If the switch is cycled fast enough, the inductor will not discharge fully in between charging stages, and the load will always see a voltage greater than that of the input source alone when the switch is opened. Also while the switch is opened, the capacitor in parallel with the load is charged to this combined voltage. When the switch is then closed and the right hand side is shorted out from the left hand side, the capacitor is therefore able to provide the voltage and energy to the load. During this time, the blocking diode prevents the capacitor from discharging through the switch. The switch must of course be opened again fast enough to prevent the capacitor from discharging too much.
Transcript of "Choppers"
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ChoppersFixed DC Supply Variable DC Output?A. K. GautamBy: Aniruddha Kr. Gautam
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• A chopper is a static device which is used toobtain a variable dc voltage from a constant dcvoltage source.• Also known as dc‐to‐dc converter.• It offers greater efficiency, faster response, lowermaintenance, smaller size and smooth control.• Widely used in trolley cars, battery operatedvehicles, traction motor control, control of largenumber of dc motors, etc…• Also used in regenerative braking of dc motors toreturn energy back to supply and also as dcvoltage regulators.A. K. Gautam
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METHODS OF CONTROL:The output dc voltage can be verified by the following methods.• Constant Frequency Control or Pulse width modulation control• Variable Frequency ControlPULSE WIDTH MODULATIONIn Pulse width modulation the pulse width ton of the output waveformis 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 CONTROLIn this method of control, chopping frequency f is varied keepingeither ton or toff constant. This method is also known as frequencymodulation.In frequency modulation to obtain full output voltage, range offrequency has to be varied over a wide range. This method producesharmonics in the output and for large toff load current may bediscontinuous.A. K. Gautam
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• Diode is reversed biased.Switch conducts inductorcurrent• This results in positiveinductor voltage, i.e.:VL=Vd-VO• It causes linear increase inthe inductor current.A. K. GautamCircuit operation when switch is turned on (closed)
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Operation when switch turned off (opened)• Because of inductive energystorage, iL continues to flow.• Diode is forward biased.• Current now flows throughthe diode and,A. K. Gautam
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Analysis for switch closed• The inductor voltage• Since the derivative if iL is apositive constant. ThereforeiL must increase linearly.A. K. Gautam
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Steady-state operation• Steady state operationrequires that iL• Steady state operationrequires that iL at the end ofswitching cycle is the sameat the beginning of the nextcycle. That is change of iLover one period is zero. i.e.A. K. Gautam
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Steady State Time Domain Analysis of Type-A ChopperA. K. GautamOutputvoltageOutputcurrentv0Vi0ImaxImintttONTtOFFContinuouscurrentOutputcurrenttDiscontinuouscurrenti0
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Expressions For Load Current iO For Continuous CurrentOperation When Chopper is ON (0 t tON)A. K. GautamVi0V0RLE+-
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Steady State RippleA. K. Gautam)26.2.........(....................11111111111)(minmaxaaonaonaaaonaonaaonaaonaaonTTTTTTTSTTTTTTTTTTTTSTTTTTTTTSeeeRVeeeeeeRVeeeeRVIIThe ripple current given by the equation is seen to be independent of loadcurrent emf E With Ton= αT and T-Ton = (1-α)T can be written asaaTaTTTTTSeeeRVII1111minmax
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Limit of Continuous Conduction• In a chopper if Ton is reduced , Toff increases for a constant chopping periodT.• As Ton gets low and Toff goes high so current I may fall to zero.• As in step down chopper current cannot go reverse so for limit current cango to only zero from Imin (Eqn. 2.20)• We can get value of α is obtain by equating the above equation.A. K. Gautam)27.2.......(....................11ln11011minaaaonaaonTTaonSTTTTTTTTSemTTTTmVEeeREeeRVI
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• So for Given Value of E, VS, T and Ta; if duty cycle is α’ thencurrent goes at just continuous point.• If actual duty cycle is less than α’ , load current will bediscontinuous.A. K. Gautam
