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CHAPTER 2

DESIGN AND MODELING OF POSITIVE BUCK BOOST

CONVERTER WITH CASCADED BUCK BOOST

CONVERTER

2.1 GENERAL

Designing an efficient DC to DC buck-boost converter is very

much important for many real-time battery powered applications. A positive

buck-boost converter is one which has the capability of switching its

operating modes according to the variation in the input supply.

In this chapter, a positive buck-boost converter and cascaded buck

boost converter had been designed with a variable battery supply. The

drawbacks associated with buck-boost converter, the different conventional

solutions and the demerits of such conventional methods are presented in this

section. The reasons behind the selection of a positive buck-boost converter

for application, instead of buck-boost converter are clearly presented here.

2.2 BUCK BOOST CONVERTER

The buck boost converter is a type of DC-DC converter that has an

output voltage magnitude, which is either greater or less than the input

voltage magnitude. It is a switch mode power supply with a circuit topology

similar to the boost converter and the buck converter. The output voltage is

adjustable, based on the duty cycle of the switching transistor. One possible

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drawback of this converter is that the switch does not have a terminal on the

ground; this complicates the driving circuitry. Also, the polarity of the output

voltage is opposite the input voltage. Figure 2.1 shows the schematic diagram

of a buck boost converter (49).

The basic operation of buck–boost converter is simple which is

given below

When in the On-state, the switch S is in the ON condition.

This time the input voltage source is directly connected to the

inductor L. This results in energy accumulating in L. In this

stage, the capacitor supplies energy to the output load.

While in the Off-state, the switch S is in the OFF condition.

This time the inductor is connected to the output load and

capacitor; so energy is transferred from L to C and R.

The output voltage can vary continuously from 0 to (for

an ideal converter). The output voltage ranges for a buck and

boost converter are respectively 0 to Vi and Vi to .

But the polarity of the output voltage is opposite to that of the

input voltage.

Figure 2.1 Buck Boost Converter – schematic Diagram

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2.2.1 Drawbacks of Using Buck Boost Converter

The various issues associated with buck–boost converter, which

prevents its use for specific applications are mentioned below (51).

The biggest problem associated with buck-boost converter is

that the output of such a converter is inverted. Of course, it

can be inverted, but it requires a transformer, which adds to

the cost and space.

The output voltage polarity is opposite to that of the input

voltage, and these types of converters are also known as

inverting regulators.

The efficiency of the converter is very low.

It produces high output transients.

2.3 EXISTING SOLUTIONS TO DEAL WITH THE BUCK

BOOST CONVERTER

The above mentioned disadvantages of a normal buck-boost

converter have been solved by using different methods. Some of these

solutions had been analyzed and the drawbacks of such solutions are

described below.

2.3.1 Problem Existing in a SEPIC Converter

A very popular buck boost topology that requires more components

but produces a non-inverting output is the Single Ended Primary Inductance

Converter (SEPIC) shown in Figure 2.2. SEPIC, a popular buck–boost circuit

has limited efficiency and requires either a transformer or two inductors (2).

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Thus, including a transformer or two inductors would occupy more space and

hence, increase the size and the cost. The use of these components would add

to the losses, and thereby degrade the efficiency of the converter.

Figure 2.2 SEPIC Converter

2.3.2 Drawbacks of a Cascaded Buck Boost Converter

The Cascaded buck boost topology applies two dc–dc converters

cascaded together as shown in Figure 2.3. Hence, the loss of the whole single

converter is actually doubled in this case, resulting in poor efficiency (66).

The number of external components, such as inductors, decoupling capacitors,

and the compensation networks needed for both controllers in this case, is

more. Due to more components, more space is occupied, which results in

higher cost. The sub harmonic problem is another issue, which prevents

utilizing cascaded converters (52-54).

Figure 2.3 Cascaded Boost Buck Converter

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2.3.3 Boost Converter Cascaded to a Low-Dropout (LDO) Voltage

Regulator

This is another cascaded topology, composed of two different

converter circuits. The first one is a boost converter, followed by an LDO

voltage regulator as shown in Figure 2.4. Here, the varying input voltage from

the battery is stepped up; using the boost converter, and then the output of the

boost converter is regulated using the LDO to obtain a voltage in the middle

range of the varying battery voltage. The biggest disadvantage of an LDO

circuit is lower efficiency, due to the fact that it is a cascaded network of two

converters, which in turn, double the number of components and losses used

in the proposed converter stage. The cost is another concern here. Thus, this

topological control is not much desired (81-83).

Figure 2.4 Cascaded Boost Converter and LDO regulator

2.4 POSITIVE BUCK BOOST CONVERTER

Proposing a battery powered application is very difficult; by using

the above mentioned buck boost converters. But introducing a positive buck

boost converter can solve most of the problems associated with the other buck

boost converters. The proposed positive buck boost converter consists of only

two switches and two diodes; thus, it reduces the total cost when compared

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with other converters. Its output is positive and it operates as both a buck and

a boost converter. The circuit topology of such a positive buck boost

converter is shown in Figure 2.5 (87).

Figure 2.5 Circuit diagram of the Positive Buck Boost Converter

In the buck boost operating mode, always two switches Q1 and Q2,

and two diodes D1 and D2, are switching in the circuit. A positive buck–boost

converter can operate as a buck converter by controlling switch Q1 and diode

D1, when Q2 is OFF and D2 is conducting. It can also work as a boost

converter by controlling switch Q2 and diode D2, while Q1 is ON and D1 is not

conducting. When the voltage of the battery is more than the output reference

voltage, the converter operates as a buck converter. As soon as the voltage of

the battery drops to a value less than the output reference voltage, the

converter should switch to the boost mode. The added advantage of the

converter is that the output of such a converter is always positive.

