How should one
design a 50kHz, 1200VA transformer
as per IEC 61558 ?
Technical specification relevant only
Electrical data and diagram
||Max. 280Vac, square-wave
|Nominal output voltage 1
Nominal output current 1
|Nominal output voltage 2
Nominal output current 2
|24Vdc, bridge rectifier with
|Nominal output voltage 3
Nominal output current 3
|24Vdc, bridge rectifier with
|Nominal output voltage 4
Nominal output current 4
|24Vdc, central tapping
rectifier with R load
|Nominal output voltage 5
Nominal output current 5
|24Vdc, central tapping
rectifier with RL load
Ambient and operating conditions:
|Mode of operation
||Non inherently short-circuit
Criteria for design
A high-frequency transformer with non inherently
short-circuit proof as per IEC 61558 is equipped with a safety.
Very often we arrive at a combined protection solution consisting
of a thermal cutout in the transformer and cut-out electronics in
the cycled mains power unit to protect against overload and
short-circuit. For this reason, short-circuit and overload are
not design criteria. The criterion for design with regard to IEC
61558 is only temperature q nominal.
temperature in test q max (° C)
temperature in nominal operating mode q nominal
Max winding temperature in nominal operating mode = 115°C
Max winding temperature in test mode = 215°C
Insulation class E is prescribed.
Criterion for design
Normally, high-frequency transformers have very low
regulation and are designed according to the prescribed
Since these transformers are manufactured almost exclusively
using ferrite, the optimum operating temperature is around
In order to protect the transistors, high-frequency
transformers should be manufactured for low scatter, with
single-chamber bobbin units. For this reason, we very often
arrive at dual-wire or interleaved windings.
Since the optimum operating temperature of ferrite for
high-frequency transformers over 100VA is around 100°C and their
ambient temperature is between 40°C and 70°C, our design
assumption must be for an temperature rise of between 30°K and
60°K. If the core losses in relation to temperature rise are not
economically acceptable, then the computer program will optimise
or reduce induction automatically. But this does indicate that
the selected ferrite quality is not optimized.
Induction and ferrite quality
High-frequency transformers are equipped almost exclusively
with ferrite. The program calculates both the active and the
reactive core losses by hypothesizing the ferrite type, the
frequency, the form of input voltage, induction and core
temperature. The induction should be selected such that the
transformer does not saturate at maximum input voltage and
maximum core temperature.
Copper additional losses
With a high-frequency transformer, the distinctions are drawn
between the following additional losses in a winding, over and
above the dc-current losses:
- Eddy current losses
- Displacement losses
- Proximity effect losses
- Losses due to circulating currents through the
Additional losses are smaller in the case of a winding that
takes up only 30-60% of the available winding space. For that
reason, one should always set the input for the filling factor
between 0.3 and 0.6 for purposes of automatic core selection.
The input for Rac/Rdc will limit the extent of additional
losses (eddy current losses and displacement losses). The
computer program selects a high enough number of
parallel-connected wires for the eddy current losses and
displacement losses to fall short of the prescribed value for Rac/Rdc.
For that reason, the input for Rac/Rdc is also used for
monitoring of parallel-connected wires. The value is normally set
between 1.5 and 5.
Proximity effects can be reduced by means of the Spread
input. Another option for reducing proximity effects is to select
wires with thicker insulation.
Losses of circulating currents through the parallel-connected
wires are not calculated. It is assumed that these additional
losses have been eliminated by suitable design precautions. In
particular, it should be ensured, for a given litz, that the
twisting for the winding is done such that a given wire has the
same position at the input and at the output of the winding.
Procedure for design
- If you are not yet acquainted with Rale design software,
please read the text "How should I design a small
transformer?". Keep a copy of this text within
convenient reach whenever performing design work.
- Fill in the design input mask as follows. If you need any
help, press function key F1. There is extensive
description for each input field.
- The Selection input field is set at 0. This
means that the program should search on-line for a
suitable core for this application, from your selected
- Save your input data file. In this specimen design
calculation, we saved the input data in input data file CAL0009E.TK1.
This input data file was supplied together with this
document. Copy it into the directory in which your Rale
demo program is installed.
- Connect up to the Rale design server.
- Load up your input data file.
- Now select the core family and the core for automatic
search by the computer program.
- Click on OK.
- Start your design work. In the system for automatic
selection of the core from your prescribed core family,
the program will offer you an adequately sized core for
your application. Click on OK in order to accept the
On completion of the design work, the following design
data will be available and can be printed on three pages:
- This is followed by checking of the design data.
- The program has reduced the entered induction from 0.2T
to 0.172T. This is an indication that at the selected
induction of 0.2T, core losses would be too high by
comparison with copper losses. An improvement could be
achieved by increasing the core cooling surface area or
by using a higher grade of ferrite.
- We now check the winding data and the filling factor
- The maximum temperature of the windings is 40°C+58.25°K
= 98.2°C < 115°C.
- The number of parallel-connected wires with 0.15 mm
diameter is 66 and 161. Commercial considerations prompt
us to select a litz of 50 wires of 0.16 mm diameter for
the primary and a litz of 175 wires with 0.16 mm for all
secondary windings. This operation must be performed
manually in the test mode.
- In the test mode, furthermore, the number of windings of
the secondary is manually rounded up from 1.9 to 2 turns.
This will result in approximately 5% higher output
- If the design data is not satisfactory, then there are
two ways by which we can implement the desired
- You can return to the input mask (function key F2),
correct the input data and redesign the transformer.
- Or you can access the test program (function key F5),
modify the designed transformer manually and redesign the
transformer by that means.
- On completion of the design work, you can print out the
design data on-line, or save it on your local PC and
print it out off-line. The output data file from this
design example, CAL0009E.TK2, is supplied together with
this document. Copy it into the directory in which your
Rale demo program is installed.
Tips & Tricks
Upon entering the input voltage, we assume that there is an
impressed voltage for circuits 1, 2 & 3.
The input current for circuits 4, 5 & 6 is impressed. For
these circuits, we have to start by determining the primary input
voltage, the secondary voltage and the secondary current manually
in order to be able to use the computer program to design the
Circuits 7 to 10 are a combination of supply with impressed
voltage and supply with impressed current, and are treated as
circuits with impressed current.
Copper strip instead of litz
A litz can be replaced by a copper strip. The strip thickness
should correspond to the wire diameter of the litz. Strip width
should be matched to the width of the bobbin. The number of
strips connected in parallel is determined in accordance with the