4 using, as initial values, the circuit component values distributed at the ratio of equal-ratio distribution determined in step  42  (step  43 ). Values of circuit components of the RL circuit in one of FIG. 2A through FIG. 2C are distributed at an equal ratio in accordance with equations 8 through 11 discussed later (step  44 ). A ratio for the equal-ratio distribution of the circuit component values is determined by minimizing equation 5 described later (step  45 ). A circuit constant vector is calculated by minimizing equation 5, using, as initial values, the circuit component values distributed at the ratio of the equal-ratio distribution determined in the step  45  (step  46 ). Then, the evaluation function defined by the equation 3 is set (step  47 ). An RCL circuit is formed by connecting in series the RC circuit composed in step  41  through step  43  and the RL circuit composed in step  44  through step  46  (step  48 ). A circuit constant vector of the RCL circuit is determined by minimizing equation 3 (step  49 ).  
         [0049]    For a tantalum solid electrolytic capacitor, a procedure for deriving a highly-accurate equivalent circuit model will be described hereinafter in detail with reference to FIG. 1.  
         [0050]    For each of the sample frequencies, impedance Z (f n ) given by:  
           Z ( f   n )= R ( f   n )+ jX ( f   n )  (Equation 1)  
         [0051]    is provided. (step  11 ),  
         [0052]    where  
         [0053]    Z is the impedance of the capacitor,  
         [0054]    R is arealpart of Z,  
         [0055]    X is an imaginary part of Z,  
         [0056]    f n  is a sample frequency (n=1, 2, . . . ,N), and  
         [0057]    j is the imaginary unit.  
         [0058]    Using frequency-independent resistances (R), capacitances (C) and inductances (L), one of an RC circuit consisting of the resistance and the capacitance, an RL circuit consisting of the resistance and the inductance, and an RCL circuit consisting of the RC circuit and the RL circuit connected in series is formed as an equivalent circuit model representing the circuit enabling a simulation in a time domain. In this embodiment, 5 (five) is chosen for both numbers N C  and N L  representing the circuit diagrams shown in FIG. 2A through FIG. 2C, to form a ten-stage RCL ladder circuit by connecting in series a five-stage RC ladder circuit and a five-stage RL ladder circuit (step  12 ).  
         [0059]    Impedance exhibited by the equivalent circuit model formed in step  12  is defined as:  
           Z   M ( f   n   ,{right arrow over (P)} )= R   M ( f   n   , {right arrow over (P)} )+ jX   M ( f   n   , {right arrow over (P)} )  (Equation 2),  
         [0060]    where  
         [0061]    Z M  is the impedance of the equivalent circuit model,  
         [0062]    R M  isarealpartof Z M ,  
         [0063]    X M  is an imaginary part of Z M ,  
         [0064]    f n  is the value of each sample frequency (n=1, 2, . . . ,N),  
         [0065]    j is the imaginary unit, and  
         [0066]    {right arrow over (P)}=(P 1 ,P 2 , . . . P K ) is a circuit constant vector having elements as values of R, C and L,  
         [0067]    An evaluation function Q({right arrow over (P)}) given by  
               Q        (     P   →     )       =       ∑     n   =   1     N          q        (         R   M          (       f   n     ,     P   →       )       ,       X   M          (       f   n     ,     P   →       )       ,     R        (     f   n     )       ,     X        (     f   n     )         )                 (     Equation                 3     )                               
 
         [0068]    is composed (step  13 ). Here,  
         q        (       R   M     ,     X   M     ,   R   ,   X     )       =         C   R            |       R   M     -   R          |   2         |   R        |   d           +       C   X            |       X   M     -   X          |   2         |   X        |   d           +       C   Z            |       Z   M     -   Z          |   2         |   Z        |   d                                   
 
