Transformers for DC–DC E-Power Conversion

7/23/2019 Transformers for DCDC E-Power Conversion

1/15

5088 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015

3-D Microtransformers for DCDC On-Chip

Power ConversionAli Moazenzadeh, Fralett Suarez Sandoval, Nils Spengler, Vlad Badilita, and Ulrike Wallrabe

AbstractWe address the miniaturization of power convertersby introducing novel 3-D microtransformers with magnetic corefor low-megahertz frequency applications. The core is fabricatedby lamination and microstructuring of Metglas 2714A magneticalloy. The solenoids of the microtransformers are wound aroundthe core using a ball-wedge wirebonder. The wirebonding pro-cess is fast, allowing the fabrication of solenoids with up to 40turns in 10 s. The fabricated devices yield the high inductanceper unit volume of 2.95 H/mm3 and energy per unit volume of133 nJ/mm3 at the frequency of 1 MHz. The power efficiency of6476% is measured for different turns ratio with coupling factorsas high as 98%. To demonstrate the applicability of our passivecomponents, two PWM controllers were selected to implement an

isolated and a nonisolated switch-mode power supply. The isolatedconverter operates with overall efficiency of 55% and maximumoutput power of 136 mW; then, we experimentally demonstratehow we increased this efficiency to 71% and output power to408 mW. The nonisolated converter can deliver an overall effi-ciency of 81% with a maximum output power of 515 mW. Finally,we benchmarked the results to underline the potential of the tech-nology for power on-chip applications.

Index TermsDCDC power conversion, magnetic layeredfilms, micromachining, transformer.

I. INTRODUCTION

THE pressure of handheld electronic devices has broughtalong an on-going trend of miniaturization applied to all

components. Among various parts, power converters are of a

great importance since they deliver the required power to each

function block of the system. Power converters generally consist

of two main parts: the control part, which is currently available

by means of integrated circuit (IC) technology, and the pas-

sive electronics part. Usually, a significant physical volume and

Manuscript received April 30, 2014; revised July 10, 2014 and September 20,2014; accepted October31, 2014. Date of publication November 6, 2014; date ofcurrent version April 15, 2015. This work was supported by the DFG GraduateSchool Embedded Microsystems under Grant 1103. The work of F. SuarezSandoval was supported by the National Council of Science and Technology

(CONACYT, Mexico), and by the General Ministry of International Affairsof the Secretariat of Public Education (DGRI, SEP, Mexico). The work of V.Badilita was supported by DFG through Contract BA 4275/2-1. Recommended

for publication by Associate Editor J. A. Cobos.A. Moazenzadeh, F. Suarez Sandoval, V. Badilita, and U. Wallrabe are

with the Laboratory for Microactuators, Department of Microsystems Engi-neering (IMTEK), University of Freiburg, 79110 Freiburg, Germany (e-mail:[email protected]; [email protected];[email protected]; [email protected]).

N. Spengler is with the Freiburg Institute of Advanced Studies, Univer-sity of Freiburg, 79104 Freiburg, Germany and also with the Laboratory forMicroactuators, Department of Microsystems Engineering (IMTEK), Univer-sity of Freiburg, 79110 Freiburg, Germany (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2014.2368252

weight of the power converter unit is reserved by the magnetic

passive components, e.g., coils and transformers [1]. The main

function of these magnetic components is to periodically store

and release the energy [2]. The more we shrink down the size

of the magnetic components, the more difficult it becomes to

maintain their power handling capability. Therefore, a solution

is needed to overcome this challenge and allow an uncompro-

mised fabrication of high-performance miniaturized converters.

One possibility to maintain the power handling capabilities

while miniaturizing the converters is to integrate a magnetic core

with the passive component [2]. Therefore, the device yields

higher inductance within the same footprint, thus being able

to drive more power. Increasing the switching frequency in the

converter offers another route which leads to further reduction

in the size of inductors [3]. A higher switching frequency means

that the converter needs to process a smaller amount of energy

during each switching cycle, for the same amount of power.

However, both solutions present further disadvantages.

Microstructuringof magnetic materials to obtain miniaturized

cores is still challenging, especially when it comes to high-

aspect-ratio structures. Electroplating and sputtering are widely

used to obtain thin magnetic layers [4][7]; however, the choice

of magnetic materials is limited for each of these processes,

respectively. Also, these techniques pose inherent limitationsin terms of thickness and the homogeneity of the magnetic

components in the bulk physical volume.

Increasing the converter switching frequency introduces addi-

tional high-frequency parasitic losses, via the skin or proximity

effects in the inductors [2]. Magnetic losses are also increased

due to hysteresis and eddy currents losses in the magnetic core

[8]. The hysteresis loss can be controlled by choosing the right

material composition as well as annealing of the core magnetic

material [9]. The eddy current losses can be minimized either

by using high resistivity magnetic materials or by splitting the

bulk magnetic core into thin layers, which are in the same thick-

ness range of the skin depth in the frequency range of interest

[10]. However, both approaches increase the complexity of the

magnetic components fabrication.

The fabrication of 3-D microcoils using an automatic ball-

wedge wirebonder was initially reported by Kratt et al. [11].

Since then, we used the technology for several unconventional

applications in microsystems [12]. In [13], we report the fabrica-

tion of SU-8 core microtransformers for the very high frequency

regime applications. Also, the preliminary results on integrating

a rod-shape multilayer magnetic core to the wirebonded coils

was reported in [14]. Although the rod-shape magnetic core is

the easiest choice for the integration with the wirebonded mi-

crocoils, this approach has some disadvantages, mainly due to

0885-8993 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications standards/publications/rights/index.html for more information.


2/15

MOAZENZADEHet al.: 3-D MICROTRANSFORMERS FOR DCDC ON-CHIP POWER CONVERSION 5089

TABLE ICOMPARISONBETWEENSTATE-OF-THE-ARTSOFTMAGNETICMATERIALSUSED FORMINIATURIZED COREFABRICATION

Type Material Fabrication Advantages Disadvantages

Ferrites NiZn [23]

MnZn [24]

Screen printing ResistanceSimplicity of fabrication

Saturation magnetization Relative permeability Coercivity Incompatible with MEMS fabrication

Metallic alloys NiFe [4][8], [28], [47]

CoNiFe [48]

Electroplating

Sputtering

Saturation magnetization

Relative permeability

Resistance Limited core thickness

Nanocrystalline

films

CoZrTa [38]

CoZrO [40]

CoFeSiB [39]

S pu tte rin g R esistan ce

Saturation magnetizationRelative permeability

Low temperature fabrication

Limited core thickness

Costly fabrication

Amorphous Metglas 2714 Lamination Relative permeability Saturation magnetization

metal ribbons Vitrovac 6025 [8], [46] Etching Resistance Sensitive to external processes

Sim pl icity of fabri cation Lim it ed ri bbons t hi ckness

Low temperature fabrication

the high demagnetization field in the open-loop magnetic core

configuration.

In this paper, we report significant progress allowing the inte-gration of a closed-loop magnetic core to our established wire-

bonded microcoils. Meanwhile, improvements in the wirebond-

ing technology enable us to fabricate the coils with twice the

turns than our previous reports [13], [14] directly on the mag-

netic core. A new assembly method has been introduced to align

and position the magnetic core structures on the final substrate in

a parallel way to speed up the fabrication process of the micro-

transformers. The applicability of our passive components has

been verified by implementing them in two different topologies

of switch-mode power supplies, an isolated and a nonisolated

topology. Altogether resulting in a new approach to fabricate

miniaturized on-chip transformers, for low-megahertz regime

(0.330 MHz) applications. In Section II, the state of the art of

soft magnetic materials used in power conversion applications

is presented. Section III reports on the design and fabrication of

the closed-loop magnetic core, as well as the integration with

our established wirebonded microcoil technology. In Section

IV, the frequency-dependent performance of the magnetic core,

as well as the fabricated microtransformers are characterized.

Meanwhile, we benchmarked the results with the state of the art

of published devices to underline the potential of the technology

for power on-chip applications. Finally, Section V reports on the

implementation of the microtransformers and microcoils in an

isolated and nonisolated dcdc converter, respectively.

