POWER ELECTRONICS

1. Shantilal Shah Engineering College

2. SUBJECT : POWER ELECTRONICS
TOPIC:DC TO DC CONVERTER
(CHOPPERS)
Branch : Electrical
Sem : 5th
Prepared By :
NO. NAME ENROLLMENT NO
1 MER VISHAL GIRISHBHAI 130430109031
2 PATEL BHAVIN 130430109039
3 SINGH SUDHIR 130430109052
GUIDED BY :
PROF. J.K.CHAVDA

3. 3
DC-DC Converters
 Convert a fixed DC Source into a Variable DC Source
 DC equivalent to an AC transformer with variable turns ratio
 Step-up and Step-down versions
 APPLICATION
 Motor Control
 Voltage Regulators

4. 4
 Step-down Operation
• Switch SW is known as a “Chopper”
• Use BJT, MOSFET, or IGBT
• Close for time t1
• VS appears across R
• Open for time t2
• Voltage across R = 0
• Repeat
• Period T = t1 + t2

5. 5
 Waveforms for the Step-Down Converter

6. 6
 Average Value of the Output Voltage
1
1
0
0
1
1
1
1
t
a O
t
a S
a S S
a S
V v dt
T
V V dt
T
t
V V ft V
T
V kV


 




7. 7
 Average Value of the Load Current
1
a S
a
V kV
I
R R
T period
t
k dutycycle
T
f frequency
 

 


8. 8
 RMS Value of the output voltage
1
2
2
0
1
2
2
0
1
1
kT
O O
kT
O S
O S
V v dt
T
V V dt
T
V kV
 
  
 
 
  
 




9. 9
 If the converter is “lossless”, Pin = Pout
0
2
0
2
2
1
1
1
kT
in O
kT
O
in
S
in
S
in
P v idt
T
v
P dt
T R
V
P kT
T R
V
P k
R







10. 10
 Modes of Operation
• Constant – frequency operation
• Period T held constant, t1 varied
• Width of the pulse changes
• “Pulse-width modulation”, PWM
• Variable -- frequency operation
• Change the chopping frequency (period T)
• Either t1 or t2 is kept constant
• “Frequency modulation”

11. G1
G2
VS
C
A2
T2
K2
L
A
D
K
AF
KF
FD
RL
LL
DC CHOPPER USING THE SCR
CIRCUIT DIAGRAM
 Working Principle For convenience the
chopper operation is divided into five
modes.
 1. Mode-1
 2. Mode-2
 3. Mode-3
 4. Mode-4
 5. Mode-5
T1

12. 30V
V
 MODE -1 OPERATION
G1
G2
VS
C
A2
T2
K2
L
A
D
K
AF
KF
FD
RL
LL
»»»
»»
»»»»
»»
»»»»
»»»
»»»»
»»»»
V
30V
Thyristor T1 is fired at t = 0.
• The supply voltage comes across the
load.
• Load current IL flows through T1 and
load.
• At the same time capacitor discharges
through T1, D1, L1, & ‘C’ and the
capacitor reverses its voltage.
• This reverse voltage on capacitor is
held constant by diode D1
T1

13. 30V
V
MODE -2 OPERATION
G1
G2
VS
C
A2
T2
K2
L
A
D
K
AF
KF
FD
RL
LL
»»»
»»
»»»»
»»
»»»»
»»
»»»»
V
30V
• Thyristor T2 is now fired to
commutate thyristor T1.
• When T2 is ON capacitor voltage
reverse biases T1 and turns if off.
• The capacitor discharges through
the load from –V to 0.
• Discharge time is known as
circuit turn-off time.
• Capacitor recharges back to the
supply voltage (with plate ‘a’
positive).
T1

14. 30V
V
MODE -3 OPERATION
G1
G2
VS
C
A2
T2
K2
L
A
D
K
AF
KF
FD
RL
LL
»»»
»»
»»»»
»»
»»»»
»»
»»»»
V
30V
FWD starts conducting and
the load current decays.
• Hence capacitor charges to a
voltage higher than supply
voltage, T2 naturally turns
off.
T1

15. 30V
V
MODE -4 OPERATION
G1
G2
VS
C
A2
T2
K2
L
A
D
K
AF
KF
FD
RL
LL
»»»
»»
»»»»
»»
»»»»
»»
»»»»
V
30V
 Capacitor has been overcharged i.e.
its voltage is above supply voltage.
 Capacitor starts discharging in
reverse direction.
 Hence capacitor current becomes
negative.
 The capacitor discharges through VS,
FWD, D1 and L.
 When this current reduces to zero D1
will stop conducting and the capacitor
voltage will be same as the supply
voltage
T1

16. 30V
V
MODE -5 OPERATION
G1
G2
VS
C
A2
T2
K2
L
A
D
K
AF
KF
FD
RL
LL
»»
»»»
V
30V
»»»
»»
 Both thyristors are off
and the load current
flows through
 the FWD.
 This mode will end once
thyristor T1 is fired.
T1

18. THANKFUL TO YOU !!