Modeling and Structure Optimization of Tapped Transformer

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 7, No. 1, February 2017, pp. 41~49
ISSN: 2088-8708, DOI: 10.11591/ijece.v7i1.pp41-49  41
Journal homepage: http://iaesjournal.com/online/index.php/IJECE
Modeling and Structure Optimization of Tapped Transformer
Abdelhadi Namoune1
, Azzedine Hamid2
, Rachid Taleb3
1,2
Electrical Engineering Department, University of Science and Technology of Oran-Mohamed Boudiaf
Laboratoire d’Electronique de Puissance Appliquée (LEPA), Oran, Algeria
3
Electrical Engineering Department, Hassiba Benbouali University
Laboratoire Génie Electrique et Energies Renouvelables (LGEER), Chlef, Algeria
Article Info ABSTRACT
Article history:
Received Jun 1, 2016
Revised Aug 23, 2016
Accepted Dec 11, 2016
In this paper, a simplified circuit model of the tapped transformer structure
has been presented to extract the Geometric and technology parameters and
offer better physical understanding. Moreover, the structure of planar
transformer has been optimized by using changing the width and space of the
primary coil, so as to enlarge the quality factor Q and high coupling
coefficient K. To verify the results obtained by using these models, we have
compared them with the results obtained by employing the MATLAB
simulator. Very good agreement has been recorded for the effective primary
inductance value, whereas the effective primary quality factor value has
shown a somewhat larger deviation than the inductance.
Keyword:
Geometrical parameters
High coupling coefficient
Quality factor
Tapped transformer
Technological parameters Copyright © 2017 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Abdelhadi Namoune,
Electrical Engineering Department,
University of Science and Technology of Oran-Mohamed Boudiaf,
Laboratoire d’Electronique de Puissance Appliquée (LEPA),
Faculté de Génie Electrique, BP 1505 El-m’naouar, Oran, Algeria.
Email: namoune.abdelhadi@gmail.com
1. INTRODUCTION
Transformers are exploited in RFIC (radio-frequency integrated circuit) functions in place of two
inductors in disparity circuits to acquire superior Q (quality factor) while absorbing less expire region [1].
They are also utilized for solitary to differential signal translation, impedance matching, signal pairing and
phase dividing. A transformer is shaped when two spiral coils, or inductors, magnetically combine due to
their close proximity. This reasons the impedance levels, distincted as the ratio of the terminal voltage to the
current flow, to modify between coils [2-4]. The transformer characteristics comprise the self-inductance,
series resistance, mutual coupling coefficient, substrate capacitances, self resonating frequencies, symmetry
and die region. Alike to inductor, the type of transformer structure influences these characteristics and is
selected based on the application usage the transformer is intended for. The three ordinary transformer
configurations are shown in Figure 1. Figure 1(a) illustrates a tapped structure consisting of an inner winding
and an outer winding. Mutual combination between adjacent conductors contributes mostly to the self-
inductance of every coil. The structure is not often useful as the mutual inductance is small due to very small
coupling [5]. Figure 1(b) shows two spirals interwound in the equal plane. The interwound spirals guarantee
electrical characteristics of primary and secondary are equal, including the similar number of turns. The
transformer terminals are situated in opposite side’s permiting easy access for layout [6]. Figure 1(c) shows
two spirals stacked in divide metal planes. The advantage of the structure is an abridged generally region
since it is implemented in different metal layers. The flux linkage between the two windings advances due to
the close coupling. The coupling coefficient, K, can be as high as 0.9 for a stacked structure [7]. However,
the use of separate metal planes consequences in an asymmetry between the primary and secondary coils