II. SOFTMAGNETICMATERIALSUSED FORMINIATURIZED

COREFABRICATION

The state-of-the-art air core inductors with high quality fac-

tors of up to 30 and inductances per unit volume of up to

700 nH/mm3 have been reported [13], [15][18]; thus, magnetic

core inductors with lower inductance per unit volume would not

represent an advantage.

Soft magnetic materials are fundamental for microtransform-

ers magnetic core. They enhance the efficiency and the power

density of transformers, thus enabling further miniaturization.

An appropriate material for the magnetic core of a power

transformer should have a high saturation magnetization, high

permeability, high resistivity, low coercivity, and high thermal

conductivity. Among the above-mentioned properties, resistiv-ity, thermal conductivity, and saturation magnetization are in-

trinsic properties of a material, while the other depend on the

external effects, such as the shape and dimension of the core

[19], fabrication process [20], operating frequency [10], extra

annealing [20], and excitation waveform [21]. All these param-

eters need to be considered when designing an efficient core.

Table I summarizes the properties of different magnetic mate-

rials used for miniaturized core fabrication.Ferritesare popular

candidates as a core material for megahertz frequency regime

applications, mainly due to their high resistivity, which is nec-

essary to damp the induced eddy current in the core at high

frequencies [22][26]. This advantage is paid by the relatively

low permeability and high coercivity, which limit the further

miniaturization of ferrite cores [4], [10]. Besides, the main dis-

advantage of ferrites is the relatively lower saturation flux den-

sity when compared to other types of core magnetic materials.

Furthermore, their typical fabrication is not compatible with

MEMS processing due to the high temperature needed for sin-

tering [23].

Magnetic metallic alloys are other candidates for the core

material. Among them, PermalloyNiFeis commonly used

as a magnetic core material [4][8], [27][35]. A core layer

thickness of up to several micrometers is achievable by elec-

trodeposition that enables manufacturing of 3-D cores [4]. The

fabrication process is well established, mainly due to the wideusage of NiFe in magnetic recording heads technology [36]. Be-

sides the MEMS compatible fabrication, metal alloys also have

good soft magnetic properties in terms of high permeability

and low hysteresis loss. However, the intrinsic low resistivity

of metals results in high eddy currents losses in the core for

high switching frequency applications [4]. Reducing the metal-

lic film thickness to the nanometer range enables the fabrication

of nanolaminated Permalloy cores, which results in lower core

losses at high frequencies [37]. However, realizing such a core

needs a complex and expensive fabrication method. In order

to improve the resistivity of magnetic alloys and make thick

alloys applicable for high-frequency applications, researchers


3/15


Fig. 1. (a) Exploded view of an UT magnetic core microtransformer chip. (b) Fabricated UT Metglas core in this paper compared to the smallest commerciallyavailable Metglas core. The UT core has been placed inside a commercial toroidal core.

TABLE I IPHYSICALPROPERTIES OFMETGLAS2714 [49], [50]

Property Value

Composition Co6 6Fe4Ni1Si1 5B1 4Sat ur at ion m agneti zati on 0.57 (T)

Max DC permeability (cast) >80 000

Saturation magnetostriction


4/15


Fig. 3. Wirebonded UT core microtransformers fabrication process flow.

III. DESIGN ANDFABRICATION

Fig. 1(a) shows an exploded view of the microtransformer

chip introduced in this paper. The magnetic core and the in-

ductive coils were designed as 3-D structures. This results in

minimizing the footprint of the chip without compromising the

transformer performance. The small footprint of 5.4 mm2 is

enough to contain the magnetic core as well as the primary and

secondary coils. Additionally, when compared to 2-D deposited

cores, the larger cross-sectional area of the 3-D core enhances

the power-handling capability without reaching saturation. Hav-

ing a 3-D shape for the inductive coils results in more inductance

density, and also more concentrated and uniform magnetic field,

which leads to stronger inductive coupling between the primaryand secondary coils than a 2-D design [13], [51].

The core is composed of two structures, one with a U-shape,

the other one with a T-shape, allowing an easy assembly to form

a closed-loop magnetic core. Winding both the primary and

secondary coils on the central post enhances the self-shielding

properties of the microtransformer. The designed magnetic cores

have the footprint of 3 mm 1 mm and the height of 2.5 mm.For the fabrication of our multilayered UT magnetic cores,

37 layers of 18-m-thick Metglas 2714A were laminated with ahand roller (DTS-HR, Asmetec). Prior to the lamination, the al-

loy surface was cleaned in an ultrasonic bath filled with pure iso-

propanol. The hand roller also removes dusts, which improves

the adhesion of the layers. Intermediate layers of 10-m-thickpolyester-based double-sided sticky tape (5601, Nitto) provided

both adhesion and electrical insulation between the Metglas lay-

ers. The lamination was performed on the surface of a heater

plate at the temperature of 80C[see Fig. 3(a)]. The whole lam-

ination process was carried out inside a clean environment of a

class II biological cabinet which provides the clean area. The

last step was to place the stack inside a hydraulic press (JAS105,

High force), for 12 h at the force of 4.5 kN and the temperature

of 45C[see Fig. 3(b)]. This last step was introduced to guar-

antee the perfect uniformity and high adhesion strength for the

37 layers of Metglas, as well as for the 36 layers of sticky tape.

By employing an electrical discharge machine (EDM) (Robifil

2020SI, GF Agie Charmilles), the magnetic stacks were pre-cisely cut into submillimeter core parts [see Fig. 3(c)]. The

cutting direction was chosen so that the easy magnetization axis

of the cores is aligned with their height. EDM cutting has the

advantage of producing good surface quality on the cut edges

while not changing the magnetic or physical properties of the

Metglas alloy. The use of EDM offers an almost unrestricted

way of structuring the laminated magnetic stacks. However,

this method is not applicable to fabricate cylindrical shape posts

and the EDM cut posts cross section are limited to square or

rectangular shapes. Fig. 2 shows the distinct proven methods to

laminate the Metglas layers as well as several cutting techniques

for the laminated stack.

To enable coil winding, the cores were arranged onto a

4 in borosilicate wafer. Borosilicate wafers were chosen to

avoid eddy currents induced in the substrate, hence minimizing

substrate-related losses. The bond pads were made by means of

standard UV lithography. After evaporating the 15 nm/150 nm

Cr/Au seed layer, a 20-m-thick mold for pads and traces waspatterned using AZ-9260 photoresist. A layer of 12 m of goldwas subsequently electroplated on top of the seed layer. In order

to define the bond positions with respect to the microtransformer

cores, 2m of ma-N 1420 negative tone photoresist was spunand patterned to structure the alignment marks on the substrate

[see Fig. 3(d)]. The substrate was finally diced into the 8 mm

8 mm chips using a wafer dicing machine (DAD321, Disco).To enable the magnetic core assembly on the processed

borosilicate substrate, a 1.5 mm-thick paper-based plastic sub-

strate was used as an alignment aid. Square through-holes were

patterned with a UV laser (TruMark 6330, Trumpf) to perfectly

accommodate the posts of the T-shape cores[see Fig. 3(e)]. The

distance between the through-holes was equal to the distance be-

tween the landing marks on the borosilicate wafer. Two vacuum

chucks fixed both the borosilicate (top) and the plastic substrate

(bottom) containing the structures to be positioned. Taking ad-

vantage of the borosilicate transparency and using precision

moving tables, the plastic substrate was aligned with respect to

the borosilicate substrate [see Fig. 3(f)]. AZ-direction moving


5/15


Fig. 4. Full view of a UT core microtransformer as well as some closeupviews of the laminated stack, the remaining gap on top of the core between theT-shape and U-shape parts, the contact area between the T-shape and U-shapecore on bottom of the core as well as the wedge and ball bonds (in clockwise

direction starting from left).

table was used to raise the plastic substrate and bring the T-shape

structures in contact with the borosilicate wafer. Fast adhesive

(4204, Loctite) served as the bonding material. After the adhe-

sive got cured, the plastic substrate was brought down leaving

behind the T-shape structures positioned[see Fig. 3(g)]. After a

short O2 -plasma cleaning step for 2 min, at 40 C, 0.3 mbar and

the power of 1000 W at 2.54 GHz, a modified, automated ball-

wedge wirebonder (3100plus , ESEC) was employed to wind theprimary and secondary solenoids directly around the post of the

T-structures. The starting height of the coil was set to 600m

above the surface of the substrate allowing for a precise 100 mclearance from the horizontal part of the T-structure core. Within

10 s, each solenoid was wound with up to 40 turns of insulated

25-m-thick copper or gold wire [see Fig. 3(h)]. After the wind-ing process, the U-shape structure was positioned following the

same methodology as used for the T-structures, in this way the

microtransformer fabrication being completed[see Fig. 3(i) and

(j)]. Fig. 4 shows a full view of a UT core microtransformer chip

as well as some closeup SEM pictures of the important regions.