         [0069]    where d is “0” for an evaluation of an absolute square error and is “2” for an evaluation of relative square error, and C R , C X , and C Z  are “0” or any positive real numbers for assigning weights to respective terms.  
         [0070]    When a real part R(f n ) of given impedance becomes a minimum value R m  at frequency f m , instead of the ten-stage RCL ladder circuit as the equivalent circuit model, a combination of an N C -stage RC ladder circuit and one resistor connected in series may be formed if m=N, or another combination of an N L -stage RL ladder circuit and one resistor connected in series may be formed if m=1  
         [0071]    In this embodiment, although 5 (five) was chosen for the numbers of stages, N C  and N L , this is not restrictive, and the numbers of stages, N C  and N L  may be different from each other. For the solid tantalum electrolytic capacitor, a condition of N C =5 and N L =5 provides the model with generally satisfying accuracy according to a result of changing the number of the stages and repeating derivation of the circuit model of this embodiment. Numbers N C  and N L  may be determined for other types of capacitors by repeating derivation in the same manner.  
         [0072]    A procedure shown in FIG. 4 is applied for determining component values of the circuit in step  14 . The procedure will be described hereafter according to FIG. 4.  
         [0073]    Values of the circuit components for the five-stage RC ladder circuit are distributed at an equal ratio (step  41 ) in accordance with:  
                 A        (     P   →     )       =       ∑     n   =   1     N          a        (         R   M          (       f   n     ,     P   →       )       ,       X   M          (       f   n     ,     P   →       )       ,       R        (     f   n     )       -     xR   0       ,     X        (     f   n     )         )           ,           (     Equation                 6     )                               
 
         [0074]    where 0:5≦x≦1,  
                 B        (     P   →     )       =       ∑     n   =   1     N          b        (         R   M          (       f   n     ,     P   →       )       ,       X   M          (       f   n     ,     P   →       )       ,       R        (     f   n     )       -       (     1   -   x     )          R   0         ,     X        (     f   n     )         )           ,           (     Equation                 7     )                               
 
         [0075]    where 0≦x≦1,  
                 Rc        (   1   )       =       (     1   -   x     )          R   0         ,         ∑     k   =   1     Nc          C        (   k   )         =       -   1       2      π                   f   1          X        (     f   1     )             ,           (     Equation                 8     )                               
 
         [0076]    and  
           Rc ( k+ 1)=α c·Rc ( k ),  C ( k+ 1)=β c·C ( k )  (Equation 9).  
         [0077]    In other words, since the minimum value of the real part of impedance R 0  is 0.018 Ω, and since a capacitance in a low frequency region is 66.5 μF according to FIG. 5B and FIG. 5C, respectively,  
           Rc ( k+ 1)=α C·R   C ( k ),  C ( k+ 1)=β C·C ( k )  (Equation 9)  
         [0078]    stand applicable for k=1, 2, 3 and 4 when x=½, under the conditions of  
           Rc        (   1   )       =       0.018   ×     10     -   3         2       ,         ∑     k   =   1     5          C        (   k   )         =     66.5   ×       10   6     .                               
 
         [0079]    Coefficients α C  and β C  are obtained by minimizing evaluation function A({right arrow over (P)}) set in the step  13 , as α C =20.6, and β C =0.37 (step  42 ).  
         [0080]    Values of the circuit components are determined by minimizing the evaluation function A({right arrow over (P)}) set in third step  13  from initial values of the component values given by the coefficients α C  and β C  obtained in the step  42  (step  43 ). Table 1 shows the determined circuit component values.  
                           TABLE 1                                       Rc(1) = 1.12 × 10 −02     C(1) = 2.29 × 10 −05             Rc(2) = 8.75 × 10 −02     C(2) = 3.12 × 10 −05             Rc(3) = 3.89 × 10 +00     C(3) = 4091 × 10 −17             Rc(4) = 8.03 × 10 +01     C(4) = 2.71 × 10 −06             Rc(5) = 1.67 × 10 +03     C(5) = 3.10 × 10 −06                                    
 
         [0081]    Values of the circuit components for the five-stage RL ladder circuit are distributed at an equal ratio (step  44 ) in accordance with equations 8 and 9 and the following equations:  
                   R   L          (   1   )       =     x   ·     R   0         ,         L        (   1   )       =       X        (     f   N     )         2      π                   f   N           ;             (     Equation                 10     )                               
 
