Buck Boost 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} .
• 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, the proposed choking coil L is chosen such that ΔI_L is 40% of the average value of the inductor current I_L, for V_{in_min} as the input voltage.
• If you should not be content with the proposed value, you can change L or Δ I_L. If this is the case, the field "proposal" is 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: Buck Boost Converter

Function principals:

During the on-time of the transistor, there is an input voltage V_in applied across the inductor L. The inductor current I_L increases linearly. Energy is loaded into the inductor.

During the blocking phase of the transistor, the current I_L continues to flow through the inductor and loads the output capacitor C_out. The inductor transfers its energy to C_out.

One can make a distinction between continuous and discontinuous mode depending on whether the inductor current I_L goes to zero or not.
For continuous mode using Faraday's Law gives:

From this it follows that:

• In continuous mode the output voltage depends only on the duty cycle t₁/T and the input voltage V_in, it is independent of the load.

Continuous Mode	Discontinuous Mode

Illustration 2: Operating modes of the Buck Boost 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.
• 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.
• 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.

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:

where V_F = 0.7 (Diode Forward-voltage) and

From this it follows that:

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

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