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Transformers
have been called "Magical Black Boxes." Transformer
engineers have been accused of practicing "Voodoo Magnetic."
In reality the transformer is the product of nineteenth
century physics. It is a passive device that performs
fundamental circuit requirements. It changes voltage,
current or impedance to circuit needs. Protecting end
users, the transformer performs the isolation required
by the international safety agencies.
The Magnetic Circuit: The classic iron filings
experiment demonstrates that a magnetic field or flux
exists in the space around the conductor carrying current.
This gives the appearance of spaced circular lines.
This is simply the effect on the iron filings sprinkled
on paper. There is no sharply defined limit to this
field, nor does it exist in lines. The field around
the coil can be thought of as "flow" of flux in the
area surrounding the coil. The magnitude of this flux
flow is determined by the product of current and the
number of turns in the coil plus the magnetic conductivity
of the area surrounding the coil. The property of flux
conduction is called permeability. The force required
creating the flow is called magnetomotive force. The
changing magnetic field is the basis of a transformer.
The magnetic field must be changed to induce current
flow into another winding. Transformers are single application
devices. Design does not offer good performance over
a wide range of operations. The transformer is a poor
performer in circuits requiring highfidelity reproduction
of audio or video signals. Transformers pose problems
to equipment in which size and weight are a premium.
A circuit that provides DC bias to the core seriously
degrades the transformer properties.
A transformer may come in many different shapes and
sizes. Both size and shape are dependent upon function.
It is composed of a core, windings and insulation. The
core size, shape and material depend on power requirements
and frequency of operation. Core types include:
Laminations: Multiple vendors supply silicon
iron and nickel alloys in stamped letter shapes like:
"EE", "El", "EL", "F" and "UI". These materials
are generally used for line frequency power magnetics.
Nickel alloys are used for audio and telecommunication
transformers.
Ferrites: Ferrites are ceramic magnetic materials
comprised of ferric oxide and combinations of manganese,
zinc or nickel. These different materials are used within
audio to megahertz frequencies. Multiple vendors supply
these materials in varying shapes and sizes. Telecommunications,
pulse and low power applications use the following shapes:
Pot Core, TouchTone Core, "RM" and "EP." High frequency
power utilizes the following shapes: "EE", "UU", "PQ"
and "ETD." RF frequency magnetic shapes generally are
Rods and Beads.
Toroids: The toroid, a circular non-radiating
magnetic structure, is popular for its relatively low
material cost and size. The largest selection of magnetic
materials is available in this toroidal shape. These
materials include: round and square loop silicon irons,
round and square loop nickel alloys, metallic glass,
cobalt alloys, ferrite and powdered irons. Toroids are
ideal for current sense transformers and non-radiating
power transformers.
Winding:
Copper is the conductor of choice in the manufacturing
of transformers. Magnet wire is extruded copper, coated
with various temperature class insulation. The wire
size is based on the American Wire Gauge (AWG). Wire
sizes vary from 0000 AWG (.460" dia.) to 50 AWG (.00099"
dia.). Copper foils of varying widths and thickness
are also used. Multiple strands of magnet wire are twisted
together to create larger conductors, called litz wire.
The size of wire used is determined by the amount of
current that it has to carry. House wiring is 12 and
14 AWG. The turn count of a winding is dependent on
the core selected, voltage applied and type of wave
form.
Insulation:
The temperature of operation determines the insulation
requirement. Insulation systems range from 105 degrees
C to 220 degrees C. Thermoset and thermoplastic materials
are used in the manufacture of bobbins to hold the wire.
Mylar, Kapton and Nomex are used as insulation within
a winding. These materials isolate one winding from
another. The dielectric withstanding voltage (hipot)
determines the amount of insulation. As the voltage
increases, so does the insulation requirement. A transformer
is essential for line voltage alterations in power applications.
Its ability to isolate circuits cannot be conveniently
matched with alternative methods. Transformers are extremely
rugged devices, capable of withstanding severe environmental
conditions. Once in service a transformer will function
for many years.
Linear Power Transformers
Linear power transformers generally operate within the
frequency range of 47 to 400 Hz from sinusoidal AC power.
This is the first component between the power system
and your equipment. These devices, call isolation, step-up,
step-down or rectifier, all function by altering the
voltage and/or current necessary for the system. They
also isolate the end user from the power source: a wall
socket or generator, as in aircraft power systems.
The major consideration of power transformer is efficiency.
Power losses are due to the core loss and the winding
resistance. Both contribute to the economics of the
system. The heat generated by these losses must be considered.