         [0082]    and  
           R   L ( k+ 1)=α L   ·R   L ( K ),  L ( k+ 1)=β L   ·L ( k )  (Equation 11).  
         [0083]    In other words, since the minimum value of the real part of impedance R 0  is 0.018 Ω, and since a capacitance in a high frequency region is 1.14 nH according to FIG. 5B and FIG. 5C, respectively, the following equations  
             R   L          (   1   )       =       0.018   ×     10     -   3         2       ,   and                         
 
           L (1)=1.14×10 −9    
         [0084]    are applicable for k=1, 2, 3 and 4 when x=½, under the conditions of  
           R   L ( k+ 1)= α   L   ·R   L ( K ),  L ( k+ 1)=β L   ·L ( k )  (Equation 11)  
         [0085]    Coefficients α L  and β L  are obtained by minimizing evaluation function B({right arrow over (P)}) set in step  13 , as α L =150.7, and β L =3.47 (step  45 ).  
         [0086]    In 4f-th step  46 , values of the circuit components are determined by minimizing evaluation function B({right arrow over (P)}) set in step  13  from initial values of the component values given by coefficients α L  and β L  obtained in 4e-th step  45 . The circuit component values are shown in Table 2.  
                           TABLE 2                                       R L (1) = 6.13 × 10 −03     L(1) = 1.57 × 10 −09             R L (2) = 1.60 × 10 −01     L(2) = 7.24 × 10 −09             R L (3) = 2.18 × 10 +00     L(3) = 9.93 × 10 −09             R L (4) = 3.41 × 10 +01     L(4) = 2.19 × 10 −06             R L (5) = 5.35 × 10 +02     L(5) = 7.15 × 10 −16                                    
 
         [0087]    The estimation function Q(P) defined by equation 3 (step 47). That is,  
           Q        (     P   →     )       =       ∑     n   =   1     m          q        (         R   M          (       f   n     ,     P   →       )       ,       X   M          (       f   n     ,     P   →       )       ,       R        (     f   n     )       -       R   0     /   2       ,     X        (     f   n     )         )           ,   and               q        (       R   M     ,     X   M     ,   R   ,   X     )       =         c   R            |       R   M     -     (     R   -       R   0     /   2       )            |   2         |   R        |   d           +       c   X            |       X   M     -   X          |   2         |   X        |   d           +       c   Z            |       Z   M     -   Z          |   2         |   Z        |   d               ,                         
 
         [0088]    where C X &lt;&lt;C Z &lt;&lt;C R , in order to assign a greater weight to a relative square error of the real part, since accuracy of the real part of impedance is not easily assured.  
         [0089]    A ten-stage RCL ladder circuit is formed by connecting in series the five-stage RC ladder circuit composed in step  41  through step  43  and the five-stage RL ladder circuit composed in step  44  through step  46  (step 48).  
         [0090]    Values of circuit components of the ten-stage RCL ladder circuit formed in step  48  are determined by minimizing evaluation function Q({right arrow over (P)}) set in step  47  (step  49 ). Table 3 shows the determined circuit component values.  
                           TABLE 3                                       R C (1) = 1.24 × 10 −02     C(1) = 2.25 × 10 −05             R C (2) = 6.00 × 10 −02     C(2) = 3.85 × 10 −05             R C (3) = 3.90 × 10 +00     C(3) = 2.07 × 10 −17             R C (4) = 8.04 × 10 +01     C(4) = 2.79 × 10 −06             R C (5) = 1.67 × 10 +03     C(5) = 4.49 × 10 −06             R L (1) = 5.24 × 10 −03     L(1) = 1.24 × 10 −09             R L (2) = 2.66 × 10 −01     L(2) = 9.59 × 10 −09             R L (3) = 2.18 × 10 +00     L(3) = 4.51 × 10 −09             R L (4) = 3.41 × 10 +01     L(4) = 3.68 × 10 −06             R L (1) = 5.35 × 10 +02     L(5) = 4.65 × 10 −16                                    
 
