Module 3. DC to DC Converters. Version 2 EE IIT, Kharagpur 1

Transcription

1 Module 3 DC o DC Converer erion 2 EE, Kharagpur 1

2 eon 17 ype of Baic DCDC Converer erion 2 EE, Kharagpur 2

3 nrucional Objecive Sudy of he following: hree baic ype of dcdc converer circui buck, boo and buckboo he expreion for he oupu volage in he above circui, wih inducive (R) and baery (or back emf = E) load nroducion n he la module (#2) coniing of eigh leon, he variou ype of circui ued in boh inglephae and hreephae acdc converer, were dicued in deail. hi include halfwave and fullwave, and alo halfconrolled and fullconrolled one. n hi leon he fir one in hi module (#3), firly, hree baic ype of dcdc converer circui buck, boo and buckboo, are preened. hen, he expreion for he oupu volage in he above circui, wih inducive (R) and baery (or back emf = E), i.e., RE, load, are derived, aug coninuou conducion. he differen conrol raegie employed are briefly decribed. Keyword: DCDC converer circui, hyrior chopper, Buck, boo and buckboo converer (dcdc), Sepdown (buck) and epup (boo) chopper, Oupu volage and curren. DCDC Converer here are hree baic ype of dcdc converer circui, ermed a buck, boo and buckboo. n all of hee circui, a power device i ued a a wich. hi device earlier ued wa a hyrior, which i urned on by a pule fed a i gae. n all hee circui, he hyrior i conneced in erie wih load o a dc upply, or a poiive (forward) volage i applied beween anode and cahode eral. he hyrior urn off, when he curren decreae below he holding curren, or a revere (negaive) volage i applied beween anode and cahode eral. So, a hyrior i o be forcecommuaed, for which addiional circui i o be ued, where anoher hyrior i ofen ued. aer, GO came ino he marke, which can alo be urned off by a negaive curren fed a i gae, unlike hyrior, requiring proper conrol circui. he urnon and urnoff ime of GO are lower han hoe of hyrior. So, he frequency ued in GObaed chopper can be increaed, hu reducing he ize of filer. Earlier, dcdc converer were called chopper, where hyrior or GO are ued. may be noed here ha buck converer (dcdc) i called a epdown chopper, wherea boo converer (dcdc) i a epup chopper. n he cae of chopper, no buckboo ype wa ued. Wih he adven of bipolar juncion ranior (BJ), which i ermed a elfcommuaed device, i i ued a a wich, inead of hyrior, in dcdc converer. hi device (NPN ranior) i wiched on by a poiive curren hrough he bae and emier, and hen wiched off by wihdrawing he above ignal. he collecor i conneced o a poiive volage. Nowaday, MOSFE are ued a a wiching device in low volage and high curren applicaion. may be noed ha, a he urnon and urnoff ime of MOSFE are lower a compared o oher wiching device, he frequency ued for he dcdc converer uing i (MOSFE) i high, hu, reducing he ize of filer a aed earlier. hee converer are now being ued for applicaion, one of he mo imporan being Swiched Mode Power Supply (SMPS). Similarly, when applicaion require high volage, nulaed Gae Bipolar ranior (GB) are preferred over erion 2 EE, Kharagpur 3