IV. CHARACTERIZATION RESULTS

A. Metglas Core PerformanceMetglas is a soft magnetic material with ultrahigh dc perme-

ability. However, its permeability is a function of different phys-

ical parameters. In order to characterize the high-frequency per-

meability of the Metglas, a rod-shape (magnetically open-loop)

cubic multilayered core was fabricated with the same process

as mentioned in Section III. The cubic core had the dimensions

of 0.8 mm0.8 mm1.2 mm (LWH). A wirebondedmicrocoil with seven turns was wound around the post. Another

core with nominal exact shape and dimension as the Metglas

cubic core was fabricated by means of a thick SU-8 lithography

process, which later was wirebonded to realize the same coil

with seven turns. Both coils, the one on the Metglas rod-shapecore and the one on the SU-8 post, were wirebonded using the

same trajectory, and as previously demonstrated [13], the coils

are supposed to be very similar due to the high precision of the

wirebonding process.

In order to show the effects of adding the rod-shape Metglas

micromachined core, both coils have been characterized in the

range of 1 MHz to 1 GHzusingan impedanceanalyzer(E4991A,

Agilent). The coil with the Metglas core resonates at 560 MHz,

while the other resonates at around 1 GHz [see Fig. 5(a)]. This

fact shows that the Metglas coil has more internal inductance

and capacitance since the resonance frequency is a function of

these two parameters:

f = 1

2LC

(1)

Fig. 5(b) shows the frequency-dependent inductance for each

coil. The SU-8 core coil shows a constant inductance over al-

most the full measurement range with an inductance of 70 nH;

the Metglas core coil, however, exhibits 4.42 times higher in-

ductance at the frequency of 1 MHz. Until 10 MHz, it shows a

slight decrease of 10 nH in the inductance value; however, the

inductance falls rapidly with further increase in frequency.

The total magnetic flux density that is generated by the Met-

glas core coil is

B= 0 (Hc+ M) =0 (Hc NdM+ M) (2)where Hc is the magnetic field produced by the coil, Nd isthe demagnetization factor of the magnetic core, and M is the

magnetization of the Metglas. The magnetic flux density of the

SU-8 core coil is

B= 0Hc . (3)

A demagnetization factor Ndz of a rectangular rod, magne-tized along its long axis (z-direction), is found to be as expressed

below [52]:

Ndz = 1

(2n + 1) (4)

where n is the dimensional ratio of the rectangular rod. Based

on the dimension of the rod-shape Metglas core, the demagne-

tization factor of the cubic Metglas core is 0.25 [52].

An electrical current Iflowing through a coil creates a mag-

netic flux proportional to the current. The proportional con-stant is defined as the inductance L of the coil [20]. The in-

ductance of the coil is only a function of its geometry and the

permeability of the surrounding medium. Since in our case,

both the rod-shape Metglas core and the SU-8 core coils were

geometrically identical, we can consider the inductance ratio

of the coils as the effective permeability e of the rod-shapeMetglas core. In order to calculate the relative permeabilityrof the Metglas core from its effective permeability, we need to

consider the demagnetization factor of the core [53]:

r =

e(1 Ndz )1 Ndze . (5)

By substituting the values of the demagnetization factor and

the effective permeability in equation (5), the relative perme-

ability of the rod-shape Metglas core is measured to be 31.57 atthe frequency of 1 MHz. For an ideal closed-loop UT Metglas


6/15


Fig. 5. Characterization result of two similar wirebonded coils, one with the rod-shape Metglas core and the other with a SU-8 (magnetically like air) core. (a)Resonance frequency of the coil with magnetic core was decreased as a result of an increase in the internal inductance and capacitance. (b) Coil inductance hasbeen increased 4.42 times at the frequency of 1 MHz.

Fig. 6. Two-port characterization setup for the frequency-dependentimpedance measurements of the microtransformers.

core, the effective permeability of the core is expected to be the

same as its relative permeability; then, both should be equal to

31.57 at the frequency of 1 MHz. However, any airgap in the

closed-loop core structure could impact its effective permeabil-

ity. Also, it should be considered that two-third of the magnetic

core volume in the both closed-loop (UT core) and open-loop

(rod-shape core) consist of Metglas and the rest was filled withthe double-sided tape, which is a nonmagnetic material.

B. UT Core Microtransformer Electrical Performance

The high-frequency characterization of the UT core mi-

crotransformers was performed using the two-port on-wafer

measurement method. The scattering parameters of the micro-

transformers were measured using a vector network analyzer

(E5071A, Agilent) connected to a probe-station (9000, Cascade

Microtech), equipped with two microprobes (SG/GS-500, Cas-

cade Microtech) as shown in Fig. 6.

Prior to the measurements, open, short, load, and through

calibrations were done using an impedance standard substrate

(106-683, Cascade Microtech). The scattering parameters of the

microtransformers were recorded simultaneously as a function

of a logarithmic frequency sweep from 300 kHz up to 300 MHz

as a S2 P file. The power efficiency of the microtransformers,defined as the output load power versus the input power, was

computed directly from the scattering parameters Sxy , using(6), wherexandy denote the port numbers [54]:

Z0 = |S21 |21 |S11 |2 (6)

where Z0 = 50 is the nominal impedance of the network ana-

lyzer which served as the load impedance of the measurements.By converting theS-parameters toZ-parameters and by extract-

ing the real and imaginary partsof the impedance for each port at

each frequency, the inductance, electrical resistance and quality

factor, as well as the coupling factor of the microtransformers

were calculated using

Lxy = [Zxy ]

2f

Rxy = [Zxy ]

Qxy = [Zxy ] [Zxy ] (7)

k=

[Z12 ] [Z21 ] [Z11 ] [Z22 ] .

On that basis, a UT core microtransformersample 1with

copper solenoids of 40 turns and turns ratio of 1:1 on a borosili-

cate substrate was characterized. The self-resonance frequency

of the microtransformer was found to be 28 MHz. The self-

inductance of the primary and secondary solenoids were mea-

sured to be L11 = 43.7 H and L22 = 41.7 H, at 300 kHz,as can be observed in Fig. 7. The self-inductance, however, de-

creased to the values of 39.91 and 37.93 H at the frequency of1 MHz as a result of the internal loss of the Metglas core.

Thus, we achieved an inductance per unit volume of up to


7/15


Fig. 7. Two-port measured frequency-dependent inductance of sample 1.

Fig. 8. Two-port measured frequency-dependent resistance of sample 1.

Fig. 9. Two-port measured frequency-dependent quality factor of the primary() and the secondary () coils and impedance of the primary () and thesecondary () coils of sample 1.

2.95 H/mm3 and an energy per unit volume of 133 nJ/mm3

at a frequency of 1 MHz, both being higher than most of the

other published on-chip power microinductors and microtrans-

formers [38], [55].The two-port frequency-dependent resistance of the sample 1

is shown in Fig. 8. For both solenoids, the resistance increased

as the frequency increased. This is caused, on one hand, by the

skin and proximity effects in the inductor and, on the other hand,

by internal losses of the magnetic core, resulting from the hys-

teresis and eddy current losses. The quality factor of the primary

solenoid reached a maximum value of 7.1, whereas it was 6.8

in the secondary solenoid (see Fig. 9), which is comparable to

state-of-the-art power microinductors [38]. The coupling factor

had a constant value of 97.3% over the whole frequency range

below the self-resonance frequency indicates that the micro-

transformers had a minimal leakage inductance (see Fig. 10).The high coupling factor is attributed to the 3-D geometry, the

Fig. 10. Two-portmeasuredfrequency-dependentefficiency () andcouplingfactor (o) of sample 1.