 ISSN: 2088-8708
IJECE Vol. 7, No. 1, February 2017 : 41 – 49
42
caused by the diverse thickness and metal characteristics of the planes. This layout is suitable for small
frequency action as big capacitance between coils due to the overlap results in a low SRF (self resonating
frequencies) [8].
Figure 1. Transformers Physical Structures (a) Tapped (b) Interleaved (c) Stacked
An on-chip transformer model is needed for chip technologies. Modeling and design of on-chip
inductors and transformers is presented in [9]. Each inductor is constructed with one metal spiral or winding.
On-chip transformers, consisting of various windings, may be designed in different ways.
The tapped transformer design is chosen for various causes. This structure simply permits for any
ratio of NP: NS, but in return, due to the large relative distance between windings of the two spirals it
consequences in a awfully small coupling coefficient, K. Clearly, because of including straight relations
between two spirals there is a superior possibility of circulating tall levels of noise between parts of the
circuit that use magnetic coupling. On the other hand, due to low surface area between the two inductors, the
resulted parasitic capacitance is extremely small which consequences in tall self resonance frequency [10].
The advantages are short port-to-port capacitance and tall Lp and Ls but have the disadvantages of organism
asymmetric and have short coupling coefficient, approximately around 0.3 to 0.7 [11].
In this paper, square shaped tapped transformer has been studied. Section 2; extract the geometrical
and technological parameters of the tapped transformer. Section 3, an equivalent-circuit model of tapped
transformers is presented. In Section 4, the structure has been optimized by changing the geometrical and
technological parameters of the tapped transformer, to enlarge the quality factor Q and high coupling
coefficient K. Moreover, simulation results will be compared with calculation results. Finally, a conclusion is
drawn in Section 5.
2. TAPPED TRANSFORMER
Figure 2 illustrate the cross Section view beside the thickness of a typical tapped transformer
contrivance. The outer diameter OD is distinct as the outmost distance between two parallel segments. The
inner diameter ID is distinct as the intimate distance between 2 parallel segments. The width is W, the
spacing is S. The example device in Figure 2 has Np=Ns = 2 turns [12], [13].
Figure 2. Cross Section View of a Tapped Transformer with the Definition of the Main Geometrical and
Technological Parameters
a b c
OD
Ss
Dielectric (Cox)
Silicon substrate
Wp
Sp Ws
Primary coil
subsub
tsub
ox tox
Secondary coil
ID
Tm=tp

IJECE ISSN: 2088-8708 
Modeling and Structure Optimization of Tapped Transformer (Abdelhadi Namoune)
43
The planned transformer model obtains into explanation an amount of geometrical parameters. They
are abridged in Table 1 [14], [15].
Table 1. Geometrical Parameters of the Tapped Transformer
Symbol Geometrical Parameters
Np
Ns
Number of primary turns
Number of secondary turns
lp Wire length of the primary turns
ls Wire length of the secondary turns
Wp Width of the metal conductor of the primary turns
Ws Width of the metal conductor of the secondary turns
tp Thickness of the metal conductor of the primary turns
ts Thickness of the metal conductor of the secondary turns
Sp Space between metal conductor of the primary turns
Ss
ID
OD
Space between metal conductor of the secondary turns
Inner diameter of the transformer
Outer diameter of the transformer
Technological parameters must be cautiously measured in the enlargement of the model so that it
can be legal for an amount of different processes parameters [16], [17]. Table 2 presents a list of the
technological parameters, which are considered in the planned model (see Figure 2).
Table 2. Technological Parameters of the Tapped Transformer
Symbol Technological Parameters
tox Distance between the substrate and the (primary, secondary) lower face
tsub Substrate thickness
ox Equivalent relative permittivity of the dielectric between substrate and conductor
sub Equivalent relative permittivity of the substrate
sub Substrate resistivity
3. TRANSFORMER MODEL
A lossy transformer can be modeled with an inductor and a resistor in series for each coil
representing the dominating series losses as shown in Figure 3. From the model, the characteristics of the
tapped transformer can be illustrated [18]. The impedances of the primary and secondary coils are:
pp LjRZ ..11  (1)
ss LjRZ ..22  (2)
The inductances of the primary and secondary windings are:
 
.
Im 11
j
Z
Lp  (3)
 
.
Im 22
j
Z
Ls  (4)
The quality factors of the windings are:
 
 11
11
Re
Im
Z
Z
Qp  (5)
 
 22
22
Re
Im
Z
Z
Qs  (6)

 ISSN: 2088-8708
44
To explanation for the inadequate coupling suitable to metal ohmic loss, substrate dissipation,
parasitic capacitance and outflow, several parameters are defined [19]. The quality factor represents the
power of the magnetic coupling and M is the mutual inductance between the primary and secondary coils as
distinct in Equation 7. For a completely joined transformer, the quality factor is harmony. But in a
characteristic process, K is between 0.3 and 0.9 for monolithic transformers [20]. The relation between
coupling coefficient, K, and mutual inductance, M, is illustrated in Equation 8.
 ..
2112
j
Z
j
Z
M  (7)
sp LL
M
k
.
 (8)
Figure 3. A Lossy Transformer Model
The model for a tapped transformer is exposed in Figure 4. This model comprises two π models, one
for each coil. The π model contains the series inductance (Ls), the series resistance (Rs), the series capacitance
(Cs), the transformer to substrate capacitance (Cox), and the substrate resistance (Rsi) and capacitance (Csi).
The transformer model also includes the spiral-to-spiral capacitances (Cov) and the mutual inductance (M).
The substrate coupling elements Rsi and Csi are neglected from the use of a patterned ground shield [21].
Figure 4. Physical Representation of Equivalen Circuit Model Parameters for Transformer [22]
Sub Sub
RsiCsiRsi
Csi
M
Cov
CoxCox
Cs
Cs
Ls
Ls
Rs Rs
Cox
Cox
RsiCsi RsiCsi
Cov
Sub Sub
MLp Ls
Rp Rs
Z22
Z11

