Integrated Power Electronics Using a Ferrite Based Low ...

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Nov 24, 2013 (3 years and 10 months ago)

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Integrated Power Electronics Using a Ferrite-Based
Low-Temperature Co-Fired Ceramic Materials System

Alex Roesler, Josh Schare and Chad Hettler
Sandia National Laboratories
Albuquerque, NM

Dave Abel, George Slama and Daryl Schofield
NASCENTechnology
Watertown, SD

Abstract
This paper discusses a new approach to making hybrid
power electronic circuits by combining a low-temperature
(850°C to 950°C) co-fired ceramic (LTCC) substrate, planar
LTCC ferrite transformers/inductors and integrated passive
components into a multilayer monolithic package using a
ferrite-based LTCC material system. A ferrite tape functions
as the base material for this LTCC system. The material
system includes physically and chemically compatible
dielectric paste, dielectric tape and conductor materials which
can be co-fired with the base ferrite LTCC tape to create
sintered devices with excellent magnetic coupling, high
permeability (~400), high resistivity (> 10
12
Ω∙cm) and good
saturation (~0.3 T). The co-fired ferrite and dielectric
materials can be used as a substrate for attaching or housing
semiconductor components and other discrete devices that are
part of the power electronics system. Furthermore, the ability
to co-fire the ferrite with dielectric and conductor materials
allows for the incorporation of embedded passives in the
multilayer structure to create hybrid power electronic circuits.
Overall this thick film material set offers a unique approach to
making hybrid power electronics and could potentially allow
a size reduction for many commercial dc-dc converter and
other power electronic circuits.
Introduction
Magnetic components, namely inductors and transformers,
are necessary for most commercial power electronic circuits
such as point-of-load (PoL) converters. Often the magnetic
components represent some of the largest and most expensive
parts in these circuits. With the continual push to shrink the
size and cost of commercial electronic devices, numerous
research efforts over the past decade have focused on new
materials and manufacturing techniques to help address these
limitations. Examples range from low profile magnetic cores
with conventional wire wound technology; to integrating
planar transformers and embedded passives into the substrate
of a printed circuit board (PCB) [1]-[6]; to thick-film
inductors and transformers using screen-printed ferrite pastes
[7] and [8] or LTCC ferrite tape [9]-[29]; to using
microfabrication and thin film techniques to incorporate
inductors and transformers directly on silicon alongside
integrated circuits (ICs) for integrated dc-dc converters [30]-
[35]. Most of these approaches look to increase power
density by embedding or incorporating the
inductor/transformer into the substrate, and in many cases
look to decrease overall circuit size by also embedding
passives within the substrate.
The present work involves a new system-in-package
approach for power electronic applications that combines an
LTCC substrate, planar LTCC ferrite transformers/inductors
and integrated passive components into a multilayer
monolithic package using a ferrite-based LTCC material
system. A ferrite tape functions as the base material for this
LTCC system. The material system includes physically and
chemically compatible co-fired ferrite, dielectric and
conductor materials that can be prepared in paste or tape
format. This material system correspondingly allows for the
ferrite magnetic component(s) to be integrated directly with
dielectric substrate materials in a multilayer structure. To
date there has been limited research on LTCC materials
systems that allow integration of both dielectric tape and
ferrite tape within a single co-fired monolithic composite
[19]-[21],[23][28][29]. The ability to co-fire both dielectric
and ferrite tape can provide performance advantages for
hybrid or system-in-package power electronics. For example,
the use of LTCC ferrite tape as a substrate for mounting
discrete components can lead to performance degradation due
to the high parasitic inductance created by locating traces on
top of a ferromagnetic material [15]. In [15] this was
addressed by incorporating magnetic shields between the
buried magnetic component and the external circuitry, which
required a discrete LTCC dielectric substrate that was fired
separately from the LTCC ferrite structure that contained the
embedded magnetic component. A co-firable dielectric tape
allows direct integration of metal shields between buried
ferrite layers and the external surfaces of the co-fired
composite structure. In addition, a dielectric paste and/or tape
that can be co-fired with the LTCC ferrite can allow both
magnetic and capacitive devices to be embedded within the
substrate. Finally, co-firable ferrite and dielectric materials
can also be combined to create sintered ferrite devices with
excellent magnetic coupling and high resistivity (> 10
12

