# Patent

## Title
Maximum power output circuit for an EHC and design method thereof

## Abstract

A maximum power output circuit for EHC and its design method are presented. The circuit is comprised of a magnetic core, that is, a primary coil and a secondary coil, with a load resistor and a capacitor parallel connected at the two ends of the secondary coil. The circuit enables the EHC to be always working at the maximum power output, thus realizing maximum power output of the energy harvesting power source.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Chinese Application No. 201510111209.X, filed on Mar. 13, 2015, the content of which is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to transmission line technology, and in particular relates to a maximum power output circuit for an EHC and a design method thereof.
Background Information
For real time monitoring of grid assets and effective reduction of grid faults, online grid monitoring systems are extensively developed at home and abroad. FIG. 1 shows the structure of such a system, where a monitoring device is directly installed on the power transmission line for monitoring of inclination angle, stress, conductor temperature and conductor current, the monitoring data thereof being transmitted wirelessly to a monitoring platform, which then accesses the status of the power transmission line with combined inputs of monitored parameters and running status of the transmission line. Practices in recent years show that power supply and communication are two bottlenecks impeding development of online monitoring solutions for power transmission lines.
Up to now, of mature harvesting solutions there are mostly solar energy, wind power, capacitive divider, laser supply, induction harvesting, differential temperature harvesting, and vibration harvesting. By comparison of the above-mentioned harvesting solutions, induction harvesting is believed to be the most suitable for transmission line energy harvesting. USI, OTLM, Hangzhou Thunderbird, and Xi'an Jinyuan have all developed commercial products based on induction harvesting. However, all the above products work on the range in excess of 50 A due to limited power supply, and hence are prevented from operating normally on most applications generally with a working current below 50 A.
For an online monitoring power source for power transmission line, it needs to be capable of adapting to big load swings in addition to posing no risk for the transmission line per se. Thus, an induction harvesting solution shall meet the following requirements for: {circle around (1)} large dynamic range; current over a power transmission line ranges from a peak current over 1000 A to a valley one of 40 A and even as low as 10 A for certain distribution networks; output power of an energy harvesting coil (hereunder abbreviated as EHC) is positively correlated with the current on the transmission line; as is shown on FIG. 2, the output power of the EHC needs to be regulated via practical means to consistently output a stable power within the wide dynamic range; {circle around (2)} high density per unit power; the weight of an online monitoring device is strictly regulated due to safety considerations, for example, the weight of a universal monitoring device is limited to 2.5 kg, that of a vibration monitoring is limited to 1 kg, and that for a distribution network monitoring device is limited to 500 g; therefore, the only way to solve the problem is to increase the power density of the energy harvester; and {circle around (3)} anti-surge capability; a transmission line is subject to impact of short circuits or lightning, which might result in a peak current of several kA, so the induction harvester shall be able to withstand such current surges.
Foreign and domestic scholars focus their research mostly on two aspects, power output model and protection of the EHC. N. M. Roscoe, M. D. Judd, L. Fraser, “A novel inductive electromagnetic energy harvester for condition monitoring sensors,” in Proc. Int. Conf. Condit. Monitor. Diagnosis, Sep. 2010, pp. 615-618, N. M. Roscoe, M. D. Judd, and J. Fitch, “Development of magnetic induction energy harvesting for condition monitoring,” in Proc. 44th Int. Univ. Power Eng. Conf., September 2009, pp. 1-5, N. M. Roscoe, Judd M. D. Harvesting energy from magnetic fields to power condition monitoring sensors.” IEEE Sensors J., vol. 13, no. 6, pp. 2263-2270, 2013, consider an EHC equivalent to a voltage source or a current source, with output power of the EHC reaching it maximum when load resistance is equal to internal resistance of the power source. In fact, output voltage of the EHC changes as the load current changes, and as the load of the EHC changes, its output voltage and current change simultaneously, and therefore the above assumption does not strictly hold.
SUMMARY OF THE INVENTION
The object of the present invention is to overcome the above deficiency of the prior art and to provide a maximum power output circuit for an EHC and a design method thereof. Said circuit enables the EHC to be always working at the maximum output power point, raises the maximum power density of the harvester, and realizes maximum power output of the harvester.
The technical solution of the present invention is as follows:
A maximum power output circuit for an EHC, characterized in that it is comprised of a magnetic core, that is, a primary coil (N1) and a secondary coil (N2), with a load resistor (R) and a capacitor (C) parallel connected at two ends of the secondary coil.
A design method for the above maximum power output circuit for the EHC, characterized in that the method comprises the following steps:
{circle around (1)} setting a power density index λ under a minimum working current;
{circle around (2)} calculating a magnetization current Iμ under the minimum working current according to a maximum output power of the energy harvesting coil with the following formula:




  
    I
    μ
  
  =
  
    
      
        I
        1
      
      /
      
        (
        
          
            C
            1
          
          ⁡
          
            (
            
              k
              +
              1
            
            )
          
        
        )
      
    
    k
  




where, IFe=C1Iμk, C1 is a transformation coefficient between the hysteresis loss current IFe and the current Iμ parallel to a magnetic flux, and k is a transformation index between the hysteresis loss current IFe and the current Iμ parallel to the magnetic flux, and I1 is a primary current;
{circle around (3)} selecting a material for the magnetic core, and calculating an outer parameter Do and a thickness h in accordance with a density w and a volume V of the magnetic core by the following formula:



  
    V
    =
    
      
        π
        ⁡
        
          (
          
            
              D
              o
              2
            
            -
            
              D
              i
              2
            
          
          )
        
      
      ⁢
      
        h
        /
        4
      
    
  




  
    
      V
      =
      
        W
        /
        w
      
    
    ,
    
      

    
    ⁢
    
      
        P
        max
      
      =
      
        μ
        ⁢
        
          
        
        ⁢
        hf
        ⁢
        
          
        
        ⁢
        ln
        ⁢
        
          
            D
            o
          
          
            D
            i
          
        
        ⁢
        
          (
          
            
              
                
                  I
                  1
                
                ⁡
                
                  (
                  
                    
                      I
                      1
                    
                    
                      
                        C
                        1
                      
                      ⁡
                      
                        (
                        
                          k
                          +
                          1
                        
                        )
                      
                    
                  
                  )
                
              
              
                1
                k
              
            
            -
            
              
                
                  C
                  1
                
                ⁡
                
                  (
                  
                    
                      I
                      1
                    
                    
                      
                        C
                        1
                      
                      ⁡
                      
                        (
                        
                          k
                          +
                          1
                        
                        )
                      
                    
                  
                  )
                
              
              
                
                  k
                  +
                  1
                
                k
              
            
          
          )
        
      
    
  



where, V is the fixed volume of the magnetic core, Di is an inner diameter, W is a weight thereof, Pmax is a maximum output power, and f is a working frequency;
{circle around (4)} calculating, in accordance with the following formulas, a load resistance R and a capacitance C:




  
    
      
        R
        =
        
          
            E
            2
          
          /
          
            I
            R
          
        
      
    
    
      
        (
        13
        )
      
    
  
  
    
      
        C
        =
        
          
            (
            
              
                I
                μ
              
              -
              
                
                  I
                  1
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                α
              
            
            )
          
          
            
              E
              2
            
            *
            2
            ⁢
            
              
            
            ⁢
            π
            ⁢
            
              
            
            ⁢
            f
            *
            
              N
              2
            
          
        
      
    
    
      
        (
        14
        )
      
    
  




where, IR is a current on the load resistor, E2 is an induction voltage of a secondary side of the energy harvesting coil, N2 is a number of the secondary coil of the energy harvesting coil, μ is an effective permeability of the magnetic core, Iμ is the magnetization current, I1 is a primary current, f is a frequency of a power source, α is an angle between the primary current I1 and the magnetization current Iμ, α=90 degrees.
The underlying principle of the present invention is:
1. CT Harvesting Model
For analysis of power output characteristics of an EHC, a diagram of a load equivalent model established for the EHC on the basis of the electro-magnetic induction theory is shown on FIG. 3.
Let the current flowing through the conductor line being is, the inner diameter of the EHC being Di, its outer diameter being Do, its width being h, its turns being N2, then, the induction voltage E2 on the secondary side of the EHC is:




  
    
      
        
          E
          2
        
        =
        
          
            N
            2
          
          ⁢
          
            
              μ
              ⁢
              
                
              
              ⁢
              h
            
            
              2
              ⁢
              
                
              
              ⁢
              π
            
          
          ⁢
          ln
          ⁢
          
            
              D
              o
            
            
              D
              i
            
          
          ⁢
          
            
              d
              ⁢
              
                
              
              ⁢
              
                I
                μ
              
            
            
              d
              ⁢
              
                
              
              ⁢
              t
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




where, μ being the effective magnetic permeability of the magnetic core, Iμ being the magnetization current.