IJECE ISSN: 2088-8708 
45
4. STRUCTURE OPTIMIZATION
4.1. Space and Width of the Primary Coil of the Tapped Transformer
The width and the space between turn’s lines of primary coil can influence the inductance and
quality factor of tapped transformers. Figure 5 shows the space Sp and width Wp of a tapped transformer.
When varying the width and space of primary coil, the inductance, quality factor and coupling coefficient
will also be changed so as the parasitic capacitance.
For the tapped transformer, if the outer diameter and number of turns is fixed, equal summation of
width Wp and space Sp leads to approximately the equal region of the tapped transformer, which is a thought
in tapped transformer design. Also, the summation constant of width and space (primary coil) are optimized,
together verify the performance of the tapped transformer.
Figure 5. Tapped Transformer Structure (the Indication of width and Space of Primary Coil)
Four tapped transformer structures are considered by care the summation of width and space of the
primary coil at 25µm. The width and space of primary coil are 9+16µm, 13+12µm, 17+8µm, and 21+4µm,
respectively. The number of turns (Np and Ns) of these four tapped transformers is fixed at 2 and OD outer
diameter is 250µm. Figure 6 shows the four structures of tapped transformer.
Figure 6. The Four Structure of Tapped Transformers with width Plus Space (Wp+Sp) as: (a) 9+16µm,
(b) 13+12µm, (c) 17+8µm, (d) 21+4µm
Figure 7 shows the inductance Lp and quality factor Qp of primary coil of these four structures. The
value primary inductances of these four structures are approximately the equal. Above 5 GHz, the value
primary inductances show divergence for the reason that of the diverse element parasitic. For the primary
quality factor Qp, it is obvious that the structure with the summation of width and space
(Wp+ Sp= 17µm + 8µm) achieve the major quality factor.
Figure 8 shows the result for the highest value primary inductance can achieve for a square tapped
transformer having the thickness of metal tp, the widths of primary coil between 2 and 20 µm and (tp / Wp)
ratios between 0.1 and 0.8 (characteristic devise values for primary coil of tapped transformer). It must be
renowned that tapped transformers, still although of easy realization, eat a lot of on-wafer region, and wary
optimization to contain the top magnitude inductance and quality factor. This optimization must think a
known primary inductance value, frequency of action and the technology parameter used to create the tapped
transformers.
(a):W+S= (9+16)µm (b):W+S= (13+12)µm (c):W+S= (17+8)µm (d):W+S= (21+4)µm
Secondary
Primary
Wp
Sp

 ISSN: 2088-8708
46
Figure 7. (a) Primary Inductance Lp and (b) Primary Quality Factor Qp as a Function of Frequency of these
Four Structures (Wp+Sp)
Figure 8. Results for the Primary Inductance of Tapped Transformer having the Width Wp between 2 and 20
µm and the Thickness of Metal tp defined by the tp / Wp Ratios between 0.1 and 0.8
4.2. Effect of Geometrical Parameters
To evaluate the inductance of the primary turn of a tapped transformer, we employ the experimental
formula [23], [24]:
ADOD
ADN
L p
p
.14.11
...5,37 22
0



(9)
 







 
 2
...5,37
.13.5,14.
1
ADNN
ADODOD
k
sp
(10)
where Np is the number of primary turns, Ns is the number of secondary turns, OD is the outer
diameter, ID is the inner diameter, W is the track width, and S is the line-to-line spacing.
In addition (W and S), outer diameter (OD) is another important geometrical parameter determining
the coupling coefficient value and surface employ of the tapped transformer. Figure 9 shows how coupling
coefficient K varys with changing outer diameter OD and number of turns of primary coil Np. The spacing of
primary coil Sp can be fixed to 2μm, the width of primary coil Wp can be fixed to 10μm and the frequency f
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 10
-5
4
5
6
7
8
9
10
11
12
13
x 10
-8
Width of primary coil (m)
InductanceLp(H)
Tp/Wp=0.8
Tp/Wp=0.5
Tp/Wp=0.2
Tp/Wp=0.1
1 2 3 4 5 6 7 8
x 10
9
2
4
6
8
10
12
14
16
18
20
Frequency F(Hz)
QualityFactoroftheprimarycoil"Qp"
(Wp+Sp)=21µm+4µm
(Wp+Sp)=17µm+8µm
(Wp+Sp)=9µm+16µm
(Wp+Sp)=13µm+12µm
1 2 3 4 5 6 7 8 9 10
x 10
9
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
x 10
-7
Frequency F(Hz)
Inductanceoftheprimarycoil"Lp(H)"
(Wp+Sp)=21µm+4µm
(Wp+Sp)=13µm+12µm
(Wp+Sp)=17µm+8µm
(Wp+Sp)=9µm+16µm
(a) (b)