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• A boost converter (step-up converter) is a DC-to-DC power converter with an output voltage greaterthan its input voltage.• It is a class of switched-mode power supply (SMPS)containing at least two semiconductor switches(a diode and a transistor) and at least one energystorage element, a capacitor, inductor, or the two incombination.• Boost converter is also used as the voltage increasemechanism in the circuit known as the Joule thief.• For a step-up chopper we can obtain an averageoutput voltage V0 greater than input voltage.A. K. Gautam
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• Step-up chopper is used to obtain a load voltage higherthan the input voltage V.• The values of L and C are chosen depending upon therequirement of output voltage and current.• When the chopper is ON, the inductor L is connectedacross the supply.• The inductor current ‘I’ rises and the inductor storesenergy during the ON time of the chopper, tON.• When the chopper is off, the inductor current I is forcedto flow through the diode D and load for a period, tOFF.• The current tends to decrease resulting in reversing thepolarity of induced EMF in L.• Therefore voltage across load is given by:A. K. Gautam
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• A large capacitor ‘C’ connected across the load, will provide acontinuous output voltage .• Diode D prevents any current flow from capacitor to thesource.A. K. Gautam. .,O OdIV V L i e V Vdt
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Operating Principle• The key principle that drives the boost converter is thetendency of an inductor to resist changes in current.Mode 1 (On State):When the switch is closed, current flows through the inductor inclockwise direction and the inductor stores the energy. Polarityof the left side of the inductor is positive.A. K. Gautam
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Mode 2 (Off State)When the switch is opened, current will be reduced as theimpedance is higher. Therefore, change or reduction in currentwill be opposed by the inductor. Thus the polarity will bereversed (means left side of inductor will be negative now). Asa result two sources will be in series causing a higher voltageto charge the capacitor through the diode D.Here the switch is open and the only path offered to inductorcurrent is through the flyback diode D, the capacitor C and theload R.A. K. Gautam
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Mode of conductionContinuous Conduction mode:• When a boost converter operates in continuous mode, the current throughthe inductor (IL) never falls to zero.• During the On-state, the switch S is closed, which makes the input voltage(Vi) appear across the inductor.• Change in current (IL) flowing through the inductor during a time period (t)by the formula:• At the end of the On-state, the increase of IL is therefore:A. K. GautamLVtI iLiDTiL VLDTdtVLI on01
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• D is the duty cycle. It represents the fraction of thecommutation period T during which the switch is On.• D ranges between 0 (S is never on) and 1 (S is always on).• During the Off-state, the switch S is open, so the inductorcurrent flows through the load. If we consider zero voltagedrop in the diode, and a capacitor large enough for its voltageto remain constant, the evolution of IL is:• Therefore, the variation of IL during the Off-period is:A. K. GautamdtdILVV LoiLTDVVLdtVVI oiTDToiLoff1)()(
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• considering that the converter operates in steady-stateconditions, so the amount of energy stored in each of itscomponents has to be the same at the beginning and at the endof a commutation cycle. In particular, the energy stored in theinductor is given by:• So, the inductor current has to be the same at the start and endof the commutation cycle. This means the overall change inthe current (the sum of the changes) is zero:A. K. Gautam221LLIE010LTDVVLDTVIIIIoiiLlLloffonoffon
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• Expression shows that the output voltage isalways higher than the input voltage (as theduty cycle goes from 0 to 1), and that itincreases with D,A. K. GautamoiioVVDDVV111
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Discontinuous Conduction mode:• If the ripple amplitude of the current is too high, theinductor may be completely discharged before theend of a whole commutation cycle.• Occurs under light loads.• Here the current through the inductor falls to zeroduring part of the period.• As the inductor current at the beginning of the cycleis zero, its maximum value:A. K. Gautam)( DTtatLDTVI iLMax
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• During the off-period, IL falls to zero after :• The load current Io is equal to the average diode current (ID).Also the diode current is equal to the inductor current duringthe off-state. Therefore the output current can be written as:A. K. GautamioiiLVVDVSoLTVVI Max,0)( 0oiioioiioiioLDoLITDVVVSoVVLTDVVVDVLDTVISoIII Maz21,22,2222
