COMPARISON OF PLANAR AND WOUND TRANSFORMERS FOR FLYBACK
FORWARD AND HALF-BRIDGE SPACE POWER CONVERTERS
Thomas Björklund (1) John Andreasen (2) Finn Brøsen (3) Erik Matthiesen (4) Ole Poulsen (5)
(1) Magnetics Engineer, Flux A/S, Industrivangen 5-11, DK 4550 Asnaes (Denmark), tb@flux.dk
(2) Magnetics Engineer, Flux A/S, Industrivangen 5-11, DK 4550 Asnaes (Denmark), ja@flux.dk
(3) Power Engineer, Flux A/S, Industrivangen 5-11, DK 4550 Asnaes (Denmark), fb@flux.dk
(4) Power Manager, Flux A/S, Industrivangen 5-11, DK 4550 Asnaes (Denmark), em@flux.dk
(5) Technical Manager, Flux A/S, Industrivangen 5-11, DK 4550 Asnaes (Denmark), op@flux.dk
ABSTRACT
Planar technology has now entered the space domain.
The big advantages of planar technology are;
•
•
•
•
•
Low profile
Excellent repeatability
Economical assembly
Mechanical integrity
Superior thermal characteristics
This is why the general power industries increasingly
are using planar magnetics in more and more
applications, and therefore also why we see a rising
demand for the usability of the planar technology
among space application developers.
The differences between wound and planar transformers
have been mapped with a detailed look on the various
parasitic component values, such as DC- and ACresistance, Leakage Inductance and stray capacitance,
and revealed the magnitude of the advantages of planar
technology.
This technical solution is proven in prototypes that have
been built in different combination of PCB’s and copper
foil, with more or less interleaving of windings.
Furthermore the transformers have been designed with
several outputs stacked together with a fairly high
number of primary turns, in order to have planar
transformers similar to the wound types that are
generally used for space applications.
demands regarding the thickness of copper, laminate
tolerances and a strict control of etching the thick
copper foils and laminating with minimal resin, in order
to get a good fill factor in the final transformer.
Using PCB as a part of a component, is breaking with
the general thinking in the technologies of space
qualified PCB - and this divagation is necessary in order
to achieve functionality that can compete with
traditionally wounded transformers. Where high current
is occurring and thicker copper layers are needed,
folded copper foil windings makes a great alternative to
the PCB, and this pioneering mix of technology within
planar transformers, gives a whole new flexible design
frame for the space engineers.
Quality assurance is of utmost importance for the planar
transformer, where thermal drain, z-axis expansion and
via or Plated Through Hole (PTH) reliability, during
thermal cycling, are some of the most critical points. It
needs to be at the same low risk as traditionally wound
transformers, so tests and verification of functionality
out of the normal ECSS specified frame of use is
needed. This points to pioneering thermal tests that can
verify the life terms of the space planar transformer. I.e.
passive burn-in, power burn-in and thermal shock, but
also endurance testing to show the limits of this new
component technology.
The different topologies are selected to represent three
different usages of the ferrite shown in Fig. 1, Fig.2 and
Fig. 3
INTRODUCTION
Obtaining the optimal design has been impossible when
engineers have developed printed circuit board (PCB)
windings using the frame of The European Cooperation
for Space Standardization (ECSS) standards for space
qualified processes. Where the requirements towards the
PCB manufacturers are driven in two main directions;
fine pitch technology for microprocessors and
impedance control for Radio Frequency (RF) domain
circuits, which is also reflected in the Interconnecting
and Packaging Electronic Circuits, now the Association
Connecting Electronics Industries (IPC) standards. Here
the Planar technology comes with greatly different
Fig. 1 B-H Curve of a flyback transformer
d) Tape & PCB vs. Coil former
Using a standardised bobbin or coil former, gives a high
reliability component since it is well tested with regards
to creepage and clearance distances. Tape and PCB
structures gives an individual design each time with
tolerances that needs verification testing dependent on
the tolerances.
Fig. 2 B-H Curve of a forward transformer
Fig. 4 Planar EE18 transformer compared with RM5
(Left side), and Planar EE22 transformer compared
with RM10 (Right side)
1.
Fig. 3 B-H Curve of a half-bridge transformer
2.
THE TRUE COMPARISON
The optimal design for transformers is normally set to
be the balance of core and winding losses Fig.5, here
planar transformers can be designed towards a higher
core loss in order to gain less winding loss, because of
the better thermal drain possibility.
