Flyback Converter

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• The values of all input fields can be changed.
• If an input field is left empty, a default value is chosen. This value is displayed after leaving the input field in question.
• The switch mode power supply operates within a certain input range i.e. between V_{in_min} and V_{in_max}.
  Note:
  • For the european mains of 230V +/-10% and behind the rectifier and the smoothing (with a voltage ripple of 10%) the input voltage range is between V_{in_min} = 250V and V_{in_max} = 360V.
  • For wide range Switch Mode Power Supplies the input voltage range of the mains is from 100Vac -10% (Japan) to 240Vac +6% (Great Britain). In this case, the DC input range of the power supply is from V_{in_min} = 110V to V_{in_max} = 360V.
  • For use of a power factor pre-regulator the input voltage range is normally from V_{in_min} =360V to V_{in_max} =400V.
• The program needs the output values V_out and I_out.
• The switching frequency  f  is the operating frequency of the transistor.
• If the field "proposal" is activated for the primary inductance L₁ the value for L₁ is proposed. This is such as the border between continuous and discontinuous mode is achieved for the average input voltage which is V_{in_avg} = (V_{in_max}+V_{in_min})/2.
• If the field "proposal" for the input field "N₁/N₂" is activated, the turns ratio N₁/N₂ is proposed. It is proposed such that N₁/N₂ = V_{in_avg}/V_out, where V_in is the average value of input voltage range. This means that for the average input voltage, the maximum voltage across the transistor is V_ds = 2V_{in_avg}.
• If you do not agree with our proposals, you can change N₁/N₂ or L₁. The field "proposal" is then deactivated automatically.
• The value V_in is the value for the calculation of the current and voltage diagrams on the right side of the display. V_in must lie between V_{in_min} and V_{in_max}.

Note 2: The calculations are made for an efficiency of 1. This is actually not realistic for a flyback converter. The efficiency is determined mainly by the coupling of the primary and secondary winding. If the number of turns N₁ and N₂ are very different (normally the case with low output voltage), then the coupling between primary and secondary will be bad and this leads to a low efficiency of about 65-70%. If the number of turns N₁ and N₂ are approximately equal, then you will reach a good coupling and the efficiency can be > 90%. To reach a realistic result for the primary current I₁ and the primary inductor L₁, you should increase the output current by a realistic factor 1/η for the efficiency.

Note 3: The flyback converter may have several separately isolated output voltages which are all regulated, e.g +5V, +15V, -15V. In this case only one of the output voltages has to be regulated (e.g. +5V) and the other output voltages are coupled by their number of turns in respect to the regulated output voltage.

Function principals

Illustration 1: Flyback Converter

The transistor works as a switch which is turned on and off by a pulse-width-modulated control voltage.
During the on time of the transistor, the primary voltage V₁ = V_in and I₁ increases linearly.
During that phase energy is loaded into the transformer.
The secondary winding does not have any current because the diode is blocking during the on-time of the transistor.
If the transistor is in blocking mode then I₁ will be cut-off and the voltages at the transformer will change because of Faraday's Law.
The diode will then be conducting and the secondary winding will then give energy to the output capacitor.

During the on-phase of the transistor the drain-source-voltage V_ds will be zero.
During the off-phase the output voltage is back transformed to the primary side such that the drain-source voltage achieves the value V_ds = V_in + V_out·N₁/N₂.
This implies that for a flyback converter which is designed for a 230V/50Hz mains, the voltage V_ds usually reaches approximately 700V. In practice the voltage is even higher because an induction voltage is added as a result of transformer leakage induction. The transistor in the flyback converter for the 230V mains must have a breakdown voltage of at least 800V.

The transformer is not a "normal" transformer. Its function is to save energy during the on-phase of the transistor and to transfer that energy to the secondary side during the off-phase. This means that the transformer is a storage-inductor with primary and secondary windings and the transformer-core has an air-gap. Transformers for flyback converters are therefore called storage transformers.
In order for the stored energy of the primary current to be transferred to the secondary winding during the off-phase of the transistor, both coils must be very well magneticly coupled.

Continuous Mode	Discontinuous Mode

Illustration 2: Operating modes of the Flyback Converter

Tips

• The larger the number of turns N₂ the smaller the drain-source voltage of the transistor V_ds.
• At the border between continuous and discontinuous mode at V_in = V_{in_min} the size of the transformer will be at its smallest. However, in this case the flyback converter works in discontinuous mode with all other input voltages. As a result of this the switching-losses through the semiconductors become quite big.
• The drain-source voltage of the transistor V_ds is at its highest when V_in = V_{in_max}.

Mathematics used in the program

V_{in_min}, V_{in_max}, V_out, I_out and  f 

Using these parameters, the program produces a proposal for L and N₁/N₂:

From this it follows that:

1. For ΔI_L1< 2I'₁ the converter is in continuous mode and it follows that:
   
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2. For ΔI_L1> 2I'_L1 the converter is in discontinuous mode and it follows that:
   
   ,
   
   
   
   
   
   

Main page |	Literature Notes        |	Application                    |	Tips
Top of page |	How to use the program |	Mathematics used in the program |	Function principals |	Help for choking coils