Fig. 11. Two-port measured frequency-dependent inductance of sample 2.

Fig. 12. Two-port measured frequency-dependent resistance of sample 2.

magnetic core, the precision of the wirebonder, and the compact

design of the transformers. The efficiency of the microtrans-

former reached a maximum value of 71% at a frequency of

1.11 MHz for the 50 load, provided by the network analyzer(see Fig. 10).

Another UT core microtransformer chipsample 2with12 primary turns and a turns ratio of 1:2.5 made of gold wire

was fabricated on a printed circuit board (PCB) substrate. This

microtransformer was used later for the implementation of an

isolated dcdc converter, presented in the next section. The

measurement results are depicted in Figs. 1113. The PCB chip

was produced on a standard FR4 substrate with the thickness

of 500 m. 200-nm-thick gold layer was deposited on top ofthe usual 35m of copper to enable wirebonding on the chip.Table III summarizes the main measurement results of the both

microtransformer chips.

To conclude this part, we benchmarked our results to previ-

ously published microinductors (pure RF microinductors wereexcluded). Fig. 14 illustrates the maximum quality factor as a


8/15


Fig. 13. Two-portmeasuredfrequency-dependentefficiency () andcouplingfactor (o) of sample 2.

TABLE I II

MEASUREMENTRESULTS OF THEMICROTRANSFORMERCHIPS, SAMPLE1ANDSAMPLE2

@ f(MHz) Sample 1 @ f(MHz) Sample 2

L1 1 (H) 1 39.9 1 3.7

L2 2 (H) 1 37.9 1 22.1Q1 1 m a x 0.320 7.1 0.548 4.3

Q2 2 m a x 0.320 6.8 0.355 6.2

(%) < 28 97.3 < 55 97.1

m a x (%) 1.11 71 2.57 66

Np r i m a r y 40 12N1 : N2 1 : 1 1 : 2.5Material Cu Au

Fig. 14. Benchmark of the state-of-the-art published microinductors in terms

of maximum quality factor as a function of the inductance per unit volume(nH/mm3). The shape of each point indicates the type of magnetic core of eachinductor, whereas the color of each point distinguishes the frequency rangewhere each prototype is applicable. The inductor presented in this paper arerepresented by the TW label.

function of the inductance per unit volume (nH/mm3) for sev-

eral miniaturized inductors. The shape of each point indicates

the magnetic material type of each inductor, whereas the color of

each point distinguishes the frequency range where each proto-

type is applicable. The inductor presented in this paper is labeled

with TW and convinces through its high inductance per unit

volume and reasonably high quality factor. We further bench-

marked the microtransformers to previously published devices

Fig. 15. Benchmark of the state-of-the-art microtransformers in terms of in-ductance per dc resistance (L/RD C ) as a function of the frequency at whichthe maximum efficiency appears. The size of the bubbles indicates the powerefficiency of each transformer, whereas the color of a bubble distinguishes be-

tween air and magnetic core transformers. The label of each bubble representsthe reference, transformer type and coupling factor. The microtransformers pre-sented in this paper are labeled with TW1 and TW2. All the devices exceptin [32] and [45] were characterized using a 50 load.

(pure RF microtransformers were excluded). Fig. 15 shows the

inductance per dc resistance (L/RDC ), which represents thelow-frequency performance of the microtransformers as a func-

tion of the frequency of the maximum efficiency. The size of

the bubbles indicates the power efficiency of each transformer,

whereas the color of a bubble distinguishes between air and

magnetic core transformers. The label of each bubble repre-

sents the reference, transformer type, and coupling factor. Thetransformers presented in this paper and represented by the

biggest bubbles convince through their high power efficiency,

high inductance per dc resistance, and high coupling factors.

For higher frequencies applications, perhaps lower inductance

is needed; then, our former microtransformer prototypes are

applicable [13], [14].

V. APPLICATION OF THEMICROTRANSFORMERS

ANDMICROCOILS: DCDC CONVERTERS

In order to assess the behavior of our microtransformers and

microinductors, we implemented them in two different switch-

mode power supplies, an isolated and a nonisolated topology,respectively.

A. Isolated Converter

The isolated power supply is an asymmetrical half-bridge

Flyback converter, as depicted in the schematic of Fig. 16.

This topology uses a primary-side controller and the output

gets regulated based on the transformer turns ratio. The primary

side of the converter is a modified synchronous Buck converter

that comprises two switches, the high-side switch SHS and thelow-side switchSLS , as well as input and output capacitorsCinand Cpr i, respectively. If the filtering inductor of a conventional

Buck converter is replaced by the transformer T1 , having turns


9/15


Fig. 16. General schematic of an asymmetricalhalf-bridge Flyback converter.

Fig. 17. Microtransformer chip connected to the Fly-buck converter PCB.

ratio ofN1 :N2 and, furthermore, if the voltage of the secondary

winding is rectified by the diode D1 , one obtains the so-calledFly-buck converter, which is a combination of the Buck and

Flyback topologies.

The pulse-width modulation (PWM) controller used in our

isolated converter is the TPS55010 (Texas Instruments). We de-

signed the converter to supply an output voltage of 5 V, from an

input voltage range of 4.55.5 V at an adjustable frequency of

up to 2 MHz. After the component values needed for a proper

operation of the controller were calculated, a PCB layout was

designed and fabricated as illustrated in Fig. 17. The total foot-

print of the demonstration board was 18 mm 63 mm includingconnectors used during characterization.

To connect our microtransformer to the converters board, we

designed a second PCB with an area of 5 mm 5 mm, shownin the zoomed-in view of Fig. 17. Bond pads for the winding

process can be found on the top layer of the chip, pads for

soldering a pin header are located on the bottom face, while

the connection between both chip layers is realized by four vias.

The converter board is equipped with a pin receptacle that allows

electrical connection to the microtransformer.

The efficiency curve as well as the percentage of load and

line regulation [56] of the Fly-buck converter were extracted us-

ing four precision digital multimeters (34401A, Agilent). These

three figures of merit for power supplies were obtained by si-

multaneously recording input and output voltages, as well as

currents of the converter. The converter was operating with a

Fig. 18. Efficiency of a 5 V output Fly-buck converter operating with sample2 at a switching frequency of 1.82 MHz.

Fig. 19. Characterization of a 2.7 V output synchronous Buck converter oper-ating with an UT microcoil with 12 turns made out of a gold wire with diameterof 25m, at a frequency of 0.75 MHz. (a) Efficiency curve for different inputvoltages. (b) Percentage of load regulation for different input voltages.

microtransformer with 12 turns in the primary winding and

turns ratio of 1:2.5sample 2at a switching frequency of

1.82 MHz. Fig. 18 shows the efficiency of the converter as a

function of the output current.

The maximum converter efficiency that we measured was

55% as shown by Fig. 18,while thepercentage of load regulation

was 37%, measured from the no-load to full-load conditions.


10/15


TABLE IVSTATE-OF-THE-ARTBUCKCONVERTERSSPECIFICATIONS

Parameter Type f [MHz] Vin [V ] Vo [V ] Po m a x[W ]

[ %]@Po m a x

m a x[% ]

Io [mA]@m a x

PLOR1 [% ] PLNR2 [% ] Powerdensity

[W/mm3]

Item nr.

