DC-DC converters are power electronic converters which convert a fixed DC voltage to a variable DC voltage.
Boost converter is a DC-DC converter which produces a DC output voltage greater than the input DC voltage. Hence boost converter is also called as step-up converter.
Read about buck converter/step down converter
Boost converter requires a large inductance in series with source for boosting the source voltage.
Operation of boost converter
The boost converter consists of a controlled switch, a diode, an inductor and a capacitor. The switch controls the flow of input power to the output by turning ON and OFF periodically.
Ton – time for which switch is ON
Toff – time for which switch is OFF
Total time T= Ton + Toff
Duty cycle D= Ton/ T
The value of duty cycle D ranges between 0 and 1.
When the switch is ON, the inductor stores the energy. The input source current is equal to the inductor current. As D is reverse biased and the input is isolated from the output side, the capacitor supplies the load current.
Find the inductor voltage and capacitor current equation
When the switch is OFF, the energy stored in the inductor is released to load.
The inductor voltage over a time period is zero under steady state conditions.
where D is called the duty cycle D.
In steady state, the average capacitor current over a time period T is always zero.
The inductor current ripple can be obtained from either ON or OFF condition of the switch. Here ON condition of the switch is considered to find the current ripple. It can be observed that current ripple is inversely proportional to the inductance and frequency. As frequency increases, filter size decreases.
There are two modes of operation in dc-dc converters. Continuous conduction mode and discontinuous conduction mode. In continuous conduction mode, the inductor current remains positive while it becomes zero for a short period in discontinuous conduction mode. Critical inductance- minimum inductance required to maintain continuous conduction mode of boost converter. If the chosen inductance is less than the critical inductance, the converter may go to discontinuous mode otherwise it may remain in continuous conduction mode.
At Boundary condition, ΔIL /2= Ilavg
Io =Vo/R
The output voltage ripple is mainly reduced by the capacitor. Normally, for converter design, the percentage of ripple allowed will be given. The charge stored on the capacitor can be used to calculate the output voltage ripple. The charge on the capacitor during the ON period of the switch is a rectangle. The area of the rectangle ( b*h) gives the stored charge.
Boost converter- step up converter
Output voltage = Vs / (1-D)
Higher values of D is not possible due to the limitation of inductor.
More inductance leads to increase in size, more losses.
Frequency plays an important role in the selection of inductor core ( iron core/ air core)
Power Electronics. Converters, Applications and Design (Ned Mohan, Tore M. Undeland, William P. Robbins), John Wiley and Sons, Inc, 2003.
Fundamentals of power electronics,(Robert Warren Erickson, Dragan Maksimović), Springer, 2001.
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