IJECE ISSN: 2088-8708 
47
can be fixed to 5 GHz. bigger outer diameter and augment number of turn Np product in elevated coupling
coefficient K.
In a tapped transformer, such as the one in design, the coupling coefficient between the primary and
secondary coils is small. Figure 10 shows the coupling coefficient K as a function of number of turns Np and
different spacing Sp, different width Wp of the primary coil for a tapped transformer. For the unchanged space
and width of the primary coil, there is a big development in coupling coefficient K as number of turn
augments.
Figure 9. Outer Diameter OD Versus Number of
Turns of Primary Coil Np for a Tapped Transformer
and a given Coupling Coefficient Value K
Figure 10. Coupling Coefficient K as a Function of
Number of Turns Np and different Metal Trace
Width Wp, and Spacing Sp for a Tapped Transformer
4.3. Effect of Substrate Thickness and Substrate Resistivity
The influence of the substrate resistivity and the substrate thickness on the performance of a 5.2 nH
nominal tapped transformer is analyzed. Two different substrate resistivities were simulated: low resistivity
silicon (LR-Si) at 0.4 W.cm and high resistivity silicon (HR-Si) at 7 KW.cm. Two different substrate
thicknesses were also simulated, Tsub = 300 µm and Tsub = 500 µm. The results are shown in Figure 11.
Increasing the thicknesses from 300 to 500 µm does not augment significantly the primary quality
factor, mostly since the resistance of the substrate is limited by the skin effect. The peak of the primary
quality factors at 2.5 GHz and 5.5 GHz. On the additional hand, rising the resistivity of the substrate can
augment the primary quality factor, signifying that at very elevated frequency the substrate connected losses
are the controling ones when employing thick substrate (Tsub > 200 µm).
Figure 11. (a) Primary Inductance and (b) Primary Quality Factor as Function of the Frequency for different
Substrate Resistivity (LR – Low Resistivity and HR – High Resistivity) and different Substrate Thicknesses
(Tsub=300 µm and Tsub=500 µm).
Number of turns of primary coil Np
OuterdiameterOD(m)
0,73
0,64
0,54
0,45
0,36
2 3 4 5 6 7 8 9 10
1
2
3
4
x 10
-4
1 2 3 4 5 6 7 8 9 10
x 10
9
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
x 10
-7
Frequency F(Hz)
LR-Si & Tsub=300µm
LR-Si & Tsub=500µm
HR-Si & Tsub=300µm
HR-Si & Tsub=500µm
1 2 3 4 5 6 7 8
x 10
9
5
10
15
20
25
30
Frequency F(Hz)
LR-Si & Tsub=300µm
LR-Si & Tsub=500µm
HR-Si & Tsub=300µm
HR-Si & Tsub=500µm
(b)(a)
2 3 4 5 6 7 8 9 10
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
Number of turns Np
Couplingcoefficientk
Wp=6µm & Sp=2µm
Wp=8µm & Sp=2µm
Wp=6µm & Sp=3µm
Wp=8µm & Sp=3µm

 ISSN: 2088-8708
48
4.4. Comparisons Between Calculation and Simulation Results
Figure 12 illustrate a comparison of the effective values of the primary inductance Lp and primary
quality factor Qp of tapped transformer taked by the calculation of the parameters physical model and by
simulation results by MATLAB.
(a) (b)
Figure 12. Comparison between Calculated and Simulated Results (a) Primary Inductance and (b) Primary
Quality Factor of a Tapped Transformer
A good accord between the calculated and simulated primary inductance value was also achieved in
the holder of tapped transformer. A vaguely bigger calculated inductance value was achieved compared with
the simulated results. The calculated inductance value at 3 GHz frequency is 10.45 nH. The simulated
inductance value at 3 GHz frequency is 7.96 nH.
The maximum primary quality factor value achieved by calculating the parameters of the equivalent
model of tapped transformer was 16.82 and was achieved at 2.2 GHz frequency. The maximum primary
quality factor value obtained by simulation was 13.15 and was obtained at 2.6 GHz frequency.
5. CONCLUSION
In this paper, an equivalent circuit model has been presented to study the tapped transformer. This
model presents a great physical perception of the tapped transformer and can be utilized to remove the
geometric and technology parameters. The correctness of the circuit model is definited by contrasting its
consequence with MATLAB simulation result. Also, the physical structure of tapped transformer has been
optimized to obtain improve performance. By contrasting the MATLAB simulation results of diverse
structures, the quality factor of tapped transformer is optimized by using appropriate, outer diameter, inner
diameter, width, space, substrate thickness and substrate resistivity. The preferred tapped transformer
specifications recognized by the designer are transformed into suitable geometrical parameter without
neglecting the process parameter.
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