[57] Commercialmodule 0.4 4 3.3 16 85 95 1200 2.77 @Io = 3.9 A 0.27 @Io = 3.9 A,Vo = 2 .25 V

0.022

[58] 1.5 3.3 1.5 12 77 89 2000 0.031[59] 0.6 5 1.5 15 82 91 3000 0.019[60] 1.5 12 3.3 1.65 82 82 500 0.6 @

Io = 0.5 A0.03 @

T = 25 C,Vo = 9.5 V

851.6 106

[61] 0.6 5.5 3.3 9.9 93.5 93.5 3000 0.108[62] Research

module

1.6 4 3.3 0.99 85 85 300 0.9 @

Io = 0.3A 0.002

[63] 1.5 5 1.2 48 85 89 20,000 0.041[64] 5 12 1.2 12 86 87 60,000 0.048[65] PwrSiP

product

5.5 4.2 1.8 1.08 85 88 200 0.51 @

Io = 0.6 A0.69 @

Io = 0.2 A,Vo = 3 V

0.157

[66] 0.8 5 2 0.6 70 80 60

0.045

[67]


11/15


TABLE VSTATE-OF-THE-ARTISOLATEDCONVERTERSSPECIFICATIONS

Parameter Type f [MHz] Topology Transformervolume

[m m 3 ]

Vin[V ]

Vo[V ]

Po m a x[W ]

[ %]@Po m a x

m a x[% ]

Io [mA]@m a x

PLOR[% ] PLNR[% ]

Item nr

[18] Researchmodule 8 Half-bridgeWith ZVS 31.24 12 5 0.5 34 34 100 [79] 15 Forward 10 10 3 0.3 33 33 100 [80] 6.5 Forward 0.819 6 1.9 0.08 3 3 42 [81] 6 Resonant 23 12 5.5 0.66 49.5 49.5 120 This work Research

module

1.82 Fly-buck Wire:

Au 25m0.136 47 55 22.5 37 @

Io = 0 .04 A3.2 @

Io = 0 .02 A ,Vo = 1 V

Fly-buck Wire:

Cu 34m7.5 5 5 0.296 62.5 67.2 42 14 @

Io = 0 .04 A1.2 @

Io = 0 .02 A ,Vo = 1 V

Fly-buck Wire:

Cu 100m0.408 64.2 71 65 11 @

Io = 0 .04 A0.8 @

Io = 0 .02 A ,Vo = 1 V

the other two cases of input voltage, because the controller wasset to enter an automatic pulsed frequency modulation (PFM)

mode. In this mode, the IC operates in either PWM or in PFM

mode. When the device is initially powered, it operates in fixed

PWM mode until completion of the soft-start. It remains in this

mode until it senses that the converter is on the verge of breaking

into discontinuous operation. At this point, the controller goes

to sleep mode until the output voltage has decreased by 2%.

The controller then starts again at a fixed PWM frequency for a

short duration and increases the output voltage. If the controller

again senses discontinuous operation, the cycle repeats. Since

the duty cycle in PFM mode is low, all losses are reduced which

results in efficiency improvement at light loads. The two mod-

ulation modes are clearly identified in Fig. 19(b) for the threeinput voltages.

The percentage of load regulation of the synchronous Buck

converter for an output current change of 150 mA is 4.63%,

0.35% and 1.78% with an input voltage of 3.3, 4, and 5 V, re-

spectively, as summarized in Table IV. As depicted in Fig. 19(b),

with an input voltage of 3.3 V and an output current greater than

140 mA, the efficiency reduces dramatically due to the decrease

in output voltages, leading to the conclusion that a compro-

mise should be made between the overall circuit efficiency and

possible output voltage for load currents greater than 140 mA.

The highest percentage of line regulation that was measured is

0.81%, at a load current of 60 mA and a change in input voltagefrom 3 to 5 V.

C. Effects of the Wire Diameter on the Microtransformer:

Impact on the Circuit Efficiency

At the efficiency maxima of the isolated converter, we have

calculated that the ohmic loss in the microtransformer accounts

for 57.5% of the total losses. In order to improve the efficiency

of the converter, as well as to increase the power density of

the microtransformer, we replaced the gold wire of 25 m indiameter with a hand-wound copper wire with a diameter of

34 m in the first case, and of 100 m in the second case.

The new efficiency curves were measured and are presented in

Fig. 20. Efficiency curve of a 5-V output Fly-buck converter operating withan UT microtransformer with 12 turns on the primary side and turns ratio of1:2.5. The transformers were made of wirebonded gold with diameter of 25 m(), hand-wound copper with diameter of 34 m () and hand-wound copperwith diameter of 100m ().

Fig. 20. We have maintained the efficiency curve from Fig. 18

to ease the comparative process.

Increasing the wire diameter as well as using copper, which

has higher conductivity than gold, reduces the ohmic loss of themicrotransformer. Fig. 20 shows that the efficiency maximum

increased from 55% to 67% and 71% for the copper wires with

34 and 100 m diameter, respectively. With increasing wirediameter, the maximum possible output current has increased

as well.

Furthermore, high converter efficiencies are maintained for

a wider range of loads at increased wire diameters. The low-

efficiency maxima of sample 2 is caused by the poor load regu-

lation of the converter when working with thin wire diameters.

The percentage of load regulation has decreased from 37% to

14% and to 11% for the output current range of 040 mA,

whereas the percentage of line regulation decreased from 3.1%


12/15


TABLE VIPOWERLOSSDISTRIBUTION OF THE FLY-BUCKCONVERTER OPERATINGWITH

DIFFERENTWIREDIAMETERS FOR THE UT M ICROTRANSFORMERS

Measured input power[mW] 172.26 399.61

Measured output power[mW] 94.77 283.8

Power losses [%] [%]

UT transformer[mW] 43.03 55.53 42.54 36.7MOSFETs [mW] 22.64 29.21 37.23 32.15

Diode [mW] 11.25 14.52 32.5 28.06

Input, primary and output capacitors[mW] 0.57 0.74 3.58 3.09

Total [mW] 77.49 100 115.81 100

to 1.1% and to 0.8%, for a variation in the input voltage from

4.5 to 5.5 V at a fixed value of output current of 20 mA. Ta-

ble V summarizes the most important characteristics of the Fly-

buck converter operating with the distinct microtransformers

in comparison with other reported results. In contrast with the

large number of references listed in Table IV, Table V shows

only a few isolated converters using miniaturized transformers.Moreover, the reported efficiencies and output power capabil-

ities of these are considerably lower than those of nonisolated

converters.

The power losses in the microtransformer made from 100-

m-diameter copper wire account for only 36% of the total lossin converter, comparable to the total power losses of the semi-

conductor devices. Table VI summarizes the different sources of

loss at the efficiency maxima for the smallest and the largest wire

diameter used. The power losses in the semiconductor devices

account for both switching and conduction losses. The losses

in the primary and secondary windings of the microtransformer

were summed and are accounted as one single value.

The high converter efficiency that we measured with the

hand-wound microtransformers clearly indicates that a mod-

erate increase in wire diameter may lead to a drastic increase

in efficiency while maintaining batch fabrication capability, as

wirebonder machines can be configured to work with wire di-

ameters of 100m.

VI. CONCLUSION

Three-dimensional microtransformers were manufactured by

combining a newly developed closed-loop micromachined mag-

netic core with wirebonded microcoils meant for on-chip power

conversion applications in the low-megahertz region. No clean-

room processes were needed, neither for the fabrication of themagnetic core nor for the fabrication of the inductive coils.

Thus, we present a process which is relatively cost effective and

straightforward to realize in a usual laboratory environment.

This fabrication technique lends itself to direct on-chip inte-

gration of the microtransformers into any converter circuits.

Former reports on wirebonded microcoils treated individual

solenoids showing low mutual inductances [82], whereas in

this study, transformers with strongly coupled microsolenoids

are presented. The effective permeability of the new cores has

increased seven times at the frequency of 1 MHz, as the result

of the closed-loop design. Moreover, improvements in the wire-

bonding technology enabled the fabrication of coils with twicemore turns directly on the magnetic core. As a result, the new

microtransformers generate at 1 MHz 7.4 times more induc-

tance density than the rod-shape core microtransformers previ-

ously reported in [14] and 26.7 times more inductance density

than our air core microtransformers reported in [13]. We bench-

marked the devices to previously published microinductors and

microtransformers, both from dc and ac performances points

of view, to underline the potential of the technology for power

on-chip applications.

We further investigated how the winding wire diameter influ-

ences the system efficiency of the power converters. The power

losses of the microtransformer made from 100-m-thick hand-wound copper wire account for the 36% of the total loss in the

circuit. This is 19% less than for the microtransformer wound

with the 25-m-thick wirebonded gold wire. These results provethat if the wirebonder machine is calibrated to wind coils made

of increased wire diameters (>25m), the system efficiency ofthe converters could be even higher than what we have achieved

so far with the hand-wound devices, because the winding preci-

sion of this machine also reduces the leakage inductance of our

microtransformers, thus reducing switching losses.Therefore, adaptation of our wirebonder-based coil wind-

ing technique to thicker copper wire appoints to more efficient

magnetic devices, consequently better system efficiency. More-

over, our laboratory style manual pick-and-place assembly of

the magnetic cores can be easily automated by the use of pick-

and-place machines to push our process toward industrial man-

ufacturing standards.

