buck boost report

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#rtr: 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#alnt circuit

1.2.1 %nal&sis !or t' S(itc' Closd: &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' Opn: &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#rtr

!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 Currnt: 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., Opn 0oop Siulation o! /uc" /oost con#rtr

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

Pa" o#rs'oot -), -86 -86 -86 -86 -86 -86 -86 -86

Sttlin+ ti .)2 2.235 2.2;8 2.297 2.233 2.238 2.2;5 2.292 2.27;

Thus if 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 .