The core loss is determined by the core material and
the specific design. The winding resistance times the
square of the current produce copper loss. The common
definition of efficiency is the power output divided
by the power input. When applying this definition to
a transformer the power factor must be stipulated as
one (1). If the load is resistive, then the current
in the load is in phase with the voltage across it.
The voltage dropped across the load, multiplied by the
current in the load produces watts. This power is referred
to as true power.
If the load is reactive (capacitive or inductive) the
voltage and current are no longer in phase. A transformer
winding always has resistance. The reactive current
of the load is in phase with the voltage dropped across
the winding resistance. The phase agreement causes excess
power to be consumed within the windings. This power
loss is a major limiting factor in transformer ratings.
This loss can occur even if the load is not consuming
watts, but only out of phase volt-amperes. The ratio
of watts to volt-amperes, or true power to apparent
power, in a load is the power factor. The volt-amperes
can never be less than the watts: The power factor must
then be one (1) or less. When the power factor is one
(1) then the load is purely resistive and consumes watts
only. If the power factor is less than one (1), then
the load is partly reactive and volt-amperes must be
considered. True transformer efficiency is voltampere
output divided by volt-ampere input plus transformer
losses. With this definition a transformer can theoretically
reach efficiencies of 100%. The practical limits to
efficiency, however, are size, weight and cost. Power
factor also leads to a brief discussion of rectified
outputs. Linear power supplies use capacitive input
filters. This type of filter will increase the current
within the winding up to two times the DC current. This
is a factor to be remembered when specifying your transformer.
The use of an inductive or choke input filter yields
a unity ratio between DC current and winding current.
Choke input filters are generally not cost effective
below 1000 VA (volt-amperes).
Switching Power Transformers
Buck, boost, converter and inverter are some of the
common names for switching power transformers. The switching
power supply is the answer to decreasing size and weight
and improving efficiency. The switcher operates from
DC power that is switched at a chosen time rate. The
common switching rate today is 100 Khz to 500 Khz. With
the advancement of magnetic materials and switching
power devices switching speeds are now reaching 1 Mhz.
Switching DC creates the required flux to induce current
into another winding. Switched DC is square wave AC
to the transformer. The switching transformer input
power is supplied by batteries, system DC power or a
rectified AC line. The common topologies in use today
are flyback, forward, push-pull or bridge. Each type
of topology presents requires a different type of transformer.
The transformer is an intrinsic part of a switching
supply. A transformer designed for one topology will
not work for another topology.
Flyback:
The flyback combines the actions of an isolating transformer,
output inductor and a flywheel diode in a single unit.
The results of this magnetic integration provides a
cost effective supply. Realistically the power capability
of a flyback is 100 VA. The flyback transformer requires
a gapped core. During the on-time of the switch, the
energy is stored in the gap. During the off-time this
energy is delivered to the load. Standard transformer
action is not used. In reality flybacks are isolated
storage inductors.
Forward:
The forward operates as a single ended transformer.
Power is transferred to the load during the switch on-time.
The core is reset during the off-time with the use of
the clamp diode and clamp winding. The practical limitation
of the forward converter is 500 VA. A disadvantage of
both the forward and flyback is that power is transferred
to the load on only half of the input cycle. This requires
a larger transformer.
Push-Pull:
Push-pull is not favored for off-line converters because
the power switches operate at collector stress voltages
of twice the supply voltage. At low input voltages the
push-pull is practical for smaller sizes and higher
output power. Pushpull delivers power to the load on
both halves of the input cycle. The transformer must
have a center tapped primary and secondary. Half of
each winding is used aiternativeiy with the input cycle.
Power levels in excess of 1 KVA can be achieved with
the push-pull topology.
Half-Bridge: This converter uses two power switches.
It is popular for off-line applications because the
voltage across the switch does not exceed the supply
voltage. (Half the stress voltage compared to a single
ended forward.) Power is delivered to the load during
both of the input cycles. This topology allows for a
smaller transformer than the forward.
Full-Bridge: The full-bridge requires four power
switches and is reserved for off-line high power applications,
utilizing a single primary. It is driven to full supply
voltage in both directions. Full-wave output rectification
provides excellent utility factor for the transformer
core and windings. The voltage stress on the switches
is sharply defined and does not exceed the supply voltage.
This topology is ideal for an off-line supply delivering
1 KVA or more to the load.
Current Transformers
The current (I) transformer is used as a sensing device.
This type generally uses a one (1) turn (N) primary.
Secondary turns are determined by I1 x N1 = I2 x N2.
The secondary sense resistor of the circuit determines
output voltage. As required accuracy increases so do
material and construction costs. Toroids manufactured
from silicon steel handle most requirements.
Nickel alloy toroids are used in high accuracy applications.