Ω•cm) [9]-[12].
Co-Fired Ferrite and Dielectric Material Properties
The co-firable material set used in this study includes
three different LTCC ferrite tapes based on NiCuZn
compositions, a dielectric tape, two dielectric pastes, a buried
silver conductor paste, a via-fill silver conductor paste, and an
external silver solderable conductor paste. References [22]
and [29] provide a good overview of the materials issues and
technical approach for ensuring physical and chemical
compatibility of the various co-fired materials, as well as
reducing the ferrite sintering temperature through the use of
sintering aids and fluxes. The three different ferrite tapes
provide different magnetic properties after firing. All three
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2010 Electronic Components and Technology Conference
ferrite tapes are compatible with the co-firable dielectric paste
and tape materials. To characterize the magnetic properties,
toroidal cores were fabricated from each tape, wound with
magnet wire, and characterized using a Wayne Kerr 3260B
Precision Magnetics Analyzer (for inductance measurements),
and a Tektronics TDS420A Digital Oscilloscope with
TCP202 Current Probe (for BH loop measurements). Note
that external wire, and not co-fired silver, was used to provide
the magnetizing current for these tests. Magnetic properties
were determined based on the as-fired toroidal core
dimensions, similar to the method described in [16]. When
fired with a peak firing temperature of approximately 900 °C
the three ferrite tapes possess a permeability of 60, 200 and
400, and are correspondingly referred to as 60μ tape, 200μ
tape and 400μ tape, respectively. The 60μ tape possesses a
Curie temperature > 350 °C; the 200μ and 400μ tapes possess
a Curie temperature > 125 °C. A B-H hysteresis loop
obtained for a toroid fabricated from the 400μ ferrite LTCC
tape is provided in Figure 1. The ferrite tape possesses a
saturation magnetization slightly above 300 mT. Unless
noted otherwise, all magnetic devices discussed in this paper
were fabricated utilizing the 400μ tape.
Figure 1: B-H loop measured on a toroid fabricated from the
LTCC ferrite tape possessing a fired permeability of 400.

Options for co-fired dielectric materials include screen
printable dielectric pastes and a cast dielectric tape, all of
which possess shrinkage and shrinkage rates that are well
matched to the LTCC ferrite tapes. (Note that the ability to
co-fire the dielectric tape has only been demonstrated with the
400μ ferrite tape.) Besides being useful as substrate materials
for mounting external circuit components, the co-firable
dielectric materials can also be used to embed capacitors
within the multilayer structure. The NiCuZn ferrite materials
can also be used as a dielectric material for embedded
capacitors when their lower insulation resistance does not
preclude their use. Note that the 400µ tape possesses a
resistivity of 3.58 x 10
12
Ω∙cm at room temperature, and the
dielectric tapes possesses a resistivity of 5.2 x 10
14
Ω∙cm at
room temperature. The dielectric tape possesses a dielectric
breakdown strength approximately double that of the ferrite
tapes, although breakdown strengths for the fired ferrite
materials exceed 5V/µm which is more than adequate for
most embedded capacitor needs since fired thicknesses are
limited to approximately 10 µm for screen printed paste.
Table 1 summarizes the permittivity of the dielectric and
ferrite materials. All measurements were taken at room
temperature, 1 MHz, using an HP4192A impedance analyzer.

Table 1: Dielectric permittivity of the co-firable ferrite and
dielectric materials.