It follows from the magnetic potential balance equation that:

İ1N1+İ2N2=İmN1  (2)

where, N1 being the number of the primary turns, and is set as 1 here, N2 being the number of the secondary turns, Im being the exciting current.

Take into account of hysteresis loss, the exciting current İm can be decomposed into a current İμ parallel to the magnetic flux and a hysteresis loss current İFe perpendicular to the magnetic flux, satisfying

İμ+İFe=İm  (3)

By ignoring the primary and secondary magnetic flux leakage and the internal resistance of the coil, the vector diagram of the load model for the EHC is shown on FIG. 4.
Referring to FIG. 4, İR being the resistive current component of the load, İc being the capacitive current component of the load, and the following equations can be deduced from FIG. 4:




  
    
      
        
          
            
              
                I
                .
              
              1
            
            ⁢
            sin
            ⁢
            
              
            
            ⁢
            a
          
          -
          
            
              
                I
                .
              
              R
            
            ⁢
            
              N
              2
            
          
        
        =
        
          
            I
            .
          
          Fe
        
      
    
    
      
        (
        4
        )
      
    
  
  
    
      
        
          
            
              
                I
                .
              
              1
            
            ⁢
            cos
            ⁢
            
              
            
            ⁢
            a
          
          +
          
            
              
                I
                .
              
              C
            
            ⁢
            
              N
              2
            
          
        
        =
        
          
            I
            .
          
          μ
        
      
    
    
      
        (
        5
        )
      
    
  
  
    
      
        
          P
          ′
        
        =
        
          
            
              (
              
                
                  
                    
                      I
                      .
                    
                    1
                  
                  ⁢
                  sin
                  ⁢
                  
                    
                  
                  ⁢
                  α
                
                -
                
                  
                    I
                    .
                  
                  Fe
                
              
              )
            
            ⁢
            
              
                μ
                ⁢
                
                  
                
                ⁢
                h
              
              
                2
                ⁢
                
                  
                
                ⁢
                π
              
            
            ⁢
            ln
            ⁢
            
              
                D
                o
              
              
                D
                i
              
            
            ⁢
            
              
                d
                ⁢
                
                  
                    I
                    .
                  
                  μ
                
              
              
                d
                ⁢
                
                  
                
                ⁢
                t
              
            
          
          =
          
            μ
            ⁢
            
              
            
            ⁢
            h
            ⁢
            
              
            
            ⁢
            f
            ⁢
            
              
            
            ⁢
            ln
            ⁢
            
              
                D
                o
              
              
                D
                i
              
            
            ⁢
            
              
                I
                μ
              
              ⁡
              
                (
                
                  
                    
                      I
                      1
                    
                    ⁢
                    sin
                    ⁢
                    
                      
                    
                    ⁢
                    α
                  
                  -
                  
                    C
                    ⁢
                    
                      
                    
                    ⁢
                    
                      I
                      μ
                      k
                    
                  
                
                )
              
            
          
        
      
    
    
      
        (
        6
        )
      
    
  




The core loss can be calculated according to the empirical Steinmetz formula:

Pv=CmfαBβ  (7)

The core loss per volume Pv is an exponential function of alternating magnetizing frequency f and the peak flux density B. Cm, α, and β are empirical parameters, and the two exponents α and β are in the ranges of 1<a<3 and 2<β<3, where the work frequency f is fixed. Thus the core loss per volume is dependent only on the peak flux density B, and by regarding the core hysteresis resistance approximately as Rm, then

Pv=RmIFe2  (8)

Comparing expression (1) and (3), the hysteresis loss current IFe can be expressed as:

IFe=C1Iμk  (9)

where, C1 is a transformation coefficient between the hysteresis loss current IFe and the current Iμ parallel to the magnetic flux, and k is a transformation index between the hysteresis loss current IFe and the current Iμ parallel to the magnetic flux. By substituting expression (9) in expression (6), the output power model of the EHC is:





  
    
      
        
          P
          ′
        
        =
        
          μ
          ⁢
          
            
          
          ⁢
          h
          ⁢
          
            
          
          ⁢
          f
          ⁢
          
            
          
          ⁢
          ln
          ⁢
          
            
              D
              o
            
            
              D
              i
            
          
          ⁢
          
            
              I
              μ
            
            ⁡
            
              (
              
                
                  
                    I
                    1
                  
                  ⁢
                  sin
                  ⁢
                  
                    
                  
                  ⁢
                  α
                
                -
                
                  
                    C
                    1
                  
                  ⁢
                  
                    I
                    μ
                    k
                  
                
              
              )
            
          
        
      
    
    
      
        (
        10
        )
      
    
  




It follows from expression (10) that α is an independent variable, the output power reaching its maximum when α=90 degrees, with İμ and İ1 differing by 90 degrees at that point; it follows at the mean time that the load of the EHC is capacitive.

