BUCK-BOOST CONVERTER 1.1 Introduction: In a large number of industrial applications, it is required to convert a dc voltage to a different dc voltage level, often with a regulated output. To perform this task, a dc-dc converter is needed. A dc-dc converter directly converts a dc voltage of one level to another. It can be used to step-down (buck, or step-up (boost a dc voltage source. !igher switching frequency would reduce the si"e of the filter used.
1.2 Basic Principl o! Buc"-Boost con#rtr: The buck-boost is a popular non-isolated inverting power stage topology, sometimes called a step-up#down power stage. $ower supply designers choose the buck-boost power stage because the required required output is inverted inverted from the input voltage, and the output output voltage can be either higher or lower than the input voltage. The input current for a buck-boost power stage is discontinuous, or pulsating, because of the power switch current that pulses from "ero to I% every every switch switching ing cycle. cycle. The The output output current current for a buck-b buck-boos oostt power power stage is also discontinuous or pulsating because the output diode only conducts during a portion of the switching cycle.
E$ui#alnt circuit
1.2.1 %nal&sis !or t' S(itc' Closd: &hen the switch is closed, the voltage across the inductor is
The rate of change of inductor current is a constant, indicating a linearly increasing inductor current. The preceding equation can be e'pressed as
1.2.2 %nal&sis !or t' S(itc' Opn: &hen the switch is open, the current in the inductor cannot change instantaneously, resulting in a forward-biased diode and current into the resistor and capacitor. In this condition, the voltage across the inductor is
Again, the rate of change of inductor current is constant, and the change in current is
1.2.) Inductor *si+n $ower absorbed by the load must be the same as that supplied by the source.
Average source current is related to average inductor current by
ubstituting for Vo derived above and solving for IL, we find
)or continuous current, the inductor current must remain positive. To determine the boundary between continuous and discontinuous current, I min is set to "ero resulting in
1.2., Output Volta+ Rippl: The output voltage ripple for the buck-boost converter is computed from the capacitor current waveform.
The converter consists of dc input voltage source * , controlled switch , inductor %, diode +, filter capacitor , and load resistance . &ith the switch on, the inductor current increases while the diode is maintained off. &hen the switch is turned off, the diode provides a path for the inductor current. ote the polarity of the diode that results in its current being drawn from the output. The condition of a "ero volt-second product for the inductor in steady state yields
V S DT
V o (/ D T
= −
−
i+ 1.) Circuit dia+ra o! /uc" /oost con#rtr
!ence, the dc voltage transfer function of the buck-boost converter is V o D M V V S / D =
= −
−
The output voltage * 0 is negative with respect to the ground. Its magnitude can be either greater or smaller (equal at + 1 2.3 than the input voltage as the name of the converter implies. The value of the inductor that determines the boundary between the 4 and +4 is
Lcric
(/ D 5 R 5 f −
=
C min
=
DV o V r Rf
DESIGN PROBLEM:
*s 1 56 * + 1 2.6 1 3 0hm % 1 52 u! 1 72 u) f 1 /22 k!" %imitation 1 2 * to 89 *(by simulation Output Volta+ :
V o V S
= −
D /
−
D
*o 1 -/9*olt.
Inductor Currnt: VsD
I% 1 ( R∗( 1− D )2 1 5.33 A Ripple current: VsDT
∆I%1
I%ma'1
I%min1
14.8 A
L
IL +
IL−
∆ IL 2
1 7.33 A
∆ IL
2
1 2.93 A
Output #olta+ rippl: D
∆*o1 RCF
1 0.01=1%
Inductor Design: Type
: Power Inductor
Inductance
: 20uH
Maximum DC current Core Material Maximum DC resistance
: 7!"mps : Powdered Iron Core : 2#m$%m
OPEN LOOP SIMULATION:
i+ 1., Opn 0oop Siulation o! /uc" /oost con#rtr
Fi 1.5 Si!"#$i&' ()*"#* +LOSED LOOP SIMULATION:
Fi 1., O"-" &#$)
PI +ONTROLLER:/T"'i' (i$# $' )((&( !)& Kp .22 2.2223 2.2223 2.222; 2.225 2.223 2.22; 2.22/ 2.2/
• • • •
Ki 1 /2 / 3 7 /2 3 7 3
Pa" o#rs'oot -), -86 -86 -86 -86 -86 -86 -86 -86
Sttlin+ ti .)2 2.235 2.2;8 2.297 2.233 2.238 2.2;5 2.292 2.27;
Thus if
3-il Codin+ !or Opn loop Buc" Boost Con#rtr: function my_ode() global A D Cf Lf Vs f = 100000; R = 5; Vo = 1; !o = Vo"R;
Vs = #$; D = Vo"(Vo% Vs); L = #0&'; C = 0&'; Lf = Lf; Cf = Cf; A = *0 '1"Lf; 1"Cf '1"(RCf)+; ,0 = *D!o; Vo+; tf = 50; tic *t-.+ = ode#/(buboost-*0 tf+-,0); toc; !L = .(2-1); VC = .(2-#); sub3lot(#-1-1)-3lot(t-!L)-g4id a,is(*0 tf 0 #0+) title(!nducto4 Cu44ent) sub3lot(#-1-#)-3lot(t-VC)-g4id a,is(*0 tf 0 1#0+) title(6ut3ut Voltage) ,label(cycles) end function d, = buboost(t- ,) global A D Cf Lf Vs iL = ,(1); 7C = ,(#); 8 = *(7C%Vs)"Lf; 'iL"Cf+; u = 095(1'sign(t'fi,(t)'D)); d,=A,%8u; end
i+ 1.4 Inductor Currnt and Capacitor Volta+
Tic starts a stopwatch timer to measure performance. The function records the internal time at e'ecution of the tic command. +isplay the elapsed time with t he toc function. %PP0IC%TIONS:
Battr&-po(rd s&sts : In battery powered systems , where the input voltage can vary widely, starting at full charge and gradually decreasing as the battery charge is used up. At full charge, where the battery voltage may be higher than actually needed by the circuit being powered, a buck regulator would be ideal to keep the supply voltage steady. !owever as the charge diminishes the input voltage falls below the level required by the circuit, and either the battery must be discarded or re-charged= at this point the ideal alternative would be the boost regulator. !ence buck boost converter will be the preferred choice In Solar PV !or 3PPT:
>uck#>oost converters make it possible to efficiently convert a + voltage to either a lower or higher voltage. >uck#>oost converters are especially useful for $* ma'imum power tracking purposes, where the ob?ective is to draw ma'imum possible power from solar panels at all times, regardless of the load .