         [0091]    The equivalent circuit model derived as above, a result of reproduction of the real parts of impedance, and a result of reproduction of the capacitances are shown in FIG. 5A, FIG. 5B, and FIG. 5C, respectively. With the equivalent circuit model, an accuracy including a relative error less than 10% is ensured in reproduction of impedances across all points of the sampling frequencies.  
         [0092]    (Exemplary Embodiment 2)  
         [0093]    In a method of deriving an equivalent circuit model of exemplary embodiment 2, an RCL circuit is formed as an equivalent circuit model wherein the real part R(f n ) of impedance becomes minimum value R 0  at sample frequency f m (f m ≠f 1  and f m ≠f n ), in step  12  of exemplary embodiment 1, and evaluation function in a low frequency region f m+1 ≦f n ≦f m  is calculated according to:  
               A   (     P   -&gt;     )     =       ∑     n   =   1     N          a        (         R   M          (       f   n     ,     P   -&gt;       )       ,       X   M          (       f   n     ,     P   -&gt;       )       ,       R        (     f   n     )       -       (     1   -   x     )          R   0         ,     X        (     f   n     )         )                 (     Equation                 4     )                               
 
         [0094]    where (0≦x≦1), instead of the evaluation function defined by equation 3 in step  13 . Further, factors x and d are set as x=½ and d=2 in calculation of evaluation function in a high frequency region f m+1 ≦f n ≦f N  according to  
               B   (     P   -&gt;     )     =       ∑     n   =   1     N          b   (         R   M     (       f   n     ,     P   -&gt;       )     ,       X   M     (       f   n     ,     P   -&gt;       )     ,       R        (     f   n     )       -       (     1   -   x     )          R   0         ,     X        (     f   n     )         )               (     Equation                 5     )                               
 
         [0095]    where 0≦x≦1. That is,  
           A        (     P   -&gt;     )       =       ∑     n   =   1     N          a        (         R   M          (       f   n     ,     P   -&gt;       )       ,       X   M          (       f   n     ,     P   -&gt;       )       ,       R        (     f   n     )       -       (     1   -   x     )          R   0         ,     X        (     f   n     )         )           ,     
                  a        (       R   M     ,     X   M     ,   R   ,   X     )       =                c   R                     R   M     -     (     R   -       R   0     /   2       )            2            R        d         +       c   X                     X   M     -   X          2            X        d         +                              c   Z                     Z   M     -   Z          2            Z        d         ,                         B        (     P   -&gt;     )       =       ∑     n   =   1     N          b        (         R   M     (       f   n     ,     P   -&gt;       )     ,       X   M          (       f   n     ,     P   -&gt;       )       ,       R        (     f   n     )       -       (     1   -   x     )          R   0         ,     X        (     f   n     )         )           ,   and                   b        (       R   M     ,     X   M     ,   R   ,   X     )       =                c   R                     R   M     -     (     R   -       R   0     /   2       )            2            R        d         +       c   X                     X   M     -   X          2            X        d         +                              c   Z                     Z   M     -   Z          2            Z        d         ,                                 
 
         [0096]    where C X &lt;&lt;C Z &lt;&lt;C R , in order to assign a greater weight to a relative square error of the real part, since it is considerably difficult to ensure accuracy of the real part of impedance.  
         [0097]    (Exemplary Embodiment 3)  
         [0098]    An RC circuit is formed as an equivalent circuit model in which a real part R(f n ) of impedance becomes minimum value R 0  at sample frequency of f m  (f m =f N ) in step  12  of embodiment 1, and an evaluation function in an entire frequency region f 1 ≦f n ≦f N  is calculated according to:  
               A        (     P   -&gt;     )       =       ∑     n   =   1     m          a        (         R   M          (       f   n     ,     P   -&gt;       )       ,       X   M          (       f   n     ,     P   -&gt;       )       ,       R        (     f   n     )       -       (     1   -   x     )          R   0         ,     X        (     f   n     )         )                 (     Equation                 6     )                               
 
