digital combination of buck and boost converters to ... · digital combination of buck and boost...

ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015

Canara Engineering College, Mangalore NJCIET-2015 418

Digital Combination of Buck and Boost Converters to Control a Positive Buck–

Boost Converter and Improve the Output Transients

Shruthi Prabhu1

1 Electrical & Electronics Department, VTU

K.V.G College of Engineering, Sullia ,India

[email protected]

Abstract. A highly efficient and novel control strategy for improving the transients in the output voltage of a dc–dc positive buck–boost converter, required for low-power portable electronic applications, is presented in this paper. The proposed control

technique can regulate the output voltage for variable input voltage, which is higher, lower, or equal to the output voltage.

There are several existing solutions to these problems, and selecting the best approach involves a trade-off among cost,

efficiency, and output noise or ripple. In the proposed method, instead of instantaneous transition from buck to boost mode,

intermediate combination modes consisting of several buck modes followed by several boost modes are utilized to distribute

the voltage transients.

Keywords: Buck–boost converter, DC–DC

1 Introduction

A very common power-handling problem, especially for portable applications, powered by batteries such as cellular phones,

personal digital assistants (PDAs), wireless and digital subscriber line (DSL) modems, and digital cameras, is the need to provide

a regulated non inverting output voltage from a variable input battery voltage. The battery voltage, when charged or discharg ed,

can be greater than, equal to, or less than the output voltage. But for such small-scale applications, it is very important to regulate

the output voltage of the converter with high precision and performance. Thus, the criterions such as cost, efficiency, and o utput

transients should be considered. A common power-handling issue for space-restrained applications powered by batteries is the

regulation of the output voltage in the midrange of a variable input battery voltage. Some of the common examples are 3.3 V

output with a 3–4.2 V Li cell input, 5 V output with a 3.6–6 V four-cell alkaline input, or a 12 V output with an 8–15 V lead–

acid battery input. With an input voltage range that is above and below the output voltage, the use of a buck or a boost conv erter

can be ruled out unless cascaded. Cascaded combination of converters results in cascaded losses and costs; therefore, this

approach is seldom used. In such a range of power demand, the transition of dc voltage from one level to another is generally

accomplished by means of DC-DC power converter circuits, such as step-down (buck) or step-up (boost) converter circuits.

There are various topologies such as inverting buck–boost converters, single-ended primary inductance converters (SEPICs),

Cuk converters, isolated buck–boost converters, and cascaded buck and boost converters, which can be implemented to maintain

a constant output voltage from a variable input voltage.

2 Existing Solutions

2.1 Demerits of Using an Ordinary Buck–Boost Converter

There are various issues about an ordinary buck–boost converter, which prevents its use for such specific applications. The

biggest problem associated with such a converter is that the output of such converter is inverted. Of course, it can be inver ted,

but it requires a transformer, which adds to the cost and space and sacrifices the ef ficiency of the converter

mailto:[email protected]



2.2 Disadvantages of a Cascaded Buck and Boost Converter

Such a topology applies two DC–DC converters cascaded together hence; the loss of the whole single converter is actually

doubled in this case, which results in poor efficiency. 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. In addition, sub harmonic problem is another issue, which prevents utilizing

cascaded converters.

Fig. 1. Cascaded boost and buck converter.

3 Proposed New Method

3.1 Operation of positive buck–boost converter

The circuit topology of a positive buck–boost converter is shown in Fig.2. In 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, 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 boost mode. The added advantage of the converter is that the

output of such a converter is always positive.

Fig. 2. Circuit topology of a positive buck–boost converter.

