DC-DC converters are power electronic converters which convert a fixed DC voltage to a variable DC voltage.
Buck converter is a type of DC-DC converter which produces a DC output voltage less than the input DC voltage. Hence a buck converter is also called as step-down converter.
A low pass LC filter is usually connected at the output of the DC-DC converter to reduce the ripple and harmonics.
The inductance helps to reduce the current ripple while the capacitor reduces the output voltage ripple.
Operation of buck converter
The buck 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.
According to Kirchoff’s current law, the inductor current IL is equal to sum of the capacitor current ic and load current Io.
When IL > Io, Capacitor charges
When IL < Io, Capacitor discharges
Find the inductor voltage and capacitor current equation
When the switch is OFF, the energy stored in the inductor is released to load.
The load current freewheels through the freewheeling diode.
The inductor voltage over a time period is zero under steady state conditions.
VL * Ton + VL * Toff = 0
(Vs – Vo )* DT + ( – Vo )* (1-D)T = 0
Vs D – Vo D – Vo + Vo D=0
Vo = DVs
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 for a short period in discontinuous conduction mode. Critical inductance- minimum inductance required to maintain continuous conduction mode of buck 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.
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 find the The charge stored on the capacitor can be used to calculate the output voltage ripple.The charge on the capacitor can be represented by a triangle and the area of the triangle ( bh/2) gives the stored charge.
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