High Frequency Transformer Guide
High Frequency Transformer Guide
High Frequency Transformer Guide
HIGH FREQUENCY
TRANSFORMERS
DESIGN CONSIDERATIONS FOR HIGH FREQUENCY TRANSFORMERS | 2
Operating at a higher frequency has many benefits, the first of which is size. For any given power rating, the
higher the frequency, the smaller the transformer can be. Second, because the transformer is smaller, less
copper wire is needed, thus reducing the losses and helping to make the transformer more efficient. Also, since
the core is typically ferrite, a wide variety of geometries are available so the transformer can be tailor-made for
the application. Whether additional shielding or a specific form factor is required, the chances are good that a
ferrite core exists to meet the requirement.
However, the benefits brought about by light weight, small size, and higher power density, pose a number of
challenges. Minimizing the issues such as skin and proximity effects are a serious concern when designing high-
frequency transformers.
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Proximity effects are also known as eddy current losses are caused by the magnetic fields from adjacent
conductors either in adjacent windings, or of more serious concern, in adjacent layers, which causes current
to flow in unintended patterns or in eddy currents. This effect creates excessive resistance within the wire and
unintentional power loss. There are a number of design considerations that will minimize proximity effects,
selecting a core allowing an increased number of turns/layers is one method. The use of foil winding layers
rather than round wire is another, as is interleaving the winding.
Since proximity effects are a result of magnetic field induced eddy currents, it should be noted that core gaps
can result in losses as well. Wherever there is an air gap in the core, stray flux lines (magnetic field) extend in
an arc, and as a result proximity effects can have an impact on the transformer winding closest to the air gap.
Again, there are a number of construction techniques that can be used to minimize this, such as using multiple
gaps on the core leg so each gap is smaller, or using a physical barrier such as tape to keep the winding away
from the gap.
As transformers become smaller, power density increases, and dispite higher efficiency, heat dissipation can
becomes a major design hurdle. In many cases active cooling is needed either with a fan to increase airflow, or
by mounting the transformer on a coooling plate. Potting in a thermally conductive material is also common.
SMPS Topology
The efficiency delivered by an SMPS is critical
for modern electronics and has facilitated the
development of a large number of topologies to meet
many design requirements. Below are basic details of
some of the most common.
Buck
Buck is a common type of topology; it is also the
simplest and cheapest to manufacture. It is non-
isolating and ideally suited to step down voltages as
a DC to DC converter. This is a relatively simple circuit
with the inductor current controlled by a transistor and
diode. The downside is that to reduce the voltage ripple, output filters typically need to be added.
Boost
This topology is similar to Buck, as it is also non-isolating, but unlike Buck, Boost steps up the voltage, rather
down. In continuous conduction mode, it draws current relatively evenly. It is also highly customizable so that it
can be applied in may applications such as power factor correction circuits.
Buck-Boost
Buck-Boost topology is designed to both step down and step up voltages and is very prevalent in battery-
powered applications that provide varying input voltages. However there are several disadvantages, the output
voltage polarity is opposite the input, and input filters are needed to reduce EMI from current ripple resulting from
switching.
Flyback
Fa Flyback provides isolation through the use of a transformer that acts as the storage inductor. The transformer
provides isolation, facilitates multiple outputs, and allows for voltage adjustment by varying the turns ratio. This
topology is widely used for low power applications, and can also be designed for higher output voltages, but,
it should not be used for anything above 10 amps. With simple circuitry and no need for a separate inductor,
Flyback is one of the most common topologies.
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Forward
Forward topology is, in essence, a transformer isolated
Buck converter and is also designed for low voltage
applications. Although it offers better efficiency than a
Flyback, it does use an extra inductor, making it slightly
unattractive for higher output currents. However, when
higher output is required, its non-pulsating output
current makes it ideal for some applications of 15
amps or more.
Push-Pull
This topology is a forward converter and uses two
primary windings that form a dual drive winding. In this
design, the transformer core operates very efficiently, and unlike Forward and Flyback topology, it lends itself well
to being scaled up and requires less filtering.
However switching control can be difficult, as both switches cannot be activated simultaneously. This scenario
results in equal and opposite flux within the transformer, causing low impedance, and high shoot-through current
potentially damaging or destroying the switch.
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For example, just a few years ago 50-100 KHz frequency range was standard. However, today frequencies of up
to 400-500 KHz and even 1 MHz are possible.
As the equation below demonstrates, these higher frequencies have allowed transformers to shrink in size,
utilizing a smaller Ae (core cross section) for any given number of turns. In practice, design engineers must
balance this equation so that as frequency increases a smaller core and fewer turns are required to operate at
the desired flux density.
N = V x108/(4.44)(F)(B)(Ae)
Additionally, when dealing with high-frequency transformers, the designer needs to make sure the wire thickness
does not exceed 2X the skin depth at that frequency. As frequency increases, the current flows more towards the
outside of the wire causing skin effect.
Not taking this into account can result in high copper loses resulting in a transfomer that runs hot. As described
earlier, Litz wire can be used to mitigate this problem.
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