Most applications require good dielectric performance
over a range of frequency and temperature. Figure 2 through
Figure 4 summarize these performance properties. Figure 2
provides change in capacitance versus frequency up to 50
MHz, normalized to the 1 MHz readings summarized in Table
1. The dissipation factor as a function of frequency (up to 50
MHz) is plotted in Figure 3. Finally, Figure 4 shows change
in capacitance versus temperature normalized to 100 kHz
room temperature measurements. As evident in these figures,
reasonable dielectric performance is offered by these
materials. For example, dielectric paste #2 offers a
capacitance density of 3.8 nF/cm
2
(assuming a fired thickness
of 10 µm), low dissipation from 10 kHz to 10 MHz, and very
stable capacitance over -55 °C to 85 °C.
Figure 2: Change in capacitance versus frequency for the
dielectric and ferrite materials. The measurements are
normalized to the 1 MHz readings in Table 1.
Design of Magnetic Components with High Coupling
As mentioned previously, the ferrite and dielectric
materials can be combined to create sintered inductive devices
with good magnetic coupling. The process used to construct
transformers with good turn coupling is outlined
chronologically in Figures 5a through 5f.
-400
-300
-200
-100
0
100
200
300
400
-1400 -1200 -1000 -800 -600 -400 -200 0 200 400 600 800 1000 1200 1400
Magnetizing Force (A/m)
Flux Density (mT)
Material Description
ε
r
@ 1 MHz
3203Y 400μ Ferrite Tape 18.6
3204Y 200μ Ferrite Tape 19.1
3205Y 60μ Ferrite Tape 15.4
3206Y Dielectric Tape 18.3
3207Y Dielectric Paste #1 5.9
3208Y Dielectric Paste #2 43
Δ Capacitance vs. Frequency
‐100.0
‐80.0
‐60.0
‐40.0
‐20.0
0.0
20.0
40.0
60.0
80.0
100.0
10,000 100,000 1,000,000 10,000,000 100,000,000
Hz
% change
3203Y
3204Y
3205Y
3206Y
3207Y
3208Y
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Figure 3: Dissipation versus frequency for the dielectric and
ferrite materials listed in Table 1.

Figure 4: Change in capacitance over temperature for the co-
firable dielectric and ferrite materials. The measurements are
taken at 100 kHz and normalized to the 20 °C readings.

First, interlayer connections are made using vias which are
punched into the ferrite tape and filled using stencils (Fig. 5a).
Conductive windings are then screen printed and dried, with
line widths down to 3 mils achievable. Next, low
permeability material is applied over the coils to direct flux
and improve coupling. The sheets are then aligned and
stacked together. High pressure pressing, or laminating, melds
all the layers into a solid mass. The matrix of transformers is
then singulated into individual pieces. Next, they are fired in a
furnace following a precise and carefully controlled
temperature profile with peak temperatures in excess of 850
°C. The firing process burns off the organic binders and
plasticizers, and then sinters the layers and printings into a
solid monolithic structure, physically bonding the particles
together.
FEM modeling can illustrate the effects of the low
permeability dielectric on the transformer performance.
Without the low-permeability dielectric, upon firing the coils
become completely embedded in ferrite with uniform
permeability throughout. This results in very poor coupling
and extremely poor transformer performance. Figure 6
provides an illustration from a finite element method
magnetics (FEMM) finite element model [36] that shows the
flux distribution in an LTCC transformer without the low
permeability layers. (The figure shows a 2-D axisymmetric
cross-section for one half of the transformer.) Note that the
design possesses an interleaved winding structure (primary
sandwiched between secondary windings) and an 8:1 turns
ratio. The image clearly shows the poor flux linkage with the
secondary, with a large portion of the flux traveling through
the regions of the transformer that contain the secondary
windings. Figure 7 provides FEMM output for the same
transformer in Figure 6, with the low-permeability layers
applied over the windings. The structure possesses noticeably
improved coupling. The inclusion of the low-permeability
layer over each winding layer creates a higher reluctance
magnetic path through the winding regions. The flux
therefore prefers the low-reluctance core path, thereby leading
to a considerable improvement in the coupling.

Figure 5: Processing steps for building LTCC ferrite devices
with good magnetic coupling.

Figure 6: Output from a FEMM model showing poor
coupling for an LTCC transformer without low-permeability
dielectric over the winding layers. A large portion of the flux
traverses through the secondary windings. The primary
current was set to 1A in the model.

