A Guide to Specifying Encapsulated Transformers for use in Power Supplies
Spencer Manley – Applications Engineer, Transonics plc.
What is a transformer?
A transformer is an electromechanical device that takes an Alternating Voltage (AC) and transforms
it to another alternating voltage via an isolated magnetic flux path.
A transformer has 2 basic functions.
1) To convert one AC voltage to another, either greater than, equal to or less than the AC
voltage applied.
2) To provide an isolation barrier for safety purposes
Transformer types are often referred to as:Step up transformer
The input voltage is increased at the output to generate a higher voltage.
This is typically used for distributing power over a longer distance as the output copper wires
will have to carry less current, reducing losses dissipated as heat.
Step down transformer
The input voltage is decreased at the output to generate a lower voltage.
This is usually used for power supplies to provide a safe and lower voltage to power low
voltage electronics.
Isolation transformer
The input and output have the same voltages and the transformer construction provides a
safety barrier.
This would be used if there was a safety requirement to prevent the end user creating a path
to ground for current to flow in the event of an electrical breakdown.
This article will consider the essential specification requirements of a transformer for use in
electronic power supplies, the “step-down” transformer design mentioned above
Virtually every product in common use requires a power supply to convert mains voltages to those
required for the end product.
In most cases this will be an AC input power source which could be converted to a higher DC voltage
for a switched mode power supply or reduced to create a low DC voltage for a simple linear power
supply. Common power supply output requirements are in the range 5-24V or 48V for telecoms
equipment
As the majority of equipment is supplied from the 230VAC mains a 50-60 Hz operating frequency will
be the most common design
The following diagram shows a basic 50-60Hz power supply diagram. The step down transformer is
highlighted in Yellow
For any power supply design the requirements of the load will have to be known
Some of the load requirements may not be obvious at the outset and not be known until prototype
testing shows there are more parameters to be considered for the final design.
Basic considerations for a power supply transformer;
The power consumed by the transformer is not equal to the power being drawn from its output,
there will be internal losses.
Power in = Power out + Losses
These losses can be very significant.
A transformer functions by winding an input “primary” coil, electromagnetically connected to a
secondary “output” coil using a “Turns Ratio” to achieve the output required relative to the input
voltage.
The voltage [Vp] applied to the primary divided by the Voltage [Vs] on the secondary will determine
the turns ratio.
Therefore Vp/Vs
= Np/Ns (N being Turns) = Turns Ratio
The output voltage is calculated using Vs=Vp x Np/Ns
For correct operation of the end power supply output Voltage, output current, line regulation, load
regulation, short circuit protection, thermal performance and efficiency all need to be considered
Output Voltage (the output voltage generated by the transformer turns ratio)
Design considerations:
Is the design a basic single voltage output?
Will more than one output be required?
Would multiple outputs need to be isolated from each other or would a common-rail design be
satisfactory?
Does the output require opposing voltages?
E.G. +/- 12V
Output Current (the current required to power the load circuit)
The load requirements and ambient conditions of the load circuit will determine the size and rating
of the transformer design.
Will the power level be constant or will it be nominally a low output current level with high output
peak currents as in a battery charging circuit or data communications system?
A design appearing to require 1VA may only require that capacity for short bursts of time allowing
size and cost saving by de-rating to possibly a 0.6VA transformer design provided the duration of the
increased current spikes doesn’t saturate the core.
Line Regulation (variation in the output voltage in response to a change in the input voltage)
A transformer is basically a potential divider that will convert the AC input voltage up or down
depending upon the ratio of the turns specified.
If a design is based around a nominal 230V input care must be taken to ensure that the output
voltage swing is catered for.
A 230V input and a 12V output gives an approximate ratio of 19:1
Clearly the input voltage can vary depending upon how clean the mains supply is and the other load
conditions on the local power feed.
For example, the input voltage may drop to 210V having the effect of reducing the output voltage to
11V or should the input rise to 250V the output will rise to 13V. This line fluctuation will need to be
considered if it will affect the intended load.
