The Cuk converter is a
step-down/step-up converter based on a switching boost-buck topology. Essentially,
the converter is composed of two sections, an input stage and an output stage.
The input voltage vg is fed into the
circuit via inductor L1. When transistor Q1 is on, current i1 builds the
magnetic field of the inductor in the input stage. The
diode CR1
is
reverse biased, and energy dissipates
from the storage elements in the
output stage. When Q1
turns off,
inductor L1
tries to
maintain the current flowing through it by reversing polarity and sourcing
current as its magnetic field collapses. It thus provides energy to the output
stage of the circuit via capacitor C1. R1 and R2 are parasitic or stray resistances of inductor.
The inductor currents are the input
and output currents, therefore, if the principle of conservation of energy is
applied:
where Ds is the duty cycle of the switch :
The voltage ratio of a Cuk converter
is the same as that of a buck-boost converter, but its main advantage over
other converters is that the input and output inductors result in a filtered
current on both sides of the converter, while buck, boost, and buck-boost converters
have a pulsating current that occurs on at least one side of the circuit i.e
either on input side or output side.
This pulsation will increase the
ripple in the circuit and due to this ripple , the efficiency of battery gets
lowered. To ensure good efficiency ripple should be reduced.
By controlling the duty cycle of the
switch , the output voltage vo can be controlled and
can be higher or lower than the input voltage vg. By using a controller to vary the duty
cycle during operation, the circuit can also be made to reject disturbances ,as
second part of circuit consists of parallel resonance circuit and it work as a
tank circuit for specific frequency (resonant frequency) , and during resonance
current will not be allowed to enter in the circuit.