Ferrite toroids are used in current mode controlled
switching power supplies. When a narrow band of current
requires sensing, laminated silicon steel will do the
job effectively.
Telecommunications
Transformers
Transformers that are used in the telecommunication
industry couple the phone line to the product. The coupler
is used to isolate the line from the system. It can
be considered a dielectric fence. Common-mode noise
rejection (longitudinal balance) is another function
of these transformers.
Determining
System Needs
Carefully examine your needs when specifying the transformer
to use in the system. The determining factor is the
efficiency absolutely required. The country in which
the system will be sold determines the supply voltage,
frequency and safety agency requirements. For example,
European transformers operate at higher voltages and
lower frequenciesThese two factors increase size. European
safety agency compliance requires increased insulation.
Environmental conditions such as temperature range,
humidity, shock and chemical exposure should also be
considered. Specify only what you need.
Extra margins increase size, weight and cost.
Tolerance Versus Cost
For transformer design purposes, tolerance is defined
as the variation allowed from a nominal value. How much
variation can your system endure? The tighter the component
values the greater the cost. Tolerance value decisions
should be based on worst-case analysis. Many tolerances
can be single ended (either minimum or maximum). Take
advantage of these whenever possible. All tolerances
should be set with customer-vendor correlation in mind.
Tolerances should never be set tighter than the test
method and precision of measurement. Care should be
taken in defining the method by which the parameters
are measured.
Typical Production Tolerances Open-Circuit Inductance:
Most applications require a minimum inductance only,
the higher the inductance, the better most circuits
work. When this is the specification, it is designed
for no cost penalty. Flyback Transformers and Inductors
require a gapped core, which is be provided with a ±
10% tolerance and no cost penalty. A tolerance of ±
5% or less requires individual tuning and is expensive.
Leakage Inductance: The coil's physical geometry
and the number of turns determine leakage inductance.
A maximum value will satisfy the majority of circuit
requirements. This is assigned after a pilot run on
automated winding equipment. If a ± I tolerance is required
it can be assigned after the pilot run and an agreed
upon measurement method.
Capacitance:
Capacity is determined by a coil's physical geometry
and the dielectric between windings (similar to leakage
inductance). A maximum value will satisfy the majority
of circuit requirements. This is assigned after a pilot
run on automated winding equipment. If a ± tolerance
is required it can be assigned after the pilot run and
an agreed upon measurement method.
Resistance: Resistance is a function of wire diameter
and length. Tolerances for resistance are determined
by the turn count and wire size. When the resistance
of any winding exceeds 10 ohms, ± 10% tolerance can
be held at no additional cost. Tighter tolerances require
specialty wire and increased costs. If the value of
resistance is 10 ohms or less, the tolerance should
be held to a maximum value
Open-Circuit
Voltage or Turns Ratio: Modern winding machines
usually can achieve ± 1 turn resolution. A ±3% tolerance
is standard for this parameter due to measurement errors
caused by meter and source impedance differences. Tighter
tolerances can be achieved with an agreed upon measurement
method.
Full-Load Voltage: After a design is completed,
variations in output voltages are caused by turns ratio,
winding resistance and leakage inductance. A 5% tolerance
for this parameter is easily attainable. Tighter tolerance
can be obtained and an agreed upon measurement method.
Mechanical Dimensions: Envelope dimensions are intended
to ensure fit. Specifying minimum or maximum dimensions
whenever possible will accomplish this. The use of reference
dimensions, which imply no tolerance control, conveys
information with no added cost. These practices will
eliminate unnecessary fixturing or custom tolerances
from raw material vendors. Row to row dimensions for
PC mountable pins can be held ± .02", (Most cases dependent
upon part size), without special handling and packaging.
Insulated flying lead lengths can be held ± 1/8", (Lead
length is 6 " or less).
Magnetics: Size Versus Economics
Transformers are large components in this age of miniaturized
PC boards. Inevitable there is pressure to reduce size,
but minimum size requirements cost more. Try to avoid
size requirements that force the design beyond common
manufacturing methods. Unique methods are usually time
consuming and expensive. Size reduction is achieved
with carefully specified requirements, leading to a
good design.
Tips for Minimizing Size and Cost
- Determine
your requirements with care.
- Ask
only for what you need, try not to over specify. If
you're not sure, ask your friendly magnetics application
engineer.
- Transformers
and inductors are extremely reliable components when
operating at full load conditions. Avoid Unnecessary
Safety Margins.
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Utilize high-temperature insulation. Take advantage
of allowable temperature rise.
References:
"Handbook of Transformer Applications;" William M. Flanagan
"Practical Transformer Design Handbook;" Eric Lowdon
"Switchmode Power Supply Handbook;" Keith Billings
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