4 BJ, a he urnon and urnoff ime of GB are lower han hoe of power ranior (BJ), hu he frequency can be increaed in he converer uing hem. So, moly elfcommuaed device of ranior family a decribed are being increaingly ued in dcdc converer. Buck Converer (dcdc) A buck converer (dcdc) i hown in Fig. 17.1a. Only a wich i hown, for which a device a decribed earlier belonging o ranior family i ued. Alo a diode (ermed a free wheeling) i ued o allow he load curren o flow hrough i, when he wich (i.e., a device) i urned off. he load i inducive (R) one. n ome cae, a baery (or back emf) i conneced in erie wih he load (inducive). Due o he load inducance, he load curren mu be allowed a pah, which i provided by he diode; oherwie, i.e., in he abence of he above diode, he high induced emf of he inducance, a he load curren end o decreae, may caue damage o he wiching device. f he wiching device ued i a hyrior, hi circui i called a a epdown chopper, a he oupu volage i normally lower han he inpu volage. Similarly, hi dcdc converer i ermed a buck one, due o reaon given laer. + S Swich + D F Fig. 17.1(a): Buck converer (dcdc) O A D v OFF i Fig. 17.1(b): Oupu volage and curren waveform he oupu volage and curren waveform of he circui (Fig. 17.1a) are hown in Fig. 17.1b. he oupu volage i ame a he inpu volage, i.e., v =, when he wich i ON, during he period,. he wich i urned on a =, and hen urned off a =. hi i erion 2 EE, Kharagpur 4

5 called ON period. During he nex ime inerval,, he oupu volage i zero, i.e., v =, a he diode, DF now conduc. he OFF period i OFF = ON, wih he ime period being = + OFF. he frequency i f = 1/. Wih kep a conan, he average value of he oupu volage i, ON 1 1 = v d = d = ON = k he duy raio i k = ( ON / ) = [ ON /( ON + OFF )], i range being 1. k.. Normally, due o urnon delay of he device ued, he duy raio (k) i no zero, bu ha ome poiive value. Similarly, due o requiremen of urnoff ime of he device, he duy raio (k) i le han 1.. So, he range of duy raio i reduced. may be noed ha he oupu volage i lower han he inpu volage. Alo, he average oupu volage increae, a he duy raio i increaed. So, a variable dc oupu volage i obained from a conan dc inpu volage. he load curren i aumed o be coninuou a hown in Fig. 17.1b. he load curren increae in he ON period, a he inpu volage appear acro he load, and i (load curren) decreae in he OFF period, a i flow in he diode, bu i poiive a he end of he ime period,. Boo Converer (dcdc) A boo converer (dcdc) i hown in Fig. 17.2a. Only a wich i hown, for which a device belonging o ranior family i generally ued. Alo, a diode i ued in erie wih he load. he load i of he ame ype a given earlier. he inducance of he load i mall. An inducance, i aumed in erie wih he inpu upply. he poiion of he wich and diode in hi circui may be noed, a compared o heir poiion in he buck converer (Fig. 17.1a). + D + S Swich Fig. 17.2(a): Boo converer (dcdc) O A D OFF Fig. 17.2(b): Waveform of ource curren (i S ) erion 2 EE, Kharagpur 5

6 he operaion of he circui i explained. Firly, he wich, S (i.e., he device) i pu ON (or urned ON) during he period,, he ON period being. he oupu volage i zero ( v = ), if no baery (back emf) i conneced in erie wih he load, and alo a aed earlier, he load inducance i mall. he curren from he ource ( i ) flow in he inducance. he value of curren increae linearly wih ime in hi inerval, wih ( di d) being poiive. A he curren hrough increae, he polariy of he induced emf i aken a ay, poiive, he lef hand ide of being +ve. he equaion for he circui i, di di = or, = d d he wich, S i pu OFF during he period, ON, he OFF period being OFF =. ( = + OFF ) i he ime period. A he curren hrough decreae, wih i direcion being in he ame direcion a hown (ame a in he earlier cae), he induced emf revere, he lef hand ide of being ve. So, he induced emf (aken a ve in he equaion given laer) i added wih he upply volage, being of he ame polariy, hu, keeping he curren ( i = i ) in he ame direcion. he curren ( i = i ) decreae linearly in he ime inerval,, a he oupu volage i aumed o be nearly conan a v, wih ( i d ) being OFF negaive, a <, which i derived laer. he equaion for he circui i, di di ( ) = + or, = d d he ource curren waveform i hown in Fig. 17.2b. A aed earlier, he curren varie linearly from 1 ( ) o 2 ( ) during he ime inerval, ON. So, uing he expreion for di d during hi ime inerval, 2 1 = = ( / ). Similarly, he curren varie linearly from 2 ( ) o 1 ( ) during he ime inerval, OFF. So, uing he expreion for di d during hi ime inerval, 2 1 = = [( )/ ] OFF. Equaing he wo equaion, ( / ) ON = [( )/ ] OFF, from which he average value of he oupu volage i, 1 1 = = = ( ) = OFF ON 1 ON / 1 k he ime period i = + OFF, and he duy raio i, k = / = / + )], wih i range a 1. k.. he ON ime inerval i ( ) ( ON [ ON ON OFF d = k. A aed in he previou cae, he range of k i reduced. hi i, becaue he imum value i higher han he imum (.), and he imum value i lower han he imum (1.), for reaon given here, which are alo valid here. A hown, he ource curren i aumed o be coninuou. he expreion for he oupu volage can be obained by uing oher procedure. n hi cae, he oupu volage i higher han he inpu volage, a conraed wih he previou cae of buck converer (dcdc). So, hi i called boo converer (dcdc), when a elf erion 2 EE, Kharagpur 6

