Version 2 EE IIT, Kharagpur 1
Module
3
DC to DC Converters
Lesson
17
Types o !asi" DC#DC
Converters
Instructional Objectives
$tudy o the ollo%ing&
Three 'asi" types o d"#d" "onverter "ir"uits ( 'u"), 'oost and 'u")#'oost
The e*pressions or the output voltage in the a'ove "ir"uits, %ith indu"tive +,#L- and 'attery
+or 'a") e. / E- load
Introduction
In the last .odule +02- "onsisting o eight lessons, the various types o "ir"uits used in 'oth
single#phase and three#phase a"#d" "onverters, %ere dis"ussed in detail1 This in"ludes hal#%ave
and ull#%ave, and also hal#"ontrolled and ull#"ontrolled ones1
In this lesson ( the irst one in this .odule +03-, irstly, three 'asi" types o d"#d" "onverter
"ir"uits ( 'u"), 'oost and 'u")#'oost, are presented1 Then, the e*pressions or the output voltage
in the a'ove "ir"uits, %ith indu"tive +,#L- and 'attery +or 'a") e. / E-, i1e1, ,#L#E, load, are
derived, assu.ing "ontinuous "ondu"tion1 The dierent "ontrol strategies e.ployed are 'riely
des"ri'ed1
Keywords& DC#DC "onverter "ir"uits, Thyristor "hoppers, !u"), 'oost and 'u")#'oost
"onverters +d"#d"-, $tep#do%n +'u")- and step#up +'oost- "hoppers, 2utput voltage and "urrent1
DC-DC Converters
There are three 'asi" types o d"#d" "onverter "ir"uits, ter.ed as 'u"), 'oost and 'u")#'oost1
In all o these "ir"uits, a po%er devi"e is used as a s%it"h1 This devi"e earlier used %as a
thyristor, %hi"h is turned on 'y a pulse ed at its gate1 In all these "ir"uits, the thyristor is
"onne"ted in series %ith load to a d" supply, or a positive +or%ard- voltage is applied 'et%een
anode and "athode ter.inals1 The thyristor turns o, %hen the "urrent de"reases 'elo% the
holding "urrent, or a reverse +negative- voltage is applied 'et%een anode and "athode ter.inals1
$o, a thyristor is to 'e or"e#"o..utated, or %hi"h additional "ir"uit is to 'e used, %here
another thyristor is oten used1 Later, 3T24s "a.e into the .ar)et, %hi"h "an also 'e turned o
'y a negative "urrent ed at its gate, unli)e thyristors, re5uiring proper "ontrol "ir"uit1 The turn#
on and turn#o ti.es o 3T2s are lo%er than those o thyristors1 $o, the re5uen"y used in 3T2#
'ased "hoppers "an 'e in"reased, thus redu"ing the si6e o ilters1 Earlier, d"#d" "onverters %ere
"alled 7"hoppers4, %here thyristors or 3T2s are used1 It .ay 'e noted here that 'u") "onverter
+d"#d"- is "alled as 7step#do%n "hopper4, %hereas 'oost "onverter +d"#d"- is a 7step#up "hopper41
In the "ase o "hopper, no 'u")#'oost type %as used1
8ith the advent o 'ipolar 9un"tion transistor +!:T-, %hi"h is ter.ed as sel#"o..utated
devi"e, it is used as a s%it"h, instead o thyristor, in d"#d" "onverters1 This devi"e +;<;
transistor- is s%it"hed on 'y a positive "urrent through the 'ase and e.itter, and then s%it"hed
o 'y %ithdra%ing the a'ove signal1 The "olle"tor is "onne"ted to a positive voltage1 ;o%#a#
days, M2$=ETs are used as a s%it"hing devi"e in lo% voltage and high "urrent appli"ations1 It
.ay 'e noted that, as the turn#on and turn#o ti.e o M2$=ETs are lo%er as "o.pared to other
s%it"hing devi"es, the re5uen"y used or the d"#d" "onverters using it +M2$=ET- is high, thus,
redu"ing the si6e o ilters as stated earlier1 These "onverters are no% 'eing used or appli"ations,
one o the .ost i.portant 'eing $%it"hed Mode <o%er $upply +$M<$-1 $i.ilarly, %hen
appli"ation re5uires high voltage, Insulated 3ate !i#polar Transistors +I3!T- are preerred over
!:Ts, as the turn#on and turn#o ti.es o I3!Ts are lo%er than those o po%er transistors +!:T-,
thus the re5uen"y "an 'e in"reased in the "onverters using the.1 $o, .ostly sel#"o..utated
devi"es o transistor a.ily as des"ri'ed are 'eing in"reasingly used in d"#d" "onverters1
Buck Converters (dc-dc)
> 'u") "onverter +d"#d"- is sho%n in =ig1 1711a1 2nly a s%it"h is sho%n, or %hi"h a devi"e
as des"ri'ed earlier 'elonging to transistor a.ily is used1 >lso a diode +ter.ed as ree %heeling-
is used to allo% the load "urrent to lo% through it, %hen the s%it"h +i1e1, a devi"e- is turned o1