Thus the maximal condition for the output power is:




  
    
      
        
          
            d
            ⁢
            
              
            
            ⁢
            P
          
          
            d
            ⁢
            
              
            
            ⁢
            
              I
              μ
            
          
        
        =
        
          
            μ
            ⁢
            
              
            
            ⁢
            h
            ⁢
            
              
            
            ⁢
            f
            ⁢
            
              
            
            ⁢
            ln
            ⁢
            
              
                D
                o
              
              
                D
                i
              
            
            ⁢
            
              (
              
                
                  
                    
                      C
                      1
                    
                    ⁡
                    
                      (
                      
                        k
                        +
                        1
                      
                      )
                    
                  
                  ⁢
                  
                    I
                    μ
                    k
                  
                
                -
                
                  I
                  1
                
              
              )
            
          
          =
          0
        
      
    
    
      
        (
        11
        )
      
    
  




From expression (11) the condition for maximum power output of the EHC is obtained as




  
    
      I
      μ
    
    =
    
      
        
          I
          1
        
        /
        
          (
          
            
              C
              1
            
            ⁡
            
              (
              
                k
                +
                1
              
              )
            
          
          )
        
      
      k
    
  
  ,




with the maximum power output being:





  
    
      
        
          P
          
            m
            ⁢
            
              
            
            ⁢
            ax
          
        
        =
        
          μ
          ⁢
          
            
          
          ⁢
          h
          ⁢
          
            
          
          ⁢
          f
          ⁢
          
            
          
          ⁢
          ln
          ⁢
          
            
              D
              o
            
            
              D
              i
            
          
          ⁢
          
            (
            
              
                
                  
                    I
                    1
                  
                  (
                  
                    
                      I
                      1
                    
                    
                      
                        C
                        1
                      
                      ⁡
                      
                        (
                        
                          k
                          +
                          1
                        
                        )
                      
                    
                  
                  )
                
                
                  1
                  k
                
              
              -
              
                
                  
                    C
                    1
                  
                  (
                  
                    
                      I
                      1
                    
                    
                      
                        C
                        1
                      
                      ⁡
                      
                        (
                        
                          k
                          +
                          1
                        
                        )
                      
                    
                  
                  )
                
                
                  
                    k
                    +
                    1
                  
                  k
                
              
            
            )
          
        
      
    
    
      
        (
        12
        )
      
    
  




Solve for Iμ from C1(k+1)Iμk=I1, substitute it in expressions (4) and (5) to obtain the maximum power and the resistance and capacitance values at the maximum power point:




  
    
      
        R
        =
        
          
            E
            2
          
          /
          
            I
            R
          
        
      
    
    
      
        (
        13
        )
      
    
  
  
    
      
        C
        =
        
          
            (
            
              
                I
                μ
              
              -
              
                
                  I
                  1
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                α
              
            
            )
          
          
            
              E
              2
            
            *
            2
            ⁢
            
              
            
            ⁢
            π
            ⁢
            
              
            
            ⁢
            f
            *
            N
            ⁢
            
              
            
            ⁢
            2
          
        
      
    
    
      
        (
        14
        )
      
    
  




with the annular magnetic core having a volume of:

V=π(Do2−Di2)h/4  (15)

By defining the unit output power density A. as the ratio of the output power over the volume, it follows that:




  
    
      
        λ
        ∝
        
          I
          ⁢
          
            
          
          ⁢
          n
          ⁢
          
            
              
                D
                o
              
              
                D
                i
              
            
            /
            
              (
              
                
                  D
                  o
                  2
                
                -
                
                  D
                  i
                  2
                
              
              )
            
          
        
      
    
    
      
        (
        16
        )
      
    
  




It can be seen that by selecting the magnetic core material, fixing its volume, the is permeability, and the primary current, the power density is proportional to




  I
  ⁢
  
    
  
  ⁢
  n
  ⁢
  
    
      
        D
        o
      
      
        D
        i
      
    
    /
    
      
        (
        
          
            D
            o
            2
          
          -
          
            D
            i
            2
          
        
        )
      
      .
    