Load regulation (variation in output voltage in response to a change in the load current)
Load regulation of a transformer is determined by measuring the voltage at the output with no load
applied and then measured again once the full load is connected.
A typical power supply specification may ask for 12V on load but could reach in excess of 20V off
load.
This can vary depending on the internal construction of the transformer and must be taken into
consideration in the design.
Components specified in a power supply will have a maximum voltage rating which must be within
the maximum off load voltage specification.
Short Circuit Proof (a fault condition created by a failure at the output of the transformer)
An encapsulated transformer could pose a fire hazard if the design is subjected to a direct short
circuit under a fault condition.
With encapsulated designs the short circuit current is limited by the resistance of the windings,
which together with the encapsulating materials used, should keep the temperature within safety
requirements and remove the possibility of a fire.
Isolation (the minimum breakdown voltage that can be applied between the input and output)
All transformers inherently provide isolation between input and output windings.
This is achieved by either designing the windings on 2 separate parts of the coil former or by putting
a barrier media (tape or plastic housings) between the windings internally.
Any barrier will eventually break down given a high enough voltage applied between input and
output allowing a high voltage a path from one side to the other.
Most specifications call for protection to around 1500V minimum. Efiter encapsulated transformers,
supplied by Transonics, have an innovative internal design and encapsulation technique and can
offer 5000V for 1 minute.
Thermal Considerations
Heat is generated inside a transformer in 2 ways, Conducted and Radiated.
Conducted heat is generated from the primary resistance of the windings. Current passing through
these windings will generate heat within the conducted element which is known as Copper Loss.
Heat generated from radiated loss is caused by the energy moving via the magnetic path from the
Primary to the Secondary inside the transformer, not from a physical resistance path.
The energy passing through the magnetic core material will be conducted as Flux and radiated as
Heat. This is referred to as “Core Losses”.
Low cost, low quality core materials produce more losses and consequently raise the internal
temperature of the transformer.
Use of a laminated core with multiple sheets of iron instead of a solid core will lower the thermal
skin effect and therefore lower the internal temperature but will add cost in manufacture.
Transformers can be deigned with very low loss cores offering higher efficiency but this may not
always be the most economical solution.
Thermal considerations and the relative cost of core designs needs to be a design consideration from
the outset.
Efficiency
The efficiency of a transformer is a function of the quality of the Copper wire used for the windings
and the material and design of the Cores used including with any gaps in the core construction.
The ideal is clearly 100% efficiency as this offers a perfect conduction path but there will always be
resistances and other losses in the transformer circuit.
Most common transformer designs running at 50Hz provide efficiency levels around 55-65% due to
these losses.
To specify transformers with greater efficiency better cores which operate at higher frequencies will
be required. For very high efficiencies changing the power supply topology to a switch-mode design
may be desirable.
Working with a trustworthy supplier
From the above it can be seen that many technical and safety critical factors are involved in the
design of a transformer making it a specialised task which most product designers outsource to a
specialist supplier.
While standard transformers may still be widely used, custom designs are very common and finding
a reliable and responsive supplier is a key part of the supply chain. Transonics have a UK based
design team and supply transformers from Efiter, a leading European based specialist transformer
manufacturer.
Efiter transformers are 20% lighter than most of their competitors as a result of using their own
specially manufactured encapsulating resin and the use of high performance, high reliability wire
and a very high precision winding process. High isolation voltages up to 5KV are available which
greatly exceeds market norms and all Efiter transformers are 100% tested on and off load, at
maximum load and are isolation tested.
With transformer solutions from 0.35VA up to over 30VA, Transonics can support multiple
configurations ranging from a single 115V or single 230V AC level to a dual 115V input and outputs in
either a single isolated, dual isolated or a dual centre tapped configuration Efiter offers the most
varied range of solutions in the market place today.
For more information on Efiter transformers from Transonics visit www.transonics.com
Transonics PLC
Breakspears Park
Breakspears Way
Hemel Hempstead
HP2 4UL
Tel: 0845 567 8899
Email: sales@transonics.com
www.transonics.com
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