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Types of Choppers• Type A Chopper or First–Quadrant Chopper• Type B Chopper or Second-Quadrant Chopper• Type-C chopper or Two-quadrant type-AChopper• Type-D Chopper or Two-Quadrant Type–BChopper• Type-E chopper or the Fourth-QuadrantChopperA. K. Gautam
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Type A Chopper or First–Quadrant ChopperA. K. Gautam
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• When chopper is ON, supply voltage V is connected across theload.• When chopper is OFF, vO = 0 and the load current continues toflow in the same direction through the FWD.• The average values of output voltage and current are alwayspositive.• Class A Chopper is a step-down chopper in which poweralways flows form source to load.• It is used to control the speed of dc motor.• The output current equations obtained in step down chopper• with R-L load can be used to study the performance of Class AChopperA. K. Gautam
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Type B Chopper or Second-Quadrant ChopperA. K. Gautam
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• When chopper is ON, E drives a current through L andR in a direction opposite to that shown in figure.• During the ON period of the chopper, the inductance Lstores energy.• When Chopper is OFF, diode D conducts, and part ofthe energy stored in inductor L is returned to the supply.• Average output voltage is positive and average outputcurrent is negative.• In this chopper, power flows from load to source.• Class B Chopper is used for regenerative braking of dcmotor.• Class B Chopper is a step-up chopper.A. K. Gautam
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Type-C chopper or Two-quadrant type-A ChopperA. K. Gautam
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• Class C Chopper is a combination of Class A and Class BChoppers.• For first quadrant operation, CH1 is ON or D2 conducts.• For second quadrant operation, CH2 is ON or D1 conducts.• When CH1 is ON, the load current is positive.• The output voltage is equal to ‘V’ & the load receives powerfrom the source.• When CH1 is turned OFF, energy stored in inductance L forcescurrent to flow through the diode D2 and the output voltage iszero.• Current continues to flow in positive direction.• When CH2 is triggered, the voltage E forces current to flow inopposite direction through L and CH2• The output voltage is zero.A. K. Gautam
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• On turning OFF CH2, the energy stored in the inductancedrives current through diode D1 and the supply Output voltageis V, the input current becomes negative and power flows fromload to source.• Average output voltage is positive• Average output current can take both positive and negativevalues.• Choppers CH1 & CH2 should not be turned ONsimultaneously as it would result in short circuiting the supply.• Class C Chopper can be used both for dc motor control andregenerative braking of dc motor.• Class C Chopper can be used as a step-up or step-downchopper.A. K. Gautam
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Type-D Chopper or Two-Quadrant Type –B ChopperA. K. Gautam
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• Class D is a two quadrant chopper.• When both CH1 and CH2 are triggered simultaneously,the output voltage vO = V and output current flowsthrough the load.• When CH1 and CH2 are turned OFF, the load currentcontinues to flow in the same direction through load, D1and D2, due to the energy stored in the inductor L.• Output voltage vO = - V• Average load voltage is positive if chopper ON time ismore than the OFF time• Average output voltage becomes negative if tON < tOFF .• Hence the direction of load current is always positive butload voltage can be positive or negative.A. K. Gautam
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Type-E chopper or the Fourth-Quadrant ChopperA. K. Gautam
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• Class E is a four quadrant chopper• When CH1 and CH4 are triggered, output currentiO flows in positive direction through CH1 andCH4 , and with output voltage vO = V.• This gives the first quadrant operation.• When both CH1 and CH4 are OFF, the energystored in the inductor L drives iO through D2 andD3 in the same direction, but output voltage vO = -V.• Therefore the chopper operates in the fourthquadrant.A. K. Gautam
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• When CH2 and CH3 are triggered, the load current iOflows in opposite direction & output voltage vO = -V.• Since both iO and vO are negative, the chopperoperates in third quadrant.• When both CH2 and CH3 are OFF, the load current iOcontinues to flow in the same direction D1 andD4 andthe output voltage vO = V.• Therefore the chopper operates in second quadrant asvO is positive but iO is negative.A. K. Gautam
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