The load is indu"tive +,#L- one1 In so.e "ases, a 'attery +or 'a") e.- is "onne"ted in series
%ith the load +indu"tive-1 Due to the load indu"tan"e, the load "urrent .ust 'e allo%ed a path,
%hi"h is provided 'y the diode? other%ise, i1e1, in the a'sen"e o the a'ove diode, the high
indu"ed e. o the indu"tan"e, as the load "urrent tends to de"rease, .ay "ause da.age to the
s%it"hing devi"e1 I the s%it"hing devi"e used is a thyristor, this "ir"uit is "alled as a step#do%n
"hopper, as the output voltage is nor.ally lo%er than the input voltage1 $i.ilarly, this d"#d"
"onverter is ter.ed as 'u") one, due to reason given later1
+
S
Switch
L
+
I
0
L
V
s
V
0
O
D
F
A
D
-
-
v
0
V
s
Fig. 17.1(a! "#c$ co%verter (dc-dc
V
0
&
O'
t
&
&
OFF
i
0
t
Fig. 17.1((! O#t)#t vo*tage a%d c#rre%t wave+or,s
The output voltage and "urrent %aveor.s o the "ir"uit +=ig1 1711a- are sho%n in =ig1 1711'1
The output voltage is sa.e as the input voltage, i1e1, v
@
= V
s
, %hen the s%it"h is 2;, during the
period,
T
ON
t @ 1 The s%it"h is turned on at t = @ , and then turned o at t = T
ON
1 This is
"alled 2; period1 During the ne*t ti.e interval,
T t T
ON
, the output voltage is 6ero, i1e1,
v
@
= @ , as the diode, D
F
no% "ondu"ts1 The 2== period is T
OFF
= T T
ON
, %ith the ti.e period
'eing T = T
ON
+ T
OFF
1 The re5uen"y is
the output voltage is,
f = 1 A T 1 8ith T )ept as "onstant, the average value o
V /
1
v
dt /
1
T
ON
V
dt / V
= T
= k V @
@
s
@ @ s
T
s
The duty ratio is k = (T
ON
A T ) = [T
ON
A(T
ON
+ T
OFF
)], its range 'eing 11@ k @1@ 1 ;or.ally, due
to turn#on delay o the devi"e used, the duty ratio +)- is not 6ero, 'ut has so.e positive value1
$i.ilarly, due to re5uire.ent o turn#o ti.e o the devi"e, the duty ratio +)- is less than 11@1 $o,
the range o duty ratio is redu"ed1 It .ay 'e noted that the output voltage is lo%er than the input
voltage1 >lso, the average output voltage in"reases, as the duty ratio is in"reased1 $o, a varia'le
d" output voltage is o'tained ro. a "onstant d" input voltage1 The load "urrent is assu.ed to 'e
"ontinuous as sho%n in =ig1 1711'1 The load "urrent in"reases in the 2; period, as the input
voltage appears a"ross the load, and it +load "urrent- de"reases in the 2== period, as it lo%s in
the diode, 'ut is positive at the end o the ti.e period, T1
Boost Converters (dc-dc)
> 'oost "onverter +d"#d"- is sho%n in =ig1 1712a1 2nly a s%it"h is sho%n, or %hi"h a devi"e
'elonging to transistor a.ily is generally used1 >lso, a diode is used in series %ith the load1 The
load is o the sa.e type as given earlier1 The indu"tan"e o the load is s.all1 >n indu"tan"e, L is
assu.ed in series %ith the input supply1 The position o the s%it"h and diode in this "ir"uit .ay
'e noted, as "o.pared to their position in the 'u") "onverter +=ig1 1711a-1
I
s
L
+
V
s
-
D
S
Switch
+
I
0
L
V
0
O
A
D
-
Fig. 17.-(a! "oost co%verter (dc-dc
I
-
I
1
0 &
O' & -&
&
OFF
Fig. 17.-((! .ave+or,s o+ so#rce c#rre%t (i
S
T
ON
T T
The operation o the "ir"uit is e*plained1 =irstly, the s%it"h, $ +i1e1, the devi"e- is put 2;
+or turned 2;- during the period, T
ON t @ , the 2; period 'eing T
ON
1 The output voltage is
6ero + v
@
= @ -, i no 'attery +'a") e.- is "onne"ted in series %ith the load, and also as stated
earlier, the load indu"tan"e is s.all1 The "urrent ro. the sour"e + i
s
- lo%s in the indu"tan"e L1
The value o "urrent in"reases linearly %ith ti.e in this interval, %ith + di d t - 'eing positive1
>s the "urrent through L in"reases, the polarity o the indu"ed e. is ta)en as say, positive, the
let
hand side o L 'eing Bve1 The e5uation or the "ir"uit is,
V
s
= L
d i
s
d t
or,
d i
s
d t
=
V
s
L
The s%it"h, $ is put 2== during the period,
T t T
ON
, the 2== period 'eing
T
OFF = T T
ON
1 + T = T
ON
+ T
OFF
- is the ti.e period1 >s the "urrent through L de"reases, %ith its
dire"tion 'eing in the sa.e dire"tion as sho%n +sa.e as in the earlier "ase-, the indu"ed e.