When comparing a standard transformer with a planar
transformer there are actual four different points of
comparison:
a)
THERMAL PERSPECTIVE
EELP vs. RM
Here the core shapes are different in standard sizes.
Designing the winding structure sets the criteria for the
needed window, which results in selecting a bigger core
than initially needed. RM cores enclose the windings
better than the EE cores, which gives a better EMI
performance.
b) PCB vs. Wire
Here the PCB is limited in winding-techniques and has
a thermal expansion factor, where the wire can be
wound in wrong number of turns from time to time, and
has pulling and bending issues.
c)
Fig. 5 Power loss balance
(1)
Flat vs. Round
A thin flat conductor doesn’t suffer from skin effects in
high frequency alternating current (AC), like a round
conductor. But Flat conductors couples better to each
other hence generates a larger parasitic capacitance.
(2)
There is also the possibility of better thermal drain of
the winding structure, when using PCB for windings.
3.
MECHANICAL INTEGRATION
One of the most obvious advantages of planar magnetics
is the different possibilities of integrating the transformer with the converter PCB. Stand-alone is the
common use of wound transformers, but sunken wound
transformers are also seen.
The winding construction of a PCB transformer can also
never reach as high a mass density as the wound transformer. This is due to the maximum possible fill factor
Fig.8 and the material density Fig.9, where polyimide
and plastics are lighter than copper.
Fig. 8 Typical Max Fill factors (Ku) for standard and
planar magnetics
Fig. 9 Density of transformer materials
Fig. 6 PCB mounting options
Planar transformers are often integrated, but here the
tradeoffs between PCB layer build-up and transformer
performance becomes an issue. Even a small modification in the transformer, demands for a whole new
layout, where a stand-alone model easily can be
prototyped to the right fit, by removing or adding a
single turn.
A big advantage with stand-alone transformers is the
possibility of using several multilayer PCBs stacked
together, and even in combination with foil or wire
windings. Here the Hybrid model also has advantages,
when high secondary current can be placed in thick
copper foil combined with primary PCB windings, or
when multiple primary windings can be placed in thin
wire combined with the lower number of turns
secondary PCB windings in the converter board.
This points on a more mechanical robust transformer,
however only if they are constructed with the same size
of core and same window area. It will then give a higher
current density in the planar transformer, and this is also
often the case in the design as the thermal transfer is
more efficient.
4.
PCB TOLERANCES
When designing transformers, it is very important to
take thermal expansion into consideration due to the
fragile core that easily breaks in stressed conditions.
Introducing PCB is actually rather troublesome, as the
glass woven fibres are preventing x-y expansion and
instead transfer all the thermal expansion to the zdirection. Which is exactly what is needed for a stable
Surface Mount Device (SMD) on a PCB, but absolutely
a bad idea for fitting into a transformer core.
3.1. Fill Factor and Robustness
When looking at the RM wound transformer and the
EELP PCB in Fig.7 from a vibration point of view, it is
clear that the Centre of Mass Point is lower on the
planar core, as the core is lower. This gives a more
robust mechanical construction, despite of the windings.
Fig. 7 Centre of Mass Point
The core dimensions comes with big tolerances of
window height shown in Fig.10 for EELP18 which is
±5%, but it is worse for the final PCB where the tolerances is ±10%.
Fig. 10 EELP18 Core tolerances
Examining the manufacturing process reveals that the
laminate and prepreg process is dependent on the resin
content, which depends on the heat and flow when
filling the bare woven fibre sheets, where the viscosity
also depends on the stock-age of the resin. For a 100µm
laminate the tolerance is as large as ±25% according to
IPC as shown in Fig.11
clearance. In Fig.14 is shown the microsection of the
EELP18 transformer windings. The ECSS standards
states max ±100µm.
Fig. 11 Raw laminate tolerances
Additional the copper plating also has tolerances and
since copper is expensive, the manufacturer aims very
precise on the minimum tolerances Fig.12.
Fig. 14 Flyback transformer microsection
5.
DIELECTRIC CONSTANT
When it is mentioned that the Planar transformer
repeatability is excellent, it is needed to take a deeper
look into predicting the parasitic capacitance according
to production tolerances.
The variations of dielectric constants for polyimide PCB
Fig.15 appears when the resin has a lower value than the
fibre glass, and the resin content varies.
