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.
• 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 ΔI_L is proposed. These values are laid out such that ΔI_L = 0.4I_out with V_{in_max} as the input voltage.
• If the field "proposal" for the input field "N₁/N₂" is activated, the turns ratio N₁/N₂ 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 N₁/N₂ or L as well as ΔI_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}.

Function principals

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 V₁ is equal to the input voltage V_in. The winding N₂ is in the same direction as N₁. When the transistor is on the voltage V₂ at N₂ is given by V₂ = V_in ·N₁/N₂. The voltage V₂ charges the output capacitor C_out through the inductor L.

During the off-time of the transistor the secondary winding N₂ is without current. The inductor L draws its current through the diode D₃. The value of the voltage V₃ 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 N₁ 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 t₁/T for this converter may never be higher than 50%.

The voltage V₃ is therefore a pulse-width-modulated voltage which jumps between 0 and V_in·N₂/N₁. The Low-Pass filter, formed by the inductor and the output capacitor, produces an average value from V₃. For continuous mode (I_L never becomes zero) this leads to:

Due to the fact that the duty cycle t₁/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 N₁/N₂ 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 ΔI_L is dependent on L and can be calculated with the help of Faraday's Law.
During continuous mode, with V_out = V_in · (N₂/N₁) ·t₁/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 < ΔI_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 V₃ 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 D₃. The voltage V₃ then oscillates and fades away.

Continuous Mode	Discontinuous Mode

Illustration 2: Operating modes of the Two Transistor Forward Converter

Tips

• The larger the chosen value of the inductor L, the smaller the current ripple ΔI_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 ΔI_L = 2I_out at V_{in_max}. However, the switching losses at the transistors are at their highest in this state.
• Choose ΔI_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 N₁/N₂ proposed by us.

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 N₁/N₂ and L:

(the factor of 0.95 is taken into account to allow for the fact that the duty cycle t₁/T = 0.5 cannot be completely reached).






  V_F = 0.7 (Diode Forward-voltage)

  ΔI_L = 0.4I_out

From this it follows that:

1. For ΔI_L< 2I_out the converter is in continuous mode and it follows that:
   
   ,
   
   
   
   
   
   
   
2. For ΔI_L> 2I_out the converter is in discontinuous mode and it follows that:
   
   ,
   
   
   
   

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