Two Transistor Forward 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 inductor
*L*, a value for*L*and the corresponding current ripple Δ*I*_{L}is proposed. These values are laid out such that Δ*I*_{L}= 0.4*I*_{out}with*V*_{in_max}as the input voltage. - If the field "proposal" for the input field "
" is activated, the turns ratio*N*_{1}/*N*_{2}*N*_{1}/*N*_{2}is proposed. This suggestion is chosen such that the required output voltage can be achieved using*V*_{in_min}as an input voltage. - If you do not agree with our proposals, you can change
*N*_{1}/*N*_{2}or*L*as well as Δ*I*_{L}. 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}.

Illustration 1: Two Transistor Forward Converter |

The two transistors are simultaneously turned on and off by a pulse-width-modulated control voltage.

The forward converter transfers the energy during the on-time of the transistor. During this time the voltage *V*_{1} is equal to the input voltage *V*_{in}. The winding *N*_{2} is in the same direction as *N*_{1}. When the transistor is on the voltage *V*_{2} at *N*_{2} is given by *V*_{2} = *V*_{in} ·*N*_{1}/*N*_{2}. The voltage *V*_{2} charges the output capacitor *C*_{out} through the inductor *L*.

During the off-time of the transistor the secondary winding *N*_{2} is without current. The inductor *L* draws its current through the diode *D*_{3}. The value of the voltage *V*_{3} is equal to zero at this time (exactly 0.7V).

During the off-time of the transistor, the magnetic flux of the transformer has to reduce to zero. The transformer core is demagnetized via *N*_{1} and the primary diodes, to the input voltage *V*_{in}. Therefore the demagnetisation needs the same time interval as the on-time of the transistor. For this the minimum off-time has to be as long as the on-time. This means that the maximum duty cycle *t*_{1}/*T* for this converter may never be higher than 50%.

The voltage *V*_{3} is therefore a pulse-width-modulated voltage which jumps between 0 and *V*_{in}·*N*_{2}/*N*_{1}. The Low-Pass filter, formed by the inductor and the output capacitor, produces an average value from *V*_{3}. For continuous mode (*I*_{L} never becomes zero) this leads to:

Due to the fact that the duty cycle *t*_{1}/*T* cannot be greater that 50%, a condition for the turns ratio emerges:

In the program, this value is multiplied by a factor of 0.95, so that the proposed value for *N*_{1}/*N*_{2} includes a small margin which guarantees the demagnetisation of the core, when the input voltage is minimal, (remember: at minimum input voltage the duty cycle reaches its maximum).

For the calculation of the inductor *L*, the same rules as for the Buck Converter can be used. One also distinguishes between **discontinuous** and **continuous mode**, depending on whether or not the inductor current falls to zero during the off-time of the transistor.

During continuous operation:

The output voltage depends only on the duty cycle and the input voltage, it is load independent. The inductor current *I*_{L} has a triangular shape and its average value is determined by the load. The change in inductor current Δ*I*_{L} is dependent on *L* and can be calculated with the help of Faraday's Law.

During continuous mode, with *V*_{out} = *V*_{in} · (*N*_{2}/*N*_{1}) ·*t*_{1}/T and a chosen switching frequency *f* it can be shown that:

The change in inductor current is load independent. The output current *I*_{out} is taken to be the average value of the inductor current *I*_{L}.

At low load current, namely if *I*_{out} < Δ*I*_{L}/2, the inductor current *I*_{L} falls to zero during every switching cycle. This mode is called **continuous mode** (see illustration 2). For this mode the calculations above are **not** valid.

In that moment, when the inductor current becomes zero, the voltage *V*_{3} jumps to the value of *V*_{out}. The diode-junction capacitance forms a resonant circuit with the inductance, which is activated by the voltage jump across the diode *D*_{3}. The voltage *V*_{3} then oscillates and fades away.

Continuous Mode | Discontinuous Mode |

Illustration 2: Operating modes of the Two Transistor Forward Converter

- The larger the chosen value of the inductor
*L*, the smaller the current ripple Δ*I*_{L}. However this results in a physically larger and heavier inductor. - The higher the chosen value of the switching frequency
*f*, the smaller the size of the inductor. However the switching losses of the transistor also become larger as*f*increases. - The smallest possible physical size for the inductor is achieved when Δ
*I*_{L}= 2*I*_{out}at*V*_{in_max}. However, the switching losses at the transistors are at their highest in this state. - Choose Δ
*I*_{L}so that it is not too big. The suggestions proposed by us have adequately small current ripple along with physically small inductor size. With a larger current ripple, the voltage ripple of the output voltage*V*_{out}becomes clearly bigger while the physical size of the inductor decreases marginally. - It is best not to alter the turns ratio
*N*_{1}/*N*_{2}proposed by us.

** V_{in_min}**,

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

(the factor of 0.95 is taken into account to allow for the fact that the duty cycle

Δ

From this it follows that:

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

,

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

,

Main page | | How to use the program | | Function principals | | Mathematics used in the program | | Help for HF transformer |

Top of page | | Application | | Tips | | Literature Notes | | Help for choking coils |