Fig. 12 Innerlayer copper thickness
When plating the drilled through holes, extra copper is
added to the entire surface. This gives an additional
thickness Fig.13
Fig. 15 Dielectric constant and resin content of prepreg
Calculations due to laminate thickness tolerances and
dielectric constant variation results in huge differences.
Fig. 13 Outerlayer copper thickness after plating (class
3 is for space and aerospace)
A final thickness of a 70µm winding is therefore in the
area of 55,7µm to 83,7µm depending on the PTH
structure.
Fig. 16 Capacitance variations of laminate thickness
Fig. 17 Capacitance variations of moist content
4.1. Milling and registration
Other PCB tolerances, such as milling of the edge and
registration of the layers in-between are also the cause
for poor fill-factor, and this is a critical point regarding
Polyimide is hygroscopic, and water has a dielectric
constant of 80,40, so within the max accepted value of
0,28% the variation is a few extra pF as in Fig.17
6.
EMI AND EMC OF CORE SHAPES
Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) are the unintentional generation, propagation and reception of electro-magnetic
energy of the transformer interacting with the surroundings. This is the almost impossible part to calculate
when designing the prototype, and it very much depends
on the core shape and usage.
6.1. Induction formulas
When looking at the standard textbook formulas for
transformers and how they are constructed it appears
that the core-shape differences are not a part of the
calculations. The induction formulas are derived from
two different systems, where the primary system is
based on a magnetic field from an endless long straight
lin shown in Eq.3-5 And Fig.20 and the self-inductance
is based on a toroidal solenoid Eq.9-10 And Fig.22
The energy propagates in a combination of magnetic (Bfield) and electric (E-field) radiation as shown in Fig. 18
Fig. 18 Electromagnetic(EM) wave
The waves are only polarised in the Far-field area,
which is more than 2 wavelengths from the source, and
at 125kHz the wavelength is 2.4km long. In the Nearfield the EM wave is much more complex and can
therefor not be measured exactly on the two kinds of
transformers.
Fig. 20 Endless long line
The E-core shapes in general are open and the windings
are wound outside the core structure. The Pot-core is
very closed and has the best EMI performance, however
the closed core makes it difficult to enter and exit the
windings. The RM core is therefore a fairly good
solution for EMI shielding of the windings. Fig. 19
shows the differences drawn with a Mean Length Turn
(MLT).
(3)
(4)
(5)
Figure 21 reveals that the distance 2 x, where x is the
radius, is the same as the Magnetic Path Length (MPL),
when calculating the transformer.
Fig. 19 Pot-, RM- and EELP cores with windings and
B-field lines indicated (Blue arrow)
Then of course there is the factor of relative ferrite
permeability µ r vs. the vacuum permeability, which is
also the permeability of air µ 0. In N87 and 3F3 the
factor is around 2000.
Fig. 21 relationship of 2 r and MPL
In the Ferroxcube handbook, Eq.6 Is given for the
magnetic field strength based on the effective MPL
denoted le. (Square root 2 is used only for a sinus)
7.
TEST RESULTS
The prototypes in Fig. 4. are defined by the winding
turns (T) shown in Fig.24
RM5
(6)
*
EELP18
*
31T
(7)
(8)
3T
16T
*
*
7T
Where Eq.7 and Eq.8, From the Fundamentals of Power
Electronics theory book, shows that this is based on a
straight endless line.
The principle of a toroidal solenoid Fig.22 gives the
base for equations where the magnetic path length is a
circle.
2T
5T
Fig. 24 Flyback Transformer turns ratio
The mass budget is:
RM5: 5,12g of which the core and coil former is 3,61g
EELP18: 8,01g of which the core is 4,81g
Dimensions are:
RM5:
H=10,52mm W=15,81mm
EELP18: H= 8,09mm W=19,23mm
L=15,95mm
L=22,52mm
Measurements have been performed on the prototypes
revealing the magnitude of the design.
Fig. 22 Toroidal solenoid
(9)
(10)
When 2 r equals the MPL, it is only true for toroidal
cores. Not for RM and EELP core structures. Hence the
differences in shape is ignored in the transformer
calculations Fig.23.
Fig. 25 Flyback Transformer measurements
Fig. 23 Transformer equation (MPL is denoted lm)
When the shape of the magnetic path length is ignored,
the contribution, due to one or another shape, cannot be
defined.
Comparison of the measurements Fig.25 shows that
they are overall the same, but the planar transformer has
lower leakage, which is around half of the standard
transformer, but also around four times as large capacitance’s. These values are also the commonly expected.