Similar to the formation of the high-reluctance path
through the windings regions, the low-permeability dielectric
can also provide a method for incorporating a high-reluctance
path through the core. This provides the same benefits as
adding an air gap to a conventional wirewound flyback
0
1
2
3
4
5
6
7
8
9
10
10,000 100,000 1,000,000 10,000,000 100,000,000
Hz
3203Y
3204Y
3205Y
3206Y
3207Y
3208Y
Δ
Capacitance vs. Temperature
‐50.0
‐40.0
‐30.0
‐20.0
‐10.0
0.0
10.0
20.0
30.0
40.0
50.0
‐60 ‐40 ‐20 0 20 40 60 80
°
C
 % change
3203Y
3204Y
3205Y
3206Y
3207Y
3208Y
a) b) c)
d) e) f)
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2010 Electronic Components and Technology Conference
transformer; namely, it prolongs the onset of saturation and
allows for increased energy storage. In the case of the LTCC
transformer, the gaps are completely monolithic and
embedded between the ferrite tape layers.

Figure 7: Output from a FEMM model showing the improved
coupling obtained when including a low-permeability
dielectric layer on top of each winding print. The dielectric
layers channel the flux to the center core area. The primary
current was set to 1A in the model.

For more details on the design of transformers with good
turn coupling, see [9]-[12].
Note that the same approach can be used to construct
inductors with good turn coupling. Prior work has shown that
coils embedded directly in LTCC ferrite possess little turn
coupling and the resulting inductance hence scales with the
length of the coil [8]. However, given the planar structure of
an LTCC inductor and process limitations such as line
spacing for the screen printed coils, highest inductance
density may actually be achieved by maximizing the length of
the coil, such as with a meander design [37].
Passive Integration
As mentioned previously, capacitors can readily be buried
within a multilayer stackup constructed using the LTCC
materials. Options for the capacitor material include both the
ferrite and dielectric tapes since the NiCuZn ferrite tapes
possess sufficiently high insulation resistance for some
applications. Figure 8 shows a layout for simple double plate
structures that were integrated within a transformer using the
200µ ferrite tape as the dielectric. The application possessed
low capacitance requirements (picofarads) but needed high
breakdown capability; therefore, two layers of the ferrite tape
(~ 100 µm fired thickness) served as the capacitor dielectric
layer, which provided sufficient voltage standoff. Figure 9
contains a picture of a fabricated transformer that incorporates
the buried capacitors. The capacitance at 4 MHz for the two
structures measured 6.51 and 18.64 pF (2R1 and 6R1,
respectively, from Figure 8). Note that design considerations
for buried capacitors include the as-fired tolerance. Based on
process limitations, the tolerance of an embedded capacitor is
limited to ±30%. If more precise capacitor tolerance is
required, a capacitor could have an external plate exposed as a
“trimming” feature.


Figure 8: Schematic drawing of a transformer with simple
double plate structures embedded in the multilayer structure.

Figure 9: Fired LTCC ferrite transformer with buried
capacitors, based on the designs shown in Figure 8.

Work was also conducted to integrate resistors with the
LTCC ferrite materials. This effort utilized commercial
resistor pastes available from Heraeus. Attempts to co-fire
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ferrite with the commercial resistor pastes did not yield good
resistors. The best results used 2 layers of screen printed
dielectric (separately fired) to insulate the resistor paste from
the ferrite substrate (this process requires 3 post firings). The
post-fired resistors showed robustness to humidity, and
additional overcoats can be used for increased environmental
protection. When post fired onto an LTCC ferrite substrate,
high value resistor designs must take into account the ferrite
insulation resistance since the parallel resistance from the
substrate can be comparable to the desired resistor value. The
effect of temperature on the insulation resistance must also be
considered. Figure 10 shows a plot of the insulation
resistance versus temperature for two conductor traces with 3
layers of ferrite insulation between them. The resistance falls
off to 100 MΩ at 80 °C, which corresponds to an 80X
reduction for the 400µ ferrite tape. A sintering aid or flux
used in the ferrite tape may be the source of this drop-off with
temperature.

Figure 10: Insulation resistance versus temperature measured
across two conductor traces separated by 3 layers of fired
ferrite tape. Three different samples were measured.

