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.

- 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
- 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
*L*_{1}the value for*L*_{1}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 "
" is activated, the turns ratio*N*_{1}/*N*_{2}*N*_{1}/*N*_{2}is proposed. It is proposed such that*N*_{1}/*N*_{2}=*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}= 2*V*_{in_avg}. - If you do not agree with our proposals, you can change
*N*_{1}/*N*_{2}or*L*_{1}. 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 *N*_{1} and *N*_{2} 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 *N*_{1} and *N*_{2} 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 *I*_{1} and the primary inductor *L*_{1}, 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.

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 *V*_{1} = *V*_{in} and *I*_{1} 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 *I*_{1} 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}·*N*_{1}/*N*_{2}.

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

- The larger the number of turns
*N*_{2}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}.

** V_{in_min}**,

Using these parameters, the program produces a **proposal for L and N_{1}/N_{2}**:

From this it follows that:

- For
**Δ**the converter is in continuous mode and it follows that:*I*_{L1}< 2*I'*_{1}

,

- For
**Δ**the converter is in discontinuous mode and it follows that:*I*_{L1}> 2*I'*_{L1}

,

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 |