The different modes of operation with the direction of current flows

are neatly sketched in Figure 2.6. When the positive buck-boost converter

works in the buck mode, switch Q2 is always in the OFF condition and only

switch Q1 conducts, while in the boost mode, switch Q1 is always in the ON

condition and only switch Q2 controls the circuit. In the buck-boost mode,

both switches Q1 and Q2 conduct. The dotted line of Figure 2.6 indicates the

direction of the current flow when the controlling switch of each mode is in

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the ON state, where as the continuous line indicates the direction of the

current when the controlling switch is in the OFF state (98-99).

Figure 2.6 Current flow directions of (a) Buck, (b) Boost and (c) BuckBoost Operating Modes of a Positive Buck Boost Converter

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2.5 HARDWARE DESCRIPTION

The hardware of a positive buck–boost converter is designed, based

on the parameters listed in Table 2.1. The converter operates at 100 kHz

switching frequency. Two n-type MOSFET switches and two Schottky barrier

diodes are used for real time buck boost converter configuration.

Table 2.1 Hardware Specifications for Positive Buck Boost Converter

Parameter Value

Digital Signal Processor (DSP) 320 F 2812

n-type MOSFET IRF540

Schottky barrier diode 1N5817

Switching frequency 100KHz

Output resistance 30

Output filter capacitance 400µF

Magnetizing inductance 110µH

Input voltage 8V – 15V

Output voltage 12V

The MOSFET switches and diodes are IRF540 and 1N5817,

respectively. A controller has been implemented using a Texas Instruments

digital signal processor (DSP) (320F2812). The output voltage reference is set

to 12 V, and input voltage varies from 15 to 8 V (66). The overall

configuration of the converter and controller is shown in Figure 2.7.

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Figure 2.7 Hardware circuit configuration

The operating modes are dependent on the mode selection signals,

applied from the DSP. G1 and G2 are the buck pulse and boost pulse,

sequentially. MD_SEL0, MD_SEL1, and MD_SEL2 determine the operation

modes: Figure 2.8 presents the experimental setup, composed of the DSP

controller, gate driver, gating logic, and converter (86, 88).

Figure 2.8 Experimental Setup of PBBC

2.6 COMPARISON OF THE NUMBER OF COMPONENTS OF

THE PBBC AND CBBC

From the above discussion, it is clear that the number of

components in the positive buck boost converter is less compared to the

cascaded buck boost converter, as shown in Table 2.1.

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Table 2.2 Components Comparison between the PBBC and CBBC

Description PBBC CBBCn-type MOSFET 02 04Schottky barrier diode 02 NilCapacitor 02 01Magnetizing inductance 01 02

2.7 OPEN LOOP RESPONSE OF CBBC AND PBBC

2.7.1 Case – i: Buck Mode - Open Loop Response of CBBC

The transient response in the output voltage is analyzed by applying

an input voltage of 6V and an output voltage of 4.35V is obtained. The ripple

content in this case is 14% and the time taken for the output voltage to reach

the steady state value is 0.014sec, as shown in Figure 2.9.

Figure 2.9 Input and Output Voltage Waveform of Buck Operation –

CBBC

Out

put v

olta

ge(V

)In

put v

olta

ge(V

)

Time (S)

Output voltage = 4.35V

Input voltage = 6V

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2.7.2 Case – ii: Boost Mode - Open Loop Response of CBBC

For an input voltage of 3.5V an output voltage of 4.56V is obtained

and the transient response in the output voltage is analyzed. The ripple

content is 15% and the time taken for the output voltage to reach the steady

state value is 0.0154 sec for this case as shown in Figure 2.10.

Figure 2.10 Input and Output Voltage Waveform of Boost Operation –

CBBC

2.7.3 Case – iii: Buck Mode - Open Loop Response of PBBC

When an input voltage of 6V is applied an output voltage of 4.59V

is obtained. The transient response in the output voltage is analyzed. The

ripple content in this case is 12% and the time taken for the output voltage to

reach the steady state value is 0.012 sec, as shown in Figure 2.11.

Out

put v

olta

ge(V

)In

put v

olta

ge(V

)

Time (S)

Output voltage = 4.56V

Input voltage = 3.5V

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Figure 2.11 Input and Output Voltage Waveform of Buck Operation –

PBBC

2.7.4 Case – iv: Boost Mode - Open Loop Response of PBBC

For an input voltage of 3.5V an output voltage of 4.76V is obtained

and the transient response in the output voltage is analyzed. The ripple

content is 10% and the time taken for the output voltage to reach the steady

state value is 0.0024 sec for this case as shown in Figure 2.12.

Figure 2.12 Input and Output Voltage Waveform of Boost Operation –

PBBC

Out

putv

olta

ge(V

)In

put v

olta

ge(V

)

Time (S)

Inpu

t vol

tage

(V)

Out

put v

olta

ge(V

)

Time (S)

Output voltage = 4.76V

Input voltage = 3.5V

Output voltage = 4.59 V

Input voltage = 6V

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Based on the analysis, the results show that the PBBC gives better

transient response compared to that of CBBC.

2.8 SUMMARY

The disadvantages of some conventional buck-boost converters are

analyzed in this chapter. The need for a novel PBBC and its designing are

discussed. The circuit diagram for the proposed model is compared with the

cascaded buck boost converter and its hardware circuit is presented and

discussed.