this is introduction to chopper circuits. basic chopper topolagies and control methods are discussed

1. DC-DC CONVERTERS
Avinash Maurya
Y.Prakash Reddy

2. Overview
What is DC-DC converter
Applications
Basic converters
a) Buck converter
b) Boost converter
c) Buck-Boost converter
 Methods of control

3. DC to DC converter is very much needed nowadays as many
industrial applications are dependent upon DC voltage source. The
performance of these applications will be improved if we use a
variable DC supply. It will help to improve controllability of the
equipment also.
Chopper is a basically static power electronics device which
converts fixed dc voltage/power to variable DC voltage or power. It
is nothing but a high speed switch which connects and disconnects
the load from source at a high rate to get variable or chopped voltage
at the output.

4. • Chopper can increase or decrease the dc voltage level at its
opposite side. So, chopper serves the same purpose in dc circuit
transfers in case of ac circuit. So it is also known as DC
transformer.
• Devices used in Chopper
• Low power application : GTO, IGBT, Power BJT, Power
MOSFET etc. High power application : Thyristor or SCR.
• These devices are represented as a switch in a dotted box for
simplicity. When it is closed current can flow in the direction of
arrow only.

5. Buck converter
5

7. • A buck converter (step-down converter) is a DC-to-DC power
converter with an output voltage less than its input voltage.
• The inductor current rises as the high-side switch turns on and
transfers energy from the input to the inductor.
• The inductor current falls as the low-side switch turns off and
transfers the inductor energy to the output load.
• Operational Equations:
Vout=D*Vin
D=Ton⁄(Ton+Toff )
• The output current is the average value of the inductor current.
• The inductor value is chosen for certain operating conditions such
as input and output voltage and output-load current and for desired
design criteria such as efficiency, tolerable output ripple noise etc.

8. Boost converter
On State Off state
N.P

10. • A boost converter (step-up converter) is a DC-to-DC power
converter with an output voltage greater than its input voltage.
• Load current rises as the low-side switch turns on and transfers
energy from the input to the inductor and it falls as the high-side
switch turns off and transfers the inductor energy to the output load.
• Inductor provides continuous conduction.
• Operational equations :
Vout=V/(1-D)
Vind=Vin-Vout
di/dt=Vin/L
• For discontinuous mode, inductor current will decrease to zero
• Filters made of capacitors (sometimes in combination with
inductors) are normally added to the output of the converter to
reduce output voltage ripple.

11. On-State
• When S is on (D is off),
capacitor energy supplies the
load voltage.
Vo= Vc (if capacitor is
charged)
• During on-state of switch S,
voltage across inductor
instantly becomes equal to
input supply voltage. Current
through it increases gradually
and stores energy in its
magnetic field.
• For very first time, when S is
closed Vo=0, as capacitor is
not charged.
Off-State
• When S is off (D is on), inductor
voltage reverses its polarity and
adds in input voltage to provide
output voltage which is equal to:
V0=Vi + VL
• During off state of S, capacitor
charges and voltage at it gradually
build up to Vi + VL
(This capacitor voltages serves as
load voltage when next time S in
on)
• If S is off forever, inductor acts as
short circuit. It does not develop
any voltage and
Vo= Vi
N.P

12. BUCK-BOOST CONVERTER

14. • The buck–boost converter is a type of DC-to-DC converter that
has an output voltage magnitude that is either greater than or less
than the input voltage magnitude.
• The output voltage is adjustable based on the duty cycle of the
switching transistor.
Vout= Vin*(D/1-D)
• If duty cycle is less than 0.5, output voltage is less than supply
voltage.
• If duty cycle is greater than 0.5,output voltage is greater than
supply voltage.
• The polarity of output voltage is reversed to that of input voltage
in buck-boost converters.

15. METHODS OF CONTROL:
The output dc voltage can be verified by the following methods.
• Constant Frequency Control or Pulse width modulation control
• Variable Frequency Control
PULSE WIDTH MODULATION
In Pulse width modulation the pulse width ton of the output waveform
is varied keeping chopping frequency ’f’ and hence chopping period ‘T’
constant. Therefore output voltage is varied by varying the ON time, ton .
Figure shows the output voltage waveform for different ON times.
VARIABLE FREQUENCY CONTROL
In this method of control, chopping frequency f is varied keeping
either ton or toff constant. This method is also known as frequency
modulation.
In frequency modulation to obtain full output voltage, range of
frequency has to be varied over a wide range. This method produces
harmonics in the output and for large toff load current may be
discontinuous.

16. V0
V
V
V0
t
t
tON
tON tOFF
tOFF
T

17. v0
V
V
v0
t
t
tON
tON
T
T
tOFF
tOFF

19. Thank you