Figure 11 shows a picture of a resistor divider integrated
onto an LTCC transformer. The picture highlights surface
termination connections to the primary (P1, P2), secondary
(S1, S2) and resistor divider (C1, C2). The resistor divider
provides feedback used to regulate the output voltage across
the secondary for a flyback dc-dc converter. Note that C1
connects directly to S1, and C2 provides the feedback signal
to the converter electronics. Because of the high resistance of
this divider (> 100 MΩ) the design required floating the C2
terminal. Otherwise, the combination of the ferrite substrate
insulation resistance in parallel with the C1-C2 resistance
would result in the divider ratio fluctuating over temperature
(recall Figure 10). The divider was also actively laser
trimmed with the required regulation voltage applied across
C1 and S2, which compensated for any change in the divider
resistance due to voltage (i.e., voltage coefficient of
resistance). Resistive divider ratios with tolerances as good
as ±1% were achieved. Although not incorporated for the
divider shown in Figure 11, fully floating C1 using dielectric
paste provided significantly improved temperature
performance since it helped isolate the high resistance divider
from the ferrite substrate insulation resistance.
Figure 11: Post-fired high value resistor divider integrated on
top of a multilayer LTCC ferrite flyback transformer.

Note that no attempts were made to co-fire or post-fire
resistors with the dielectric tape to date. Using the dielectric
tape should help mitigate the issues encountered when
integrating high value resistors with buried magnetic
components.
Integrated Power Converter Example
Using the compatible LTCC materials allows fabrication
of composite structures with integrated, embedded
components to support miniaturization of power electronic
circuits. The ability to co-fire the ferrite with both dielectric
and conductor materials thus offers a unique approach to
making hybrid or system-in-package power electronic
modules and could potentially allow a size reduction for many
commercial dc-dc converter and other power electronic
circuits. Figure 12 shows an example dc-dc converter built
using the flyback topology. A multilayer flyback transformer
is embedded in the substrate, and the ferrite/dielectric
composite includes a cavity for mounting a large surface
mount (SMT) component on the backside as well as topside
metallization for mounting the remaining SMT components.
Note that both the ferrite tapes and the dielectric tape provide
good insulation resistance for surface mounting components
and there are no issues with resistive bypass of SMT
components through the substrate, such as described in [28].
When co-firing dielectric and ferrite materials, a residual
strain caused by mismatch between the dielectric and ferrite
thermal expansion coefficients can result in quenching of the
ferrite permeability due to magnetostriction [28]. The
composite dielectric/ferrite devices characterized in the
present work did not exhibit any apparent quenching of the
ferrite permeability due to residual strain. This could result
from better matching of the ferrite and dielectric thermal
expansion, or from a lower magnetostriction effect for the
NiCuZn compositions used in this study, or a combination of
the two.
0
1000
2000
3000
4000
5000
6000
7000
8000
20 30 40 50 60 70 80 90
Temperature (Celsius)
Resistance (MOhms)
P2
P1
S2
S1C1
C2
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Figure 12: Integrated flyback dc-dc converter built using the
ferrite-based LTCC materials system. A cavity is
incorporated into the multilayer structure for accommodating
the SMT component shown in the bottom of the figure.

Future Work
Although the developed LTCC ferrite materials offer good
performance, other research suggests that significant material
improvements may be possible. For example, work by
Murthy points to possible NiCuZn compositions that provide
increased saturation magnetization (600 mT) and permeability
(600) and improved insulation resistance (> 10
14
Ω•cm) [24].
Since no additional sintering aids or fluxes were added to the
NiCuZn formulations characterized by Murthy, these
compostions may also allow increased density of the magnetic
phase when designed into an LTCC material system.
Improved temperature dependence of the ferrite insulation
resistance should also be possible with slight material
modifications to the existing tape system.
Summary
A new ferrite-based LTCC materials system allows co-
firing of ferrite, dielectric and conductor materials to
construct composite structures with embedded magnetic and
capacitive components. The dielectric and ferrite materials
possess good insulation properties and hence work well as a
substrate material for attaching or housing semiconductor
components and other discrete devices that are part of the
power electronics system. Overall, this materials system can
enable new system-in-package approaches for power
electronic applications.


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