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How should one design
a 70kHz, 60W-120VA flyback transformer with voltage controller, for input voltage range 125V to 700V ,
as per IEC 61558 ?

Technical specification relevant only to design

Electrical data and diagram:

Ambient and operating conditions:

Specification

Insulation class E

The following flyback transformer diagram illustrates only parameters relating to design. The windings for protection and for measurement cannot be calculated by program.

With a flyback transformer, the distinction is drawn between two modes of operation:

• Mode with continuous secondary current ( Fig. 1, t3=0).
• Mode with intermittent DC secondary current (Fig. 2, t3>0).

Continuous mode (Fig.1)
In this mode, secondary current ripple is less than 100%:

Ripple = 100* (Ismax-Ismin)/(Ismax+Ismin) =(Bmax-Bmin)/(Bmax+Bmin) < 100

In the flyback transformer’s continuous mode, the output voltage is "impressed" for practical purposes, and is not greatly dependent upon load. This mode is easy to regulate and is preferred by electronics engineers. Secondly, the flyback transformer is larger with less ripple in the primary current. For that reason, the choice of ripple (10% to 100%) of the secondary current has to be harmonized between the transformer manufacturer and the electronics engineer.

Intermittent mode (Fig.2)
This mode is essentially utilized for generation of the current pulses or a high output voltage.The following conditions are selected upon sizing:

• t1>>t2
• Ripple => 100%.

Criteria for design

IEC 61558
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.

Insulation class
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 temperature rise.
Since these transformers are manufactured almost exclusively using ferrite, the optimum operating temperature is around 100°C.

Bobbin unit
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.

Ferrite quality
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 (the ripple of the input current) of the 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:

1. Eddy current losses
1. Displacement losses
1. Proximity effect losses
1. Losses due to circulating currents through the parallel-connected wires.

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 follows:

In the design of a flyback 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 flyback transformer with an automatic controller of output voltage is normally designed with the following parameters:

• "Nominal" input voltage Upnom= (Upmin*Upmax)1/2 = (125*700) 1/2= 296V. At this input voltage the relative switch-on period q_nom will be 0.5.
• This flyback transformer has to be designed at the input voltage Upmin = 125V. At this input voltage the relative switch-on period has to be prescribed via the form factor:
  q_max = Unom/(Umin + Unom) = 3296/(125+296) = 0.7.
• The form factor = 1/(2* q_max) = 1/(2*0.7) = 0.711
• Th relative switch-on period at the input voltage Upmax = 700V will be:
  q_min = Upnom/(Upmax+Upnom) = 296/(700+296) = 0.3
• In order to have the ripple smaller than 100% at the maximal input voltage, you have to prescribe the ripple at the minimal input voltage as follows:

Ripple_min = Ripple_max *(Upmin*q_max/Upmax/q_min)^2 = 100*(0.7*125/750/0.3)^2 < 17%

Procedure for design

1. 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.
2. 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.

3. 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 core family.
4. Save your input data file. In this specimen design calculation, we saved the input data in input data file CAL0010E.TK1. This input data file was supplied together with this document. Copy it into the directory in which your Rale demo program is installed.
5. Connect up to the Rale design server.
6. Load up your input data file.

7. Now select the core family and the core for automatic search by the computer program.
   
   

8. Click on OK.
9. 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 core.
10. 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 and for idle are not applicable to the flyback mode for this transformer (and cannot be used for that context). However, there are two modes which relate exclusively to the flyback transformer: primary winding inductance and the gap for calibration of the primary winding inductance.
11.  On completion of the design work, the following design data will be available, which can be printed on 3 pages.:

    

    

    

    

    

    

    

    

    

    

12. Checking of the design data follows this.

• We now check the winding data and the filling factor (36.7%<100%).
• The maximum temperature of the windings is 40°C+57.9°K = 97.9°C < 115°C.
• The number of parallel-connected wires with 0.16 mm diameter is 9 and 123. Commercial considerations prompt us to select a litz of 10 wires of 0.16 mm diameter for the primary and a litz of 125 wires of 0.16mm for both secondary windings. This operation must be performed manually in the test mode.

13. This is followed by checking of the output voltage for the maximal input voltage of 700V and the relative switch-on period of 0.3: Uin = 700/125 = 5.6 and form factor = 1/(2*0.3) = 1.68.

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 1.68 (q = 0.3)
• Press F6 to recalculate
• Increase the input voltage to 700V : Uin = 5.6
• Press F6 to recalculate

or

• Decrease the input voltage to 125V : Uin = 1
• Press F6 to recalculate
• Decrease the form factor to 0.711 (q = 0.7) Press F6 to recalculate
• Press F6 to recalculate

• Secondary current ripple is: Bn/(B0-Bn) = 100*0.091/(.197-0.091) = 86%<100%

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.

In order to get the constant total losses in the whole range of the input voltage, select the ferrite and the operation frequency at the nominal input voltage so, that you get approximately Pfe = Pcu

11. f the design data is not satisfactory, then there are two ways by which we can implement the desired correction:

• 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.

12. 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, CAL0010E.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 "normal" transformer.

• You start by modifying the induction or ripple of the secondary current until the desired number of secondary windings is achieved.
• 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 following illustration.

Calibrating the inductance of the primary winding

The inductance of the first tapping of the primary winding must be calibrated at the operating frequency of the flyback transformer. The calculated gap is a guideline value only, and has to be modified during calibration so that the designed (prescribed) inductance ( 2.63 mH) is correct.

How do I design a flyback transformer, without voltage controller, for input voltage range +-20% and load variation +10% to 100%?

• Input voltage = (Upmin*Upmax)^0.5 at
• Form factor = 1 (q= 0.5)
• dI/Io = 10% (Ripple of the current)

Note that the output voltage alters proportional to the input voltage and does not depend of the load

How do I design a flyback transformer for "constant output power", without voltage controller, for input voltage range +-20% and load variation +10% to 100%?

• Input voltage = Upmax
• Maximal otput voltage
• Maximal output current
• Form factor = 1 (q= 0.5)
• dI/Io = 100% (Ripple of the current)

Note that the output voltage alters proportional to the input voltage. If you increase in test mode the resitaance of the load, the output voltage alters so, that the output power stays "constant".

.How do I design a flyback transformer , with voltage controller, for input voltage range +-10% and for constant resistance of the load?

• Input voltage = Umin
• Nominal otput voltage
• Nominal output current
• Form factor = 1 (q= 0.5)
• dI/Io = 100% (Ripple of the current)

How do I design a smoothing choke for current ripple of 20%, output voltage 12Vdc and output current 5Adc?

• Input voltage = Uout + Udiode = 12 + 0.8 = 12.8
• Form factor = 1 (q=0.5)
• Secondary circuit = 35
• Nominal output voltage = 12V
• Nominal output current = 5A
• dI/Io = 20% (Ripple of the current)
• Udiode = 0.8V
• Induction = 0.20 - 0.25T
• Not high quality ferrite

After the design you have to:

• increase the number of the in parallel (by program calculated) wires of the primary winding for the factor 1.41
• leave out the secondary winding

Note that this choke can be used in the rectifier circuit of a forward transformer.

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