Boost Converter

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

How to use the program

Reference: The shapes of current and voltage curves are calculated using Faraday's Law. They do not represent an incremental simulation like it is done normally by programs like P-Spice. In the calculations the forward voltages of the diodes are considered with VF = 0.7V, and the transistors are interpreted as ideal switches.

Top of page


The Boost Converter converts an input voltage to a higher output voltage. It is also named the step-up converter. Boost converters are used in battery powered devices, where the electronic circuit requires a higher operating voltage than the battery can supply, e.g. notebooks, mobile phones and camera flashes.

Top of page

Function principals:

Illustration 1: Boost Converter

The transistor works as a switch which is turned on and off by a pulse-width-modulated control voltage. The ratio between on-time and the period t1/T is called the Duty Cycle.

For the following analysis it will be assumed that the transistor is simplified as an ideal switch and the diode has no forward voltage drop. In the program itself, the diode will take into account a forward voltage drop VF = 0.7V.

During the on-time of the transistor, the voltage across L is equal to Vin and the current IL increases linearly. When the transistor is turned off, the current IL flows through the diode and charges the output capacitor.
The function of the boost converter can also be described in terms of energy balance:
During the on-phase of the transistor, energy is loaded into the inductor.
This energy is then transferred to the output capacitor during the blocking phase of the transistor.

The output voltage is always larger than the input voltage. Even if the transistor is not switched on and off the output capacitor charges via the diode until Vout = Vin. When the transistor is switched the output voltage will increase to higher levels than the input voltage.

A distinction is drawn between discontinuous and continuous mode depending on whether the inductor current IL reduces to zero during the off-time or not.
With the help of Faraday's Law the continuous mode and steady state conditions can be established.


From this it follows that:


In discontinuous mode, the inductor current IL will go to zero during every period. At the moment when the inductor current becomes zero, i.e. t2, the voltage V1 jumps to the value of Vout because in this case VL = 0. The drain-source capacitance in parallel with the diode-junction capacitance forms a resonant circuit with the inductance L. This is stimulated by the voltage jump across the diode. The voltage V1 then oscillates and fades away.

Continuous Mode

Discontinuous Mode

Illustration 2: Operating modes of the Boost Converter

Top of page


Top of page

Mathematics used in the program

The following parameters must be entered into the input fields:

Vin_min, Vin_max, Vout, Iout and  f 

Using these parameters, the program produces a proposal for L:

For the calculation of the curve-shapes, and also for the calculation of "ΔIL with Vin_min", two cases have to be distinguished, i.e. continuous mode and discontinuous mode:

Eqn 3.2

Eqn 3.3

From this it follows that:

  1. For ΔIL< 2Iin the converter is in continuous mode and it follows that:

    Eqn 4.0

    Eqn 4.1

    Eqn 4.2

  2. For ΔIL> 2Iin the converter is in discontinuous mode and it follows that:

    Eqn 5.0

    Eqn 5.1

    Eqn 5.2

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