         [0099]    where 0≦x≦1, instead of the evaluation function in step  13 . A new RC circuit is then completed by connecting additionally a resistance xR 0  in series to the above RC circuit.  
         [0100]    (Exemplary Embodiment 4)  
         [0101]    An RL circuit is formed as an equivalent circuit model in which a real part R(f n ) of impedance becomes minimum value R 0  at sample frequency of f m (f m =f N ), in step  12  of embodiment 1, and an evaluation function in an entire frequency region f 1 ≦f n ≦f N  is calculated according to:  
               B        (     P   -&gt;     )       =       ∑     n   =   1     m          b        (         R   M          (       f   n     ,     P   -&gt;       )       ,       X   M          (       f   n     ,     P   -&gt;       )       ,       R        (     f   n     )       -       (     1   -   x     )          R   0         ,     X        (     f   n     )         )                 (     Equation                 7     )                               
 
         [0102]    where 0≦x≦1, instead of the evaluation function in third step, and further, a new RL circuit is completed by connecting additionally a resistance (1-x)R 0  in series to the above RL circuit.  
         [0103]    (Exemplary Embodiment 5)  
         [0104]    In addition to embodiment 2, in this embodiment, evaluation function A({right arrow over (P)}) is minimized in any of a single-stage RC ladder circuit having a first resistance and a first capacitance connected in series, a two-stage RC ladder circuit having a series circuit consisting of a second resistance and a second capacitance connected in parallel with the first capacitance, and an Nc-stage RC ladder circuit (“N C ” is a natural number) formed in the same manner, and evaluation function B({right arrow over (P)}) is minimized in any of a single-stage RL ladder circuit having a first resistance and a first inductance connected in series, a two-stage RL ladder circuit having a series circuit consisting of a second resistance and a second inductance connected in parallel with the first inductance, and an N L -stage RL ladder circuit (“N L ” is a natural number) formed in the same manner.  
         [0105]    (Exemplary Embodiment 6)  
         [0106]    In addition to embodiment 3, in this embodiment, evaluation function A({right arrow over (P)}) in any of a single-stage RC ladder circuit having a first resistance and a first capacitance connected in series, a two-stage RC ladder circuit having a series circuit consisting of a second resistance and a second capacitance connected in parallel with the first capacitance, and an N C -stage RC ladder circuit (“N C ” is a natural number) formed in the same manner.  
         [0107]    (Exemplary Embodiment 7)  
         [0108]    In addition to embodiment 4, in this embodiment, evaluation function B({right arrow over (P)}) is minimized in any of a single-stage RL ladder circuit having a first resistance and a first inductance connected in series, a two-stage RL ladder circuit having a series circuit consisting of a second resistance and a second inductance connected in parallel with the first inductance, and an N L -stage RL ladder circuit (“N L ” is a natural number) formed in the same manner.  
         [0109]    The foregoing embodiments can be implemented in combination, and a number of stages in the RC ladder circuit and the RL ladder circuit can be set freely as desired.  
         [0110]    Although methods of deriving the equivalent circuit models for capacitors are explained, a simulator for deriving an equivalent circuit model according to these methods can be conducted. Furthermore, another simulator for analyzing frequency response and/or time response of a circuit with using the equivalent circuit model for capacitors can be conducted.  
         [0111]    In addition, a computer-readable recording medium storing a program containing the function of deriving an equivalent circuit model based on these methods of deriving equivalent circuit model can be provided. Moreover, another computer-readable recording medium storing a program containing the function of analyzing frequency response and/or time response of a circuit in the similar manner with using the equivalent circuit model for capacitors can be provided.  
       INDUSTRIAL APPLICABILITY  
       [0112]    By a method of deriving an equivalent circuit model for capacitors according to the present invention and a circuit simulation using the equivalent circuit model yealize accurate prediction for operation of a circuit including capacitors. This improves efficiency of designing electronic circuits. In addition, the method of the invention is applicable not only to the capacitors but also to other passive components, such as resistors and inductors.