UNIT-7 CHOPPERS 12/1/2018

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 12/1/2018

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 12/1/2018

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 12/1/2018

The nature of outptut current is same as output voltage. 12/1/2018

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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 12/1/2018

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’ 12/1/2018

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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 12/1/2018

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 12/1/2018

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 12/1/2018

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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 12/1/2018

Classification Of Choppers Choppers are classified as : Class A Chopper Class B Chopper Class C Chopper Class D Chopper Class E Chopper 12/1/2018

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. 12/1/2018

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. 12/1/2018

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. 12/1/2018

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. 12/1/2018

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) 12/1/2018