  




In comparison with prior art, the present invention is effective in that:
The present invention, by demonstrating both theoretically and experimentally the effects of the magnetic core shape and the number of secondary turns on the output power of the EHC, by establishes an output power model for the EHC based on the capacitance-resistance model, by more than doubling its unit power density, by further establishing power output characteristics for the harvester comprising the EHC and the power management module, and by enabling the EHC to be always working at the maximum power output point, realizes maximum power output for the EHC.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention description below refers to the accompanying drawings, of which:
FIG. 1 is a schematic diagram of a prior art online monitoring system for a power transmission line.
FIG. 2 exhibits a prior art method for controlling the output power of an EHC to be outputting a stable power output in a wide dynamic range.
FIG. 3 is a schematic diagram of the equivalent load model of the EHC of the present invention.
FIG. 4 is a vector diagram of the resistance-capacitance model by taking into account the hysteresis loss.
FIG. 5 is a schematic diagram of an embodiment of the maximum power output of the EHC of the present invention.
FIG. 6 shows a three dimensional display of a load resistance R of an embodiment of the maximum power output of the EHC of the present invention under dynamic adjustment of a sliding rheostat.



DETAILED DESCRIPTION
Referring to FIG. 3, a schematic diagram of the equivalent load model of the EHC of the present invention, it can be seen therefrom that the maximum power output circuit of the EHC of the present invention is comprised of a magnetic core, that is, a primary coil N1 and a secondary coil N2, with a load resistor R and a capacitor C parallel connected at the two ends of the secondary coil.
The design method for an embodiment of the maximum power output circuit of the EHC of the present invention comprises the following steps:
1) setting the power density λ of the magnetic core as 1.38 mW/g@10 A, that is, requiring the 1 kg magnetic core be capable of outputting 1380 mW power with a 10 A primary current;
2) selecting silicon steel as the material for the magnetic core of the embodiment, with a density of 7.35 g/cm3, C1 being 0.22, k being 0.95, the effective permeability being 0.01, I1=10 A, and obtaining Iμ as 27.5 A according to




  
    
      I
      μ
    
    =
    
      
        
          I
          1
        
        /
        
          (
          
            
              C
              1
            
            ⁡
            
              (
              
                k
                +
                1
              
              )
            
          
          )
        
      
      k
    
  
  ;




substituting Iμ in




  
    
      P
      
        m
        ⁢
        
          
        
        ⁢
        ax
      
    
    =
    
      μ
      ⁢
      
        
      
      ⁢
      h
      ⁢
      
        
      
      ⁢
      f
      ⁢
      
        
      
      ⁢
      ln
      ⁢
      
        
          D
          o
        
        
          D
          i
        
      
      ⁢
      
        (
        
          
            
              
                I
                1
              
              (
              
                
                  I
                  1
                
                
                  
                    C
                    1
                  
                  ⁡
                  
                    (
                    
                      k
                      +
                      1
                    
                    )
                  
                
              
              )
            
            
              1
              k
            
          
          -
          
            
              
                C
                1
              
              (
              
                
                  I
                  1
                
                
                  
                    C
                    1
                  
                  ⁡
                  
                    (
                    
                      k
                      +
                      1
                    
                    )
                  
                
              
              )
            
            
              
                k
                +
                1
              
              k
            
          
        
        )
      
    
  
  ,




to obtain the maximum power output as





  
    67.02
    *
    h
    ⁢
    
      
    
    ⁢
    ln
    ⁢
    
      
        D
        o
      
      
        D
        i
      
    
  
  ,




with the power density for the magnetic core being:





  λ
  =
  
    67.02
    *
    h
    ⁢
    
      
    
    ⁢
    ln
    ⁢
    
      
        
          D
          o
        
        
          D
          i
        
      
      /
      
        (
        
          
            π
            ⁡
            
              (
              
                
                  D
                  o
                  2
                
                -
                
                  D
                  i
                  2
                
              
              )
            
          
          ⁢
          
            h
            /
            4
          
        
        )
      
    
  




3) setting the inner diameter of the magnetic core Di as 55 mm, its weight as 450 g, with λ>1.2 mW/g, an calculation would show that Do<75 mm
The shape of the magnetic core shall be as D0=75 mm, Di=55 mm, h=30 mm,
4)




  
    
      
        R
        =
        
          
            E
            2
          
          /
          
            I
            R
          
        
      
    
    
      
        (
        13
        )
      
    
  
  
    
      
        C
        =
        
          
            (
            
              
                I
                μ
              
              -
              
                
                  I
                  1
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                α
              
            
            )
          
          
            
              E
              2
            
            *
            2
            ⁢
            
              
            
            ⁢
            π
            ⁢
            
              
            
            ⁢
            f
            *
            N
            ⁢
            
              
            
            ⁢
            2
          
        
      
    
    
      
        (
        14
        )
      
    
  




Calculations with expressions (13) and (14) will obtain C=17.1 uF, and R=1050 ohm.














