1. UNIT-7
CHOPPERS
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2. Principle of DC to DC conversion Step down and step up chopper
Introduction
Principle of DC to DC conversion
Step down and step up chopper
Chopper control techniques
Choppers with RL and RLE loads
Performance parameters of choppers
Classification of choppers
Impulse commutated chopper
Applications of chopper
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3. DC chopper is also called as dc to dc converter
Introduction
The DC chopper converts input DC voltage into fixed or variable DC output
DC chopper is also called as dc to dc converter
The output can be greater or less than the input
It uses switching principle and have high efficiency
Dynamic response is very fats due to switching nature of the device
Types of chopper
Step down chopper
Step up chopper
Applications
Dc traction drives,Trolley cars,marine lifts
Used in voltage regulators
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4. Principle of Step down chopper
The switch can be a power transistor,SCR,GTO,power MOSFET,IGBT
The drop in the switch is very small and it is neglected
The thyristor can be act as a switch
When the thyristor is ON , the supply voltage appears across the load
When the thyristor is OFF , the voltage across the load is zero
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5. The nature of outptut current is same as output voltage.
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9. Chopper control techniques
The chopper can be operated as constant frequency or variable frequency and it is called Time Ratio Control.
1.Constant Frequency operation
Chopper frequency is kept constant and hence total period T remains constant
Ton and Toff both are varied to vary the duty cycle
Advantage is that filter components are easy to design
2.Variable Frequency Operation
The frequency of the chopper varies when duty cycle is to be varied
When Ton is varied Toff is kept constant and viceversa
The filter design is difficult for this type of chopper
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10. Step down chopper with RL load
Chopper is ON , supply is connected across the load
Current flows from supply to the load
Chopper is OFF,load current continues to flow in the same direction through FWD due to energy stored in the inductance
Load current can be continues or discontinuos depending on the inductance L and duty cycle ‘d’
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19. PRINCIPLE OF STEP UP CHOPPER
The average value of the output voltage is always less than or equal to supply voltage
It is used to obtain load voltage higher than the input voltage
Chopper is ON, supply is connected across the load
The inductance current increase and it started to store the energy
Chopper is OFF,the inductor current started to flow through the diode and load for some period of time
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20. Vo =V+L di/dt… i.e Vo>V
The current tends to decrease resulting in reversing the polarity of induced EMF in L
Therefore the voltage across the inductance is given by,
Vo =V+L di/dt… i.e Vo>V
The capacitor is connected across the load, inorder to get continuous output voltage
Diode prevents the current flow back to supply
Step up chopper are used for regenerative braking of DC motors
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21. EXPRESSION FOR OUTPUT VOLTAGE When the chopper is ON
Voltage across the inductor, L=V
The energy stored in the inductor is =V.I.Ton
Ton= ON period of chopper
When the chopper is OFF
Voltage across L=Vo-V
Energy supplied by the inductor L= (Vo-V).I.Toff
Toff=OFF period of chopper
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23. Performance Parameters
The thyristor requires minimum time to turn ON and turn OFF
Duty cycle ‘d’ can be varied between min and max value,limiting the min & max value of the output voltage
Ripple in the load current depends inversely on the chopper frequency’f’
To reduce the load ripple current,frequency should be high as possible
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24. Classification Of Choppers
Choppers are classified as :
Class A Chopper
Class B Chopper
Class C Chopper
Class D Chopper
Class E Chopper
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25. Class A chopper with highly inductive load is shown in figure.
When switch is on Vo = Vs .
When switch is off load current is carried by freewheeling diode.
Thus load voltage and current always remains in same direction resulting in first quadrant operation. Power flow always from source to load.
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26. Thus the average output is always positive but for current.
CLASS B CHOPPER
When chopper is on Vo= 0 and E drives current through L and R in opposite direction allowing inductance to store energy through on period.
When chopper is off, diode D conducts ,Vo=V and part of energy stored in inductor is returned to source.
Thus the average output is always positive but for current.
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27. Class C chopper is a combination of a Class A and B chopper.
For first quad operation CH 1is on or D2 conducts.
For second quad operation CH2 is on or D1 conducts.
CH 1is on , i0 is positive and V0= v and the load receives power from the source.
CH1 turned off , energy stored in L forces current to flow through d2 and V0=0 but i0 flows in +ve direction.
CH2 on ,E forces io to flow in opposite direction through L and CH2. hence average V0 is +ve but current becomes –ve.
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28. When both ch1 and ch2 are off , io flows through load , D1 , D2.
CLASS D CHOPPER
When both ch1 and ch2 are triggered simultaneously ,V0=V and output current i0 flows through the load.
When both ch1 and ch2 are off , io flows through load , D1 , D2.
Due to energy stored in L, V0= -V.
Thus average Vo is +ve if Ton is more and –ve if Ton is less.
Load current always +ve but for load voltage.
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29. CLASS E CHOPPER
When CH1 and CH4 are on , io flows in +ve direction with V0=v.(Ist quad operation)
When CH1 and CH4 are off energy stord in L drives io thro’ D3 and D2 in same irection but V0= -v.(IVth quad operation). Direction of E reversed in this mode.
When CH2 and CH3 are on, both io and Vo are –ve.(IIIrd quad operation).
When CH2 and CH3 are off , io flows thro’ D1 and D4 and V0=v and i0 is –ve.(IInd quad operation)
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