7 commuaed device i ued a a wich. nead, if hyrior i ued in i place, hi i ermed a epup chopper. he variaion (range) of he oupu volage can be eaily compued. BuckBoo Converer (dcdc) A buckboo converer (dcdc) i hown in Fig Only a wich i hown, for which a device belonging o ranior family i generally ued. Alo, a diode i ued in erie wih he load. he connecion of he diode may be noed, a compared wih i connecion in a boo converer (Fig. 17.2a). he inducor, i conneced in parallel afer he wich and before he diode. he load i of he ame ype a given earlier. A capacior, C i conneced in parallel wih he load. he polariy of he oupu volage i oppoie o ha of inpu volage here. When he wich, S i pu ON, he upply curren ( i ) flow hrough he pah,, S and, during he ime inerval, ON. he curren hrough boh ource and inducor ( i ) increae and are ame, wih ( d i d ) being poiive. he polariy of he induced volage i ame a ha of he inpu volage. he equaion for he circui i, di di = or, = d d + S Swich O A D C Fig. 17.3(a): Buckboo converer (dcdc) OFF Fig. 17.3(b): nducor curren (i ) waveform hen, he wich, S i pu OFF. he inducor curren end o decreae, wih he polariy of he induced emf revering. ( i d ) i negaive now, he polariy of he oupu volage, d being oppoie o ha of he inpu volage, combinaion of load & C, and diode D, during he ime inerval, remain nearly conan, a he capacior i conneced acro he load.. he pah of he curren i hrough, parallel OFF. he oupu volage erion 2 EE, Kharagpur 7

8 he equaion for he circui i, di di = or, = d d he inducor curren waveform i hown in Fig. 17.3b. A aed earlier, he curren varie linearly from o during he ime inerval,. Noe ha and are he imum 1 2 ON 1 2 and imum value of he inducor curren repecively. So, uing he expreion for /. during hi ime inerval, 1 ( ) ON 2 = Similarly, he curren varie linearly from o during he ime inerval,. So, uing 2 1 he expreion for di d during hi ime inerval, 2 1 = ( / ) OFF. Equaing he wo equaion, ( / ) ON = ( / ) OFF, from which he average value of he oupu volage i, ON ON ( ON / ) k = = = ( ) = OFF ON 1 ON / 1 k he ime period i = + OFF, and he duy raio i, k = ( ON / ) = [ ON /( ON + OFF )]. he ON ime inerval i = k. may be oberved ha, for he range k >.5, he oupu volage i lower han he inpu volage, hu, making i a buck converer (dcdc). For he range.5 > k 1., he oupu volage i higher han he inpu volage, hu, making i a boo converer (dcdc). For k =.5, he oupu volage i equal o he inpu volage. So, hi circui can be ermed a a buckboo converer. Alo i may be called a epup/down chopper. may be noed ha he inducor curren i aumed o be coninuou. he range of k i omewha reduced due o he reaon given earlier. he expreion for he oupu volage can be obained by uing oher procedure. Conrol Sraegie n all cae, i i hown ha he average value of he oupu volage can be varied. he wo ype of conrol raegie (cheme) are employed in all cae. hee are: (a) imeraio conrol, and (b) Curren limi conrol. imeraio Conrol n he ime raio conrol he value of he duy raio, k = / ON i varied. here are wo way, which are conan frequency operaion, and variable frequency operaion. Conan Frequency Operaion OFF di d n hi conrol raegy, he ON ime, ON i varied, keeping he frequency ( f = 1/ ), or ime period conan. hi i alo called a pule widh modulaion conrol (PWM). wo cae wih duy raio, k a (a).25 (25%), and (b).75 (75%) are hown in Fig Hence, he oupu volage can be varied by varying ON ime,. erion 2 EE, Kharagpur 8