Parameters
Value









Density
7.35 g/cm3



Turns
200



μ
0.01



C1
0.22



K
0.95










The experiment model is shown on FIG. 5. The current of the current generator is set as 10 A, the load capacitance C is increased in steps starting from 0 to 5 uF, the load resistor R is dynamically adjusted via a sliding rheostat, with the power output on the load resistor being displayed on FIG. 6, wherefrom it can be known that the maximum power output is 600 mW, conforming to theoretical calculation.

## Claims

1. A maximum power output circuit for an energy harvesting coil, comprising
a primary coil, the primary coil being a magnetic core having an outer parameter D0, an inner diameter Di, a width h, a density w, a weight W, an effective permeability μ, a maximum power output Pmax, and a fixed volume V,
a secondary coil, the secondary coil being wound around the primary coil with N2 number of turns,
a load resistor having a resistance R, and
a capacitor having a capacitance C,
wherein the load resistor and the capacitor are separately parallel connected to two ends of the secondary coil;
the magnetic core has the fixed volume V being




  
    V
    =
    
      
        π
        ⁡
        
          (
          
            
              D
              o
              2
            
            -
            
              D
              i
              2
            
          
          )
        
      
      ⁢
      
        h
        /
        4
      
      ⁢
      
        
      
      ⁢
      and
    
  




  
    
      V
      =
      
        W
        /
        w
      
    
    ,
    and
  




  
    
      
        P
        
          m
          ⁢
          
            
          
          ⁢
          ax
        
      
      =
      
        μ
        ⁢
        
          
        
        ⁢
        h
        ⁢
        
          
        
        ⁢
        f
        ⁢
        
          
        
        ⁢
        ln
        ⁢
        
          
            D
            o
          
          
            D
            i
          
        
        ⁢
        
          (
          
            
              
                
                  I
                  1
                
                (
                
                  
                    I
                    1
                  
                  
                    
                      C
                      1
                    
                    ⁡
                    
                      (
                      
                        k
                        +
                        1
                      
                      )
                    
                  
                
                )
              
              
                1
                k
              
            
            -
            
              
                
                  C
                  1
                
                (
                
                  
                    I
                    1
                  
                  
                    
                      C
                      1
                    
                    ⁡
                    
                      (
                      
                        k
                        +
                        1
                      
                      )
                    
                  
                
                )
              
              
                
                  k
                  +
                  1
                
                k
              
            
          
          )
        
      
    
    ,
  


wherein f is a frequency of a power source, I1 is a primary current, C1 is a transformation coefficient between a hysteresis loss current IFe and a current Iμ parallel to a magnetic flux, and k is a transformation index between the hysteresis loss current IFe and the current Iμ parallel to the magnetic flux;
the load resistor has the resistance R being


R=E2/IR, 


wherein E2 is an induction voltage of a secondary side of the energy harvesting coil, and IR is a current on the load resistor; and
the capacitor has the capacitance C being





  
    C
    =
    
      
        (
        
          
            I
            μ
          
          -
          
            
              I
              1
            
            ⁢
            cos
            ⁢
            
              
            
            ⁢
            α
          
        
        )
      
      
        
          E
          2
        
        *
        2
        ⁢
        
          
        
        ⁢
        π
        ⁢
        
          
        
        ⁢
        f
        *
        
          N
          2
        
      
    
  
  ,



wherein α is an angle of 90 degrees between the primary current I1 and the magnetization current Iμ.

2. A method for designing the maximum power output circuit for the energy harvesting coil of claim 1, comprising
setting a power density index λ under a minimum working current;
calculating the magnetization current Iμ, under the minimum working current according to a maximum output power of the energy harvesting coil as follows:





  
    I
    μ
  
  =
  
    
      
        I
        1
      
      /
      
        (
        
          
            C
            1
          
          ⁡
          
            (
            
              k
              +
              1
            
            )
          
        
        )
      
    
    k
  



and


IFe=C1Iμk,


selecting a material for the magnetic core and calculating the outer parameter Do and the width h in accordance with the density w and the volume V of the magnetic core
calculating the resistance R and selecting the load resistor having the resistance R; and
calculating the capacitance C and selecting the capacitor having the capacitance C.