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Four Switch Buck-Boost Converter for Photovoltaic

DC-DC Power Applications

S.Senthilkumar1, R.Venkatesh2, P.Ganga3, G.Vijayakumar4

1Assistant professor, Department of EEE, AnnapooranaEngg. College, Salem

2Professor & Head, Department of EEE, AnnapooranaEngg. College, Salem



[email protected], [email protected],[email protected], [email protected]

Abstract

Photovoltaic energy is a wide kind of green energy. Ahigh performance on these systems is

needed to make the most of energy produced by solar cells. Also, there must be a constant adaptation

due to the continuous variation of power production. Control techniques for Power Converters like

the MPPT algorithm (Maximum Power Point Tracking) present very good results on photovoltaic

chains. Nevertheless, losses on power elements reduce global performance and the voltage/current

adaptation is not always possible. In this paper, the use of the high efficiency structure Four Switch

Buck-Boost for Photovoltaic systems is addressed. FSBB Converters present some advantages

regarding other Power Converter structures: it has few electrical components, output voltage can be

higher or lower than input voltage, and it presents a non-inverted output voltage as happens with

others Buck-Boost Converters. A prototype of FSBB has been developed in order to exploit these new

capabilities for a Photovoltaic DC-DC power application (solar module: 0V to 20V input voltage and

different resistive loads). The implemented MPPT digital control chooses between a Buck or Boost

mode, depending on the working conditions. Firstly, an introduction and the FSBB operating mode

are addressed. Then, the adaptive MPPT control algorithm is detailed. And finally, the experimental

results and conclusion are presented.

Index Terms—FSBB(Four Switch Buck-Boost),MPPT(MaximumPowerPointTracking),

Photovoltaic,PowerConverters.

1. Introduction

FSBB Power Converter is different from other Power Converters because it has two duty cycles to

be controlled, that is, two freedom degrees. This means that for a same working point, different values

for both duty cycles can be used. Furthermore, due to its simple Buck-Boost structure, it presents high

performance and high adaptability to system voltage changes.

Journal of Information and Computational Science

Volume 9 Issue 4 - 2019

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All these advantages make the FSBB Power Converter suitable for applications where the load

may be variable and where a high efficiency is required, as the Photovoltaic applications or power

supply applications [1]. To fully determine the operating mode of this new power structure, a

theoretical study has been done and compared with experimental results.

The structure of a FSBB Power Converter is presented in Fig. 1. It has four switches which commute two by two. Both switches couples (D1, /D1 and /D2, D2) are independent:

Figure 1.Electric schema of the FSBB Power Converter

The four switches (synchronous structure instead of asynchronous) [2], increase the converter

efficiency since the commutations are done very quickly compared to natural diode commutations.

Nevertheless, the control must be done carefully in order to avoid short-circuits on the inductor and

defective functioning.

Using the average model [3], the equations system for the power structure shown in Fig. 1 is:

whereD1 and D2 are the duty cycles, Vin and Vout the input and output voltages respectively, IL the

inductor current and RL theinductor equivalent resistor, which can be seen as an average of losses

(function of several parameters depending on the working point).

Thanks to the state equations presented in (1), the output voltage gain and the current through the

inductor gain in steady-state for constant duty cycles, (2) and (3), can be calculated (i.e. using Laplace

Transform and applying the final value theorem):

If RL tends to zero, the lossless expression for the voltage gain is found:



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It can be observed how if D2 tends to 0, the FSBB behaves like a Buck Converter and how if D1

tends to 1, the FSBB behaves like a Boost Converter. Other possibilities for D1 and D2lead to different

intermediate working modes.

The two freedom degrees of a FSBB power converter let choose the same working point in too

many different ways. As it can be deduced thanks to (2), for the same voltage amplification GV,

different duty cycles can be chosen:

Or working out D2, two solutions are found:

but only the negative solution is considered here since the positive one leads to very high currents

(i.e. if RL tends to 0, D2would be 1 and gains would be infinite).

Among of all possible duty cycles, it must be chosen those which minimize losses. Because losses

are proportional to current or square current, the optimum corresponds to the couple of duty cycles

which minimize current through the converter. For a fixed input and output voltage (Vin and Vout),

introducing expressions (5) and (6) into (3), an optimum for D1and D2can be found which minimize

IL:

This means that the current is minimized when D1 and D2 are fixed to 1 or 0 respectively. Thus, the

most interesting working zones are the pure Buck and pure Boost mode, defined as:

Those working modes are represented graphically in Fig. 3 for different values of RL/R:



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Figure 3. Voltage Gain versus D1 and D2, on the best configuration.