reverses, the let hand side o L 'eing #ve1 $o, the indu"ed e. +ta)en as Cve in the e5uation
given later- is added %ith the supply voltage, 'eing o the sa.e polarity, thus, )eeping the
"urrent + i
s
= i
@
- in the sa.e dire"tion1 The "urrent + i
s
= i
@
- de"reases linearly in the ti.e
interval,
T
OFF , as the output voltage is assu.ed to 'e nearly "onstant at v
@
V
@
, %ith + d i
s
d t - 'eing
negative, as V
s
< V
@
, %hi"h is derived later1
The e5uation or the "ir"uit is,
V = V
+ L
d i
s
or,
d i
s
=
(V
s
V
@
)
s @
d t d t L
The sour"e "urrent %aveor. is sho%n in =ig1 1712'1 >s stated earlier, the "urrent varies
linearly ro.
I
1
+ I
.in
- to I
2
+ I
.a*
- during the ti.e interval, T
ON
1
$o, using the e*pression or d i
s d t during this ti.e interval,
I
2
I
1
= I
.a*
I
.in
= (V
s
A L) T
ON
1
$i.ilarly, the "urrent varies linearly ro. I
2
+ I
.a*
- to I
1
+ I
.in
- during the ti.e interval, T
OFF
1
$o, using the e*pression or d i
s d t during this ti.e interval,
I
2
I
1
= I
.a*
I
.in
= [(V
@
V
s
)A L] T
OFF
1
E5uating the t%o e5uations, (V
s
value o the output voltage is,
A L ) T
ON
= [(V
@
V
s
) A L ] T
OFF
, ro. %hi"h the average
T
T
1
1
V
@
= V
s
T
= V
s
T T
= V = V
A T )
s
1 k
OFF ON
ON
The ti.e period is T = T
ON
+ T
OFF
, and the duty ratio is,
k = (T
ON
A T ) = [T
ON
A(T
ON
+ T
OFF
)], %ith its range as 11@ k @1@ 1 The 2; ti.e interval is
T
ON
= k T 1 >s stated in the previous "ase, the range o k is redu"ed1 This is, 'e"ause the
s
.ini.u. value is higher than the .ini.u. +@1@-, and the .a*i.u. value is lo%er than the
.a*i.u. +11@-, or reasons given there, %hi"h are also valid here1 >s sho%n, the sour"e "urrent
is assu.ed to 'e "ontinuous1 The e*pression or the output voltage "an 'e o'tained 'y using
other pro"edures1
In this "ase, the output voltage is higher than the input voltage, as "ontrasted %ith the
previous "ase o 'u") "onverter +d"#d"-1 $o, this is "alled 'oost "onverter +d"#d"-, %hen a sel#
"o..utated devi"e is used as a s%it"h1 Instead, i thyristor is used in its pla"e, this is ter.ed as
step#up "hopper1 The variation +range- o the output voltage "an 'e easily "o.puted1
Buck-Boost Converters (dc-dc)
> 'u")#'oost "onverter +d"#d"- is sho%n in =ig1 17131 2nly a s%it"h is sho%n, or %hi"h a
devi"e 'elonging to transistor a.ily is generally used1 >lso, a diode is used in series %ith the
load1 The "onne"tion o the diode .ay 'e noted, as "o.pared %ith its "onne"tion in a 'oost
"onverter +=ig1 1712a-1 The indu"tor, L is "onne"ted in parallel ater the s%it"h and 'eore the
diode1 The load is o the sa.e type as given earlier1 > "apa"itor, C is "onne"ted in parallel %ith
the load1 The polarity o the output voltage is opposite to that o input voltage here1
8hen the s%it"h, $ is put 2;, the supply "urrent + i
s
- lo%s through the path, V
s
, $ and L,
during the ti.e interval,
T
ON
1 The "urrents through 'oth sour"e and indu"tor + i
L
- in"rease and
are sa.e, %ith + d i
L d t - 'eing positive1 The polarity o the indu"ed voltage is sa.e as that o
the input voltage1 The e5uation or the "ir"uit is,
V L
d i
L
d i
L
=
V
s
s
=
d t
or,
d t L
+
I
s
S
V
s
Switch
I
0
L
L
O
I
L A
D
-
V
0
/
-