ACKNOWLEDGMENT

The authors would like to thank S. M. Torres Delgado

(IMTEK, Laboratory of Simulations) for access to electrical

characterization equipment, J. Hempel (IMTEK, Laboratory ofElectrical Instrumentation) for access to probe station, and A.

Gehringer (IMTEK, Laboratory for Process Technology) for

EDM cut of the magneticcores.They would alsoliketo thank M.

Pauls (IMTEK, Laboratory for Microactuators) and the Gisela

and Erwin Sick Chair for Micro-optics for access to the SEM.

They further acknowledge Prof. D. P. Arnold (University of

Florida) for the valuable technical discussion on the demagne-

tization effect.

REFERENCES

[1] D. Yao,C. G. Levey,and C. R. Sullivan, Microfabricated V-groove powerinductors using multilayer Co-Zr-O thin films for very-high-frequency

DC-DC converters, inProc. IEEE Energy Convers. Congr. Expo. , 2011,pp. 18451852.[2] C. R. Sullivan, Integrating magnetics for on-chip power: Challenges and

opportunities, in Proc. IEEE Custom Intergr. Circuits Conf. , Sep. 2009,

pp. 291298.[3] T. ODonnell, N. Wang, R. Meere, F. Rhen, S. Roy, D. OSullivan,

and S. C. OMathuna, Microfabricated inductors for 20 MHz dc-dc converters, in Proc. Appl. Power Electron. Conf. Expo., 2008,pp. 689693.

[4] J. Y. Park and M. G. Allen, Ultralow-profile micromachined power in-ductors with highly laminated Ni/Fe cores: Application to low-megahertzDC-DC converters,IEEE Trans. Magn., vol. 39, no. 5, pp. 31843186,Sep. 2003.

[5] C. R. Sullivan and S. R. Sanders, Design of microfabricated transformersand inductors for high-frequency power conversion,IEEE Trans. Power

Electron., vol. 11, no. 2, pp. 228238, Mar. 1996.[6] R. J. Rassel, C. F. Hiatt, J. DeCramer, and S. A. Campbell, Fabrication

and characterization of a solenoid-type microtransformer, IEEE Trans.Magn., vol. 39, no. 1, pp. 553558, Jan. 2003.


13/15


[7] M. Brunet, T. ODonnell,J. OBrien, P. McCloskey, and S. C. OMathuna,Design study and fabrication techniques for high power density mi-

crotransformers, in Proc. Appl. Power Electron. Conf. Expo., 2001,pp. 11891195.

[8] D. Flynn, A. Toon, L. Allen, R. Dhariwal, and M. P. Y. Desmulliez,Characterization of core materials for microscale magnetic componentsoperating in the megahertz frequency range,IEEE Trans. Magn., vol. 43,no. 7, pp. 31713180, Jul. 2007.

[9] R. M. Bozorth, Ferromagnetism. New York, NY, USA: Wiley, 1993,

p. 968.[10] D. P. Arnold, I. Zana, and M. G. Allen, Analysis and optimization

of vertically oriented, through-wafer, laminated magnetic cores in sil-icon, J. Micromech. Microeng., vol. 15, no. 5, pp. 971977, May2005.

[11] K. Kratt, V. Badilita, T. Burger, J. G. Korvink, and U. Wallrabe, A fully

MEMS-compatible process for 3D high aspect ratio micro coils obtainedwith an automatic wire bonder, J. Micromech. Microeng., vol. 20, no. 1,pp. 015021-1015021-11, Jan. 2010.

[12] A. C. Fischer, J. G. Korvink, N. Roxhed, G. Stemme, U. Wallrabe, andF. Niklaus, Unconventional applications of wire bonding create oppor-tunities for microsystem integration, J. Micromech. Microeng., vol. 23,no. 8, pp. 083001-1083001-18, Aug. 2013.

[13] A. Moazenzadeh, N. Spengler, R. Lausecker, A. Rezvani, M. Mayer,J. G. Korvink, and U. Wallrabe, Wire bonded 3D coils render air coremicrotransformers competitive,J. Micromech. Microeng., vol.23,no.11,

pp. 114020-1114020-11, Nov. 2013.[14] A. Moazenzadeh, N. Spengler, and U. Wallrabe, High-performance, 3D-

microtransformers on multilayered magnetic cores, inProc. IEEE MicroElectro Mech. Syst., 2013, pp. 287290.

[15] J. Kim, F. Herrault, X. Yu, M. Kim, R. H. Shafer, and M. G. Allen,Microfabrication of air core power inductors with metal-encapsulatedpolymer vias, J. Micromech. Microeng., vol. 23, no. 3, pp. 035006-1035006-7, Mar. 2013.

[16] C. D. Meyer, S. S. Bedair, B. C. Morgan, and D. P. Arnold, Microma-chined wiring board with integrated microinductor for chip-scale powerconversion, IEEE Trans. Power Electron., vol.29, no.11, pp.60526063,Jul. 2014.

[17] C. D. Meyer, S. S. Bedair, B. C. Morgan, and D. P. Arnold, High-inductance-density, air-core, power inductors, and transformers designedfor operation at 100500 MHz, IEEE Trans. Magn., vol. 46, no. 6,pp. 22362239, Jun. 2010.

[18] S. Tang, S. Hui, and H. Chung, A low-profile low-power converter

with coreless PCB isolation transformer, IEEE Trans. Power Electron.,vol. 16, no. 3, pp. 311315, May 2001.

[19] B. Jamieson, T. ODonnell, S. Kulkarni, and S. Roy, Shape-independentpermeability model for uniaxially-anisotropic ferromagnetic thin films,

Appl. Phys. Lett., vol. 96, no. 20, pp. 202509-1202509-4, 2010.[20] B. D. Cullity and C. D. Graham,Introduction to Magnetic Materials, 2nd

ed. New York, NY, USA: Wiley, 2009, p. 832.[21] W. Wieserman, G. Schwarze, and J. Niedra, Magnetic and electrical

characteristics of cobalt-based amorphous materials and comparison to apermalloy type polycrystalline material, in Proc. 3rdInt. Energy Convers.

Eng. Conf., 2005, pp. 5720-15720-17.[22] P. Galle, X. Wu, L. Milner, S.-H. Kim, P. Johnson, P. Smeys, P. Hop-

per, K. Hwang, and M. G. Allen, Ultra-compact power conversionbased on a CMOS-compatible microfabricated power inductor with min-imized core losses, in Proc. Electron. Compon. Technol. Conf., 2007,pp. 18891894.

[23] J. Y. Park and M. G. Allen, Development of magnetic materials andprocessing techniques applicable to integrated micromagnetic devices, J.Micromech. Microeng., vol. 8, pp. 307316, 1998.

[24] I. Kowase, T. Sato, K. Yamasawa, and Y. Miura, A planar inductorusing Mn-Zn ferrite/polyimide composite thick film for low-Voltage andlarge-current DC-DC converter, IEEE Trans. Magn., vol. 41, no. 10,pp. 39913993, Oct. 2005.

[25] M. Raimann, A. Peter, D. Mager, U. Wallrabe, and J. G. Korvink,Microtransformer-based isolated signal and power transmission, IEEETrans. Power Electron., vol. 27, no. 9, pp. 39964004, Sep. 2012.

[26] Y. Fukuda, T. Mizoguchi, S. Yatabe, and T. Tachi, Planar inductor withferrite layers for DCDC converter, IEEE Trans. Magn., vol. 39, no. 4,pp. 20572061, Jul. 2003.

[27] T. O. Donnell, N. Wang, M. Brunet, S. Roy, A. Connell, J. Power,C. O. Mathuna, and P. Mccloskey, Thin film micro-transformers forfuture power conversion, in Proc. Appl. Power Electron. Conf. Expo.,2004, pp. 939944.

