DC-DC Converters PDF

DC-DC BUCK CONVERTER
SHORT REPORT
OBJECTIVE
To understand and design a DC to DC Buck Converter.
COMPONENTS
 DC power supply
 Resistors
 Diodes
 Inductor
 Capacitors
 MOSFET
THEORY
DC-DC CONVERTER:
A DC-DC converter converts directly from dc to dc, it can be used to step down or step up a dc voltage
source. The output voltage of the DC to DC converter is less than or greater than the input voltage and
the magnitude depends on the duty cycle.
BUCK CONVERTER:
A buck converter is a DC to DC power converter which steps down voltage from its input to its output. The
Buck converter circuit consists of the switching transistor, together with the flywheel circuit. While the
transistor is on, current is flowing through the load via inductor that also acts as a store of energy. This
stored energy is later released back into the circuit as back e.m.f as current from the transistor is rapidly
switched off. MOSFET can be turned ON by applying a Voltage greater than Vgs threshold across gate and
source. Similarly, MOSFET can be turned OFF by disconnecting the gate source voltage. Gate pulse should
be capable of turn on MOSFET within very smaller time as compared to total switching.
WORKING
When switch S is ON, diode D is reverse biased and voltage across inductor will be Vs-Vo. So inductor
current increases and attains a peak. When switch S is OFF, diode D is forward biased and voltage across
inductor will be -Vo. So inductor current decreases. Hence inductor with diode ensures an uninterrupted
current flow and hence a constant output voltage with capacitor for removing voltage ripples. Output
voltage is given by
𝑉𝑜 = 𝐷 𝑉𝑠
Where D is duty cycle and VS is input voltage.
Figure 1 Circuit and waveforms of a Buck Converter
Inductor
Capacitor
Load Resistance
SIMULATION Using LTspice

CIRCUIT DESIGN:
Figure 2 LTspice Circuit Design of a Buck Converter

WAVEFORMS:
Figure 3 Waveforms by LTspice
RESULT
A Buck Converter is designed and output waveform is obtained.
DC-DC BUCK BOOST CONVERTER
SHORT REPORT
OBJECTIVE
To understand and design a circuit of a Buck Boost/Cuk converter.
COMPONENTS
 Inductances
 Resistors
 capacitors
 Diodes
 switches
Theory
A buck-boost converter is a type of DC to DC converter in which the input voltages are step-up/down to
some level of more than or less than the input voltage. These converters are also known as the step up
and step down transformers. For the step up mode (Vin < Vout) and (Iin > Iout). In the step down mode
(Vin > Vout) and (Iin < Iout). The buck boost converter is equal to the fly back circuit and single inductor
is used in the place of the transformer.
WORKING
The working operation of the DC to DC converter is the single inductor in the input resistance has the
unexpected variation in the input current. This is a switched-mode power supply and the output voltage
is adjustable based on the duty cycle of the switching transistor. If the switch is closed it discharges the
energy. A steady state output voltage presents at the load terminal.
Figure 4 Circuit of a Buck Boost Converter
A non-inverting buck-boost converter combining, a buck (step-down) converter and a boost (step-up)
converter use a single inductor which is used for both the buck inductor mode and the boost inductor
mode, using switches instead of diodes. In a Buck Boost converter, the polarity of the output voltage is
opposite to that of the input.
SWITCH STATES
SWITCH ON-STATE
When the switch is in the On-State, the input voltage
source is directly connected to the inductor (L). This
results in accumulating energy in L. In this stage, the
input voltage source supplies current to the inductor,
and the capacitor supplies current to the resistor (output
load). Figure 5 On-State
SWITCH OFF-STATE
When in the Off-state, the inductor is connected to the
output load and capacitor. So the switch is opened and
the inductor supplies current to the load C and R via the
diode D. Figure 6 Off-State
MODES OF BUCK BOOST CONVERTER

Continuous Conduction Mode
In the continuous conduction mode the current from
end to end of inductor never goes to zero. Hence the
inductor partially discharges earlier than the switching
cycle.
Figure 7 Waveforms operating in Continuous mode
Discontinuous Conduction Mode

In this mode the current through the inductor goes to
zero. Hence the inductor will totally discharge at the end
of switching cycles.
Figure 8 Waveforms operating in Discontinuous mode

Advantages of Buck Boost Converter
 It gives higher output voltage.
 Low operating duct cycle.
 Low voltage on MOSFETs
Applications of Buck boost converter

 It is used in the Battery power systems.
 Adaptive control applications and power amplifiers.
SIMULATION AND CALCULATION USING MATLAB
FOR BUCK
Code MATLAB Circuit Design
clear all
clc
% initialization
vs=48
vo=15
R=10
r=1/100
f=40000
DFWD=0.76
%parameter
computation
D=vo/(vo+vs)
L=R*(1-D)
/(2*f)
Lnom=1.25*L
Cnom=
(vo*D)
/(2*R*f*r)
%ripple current
sim('buckboost
converter.slx')
Figure 9 MATLAB Circuit Design
WAVEFORMS:
Figure 10 Buck Converter Simulation

FOR BOOST
Code MATLAB Circuit Design
clear all
clc
%initialization
vs=48
vo=100
R=10
r=1/100
f=40000
DFWD=0.76
%parameter
computation
D=vo/(vo+vs)
L=R*(1-D)/(2*f)
Lnom=1.25*L
Cnom=(vo*D)/
(2*R*f*r)
%ripple current
sim('buckboost
converter1.slx')
Figure 11 MATLAB Circuit Design
WAVEFORM
Figure 12 Boost Converter Simulation
RESULT
Circuit designed for a buck boost DC-DC Converter and output waveforms are obtained.

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DC-DC BUCK CONVERTER

SIMULATION Using LTspice

Figure 2 LTspice Circuit Design of a Buck Converter

Figure 3 Waveforms by LTspice

Figure 4 Circuit of a Buck Boost Converter

MODES OF BUCK BOOST CONVERTER

Figure 7 Waveforms operating in Continuous mode

Discontinuous Conduction Mode

Figure 8 Waveforms operating in Discontinuous mode

Applications of Buck boost converter

Figure 10 Buck Converter Simulation

Figure 11 MATLAB Circuit Design

Figure 12 Boost Converter Simulation

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