Introdruction which is generally a MOSFET used to control

Introdruction :-

Switching power
converters consists of switching regulators for converting electrical power
either from AC to DC or from DC to DC. There are different types of switching
power converters like buck converter, boost converter and buck-boost
converters. We will be discussing each of these power converters.

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Buck
converters :- This is a power converter which converters DC to
DC where the input voltage is stepped-down to get the output voltage. It is
widely used in computers where the main supply voltage (12 V) is stepped down
and given to USD, CPU and DRAM. The figure 1 below shows the circuit diagram of
a buck converter.

The circuit consists of
a switch which is generally a MOSFET used to control the transfer of energy. It
consists of inductor which is used to store the energy and smoothen the
current. Capacitor is used to maintain the output voltage and a resistor to
draw the current from the circuit. It consists of a diode which acts as a
passive switch and assures the current flow is continuous in the circuit. The
switch in the circuit i.e the MOSFET is driven by the PWM signal. Ton is the
time when the switch is on and Toff is the time when the switch is off. We can
define the duty cycle as D=Ton/Ton+Toff is the fraction of time period when the
MOSFET is on.

When the switch is on,
the input voltage is given to the inductor and the inductor gets charged. The
diode which is a passive switch turns off because of the reverse bias and the
circuit reduces to the figure 2 as shown below.

When the switch is off,
if the diode is ideal then its voltage drops to zero and diode acts as parallel
connection between the ground and the inductor. So the current in the inductor
decreases. Here we have two conditions. In continuous conduction mode, the
inductor current in steady state does not reaches to zero. In discontinuous
conduction mode, the inductor current in steady state reaches to zero. The
figure 3 shows the buck converter when switch is off.

The output voltage in continuous conduction mode is given
by

where D is the duty cycle of the time period.

The output voltage in discontinuous conduction mode
is by

Where Io is output current and T is total time period.

Boost converter :-
This is a power converter which converters DC to DC where the input voltage is
stepped-up to get the output voltage. They are used in power amplifiers,
battery power applications and adaptive control applications etc. The figure 4
shows the circuit diagram of the boost converter.

When switch is on, the
input voltage is connected to the inductor and the current flows in clockwise
direction and the inductor gets charged. Diode will be off because of reverse
bias. The inductor’s left side will have positive polarity. The figure 5 shows
the boost converter when switch is on.

When switch is off, the
inductor current starts to decrease and the inductor’s left side will have
negative polarity. Boost converter also works in two different modes i.e.
continuous conduction mode and discontinuous conduction mode. Figure 6 shows
the boost converter when switch is off.

 

The output voltage in continuous conduction mode is
given by

The output voltage in discontinuous conduction mode
is by

Where K=2L/RT

Buck-Boost
converter :- This is a power converter which converters DC to
DC where the output voltage is either more or less than the input voltage.