[28] N.Wang, T. ODonnell,S. Roy, P. McCloskey, andC. OMathuna,Micro-inductors integrated on silicon for power supply on chip, J. Magn. Magn.

Mater., vol. 316, no. 2, pp. e233e237, Sep. 2007.[29] N. Wang, T. ODonnell, S. Roy, S. Kulkarni, P. Mccloskey, and S. C.

OMathuna, Thin film microtransformer integrated on silicon for signalisolation,IEEE Trans. Magn., vol. 43, no. 6, pp. 27192721, Jun. 2007.

[30] C. H. Ahn, Y. J. Kim, and M. G. Allen, A fully integrated planar toroidalinductor with a micromachined nickel-iron magnetic bar, IEEE Trans.Compon. Packag. Manuf. Technol., vol. 17, no. 3, pp. 463469, Sep. 1994.

[31] M. Xu and T. Liakopoulos, A microfabricated transformer for high-frequency power or signal conversion, IEEE Trans. Magn., vol. 34,no. 4, pp. 13691371, Jul. 1998.

[32] M. Brunet, T. C. ODonnell, L. Baud, J. OBrien, P. McCloskey, and S.C. OMathuna, Electrical performance of microtransformers for DC-DCconverter applications,IEEE Trans. Magn., vol.38, no.5, pp. 31743176,

Sep. 2002.[33] T. M. Liakopoulos and C. H. Ahn, 3-D microfabricated toroidal planar

inductors with different magnetic core schemes for MEMS and powerelectronic applications, IEEE Trans. Magn., vol. 35, no. 5, pp. 36793681, Sep. 1999.

[34] A. Zolfaghari, A. Chan, and B. Razavi, Stacked inductors and transform-ers in CMOS technology, IEEE J. Solid-State Circuits, vol. 36, no. 4,pp. 620628, Apr. 2001.

[35] T. M. Andersen, C. M. Zingerli, F. Krismer, J. W. Kolar, N. Wang, andC. OMathuna, Modeling and Pareto optimization of microfabricated

inductors for power supply on chip, IEEE Trans. Power Electron., vol.28, no. 9, pp. 44224430, Sep. 2013.

[36] T. Osaka, Recent development of magnetic recording head core materialsby plating method,Electrochim. Acta, vol. 44, pp. 38853890, Jun. 1999.

[37] J. Kim, M. Kim, P. Galle, F. Herrault, R. Shafer, J. Y. Park, andM. G. Allen, Nanolaminated permalloy core for high-flux, high-frequency ultracompact power conversion,IEEE Trans. Power Electron.,vol. 28, no. 9, pp. 43764383, Sep. 2013.

[38] D.S. Gardner, G.Schrom, F. Paillet,B. Jamieson,T.Karnik, andS. Borkar,Review of on-chip inductor structures with magnetic films, IEEE Trans.

Magn., vol. 45, no. 10, pp. 47604766, Oct. 2009.[39] H. Kurata, K. Shirakawa,O. Nakazima,and K. Murakami, Solenoid-type

thin-film micro-transformer, IEEE Transl. J. Magn. Jpn., vol. 9, no. 3,pp. 9094, May 1994.

[40] S. Prabhakaran, Y. Sun, P. Dhagat, W. D. Li, and C. R. Sullivan, Mi-crofabricated V-groove power inductors for high-curren t low-voltage fast-

transient DC-DC converters, in Proc. IEEE 36th Conf. Power Electron.

Spec., 2005, pp. 15131519.[41] D. W. Lee, K. Hwang, and X. W. Wang, Fabrication and analysis of

high-performance integrated solenoid inductor with magnetic core,IEEETrans. Magn., vol. 44, no. 11, pp. 40894095, Nov. 2008.

[42] D. Yao, C. Levey, R. Tian, and C. Sullivan, Microfabricated V-groovepower inductors using multilayer CoZrO thin films for very-high-frequency DCDC converters, IEEE Trans. Power Electron., vol. 28,no. 9, pp. 43844394, Sep. 2013.

[43] M. Yamaguchi, K. Suezawa, K. I. Arai, Y. Takahashi, S. Kikuchi, Y.Shimada, W. D. Li, S. Tanabe, and K. Ito, Microfabrication and charac-teristics of magnetic thin-film inductors in the ultrahigh frequency region,

J. Appl. Phys., vol. 85, no. 11, pp. 79197922, 1999.

[44] M. Mino, T. Yachi, A. Tago, K. Yanagisawa, and K. Sakakibara, Pla-nar microtransformer with monolithically-integrated rectifier diodes formicro-switching converters,IEEE Trans. Magn., vol. 32, no. 2, pp. 291296, Mar. 1996.

[45] K. Yamaguchi, S. Ohnuma, T. Imagawa, J. Toriu, H. Matsuki, andK. Murakami, Characteristics of a thin film microtransformer with circu-lar spiral coils, IEEE Trans. Magn., vol. 29, no. 5, pp. 22322237, Sep.1993.

[46] O. Dezuari, S. E. Gilbert, E. Belloy, and M. A. M. Gijs, High inductanceplanar transformers,Sens. Actuators A Phys., vol. 81, pp. 355358, Apr.2000.

[47] B. Orlando, R. Hida, R. Cuchet, M. Audoin, B. Viala, X. Gagnard, P. An-cey, X. U. M. R. Cnrs, and L. Cedex, Low-resistance integrated toroidal

inductor for power management, IEEE Trans. Magn., vol. 42, no. 10,pp. 33743376, Oct. 2006.

[48] R. W. Filas, T. M. Liakopoulos, and A. Lotfi, Micromagnetic devicehaving alloy of cobalt, phosphorus and iron, U.S. Patent 6 624 498B22003, 2003.

[49] (2014, Jan. 31). [Online]. Available: http://www.hitachimetals.com/product/amorphous/magampsquareloopcores/documents/magamp_opt.pdf


14/15


[50] S. Brugger and O. Paul, Field-concentrator-based resonant magneticsensor with integrated planar coils, J. Microelectromech. Syst., vol. 18,

no. 6, pp. 14321443, Dec. 2009.[51] K. Ehrmann, N. Saillen, F. Vincent, M. Stettler, M. Jordan, F. M. Wurm,

P.-A. Besse, and R. Popovic, Microfabricated solenoids and Helmholtzcoils for NMR spectroscopy of mammalian cells, Lab Chip, vol. 7,no. 3, pp. 37380, Mar. 2007.

[52] M. Sato and Y. Ishii, Simple and approximate expressions of demagne-tizing factors of uniformly magnetized rectangular rod and cylinder, J.

Appl. Phys., vol. 66, no. 2, pp. 983985, 1989.[53] S. Tumanski, Induction coil sensorsA review, Meas. Sci. Technol.,

vol. 18, no. 7, pp. 3146, Mar. 2007.[54] D. M. Pozar,Microwave Engineering, 4th ed. Amherst, MA, USA: Wiley,

2011, pp. 559570.[55] S. C. OMathuna, N. Wang, S. Kulkarni, and S. Roy, Review of in-

tegrated magnetics for power supply on chip (PwrSoC), IEEE Trans.Power Electron., vol. 27, no. 11, pp. 47994816, Nov. 2012.

[56] M. K. Kazimierczuk,Pulse-Width Modulated DC-DC Power Converters.Chichester, U.K.: Wiley, 2008, pp. 69.

[57] Vishay product, FX5545G108.[58] Linear Tec. product, LTM4608.[59] ISL8201M.[60] ROHM product, BP5275-50.[61] PicoTLynx, APTH003A0X-SRZ.[62] M. Mino, K. Tsukamoto, K. Yanagisawa, A. Tago, and T. Yachi, A

compact buck-converter using a thin-film inductor, in Proc. Appl. PowerElectron. Conf., 1996, vol. 1, pp. 422426.

[63] Q. Li, Y. Dong, F. Lee, and D. Gilham, High-density low-profile cou-pled inductor design for integrated point-of-load converters,IEEE Trans.Power Electron., vol. 28, no. 1, pp. 547554, Jan. 2013.

[64] F. C. Lee and Q. Li, High-frequency integrated point-of-load converters:Overview, IEEE Trans. Power Electron., vol. 28, no. 9, pp. 41274136,Sep. 2013.