+
Fig. 17.0(a! "#c$-(oost co%verter (dc-dc
I
L-
I
L1
&
O' & -&
&
OFF
Fig. 17.0((! I%d#ctor c#rre%t (i
L
wave+or,
Then, the s%it"h, $ is put 2==1 The indu"tor "urrent tends to de"rease, %ith the polarity o
the indu"ed e. reversing1 + d i
L
d t - is negative no%, the polarity o the output voltage, V
@
'eing opposite to that o the input voltage,
V
s
1 The path o the "urrent is through L, parallel
"o.'ination o load D C, and diode D, during the ti.e interval,
re.ains nearly "onstant, as the "apa"itor is "onne"ted a"ross the load1
T
OFF
1 The output voltage
The e5uation or the "ir"uit is,
d i
L
d i
L
V
@
L = V
@
d t
or, =
d t L
The indu"tor "urrent %aveor. is sho%n in =ig1 1713'1 >s stated earlier, the "urrent varies
linearly ro.
I
L1
to I
L 2
during the ti.e interval, T
ON
1 ;ote that I
L1
and I
L 2
are the .ini.u.
and .a*i.u. values o the indu"tor "urrent respe"tively1 $o, using the e*pression or
d i
L
d t
during this ti.e interval,
I
L 2
I
L1
= (V
s
A L) T
ON
1
$i.ilarly, the "urrent varies linearly ro.
I
L 2
to I
L1
during the ti.e interval, T
OFF
1 $o, using
the e*pression or d i
L d t during this ti.e interval, I
L 2
I
L1
= (V
@
A L) T
OFF
1
E5uating the t%o e5uations, (V
s
the output voltage is,
A L ) T
ON
= (V
@
A L ) T
OFF
, ro. %hi"h the average value o
T
2;
T
2;
(T
2;
A T )
k
V
@
= V
s
T
= V
s
T T
= V = V
A T )
s
1 k
OFF ON
ON
The ti.e period is T = T
ON
+ T
OFF
, and the duty ratio is,
k = (T
ON
A T ) = [T
ON
A(T
ON
+ T
OFF
)]1 The 2; ti.e interval is T
ON
= k T 1 It .ay 'e o'served that,
or the range
@ k > @1E , the output voltage is lo%er than the input voltage, thus, .a)ing it a
'u") "onverter +d"#d"-1 =or the range @1E > k 11@ , the output voltage is higher than the input
voltage, thus, .a)ing it a 'oost "onverter +d"#d"-1 =or k = @1E , the output voltage is e5ual to the
input voltage1 $o, this "ir"uit "an 'e ter.ed as a 'u")#'oost "onverter1 >lso it .ay 'e "alled as
step#upAdo%n "hopper1 It .ay 'e noted that the indu"tor "urrent is assu.ed to 'e "ontinuous1
The range o k is so.e%hat redu"ed due to the reasons given earlier1 The e*pression or the
output voltage "an 'e o'tained 'y using other pro"edures1
Control Strategies
In all "ases, it is sho%n that the average value o the output voltage "an 'e varied1 The t%o
types o "ontrol strategies +s"he.es- are e.ployed in all "ases1 These are&
+a- Ti.e#ratio "ontrol, and +'- Current li.it "ontrol1
Time-ratio Control
In the ti.e ratio "ontrol the value o the duty ratio, k = T
ON
A T
is varied1 There are t%o %ays,
%hi"h are "onstant re5uen"y operation, and varia'le re5uen"y operation1
Constant Frequenc O!eration
In this "ontrol strategy, the 2; ti.e,
T
ON
is varied, )eeping the re5uen"y + f
= 1A T -, or
ti.e period T "onstant1 This is also "alled as pulse width modulation control +<8M-1 T%o "ases
%ith duty ratios, k as +a- @12E +2EF-, and +'- @17E +7EF- are sho%n in =ig1 171G1 Hen"e, the
output voltage "an 'e varied 'y varying 2; ti.e, T
ON
1
s
Load-vo*tage
&
O'
&
OFF
V
0
$ 1 0.-2
&
t
v
0
&
O'
&
OFF
V
0
$ 1 0.72
&
t
Fig. 17.3! 4#*se-width ,od#*atio% co%tro* (co%sta%t +re5#e%cy
"ariable Frequenc O!eration
In this "ontrol strategy, the re5uen"y + f
