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The pros & cons
of toroidal
power transformers
This article describes how toroidal power
transformers can yield lower hum, less weight
and size, and improved efficiency compared
with conventional E-I laminated transformers.
The disadvantages include slightly higher cost
and greater inrush currents at switch on.
By MICHAEL LARKIN*
Why choose a linear power supply?
Since most digital/analog circuits
cannot operate from rectified, filtered,
power line voltage, a step-down conversion unit must be used. Electronic
equipment designers have two main
methods for powering their equipment: switching and linear supplies.
If the product’s operational and
environmental constraints permit high
levels of radiated and conducted EMI
(electromagnetic interference), slower
closed-loop response to load variations and reduced reliability, then the
cost, power density and efficiency of a
switching supply becomes attractive.
By many designs cannot tolerate
the characteristics of switching supplies, making linear supplies the only
viable alternative. Examples of these
products include high quality audio
mixers and amplifiers, lighted matrix
displays, and video processing and
display equipment.
Toroidal transformers are inherently time-consuming and tedious to make. The
total length of wire for each winding must first be loaded onto a bobbin which is
then wound onto the core, together with inter-layer insulation.
12 Silicon Chip
A transformer is required to step
down the AC voltage from that of
the power line to the rectification,
filtering and regulation circuits in a
linear supply.
The inherent advantages of the
toroidal (donut-shaped core) transformer, relative to other core configurations, may be summarised generally
as a nearly ideal magnetic circuit,
which results in lower stray magnetic
field, smaller volume and weight, less
audible hum and higher efficiency.
Which benefits are of interest in a
particular application depend on the
type of product and sensitivity of other
circuitry to stray magnetic fields.
Ideal magnetic circuit
The toroidal transformer has a
nearly ideal magnetic circuit. In an
E-I laminated transformer it is not
possible to align the grain structure
of the stamped laminations with the
flux path over the entire magnetic
path. This inability leads to higher
core losses and less efficient operation
when compared to toroids.
Fig.1 shows a comparison of grain
alignment with flux path for a toroidal
and an E-I transformer.
Conventional laminated transformer designs employ a bobbin-wound
coil placed over a stack of “E” shaped
laminations. An “I” shaped stack
is butted onto the “E” , completing
the magnetic path. The connection
between the “E” and “I” is never a
perfect junction, causing a discontinuity or air gap in the magnetic flux
path. This gap has higher reluctance
and so causes a greater radiated magnetic field.
Similarly, in “C” cores, where strip
steel is wound into an oblong shape
then cut into two identical “C” shapes,
air gaps are present at the junctions
where the cut faces of the “C” pieces
meet to complete the magnetic circuit.
In any core with a gap, the properties
of the gap are unpredictable and depend on pressure and the quality of
the mating surfaces.
A second feature which gives rise
to leakage flux in E-I and C-core
transformers is the discontinuity in
the windings which surround the flux
path. The windings are concentrated
in short regions of the laminates,
which leaves large portions of the flux
path exposed.
The abrupt transition from windings
to bare laminates creates opportunities
for the flux to escape the confinement
of the core and form linkage paths outside the transformer. The transitions
in the windings can also lead to high
leakage inductances for the device.
By contrast, there is no air gap in the
core of a toroidal transformer. The core
is tightly wound onto a mandrel, like a
clock spring, from a continuous strip
of grain-oriented steel. Spot welding
at the beginning and ends prevents
loosening. The stresses introduced by
de-reeling and winding, which could
result in unacceptable core losses, are
relieved by annealing the wound core
in a dry nitrogen atmosphere. The result is a stable, predictable core, free
from discontin
uities, holes, clamps
and gaps.
Fig.2 compares the stray magnetic
field at 100mm from E-I laminated and
toroidal transformers of equal power
rating. If shielding with a copper strap
and careful orientation of the E-I power transformer and sensitive devices
can improve stray field immunity
sufficiently, without undue expense,
then a toroidal transformer may not
be justified.
However, the toroid’s substan
tially lower stray field may mean the
difference between accept
able and
unacceptable operation of sensitive
circuitry.
Reduced weight and size
In the E-I core structure, the magnetic flux is not aligned with the grain
of the steel for approximately 25% of
the flux path (see Fig.1). This misalignment causes higher magnetisation
losses and reduces the maximum flux
density that can be utilised in the core.
Higher efficiencies have been made
possible by using high grades of
grain-oriented steel which increases
flux densities while minimising loss-
Fig.1: comparison of grain alignment with flux path for a toroidal and an
E-I transformer. In the toroidal transformer, the grain alignment is always
optimum.
Fig.2: this graph compares the stray magnetic field at 100mm from E-I
laminated and toroidal transformers of equal power rating. If the E-I
transformer has a single section bobbin (or no bobbin) it may be possible
to fit a copper shielding strap to greatly reduce the stray field.
es. However, maximum utilisation of
these properties occurs only when the
flux in the steel is parallel to the grain
direction.
It can be seen in Fig.1 that the flux
in a toroidal core is 100% aligned
with the grain of the steel. The typical
working flux density of E-I laminates is
from 1.2 to 1.4 Tesla, whereas toroids
typically operate from 1.6 to 1.8 Tesla.
For a given core cross-section, the
voltage induced in a winding is directly proportional to the flux density
and the number of turns. The higher
allowable flux density of a toroid thus
requires fewer turns of wire in all
windings to achieve the same result.