3.2 Digital combination of power converters

Control approach based on DCPC improves the dynamic response of the converter during transients by switching between

different converter topologies to spread out the voltage spikes that are an inevitable part of the transients. Fig. 3 demonstrates the

block diagram of the proposed approach. Here converter topology control (CTC) controls the operating topologies of the

converter. In addition, transition control (TC) units control the transitions between various topologies. For instance, TCi c ontrols

the transition behaviour of the converter from the ith converting topology to the (i + 1)th topology. In the transition from ith

topology to the (i + 1)th topology, instead of instantaneous transition from ith topology to (i + 1)th topology, for αi switc hing

cycle, converter operates in ith topology and for βi switching cycle, it operates in the (i + 1)th topology. Operation with αi and βi

switching cycles in transition mode will be repeated for γi times. For a specific case, where αi = mi, βi = 1 and γi = mi − 1, in the

transition from the ith topology to the (i + 1)th topology, for the mi switching cycle, the converter operates in ith topolog y and

for one switching cycle, it switches to the (i + 1)th topology. Then, in the next cycle, TCi tries to increase the num ber of cycles



of operation in the (i + 1)th topology and decrease the number o f cycles of operation in the ith

topology. Then, in the next cycle,

TCi tries to increase the number of cycles of operation in the (i + 1)th topology and decrease the number of c ycles of operation

in the ith topology. Finally, before the complete transition to the (i + 1)th topology, for one switching period, it operates in the ith

topology and, for mi switching cycle, it switches to the (i + 1)th topology. DCPC imposes the fact t hat the proposed converter

topology should have the capability of operating in various converter topologies with only controlling the switching componen ts

of the circuit.

Fig. 3. . Block diagram of the theory of digital combination of power converters

3.3 Parameters of Positive Buck–boost Converter

Variable Parameter Value

L Magnetizing Inductance 100 μH

C Output Filter Capacitance 330 μF

Vin Input Voltage 3.6-6 V

Vref Output Voltage 5 v

f Switching Frequency 100 KHz

4 Simulation Results

4.1 Simulation Block

Simulations are carried out on the positive buck–boost converter using the conventional methods. In this method, the converter

initially works in the buck mode, when the input voltage is greater than the output voltage, followed by the buck–boost mode

when the voltages are almost equal. Finally, the converter works in the boost mode when the input voltage is lower than the

output voltage. The simulation results indicate a decrease in the presence of spikes in the output voltages.



Fig. 4. . Simulation Block.

4.2 Simulation Results

Fig.5 Waveform for Buck Operation. The above simulation waveform shows the input voltage given to the converter to check the buck operation since the output is 6v, it has to reduce the output voltage to 5.

Fig.6 Waveform for Boost Operation. shows the input voltage to check the boost operation of the converter. Here the input is set to 3.6v, the

output should be 5 v.



Fig.7 Waveform for Buck-Boost Operation. The above diagram shows the input voltage to check the buck- boost operation of the converter.

Here the input is set to 5v, the output should be 5 v.

5. Hardware Implementation

5.1. Components used

Transformer 750mA/12V, Voltage Regulator (LM 7805, 5V), Bridge Rectifier, Crystal Oscillator, Micro controller (PIC

16F877A), Gate Driver (IR 2110), DC Supply 5V, 2 MOSFETS (IRF840), Inductor (10mH), 2 Diodes (IN5408), Filter

Capacitor (1000 uF), Feedback loop.

5.2. Positive Buck-Boost Converter

Fig.8. Implemented Hardware of the Positive Buck-boost Converter.

The circuit topology of a positive buck boost converter is as shown in the figure above. In buck–boost operating mode always

two switches A positive buck–boost converter can operate as a buck converter by controlling switches and the diode when one is

conducting and the other is in OFF mode. Similarly it can also operate in the boost mode by controlling the switching action of

the MOSFET and diode respectively. When the voltage of the battery is more than the output reference voltage converter

operates as a buck converter. As soon as the voltage of the battery drops to a value less than the output reference volt age the

converter should switch to boost mode. The added advantage of the converter is that the output of such a converter is always

positive .The overall system level closed loop control strategy of the proposed method. The figure also shows the Switching I C

IRZ1101. The pulses to this IC are provided to by the micro controller which gives pulses to the gate of the MOSFETS. Hence it

drives the circuit to operate in required modes using the PWM method of control.



5.3. MICROCONTROLLER (PIC 16F877A)

Fig.9. Hardware of Microcontroller (Pic 16F877A)

The implemented Micro controller consists of an IC which is used to drive the gate of the switch. The supply is provided by a

750mA/12V step down Transformer and is fed to the bridge rectifier. The heat sink dissipates the excess heat. The duty cycle is

controlled by the micro controller. The microcontroller has an advantage of communication with external devices.