9 oadvolage v OFF k =.25 v OFF k =.75 Fig. 17.4: Pulewidh modulaion conrol (conan frequency) ariable Frequency Operaion n hi conrol raegy, he frequency ( f = 1/ ), or ime period i varied, keeping eiher (a) he ON ime, conan, or (b) he OFF ime, conan. hi i alo called a ON frequency modulaion conrol. wo cae wih (a) he ON ime, conan, and (b) he OFF ime, conan, wih variable frequency or ime period ( ), are hown in Fig he OFF oupu volage can be varied in boh cae, wih he change in duy raio, k = / ON. OFF erion 2 EE, Kharagpur 9

10 v k =.25 v OFF (a) Conan k =.75 v OFF k =.25 v oad volage OFF k =.75 (b) Conan OFF Fig. 17.5: Oupu volage waveform for variable frequency yem here are major diadvanage in hi conrol raegy. hee are: (a) he frequency ha o be varied over a wide range for he conrol of oupu volage in frequency modulaion. Filer deign for uch wide frequency variaion i, herefore, quie difficul. (b) For he conrol of a duy raio, frequency variaion would be wide. A uch, here i a poibly of inerference wih yem uing cerain frequencie, uch a ignaling and elephone line, in frequency modulaion echnique. (c) he large OFF ime in frequency modulaion echnique, may make he load curren diconinuou, which i undeirable. hu, he conan frequency yem uing PWM i he preferred cheme for dcdc converer (chopper). erion 2 EE, Kharagpur 1

11 Curren imi Conrol A can be oberved from he curren waveform for he ype of dcdc converer decribed earlier, he curren change beween he imum and imum value, if i (curren) i coninuou. n he curren limi conrol raegy, he wich in dcdc converer (chopper) i urned ON and OFF, o ha he curren i mainained beween wo (upper and lower) limi. When he curren exceed upper (imum) limi, he wich i urned OFF. During OFF period, he curren freewheel in ay, buck converer (dcdc) hrough he diode, D F, and decreae exponenially. When i reache lower (imum) limi, he wich i urned ON. hi ype of conrol i poible, eiher wih conan frequency, or conan ON ime,. hi i ued only, when he load ha energy orage elemen, i.e. inducance,. he reference value are load curren or load volage. hi i hown in Fig n hi cae, he curren i coninuou, varying beween and, which decide he frequency ued for wiching. he ripple in he load curren can be reduced, if he difference beween he upper and lower limi i reduced, hereby making i imum. hi in urn increae he frequency, hereby increaing he wiching loe. i v OFF Fig. 17.6: Curren limi conrol n hi leon, fir one in hi module (#3), he hree baic circui buck, boo and buckboo, of dcdc converer (chopper) are preened, along wih he operaion and he derivaion of he expreion for he oupu volage in each cae, aug coninuou conducion. he differen raegie employed for heir conrol are dicued. n he nex leon econd one, he expreion for he imum and curren for coninuou conducion in buck dcdc converer will be derived. erion 2 EE, Kharagpur 11