= 1A T -, or ti.e period T is varied, )eeping either
+a- the 2; ti.e,
T
ON
"onstant, or +'- the 2== ti.e, T
OFF
"onstant1 This is also "alled as
frequency modulation control1 T%o "ases %ith +a- the 2; ti.e,
T
ON
"onstant, and +'- the 2==
ti.e,
T
OFF
"onstant, %ith varia'le re5uen"y or ti.e period + T -, are sho%n in =ig1 171E1 The
output voltage "an 'e varied in 'oth "ases, %ith the "hange in duty ratio, k = T
ON
A T 1
v
0
t
v
0
&
O'
&
&
OFF
(a /o%sta%t &
O'
$ 1 0.72
t
v
0
&
OFF
&
O'
$ 1 0.-2
& t
v
0
Load vo*tage
&
OFF
&
O'
&
$ 1 0.72
t
(( /o%sta%t &
OFF
Fig. 17.2! O#t)#t vo*tage wave+or,s +or varia(*e +re5#e%cy syste,
There are .a9or disadvantages in this "ontrol strategy1 These are&
+a- The re5uen"y has to 'e varied over a %ide range or the "ontrol o output voltage in
re5uen"y .odulation1 =ilter design or su"h %ide re5uen"y variation is, thereore, 5uite
dii"ult1
+'- =or the "ontrol o a duty ratio, re5uen"y variation %ould 'e %ide1 >s su"h, there is a
possi'ly o intereren"e %ith syste.s using "ertain re5uen"ies, su"h as signaling and
telephone line, in re5uen"y .odulation te"hni5ue1
+"- The large 2== ti.e in re5uen"y .odulation te"hni5ue, .ay .a)e the load "urrent
dis"ontinuous, %hi"h is undesira'le1
Thus, the "onstant re5uen"y syste. using <8M is the preerred s"he.e or d"#d" "onverters
+"hoppers-1
&
O'
$ 1 0.-2
Current #imit Control
>s "an 'e o'served ro. the "urrent %aveor.s or the types o d"#d" "onverters des"ri'ed
earlier, the "urrent "hanges 'et%een the .a*i.u. and .ini.u. values, i it +"urrent- is
"ontinuous1 In the "urrent li.it "ontrol strategy, the s%it"h in d"#d" "onverter +"hopper- is turned
2; and 2==, so that the "urrent is .aintained 'et%een t%o +upper and lo%er- li.its1 8hen the
"urrent e*"eed upper +.a*i.u.- li.it, the s%it"h is turned 2==1 During 2== period, the "urrent
ree%heels in say, 'u") "onverter +d"#d"- through the diode, D
F
, and de"reases e*ponentially1
8hen it rea"hes lo%er +.ini.u.- li.it, the s%it"h is turned 2;1 This type o "ontrol is
possi'le, either %ith "onstant re5uen"y, or "onstant 2; ti.e, T
ON
1 This is used only, %hen the
load has energy storage ele.ents, i1e1 indu"tan"e, L1 The reeren"e values are load "urrent or
load voltage1 This is sho%n in =ig1 171I1 In this "ase, the "urrent is "ontinuous, varying 'et%een
I
.a*
and I
.in
, %hi"h de"ides the re5uen"y used or s%it"hing1 The ripple in the load "urrent "an
'e redu"ed, i the dieren"e 'et%een the upper and lo%er li.its is redu"ed, there'y .a)ing it
.ini.u.1 This in turn in"reases the re5uen"y, there'y in"reasing the s%it"hing losses1
I
,a6
i
0
I
,i%
t
v
0
&
O'
&
OFF
&
t
Fig. 17.7! /#rre%t *i,it co%tro*
In this lesson, irst one in this .odule +03-, the three 'asi" "ir"uits ( 'u"), 'oost and 'u")#
'oost, o d"#d" "onverters +"hoppers- are presented, along %ith the operation and the derivation
o the e*pressions or the output voltage in ea"h "ase, assu.ing "ontinuous "ondu"tion1 The
dierent strategies e.ployed or their "ontrol are dis"ussed1 In the ne*t lesson ( se"ond one, the
e*pression or the .a*i.u. and "urrents or "ontinuous "ondu"tion in 'u") d"#d" "onverter %ill
'e derived1