The prototypes shown in Fig. 4. are defined by the
windings in Fig.26
RM10
*
EELP22
*
5T
3T
4T
*
3T
*
7T
7T
5T
Comparison of the measurements Fig.27 shows that the
two forward transformers over all are the same.
However here the wound transformer has lower leakage
and higher capacity than the planar transformer Fig.28.
This was anticipated, but not the most intuitive result.
7T
4T
7T
Fig. 26 Forward Transformer turns ratio
Fig. 28 Forward Planar Transformer
The mass budget is:
RM10: 33,75g of which core and coil former is 27,97g
EELP22: 21,74g of which the core is 12,96g
Dimensions are:
RM10: H=18,61mm W=29,61mm
EELP18: H=11,76mm W=24,15mm
The reason for having a better performance on the
wound transformer lies within the possibility of more
complex interleaving structure Fig.29 and Fig.30.
L=39,50mm
L=33,96mm
Measurements have been performed on the prototypes
to reveal the magnitude of the design.
Fig. 29 Interleaving structure of Planar Transformer
Fig. 30 Interleaving structure of RM Transformer
(T is short for Turn)
7.1. Measuring tolerances
When measuring the leakage, it is ideal to short the
secondaries, so that only the none-coupled inductance is
measured Fig.31. Shorting with a wire adds inductance
to the circuit, hence the measurement has a large
tolerance relating from the inductance of the short.
Fig. 27 Forward Transformer measurements
Fig. 31 Simple and complex attempt to achieve a perfect
ac short
8.
DISCUSSION
For both traditional and planar transformers, it is
possible to have different structures of interleaving,
which is a choice of leakage vs. stray capacitance.
Prototyping the planar transformer is very costly, but
here simulation SW really comes in handy.
•
•
•
Low building height
Mechanical integrity
Superior thermal characteristics
- The drawbacks are; EMC and winding area
When looking on introducing PCB windings, the thin
layers and photo-etching process has its advantages:
•
•
•
•
•
Low building height
Excellent repeatability
Economical assembly
Mechanical integrity
High frequency performance (to some extend)
- The drawbacks are; prototype costs, reduces possible
number of turns. PTH or VIA reliability. Moist sensitivity, thermal expansion and production tolerances.
10. REFERENCES
Fig. 32 Test converter mock-up
Test converters Fig.32 has been built so that the RM
transformer and the EELP transformer can be swapped.
This gives the possibility of testing load and efficiency
capabilities, and the general picture is as expected; the
one with lower leakage performs better. But as they
were designed on the same criteria they are overall the
same on performance.
Simulating the transformer makes it possible to predict
the leakage and stray capacitance by Finite Element
Analysis (FEA) but here tolerances are not included,
and Monte-Carlo plots is not possible due to standard
Central Processing Unit (CPU) performance.
1. Robert W. Erickson & Dragan Maksimovic.
Fundamentals of Power Electronics, Springer; 2nd
edition (January 2001).
2. Colonel Wm. T. McLyman. Transformer and
Inductor design Handbook, Third Edition (2004)
3. Soft Ferrites and Accessories Data Handbook.
Ferroxcube (2008)
4. Hugh D. Young, Roger A. Freedman. University
Physics with Modern Physics with Mastering
Physics. Addison Wesley; 11 edition (August 8,
2003)
5. Generic Standard on Printed Board Design. IPC2221A, IPC (May 2003)
6. Chet Guiles, Rancho Cucamonga. Everything You
Ever Wanted to Know About Laminates, but Were
Afraid to Ask. Arlon-Materials for Electronics, 9th
Edition (Nov 2008)
Fig. 33 Capacitance model of a transformer
The EMC performance of the actual core can neither be
calculated nor measured precisely. The only way of
verifying the differences, due to core shapes, is to
design a PCB for each converter, with individually
optimised snubber circuits due to the different pin
positions on the RM and EELP transformer and the
layout. This part must be an issue that needs to wait for
a future study.
9.
7. Rafael Asensi, Roberto Prieto, José A. Cobos, and
Javier Uceda. Modeling High-Frequency
Multiwinding Magnetic Components Using FiniteElement Analysis. UPM, División de Ingeniería
Electrónica, Madrid 2008
Thomas Björklund
Magnetics Engineer, Flux A/S
www.flux.dk
CONCLUSION
When looking on the core structure, the lower and plane
core has its mechanical and thermal advantages:
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