5.4 Results and Discussions

Fig.10. Implemented Hardware along with the results

The results are obtained for the respective modes. By varying the input voltage using a regulated power supply. And the output

is viewed through a multi-meter. Input voltage is varied from 2V-7V .When input voltage is 3.6V then the PWM starts boosting..

As input voltage is increased the PWM starts bucking. So however when input voltage is increased or decreased the output

voltage remains constant at 5.0V.



6. Conclusion and Future scope

This simulation presents a simple but efficient system. It models each component and simulates the system using MATLAB.

Correct modeling of the DC-DC converter is an important area of study, and various difficulties remain in the current study. A

more realistic model of the DC-DC converter would involve a diode loss, a switching loss in a Power-MOSFET, and resistive

losses in inductors and capacitors. SimPower Systems provide components to build electric circuits in SIMULINK and allow

including such losses. At the initial phase of simulation design, attempts to build Buck -Boost converters in SIMULINK faced

unsolvable difficulties. Building the whole system in SIMULINK, however, could open avenues of study such as stability

analysis of system and implementations of more advanced control methods. Hardware setup was done using positive buck-boost

converter. The Buck-Boost converter is highly efficient and reduces the ripples in the output voltage Less labour and low

maintenance cost. Non polluting to the environment. Easy to remove, transport and store.

6.1 Future Scope

This project can also be carried out by implementing it to solar panel systems. Solar panel is used be cause it uses minimum

energy and gives maximum output. In our project, we have used a 5V power supply for the PIC microcontroller. This is the main

limitation. This can be eliminated by giving a feedback from the battery to a 5V regulator. Project can be a lso be modeled in

various applications such as solar-wind hybrid power system, DC pump etc.

References

1. A. Chakraborty, A. Khaligh, A. Emadi, and A. Pfaelzer, “Digital combination of buck and boost converters to control a positiv e buck–boost

converter,” in Proc. IEEE Power Electron. Spec. Conf., Jun. 2006, vol. 1, pp. 1–6. 2. A. Chakraborty, A. Khaligh, and A. Emadi, “Combination of buck and boost modes to minimize transients in the output of a posi tive buck–

boost converter,” in Proc. IEEE 32nd Ind. Electron. Annu. Conf., Paris, France, Nov. 2006, pp. 2372–2377.

3. R.W.Erickson, Fundamentals of Power Electronics, 4th ed. Norwell, MA: Kluwer, 1999.

4. L. S. Yang, T. J. Liang, and J. F. Chen, “Analysis and design of a novel three phaseAC–DCbuck–boost converter,” IEEE Trans. Power Electron.,vol. 23, no. 2, pp. 707–714, Mar. 2008.

5. B. Sahu and G. A. Rincon-Mora, “A low voltage, dynamic, noninverting, synchronous buck–boost converter for portable applications,”

IEEE Trans. Power Electron., vol. 19, no. 2, pp. 443–452, Mar. 2004. 6. B. Bryant and M. K. Kazimierezuk, “Derivation of the buck–boost PWM DC–DC converter circuit topology,” in Proc. Int. Symp. Circuits

Syst., May 2002, vol. 5, pp. 841–844.

7. Y. Zhang and P. C. Sen, “A new soft-switching technique for buck, boost, and buck–boost converters,” IEEE Trans. Ind. Appl., vol. 39, no.

6, pp. 1775–1782, Nov./Dec. 2003.

8. C. Jingquan, D. Maksimovic, and R. Erickson, “Buck–boost PWM converters having two independently controlled switches,” in Proc. IEEE

32nd Power Electron. Spec. Conf., Jun. 2001, vol. 2, pp. 736–741.

9. D. Adar, G. Rahav, and S. Ben-Yaakov, “A unified behavioral average model of SEPIC converters with coupled inductors,” in Proc. IEEE

Power Electron. Spec. Conf., Jun. 1997, vol. 1, pp. 441–446.

.

digital combination of buck and boost converters to ... · digital combination of buck and boost...

Documents