[65] Texas instruments product, TPS 82671, 2011.[66] F. Sato, T. Ono, N. Wako, S. Arai, T. Ichinose, Y. Oba, S. Kanno, E. Sug-

awara, M. Yamaguchi, and H. Matsuki, All-in-one package ultracompactmicropower module using thin-film inductor, IEEE Trans. Magn., vol.40, no. 4, pp. 20292031, Jul. 2004.

[67] S. Sugahara, K. Yamada, M. Edo, T. Sato, and K. Yamasawa, 90% highEfciency and 100-W/cm3 high powerdensity integrated DCDCconverterfor cellular phones,IEEE Trans. Power Electron.,vol.28,no.4,pp.1994

2004, Apr. 2013.

[68] Z. Hayashi, Y. Katayama, M. Edo, and H. Nishio, High-efficiency DC-DC converter chip size module with integrated soft ferrite, IEEE Trans.

Magn., vol. 39, no. 5, pp. 30683072, Sep. 2003.[69] H. J. Bergveld, K. Nowak, R. Karadi, S. Iochem, J. Ferreira, S. Ledain,

E. Pieraerts, and M. Pommier, A 65-nm-CMOS 100-MHz 87%-efficientDC-DC downconverter basedon dual-die system-in-packageintegration,inProc. IEEE Energy Convers. Congr. Expo., 2009, pp. 36983705.

[70] K. Onizuka, H. Kawaguchi, M. Takamiya, and T. Sakurai, Stacked-chipimplementation of on-chip buck converter for power-aware distributedpower supply systems, in Proc. IEEE Asian Solid-State Circuits Conf. ,2006, pp. 127130.

[71] N. Wang, J. Hannon,R. Foley,K. McCarthy, T. ODonnell,K. Rodgers, F.

Waldron, and C. O Mathuna, Integrated magnetics on silicon for powersupply in package (PSiP) and power supply on chip (PwrSoC), in Proc.3rd Electron. Syst. Integr. Technol. Conf., 2010, pp. 16.

[72] M. Bathily, B. Allard, and F. Hasbani, A 200-MHz integrated buck

converter with resonant gate drivers for an RF power amplifier, IEEETrans. Power Electron., vol. 27, no. 2, pp. 610613, Feb. 2012.[73] Y. Ahn, H. Nam, and J. Roh, A 50-MHz fully integrated low-swing buck

converter using packaging inductors, IEEE Trans. Power Electron., vol.27, no. 10, pp. 43474356, Oct. 2012.

[74] J. Wibben and R. Harjani, A high efficiency DC-DC converter using 2nHon-chip inductors, inProc. IEEE Symp. VLSI Circuits, 2007, pp. 2223.

[75] M. Wens and M. S. J. Steyaert, A fully integrated CMOS 800-mW four-phase semiconstant on/off-time step-down converter,IEEE Trans. Power

Electron., vol. 26, no. 2, pp. 326333, Feb. 2011.[76] Y. M. Nguyen, M. Brunet, J.-P. Laur, D. Bourrier, S. Charlot, Z. Valdez-

Nava, V. Bley, and C. Combettes, Low-profile small-size ferrite cores forpowerSiP integrated inductors, inProc. 15th Eur. Conf. Power Electron.

Appl., 2013, pp. 17.[77] M. Alimadadi, S. Sheikhaei, G. Lemieux, S. Mirabbasi, W. G. Dunford,

and P. R. Palmer, A fullyintegrated 660 MHz low-swingenergy-recycling

DCDC converter,IEEE Trans. Power Electron., vol.24, no.6, pp. 14751485, Jun. 2009.

[78] J.Ni, Z. Hong,and B.Y.Liu,Improved on-chipcomponentsfor integratedDC-DC converters in 0.13m CMOS, in Proc. ESSCIRC, 2009, pp. 448451.

[79] M. Mino, T. Yachi, K. Yanagisawa, A. Tago, and K. Tsukamoto, Switch-ing converter using thin film microtransformer with monolithically-integrated rectifier diodes, in Proc. Power Electron. Spec. Conf., 1995,vol. 2, pp. 665670.

[80] K. Yamaguchi, Y. Naitou, O. Nakajima, H. Matsuki, and K. Murakami,Characteristics of a DC-DC converter using a thin film microtransformerand a microinductor,IEEE Transl. J. Magn. Jpn., vol. 9, no. 6, pp. 8489,Nov. 1994.

[81] K. Yamasawa, A DC-DC converter using a microtransformer, IEEE

Transl. J. Magn. Jpn., vol. 9, no. 4, pp. 120126, Jul. 1994.[82] J. Lu, H. Jia, X. Wang, K. Padmanabhan, W. G. Hurely, and Z. J. Shen,

Modeling, design, and characterization of multiturn bondwire inductorswith ferrite epoxy glob cores for power supply system-on-chip or system-in-package applications, IEEE Trans. Power Electron., vol. 25, no. 8,pp. 20102017, Aug. 2010.

Ali Moazenzadeh was born in Shiraz, Iran. Hestarted his academic studies from 2003 in the field of

physics. He received the M.Sc. degree in photonicsfrom theLaser andPlasma Research Institute, ShahidBeheshti University,Tehran,Iran, in 2010. In Novem-ber2010,he joinedthe Laboratoryfor Microactuatorsas a Ph.D. student of the Graduate School EmbeddedMicrosystems, Department of Microsystems Engi-neering (IMTEK), University of Freiburg, Freiburg,Germany.

His research interests include the development ofmicromagnetic devices rapid manufacturing methods.

Fralett Suarez Sandoval was born in Morelia, Mi-choacan Mexico, in 1988. Shereceivedthe B.Eng. de-gree in electronicengineering fromthe Morelia Insti-tute of Technology, Morelia, Mexico, in 2011, andthe

M.Sc. degree in microsystems engineering from theUniversity of Freiburg, Freiburg, Germany, in 2013.Since March 2014, she has been working toward thePh.D. degree at the Laboratory for Microactuators,Department of Microsystems Engineering (IMTEK),University of Freiburg.

Her research interests include power electronicsand wireless power transmission with magnetic microdevices.

Nils Spengler was born in Berlin, Germany. He re-ceived the M.S. degree in microsystems engineeringfrom the University of Freiburg, Freiburg, Germany,in 2010. After one-year research at the Palo Alto Re-search Center (PARC), Palo Alto, CA, USA, he hasbeen working toward the Ph.D. degree at the Labora-toryfor Microactuators, Departmentof MicrosystemsEngineering (IMTEK), University of Freiburg, since2011.


15/15


Vlad Badilitareceived the B.Sc. and M.Sc. degreesfrom the University of Bucharest, Bucharest, Roma-

nia, in 1997 and 1999, respectively, and the Ph.D. de-gree in microoptoelectronics with a thesis focused onthe physics of coupled-cavity surface emitting lasersfrom the Ecole Polytechnique Federale de Lausanne,Lausanne, Switzerland, in 2004.

In 2007, after two years as a Postdoctoral Re-search Associate at the University of Maryland, Col-

lege Park, MD, USA, he joined the University ofFreiburg, Freiburg, Germany, as a Group Leader for

magnetic microsystems. His research interests include the broader area of minia-turized electromagnetic devices with a focus on electromagnetic actuators andmagnetic resonance detectors for sample- and volume-limited samples.

Ulrike Wallrabe received the Ph.D. degree in me-chanical engineering of microturbines and micromo-

tors from Karlsruhe University, Karlsruhe, Germany,in 1992.

From 1989 to 2003, she was with the Institutefor Microstructure Technology, ForschungszentrumKarlsruhe (today KIT), working on microactuatorsand optical microelectromechanical systems. She hasbeen a Professor of microactuators at the Department

of Microsystems Engineering, IMTEK, University ofFreiburg,Freiburg,Germany, since2003. In 2010, she

received an internal fellowship at the Freiburg Institute of Advanced Studies,FRIAS. She has published more than 110 papers in the field of microsystemstechnology. Her work focuslies in magnetic microstructures including processesfor magnetic materials and microcoils, in adaptive optics, using piezoactuators

to tune elastic lenses and mirrors, and in microenergy harvesting.

Documents

Transformers for DC–DC E-Power Conversion