Comparison of a conventional
960VA transformer with an equivalent
toroid shows the weight and volume
of the toroid to be only half.
In an existing product which uses
an E-I transformer, it is often possible
to fit a toroid which has close to the
same footprint but is only 60% as
tall. In the case where it is desirable
to increase the power supply power
rating without increasing its size, the
E-I transformer might be replaced with
a toroid which is the same size but has
1.5 to 2-times the power rating.
It is true that the empty centre hole
in a toroidal trans
former, which is
needed to enable winding, occupies
some wasted volume. This wasted volume deficit is overcome by the toroid’s
December 1995 13
Fig.3: this circuit can
be used to substantially
reduce the inrush current
for a toroidal transformer.
The relay coil is energised
by the AC input voltage
and the delay in the
relay operation, which
switches R1 out of circuit,
is sufficient to reduce the
inrush current to a safe
value.
volume advantage at roughly 50VA
and above. So, in transformers rated
less than 50VA, the size is not reduced
but the other advantages remain.
Audible hum
Audible hum in transformers is
caused by vibration of the windings
and core layers due to forces between
the coil turns and core laminations and
due to magnetostriction in the core
itself, which is manifest at any gaps
in the core. Clamps, bands, rivets and
welds cannot bind the entire structure
and varnish penetrates the laminations
only partially. As a result, laminations
tend to loosen over time and produce
increasing noise.
On the other hand, the nature of the
toroidal transformers’s construction
helps to dampen acoustic noise. The
core is tightly wound in clock spring
fashion, spot welded, annealed and
coated with epoxy resin.
Audible hum, heard immediately
after application of power, may be
noticeable in the toroid and then die
down to a quieter level a few seconds
after power is applied. This is a result
of the toroid’s greater inrush current,
which is discussed below.
Efficiency
The efficiency of a transformer is
stated as:
E = Pout/Pin
where Pout is the power delivered to
the load and Pin is the power input
to the transformer. The difference between Pin and Pout is represented by
the losses in the core and windings.
The ideal magnetic circuit of the toroid
and its ability to run at higher flux
density than E-I laminates reduces the
number of turns of wire required and/
or the core cross-sectional area. Both
benefits reduce the losses. Toroidal
transformers are typically 90-95%
14 Silicon Chip
efficient, whereas E-I laminated types
are typically less than 90% efficient.
Inrush current
The characteristics which give the
toroidal transformer advantages also
contribute to a disadvantage: high
inrush current with the initial application of power.
The absence of a gap in the toroidal
core means that the maximum possible
remanence (residual magnetisation of
the core in a particular direction and
magnitude) can be substantially more
pronounced in a toroid than in an E-I
type. The core “stores” the static bias
when the power is switched off.
If the removal of power occurs at an
unfavourable time, the strongest magnetic remanence will be stored in the
core. When power is again applied to
the primary , the peak inrush current
may be as great as Vpk divided by Rp,
where Vpk is the peak primary voltage and Rp is the primary resistance,
depending on the power capability of
the transformer and how strongly the
core was magnetised.
To cope with these very high surge
currents, a fuse or circuit breaker with
an appropriate time delay is needed;
a fast blow fuse will not last for more
than a few off-on power cycles.
In high power applications, more
exotic means may be re
quired to
ensure protection that will survive
inrush, yet still protect in fault situations. One method involves a relay
with its coil across the switched
power line. Prior to application of
power, a resistance is present in series
with the transformer primary. After
power is applied to the relay coil and
transform
er, the electromechanical
relay begins to move from the deener
gised to the energised contact
position.
If the relay takes long enough to
operate, then the inrush current has
been limited by the series resistor
and the core’s magnetic bias has been
eliminated. An example of this circuit
is shown in Fig.3.
Higher cost
Toroidal transformers are manufactured individually and have a high
labour content. Conversely, the plastic
bobbins of small E-I laminated transformers can be wound on machines
which handle several bobbins at once
and operate nearly unattended.
The process of applying inter-winding insulation is also more labour
intensive in toroidals. E-I laminate
insulation consists of one wrap of
adhesive tape or Kraft paper, whereas insulation in toroids is applied in
an overlapping spiral fash
ion. This
conforms best to the curved surfaces.
The difference in labour content is
reduced for power ratings of 500VA
and above. Large transformers are
usually made in small quantities and
become more difficult to wind as the
core becomes larger and the wire
gauges thicker.
3-phase toroidal transformers
Employing toroidal construction for
3-phase transformers does not offer a
volume advantage over the E-I laminated type. The E-I configuration is
more suited to 3-phase transformers,
since the three legs of the E portion
of the core can be used for the a-b-c
phase windings and flux is then efficiently (except for the aforementioned
non-alignment of flux to steel grain)
linked a to b, b to c and c to a.
Providing a 3-phase transformer
using standard toroidal cores and
winding techniques requires three separate transformers. This is inefficient
use of volume. In some applications,
where a very low profile transformer is
required and the real estate for 3-phase
transformers is available, a toroidal
3-phase transformer set is beneficial.
Summary
The choice between toroidal and E-I
laminated transformers depends on
the application. Toroidal transformers
offer low stray fields, smaller size and
weight and higher efficiency, which
may be required or desirable in many
products.
*Michael Larkin is the Managing DiSC
rector of Tortech Pty Ltd.
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