How should one
a 250kHz, 120W forward transformer with voltage controller,
input voltage range 200V to 350V,
as per IEC 61558?
Technical specification relevant only
Electrical data and diagram:
|Input voltages range
||200Vdc - 350Vdc (bridge rectifier with
RC-load, for input voltage 230Vac+10%-20%, 50Hz)
|Nominal output voltage
||2 x 12Vdc, automatic controlled on the
|Nominal output current
||2 x 5Adc, -50%
|Ripple of the input current
||Max. 50% at the maximal input voltage
and the nominal load
Ambient and operating conditions:
||Non inherently short-circuit proof
|Mode of operation
Insulation class E
The following forward transformer diagram illustrates only
parameters relating to design. The windings for rekuperation and
for measurement cannot be calculated by program.
During the initial half period a normal AC transformer is
powered by positive voltage, and during the second half period by
negative voltage. The values of the voltage-time surfaces of both
voltages are the same. This changes the induction in the core of
the AC transformer from
-Bmax to +Bmax.
In the case of a forward transformer, the start of the primary
development Wp during the initial switching period q*T, which is
not necessarily the same as the second switching period, is fed
to the input voltage Up. In this time the induction rises from Br
to Bmax. During the second switching period the end of the
recuperation development Wr is fed to the same input voltage. In
this time the induction falls from Bmax to Br.
In this frequently-used forward transformer switch, magnetic
energy stored during the switch-on phase q*T is returned to the
feed source via the feed source recuperation development Wr.
There are several layouts which can be used to realize the
recuperation. For this reason, and because the structural
performance of this winding is normally only approx. 10%-20% of
the structural performance of the winding Wp, this winding is not
calculated with the program. The user calculates it manually.
In the case of forward transformers, one distinguishes between
2 main phases during a switching period: during the first phase
the primary voltage Up is fed to the primary winding Wp.
The primary voltage increases in direct proportion to the voltage
IL through the choke L. In addition comes the
magnetizing voltage io. The voltage lies in the secondary
Us = (Uo+Udiode) / q.
During the second phase the primary winding WP is
separated from the primary voltage. The magnetic energy which is
stored in this is returned via the winding Wr. The number
of windings Wr in this winding is chosen to ensure that
q + r < 0.8-0.9
r = q * (Wr/Wp)
are constantly fulfilled. The relative switch-on duration q
is chosen to be between 0.40 and 0.60 at minimum
input voltage. This produces:
r < 0.3 - 0.6
Wr = (0.5 - 0.66) * Wp
During the switching period T, the induction changes by
pulsing between Br and Bmax. The choice of Bmax
depends on the ferrite type and on the operating temperature. The
residual induction Br can be set practically at a value
between 5% and 10% of the Bmax induction by an air gap in
the core. With an external source or a permanent magnet it would
be possible to set a negative residual induction.
The ripple of the voltage through the choke L affects
the effective value of the transformer voltage.
Ripple = 100 * (Ilmax-Ilmin)/Ilmax+Ilmin)
This value must be notified by the user.
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 cutout electronics in the cycled
mains power unit to protect against overload and short-circuit.
For this reason, short-circuit and overloads are not design
criteria. The criterion for design with regard to IEC 61558 is
only temperature q nominal.
|Max winding temperature in
nominal operating mode q nominal
Max winding temperature in nominal operating mode = 115°C
Insulation class E is prescribed.
Criterion for design
Normally, high-frequency transformers have very low regulation
and are designed according to the prescribed temperature rise.
Since these transformers are manufactured almost exclusively
using ferrites, the optimum operating temperature is around
In order to protect the transistors, high-frequency transformers
should be manufactured for low leaking reactance, with
single-chamber bobbin units. For this reason, we very often
arrive at bifilar 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 a 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 optimize
or reduce the AC-component of the induction automatically. But
this does indicate that the selected ferrite quality is not
Induction and ferrite quality
High-frequency transformers are equipped almost exclusively with
ferrites. 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.
Nominal input voltage and relative switch-on period
The relative primary voltage switch-on period is defined as
In the design of a forward transformer, the duration of the
relative switch-on period (q = t1/(t1+t2) ) is taken into
account indirectly via the input mode of the form factor:
Form factor = 1/(2*q)
A forward transformer with an automatic controller of output
voltage is normally designed with the following parameters:
- "Nominal" input voltage Upmin = 200V.
- At this input voltage the relative switch-on period qmax
will be 0.5 and the relative recuperation period rmin
- The form factor = 1/(2* qmax) = 1/(2*0.5) = 1
- Th relative switch-on period at the input voltage Upmax =
350V will be:
qmin = qmax * Upmin/Upmax = 0.5 *
200 / 350 = 0.285
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 keys 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 CAL0011E.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 your design work, the following design
data is available. We must not omit to mention at this
point that the calculated data for short-circuit
is not applicable to the forward transformer (and cannot
be used for that context).
On completion of the design work, the following
design data will be available, which can be printed on 3
- Checking of the design data follows this.
- We now check the winding data and the filling factor
- The maximum temperature of the windings is 40°C+57.1°K
= 97.1°C < 115°C.
- The number of parallel-connected wires with 0.16 mm
diameter is 6 and 27. Commercial considerations prompt us
to select a litz of 5 wires of 0.16 mm diameter for the
primary and a litz of 30 wires of 0.16mm for both
secondary windings. This operation must be performed
manually in the test mode.
- There now follows the configuration of the recuperation
Wr = Wp * rmin/qmax=
23 * 0.285 / 0.5 = 13 windings
The number of parallel-switched wires is smaller than the
number of parallel-switched wires in the primary winding
Wp by the factor Iporms/Iprms.
5 * 0.218/0.99= 1 wire
Iporms => No-load voltage
Iprms => Primary nominal voltage
- This is followed by checking of the output voltage for
the maximal input voltage of 350V and the relative
switch-on period of 0.285: Uin = 350/200 = 1.75 and form
factor = 1/(2*0.285) = 1.754.
Note that the program controls your input in order to avoid
the operation in the saturation of the core. If you get any
problem with your input, follow these procedures.
- Increase the form factor to 1.754 (q = 0.285)
- Press F6 to recalculate
- Increase the input voltage to 350V : Uin = 1.75
- Press F6 to recalculate
- Decrease the input voltage to 200V : Uin = 1
- Press F6 to recalculate
- Decrease the form factor to 1.0 (q = 0.5)
- Press F6 to recalculate
The following table shows the summery of the most important
parameters, calculated by program in the test mode. Note that the
relative switch-on period (q) was changed in order to get the
nominal input voltage as by a voltage controller.
- 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, CAL0011E.TK2, is supplied together
with this document. Copy it into the directory in which
your Rale demo program is installed.
Tips & Tricks
Rounding off the number of windings
With a flyback transformer, the procedure for rounding off the
number of windings differs from that employed with a
- Next, we correct the nominal primary voltage until the
desired number of primary windings is reached.
- In the test program, finally, the number of windings is
rounded off manually.
Copper strip instead of litz
A copper strip can replace a litz. 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