As it can be seen, the output voltage can be either bigger or lower than the input voltage (without

polarity inversion) and the transition between both working modes is smooth since the global

piecewise function is continuous and derivable:

III. FSBB OPERATING MODE

A. General Setup

The photovoltaic application dealt with on this paper is presented in Fig. 4. It can be

observed how the different elements are connected in order to transfer the maximumpower

produced by the solar module to the load:

Figure 4.Electric schema for the photovoltaic application.

A classic MPPT Algorithm with only one freedom degree to look for the Maximum Power Point

(MPP) of the solar module, that is, the optimal voltage Vopt and the optimal current Iopt that will be

adapted to the load [4]. Nevertheless, the adaptation cannot always be done, for example, when the

input voltage exceeds the output voltage if a Boost Converter is used (i.e. if the optimal voltage Vopt of

solar modules is higher than the load voltage VCh) and vice-versa for a Buck Converter. This

adaptation problem can be avoided by using a FSBB Converter, which can work as a Buck or a Boost



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converter. Furthermore, once the working zones have been fixed, the FSBB becomes a one freedom

degree converter by measuring the input voltage Vin and the output voltage Vout to calculate the voltage

gain GV. Like this, the MPPT Algorithm knows whether to work on Buck or Boost mode.

The system characteristics are: Solar Module that can produce 0V to 20V in function of sunlight

and temperature (BP585: ISC=5A and VOC=22.1V), and a variable resistive load. So, the FSBB must

be able to adapt different output voltages and also to change between the Buck and Boost modes if

needed.

B. FSBB Electric Setup and Control Characteristics

The chosen frequency for all the commutation signals has been fixed to 200 kHz. The FSBB

control is carried on by a DSPIC30F30. Two couples of complementary PWM signals are generated

in function of the MPPT control algorithm.

Attending to the commutation frequency, the elements on the converter have been chosen as

shown in the next table:

TABLE I.CONVERTER ELEMENTS VALUES

L 22uH

RL 0.02Ω

C 20uF

To be sure that the commutations on transistors are done properly, four drivers have been used to

reinforce the PWM control signals. The electric schema built up can be observed in Fig. 5:

Figure 5. Electric schema of the FSBB Power Converter.

The extra diodes, placed in parallel to the mosfets are freewheeling diodes. They have two

advantages regarding the transistor parasite diodes: they commute faster and the drop voltage during

the conduction mode is smaller.

Commutation on the four switches must be done carefully since the solar module or the load must

never be short-circuited. To avoid this problem, a dead time of 30ns has been implemented on

commutation signals. This is a little time compared to the time taken by diodes to commute naturally,

which improves the performance. In Fig. 6, it can be observed the commutation signals and the

inductor current simulated when /D1 = 5% and D2 = 50%:



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Figure 6.FSBB simulation: commutation signals and inductor current.

III. ADAPTIVE MPPT ALGORITHM

To achieve the MPP of a solar module, a classic MPPT Algorithm needs to know the variations of

the input power respect to the input voltage. This is enough to decide the duty cycle for the next

control action. The developed MPPT Extreme Seeking Control Algorithm [5], based on a Perturb &

Observe Algorithm, also needs to know the voltage amplification (Vout/Vin) to fully determine both

duty cycles. The control schema can be found in Fig. 7:

Figure 7. MPPT Algorithm control schema for the FSBB converter.

This MPPT Algorithm has two different working modes which improves efficiency as it has been

said before (pure Buck and pure Boost modes). Once the gain (Gain[k]) is calculated, it chooses

which duty cycle will be fixed and which will be constantly changing to look for the MPP by the

MPPT Extreme Seeking Control Algorithm. The switching between both modes is not brusque since

Gain[k] = 1 means that D1 = 1 and D2 = 0.



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IV. EXPERIMENTAL RESULTS

A. FSBB Characteristics

In Fig. 8, a picture of the FSBB circuit with its different elements is presented:

Figure 8.Picture of the FSBB circuit.

The current through the inductor is shown in Fig. 9. These waveforms correspond to the simulated

curves presented in Fig. 6:

Figure 9. Inductor current and commutation signals on Boost mode.

Inductor current behaves almost like the inductor current in a Boost converter since D2 = 40% and

/D1 = 5%.Zooming commutations on D2 and /D2, it can be observed the dead time to avoid short-

circuits:



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Figure 10. Dead time on commutation signals.

B. FSBB Efficiency

Several points have been measured for some determined working points: fixed output voltage (Vout

= 20V) and fixed input voltage (fixed amplification GV), and fixed output current (fixed resistive load,

R = 13.3Ω). In Fig. 11, a 2D graphical representation shows the difference between the theoretical

values (with RL=0.02Ω) and the measured data for the duty cycles D1=1 and D2=0:

Figure 11. Theoretical values versus measured data.

It can be seen that D1 and D2 have a linear dependency since RL is very small, as predicted by (5).

Nevertheless, commutation losses create a constant offset which is proportional to the voltage gain

GV.

Each measured point does not present the same efficiency. In order to achieve the same gain, more

or less power is consumed from the input. The markers size in Fig. 10 represents measured efficiency

(η (%) = Pout/Pin x 100%). It can be observed how the converter efficiency decreases towards the



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intermediate working modes. This demonstrates that the best performance is obtained when working

on the pure Buck or pure Boost mode.

For these two modes and for normal working conditions, efficiency tests on the FSBB can be

performed measuring the input power Pin and the output power Pout and calculating Pout/Pin. The

procedure is:

• VinandVoutare fixed depending on the workingmode.

• Different values for the resistive load are applied to the power converter.

• The duty cycled D1 and D2 are chosen for each load value in order to satisfy the input and

output voltage Vin and Vout.

As it can be seen in Fig. 12, for the Boost mode, a maximum efficiency of 93% is obtained for

an input power of 60W:

Figure 12. FSBB efficiency on Boost mode

The conditions are: Vin = 20V, Vout = 24V

In Fig. 13, it is shown that, when working in Buck mode, a maximum efficiency of 91% is

obtained for an input power of 45W:

Figure 13.FSBB efficiency on Buck mode.

The conditions are: Vin = 20V, Vout = 12V

C. MPPT Algorithm

To prove that the implemented MPPT Algorithm works, it is necessary to connect the FSBB to a

solar module and to a load as shown in Fig. 4.



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In Buck mode, to be sure that the adaptation is possible, load resistance R must be small since the dynamical impedance of a Buck converter Vin/Iin, ideally, can only vary

betweenR and ∞. If Vopt/Iopt from the solar module does not belong to the interval [R,∞[ the PPM will not be achieved.

That is, in a Buck converter, it can be written:

Vout Vin D (13)

I in I out D (14)

rearranging terms, if Vin and Iin are Vopt and Iopt from the solar module:

then, R must be smaller than Zopt since D (the duty cycle) can only vary between 0 and 1.

In Fig. 14, it the MPPT Algorithm working in Buck mode with a 0.9Ω load resistance is shown:

Figure 14.MPPT working in Buck mode.

In Boost mode, the adaptation will be only possible ifVopt/Ioptfrom the solar module belongs to the

interval [0, R] (dynamical impedance of a Boost converter), since:

rearranging terms, and again, if Vin and Iin are Vopt and Iopt from the solar module:

then, R must be bigger than Zopt since D (the duty cycle) can only vary between 0 and 1.

In Fig. 15, it the MPPT Algorithm working in Boost mode with a 16.5Ω load resistance is shown



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Figure 15.MPPT working in Boost mode.

V. CONCLUSION

In this paper, a new Power Converter structure has been presented: the Four Switch Buck-Boost

Converter (FSBB). Its characteristics made it suitable for photovoltaic applications: high performance

and the possibility of adaptation to different input and output time-varying voltage values.

Among all the possibilities to fix a single working point, it has been determined that the best

efficiency is found when the FSBB behaves like a pure Buck or like a pure Boost Converter.

A MPPT Extremum-Seeking Algorithm has been adapted to: constantly look for the maximum

power on solar modules and to choose between Buck or Boost mode in function of working

conditions.

REFERENCES

[1] BiranchinathSahu and Gabriel A. Rincón-Mora. A Low Voltage,

Dynamic, Noninverting, Synchronous Buck-Boost Converter for Portable Applications. Power Electronics, IEEE Transactions. Volume19. 2004, pages: 443 – 452

[2] XiaoyongRen, XinboRuan, HaiQian, Mingqiu Li and Qianhong Chen,

Three-Mode Dual-Frequency Two-Edge Modulation Scheme for Four-Switch Buck–Boost Converter. Power Electronics, IEEETransactions. Volume 24. 2009, pages: 499 – 509.

[3] Robert W. Erickson and DraganMaksimovic. Fundamentals of PowerElectronics. State-Space

Averaging, pages: 213 – 217.

[4] C. Cabal, C. Alonsol, A. Cid-Pastor, B. Estibals, L Seguier, R. Leyva G. Schweitz and J. Alzieu, Adaptive digital MPPT control for photovoltaicapplications. Industrial Electronics, 2007. ISIE 2007. IEEE InternationalSymposium. 2007, pages: 2414 – 2419.



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