A bidirectional resonant converter based on wide input range and high efficiency for photovoltaic application

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 10, No. 3, Sep 2019, pp. 1469~1475
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v10.i3.pp1469-1475  1469
Journal homepage: http://iaescore.com/journals/index.php/IJPEDS
A bidirectional resonant converter based on wide input range
and high efficiency for photovoltaic application
Ibrahim Alhamrouni1
, M. R. Bin Hamzah2
, Mohamed Salem3
, Awang Jusoh4
, Azhar Bin Khairuddin5
,
T. Sutikno6
1,2 Electrical Engineering Section, University Kuala Lumpur (UniKL BMI), Gombak 53100, Malaysia
3 School of Electrical and Electronic Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia
4,5 School of Electrical Engineering, Universiti Teknologi Malaysia, 81300 Skudai, Malaysia
6 Department of Electrical Engineering, Universitas Ahmad Dahlan, Yogyakarta, Indonesia
Article Info ABSTRACT
Article history:
Received Dec 17, 2018
Revised Feb 27, 2019
Accepted Apr 20, 2019
This work highlights a modular power conditioning system (PCS) in
photovoltaic (PV) applications which consists with a DC-DC converter. The
converter is able to regulate and amplify the input DC voltage produced by
the PV panal. The implementation of Mosfet as bidirectional switch on the
converter yields greater conversion ratio and better voltage regulation than a
conventional DC-DC step up converter and PWM resonant converter. It also
reduces the switching losses on the output DC voltage of the converter, as the
MOSFET switches on primary winding of converter switch on under ZVS
conditions. The proposed resonant converter has been designed, with the
modification of series resonant converter and PWM boost converter that
utilizes the high frequency of AC bidirectional switch to eliminate the
weaknesses of used converters. The topology of the proposed converter
includes the mode of operations, designing procedure and components
selection of the new converter elements. This topology provides a DC output
voltage to the inverter at range of about 120Vac-208 Vac.
Keywords:
Bidirectional Converter
Photovoltic (PV)
Power Condititiong System
(PCS)
Resonant Converter
Copyright © 2019 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Ibrahim Alhamrouni,
Electrical Engineering Section,
University Kuala Lumpur British Malaysian Institute (UniKL BMI),
Gombak 53100, Malaysia.
Email: ibrahim.mohamed@unikl.edu.my
1. INTRODUCTION
Photovoltaic (PV) energy is one of the renewable energies available in daily life as an option or
backup to the other electrical energy supply. Besides, PV energy generation causes no pollution to
environment as it is green technology. In addition, the PV energy usage and demand have achieved
significant growth in recent years due to constant researches and developments and the civilians’ awareness
towards clean environment. PV energy is gained by harnessing the solar energy of sunlight’s photon using
PV modules or solar panel [1, 2]. Each PV module has different direct current (DC) output, thus given the
wide input current range to the Power Conditioning System (PCS). In PV system, PCS acts as an inverter that
converts the DC output from PV module to alternate current (AC). The PCS is connected to the electric
power grid and equipped with Maximum Power Point Tracker (MPP Tracker). The purpose of this tracker is
to improve the efficiency of solar power generation. This tracker has Maximum Power Point Tracking
(MPPT) algorithm to maximize the power extraction from PV modules under certain factors and conditions
[3, 4]. There are 3 types of PV PCS configuration; modular, string and centralized. Among these
configurations, the PV PCS modular type has been considered as the best and commonly used in PV
installation because its efficiency in harvesting the energy. What makes PV PCS modular type is the best are
the implementation of Module Integrated Converter (MIC) on each PV module for MPPT and it varies in

 ISSN: 2088-8694
Int J Pow Elec & Dri Syst, Vol. 10, No. 3, Sep 2019 : 1469 – 1475
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connection of output AC to the grid (called as micro-inverter) or output AC to output DC (called as micro-
converter)[5]. In order to obtain the best from solar energy due to many factors affecting the sunlight, the
modular PV PCS has to work with high efficiency over wide input range. There are several electronic
approaches used to make PV PCS modular type working in high efficiency condition, as example, flyback
converter variants, LLC converter, isolated Pulse Width Modulation (PWM) converter and series resonant
converter, to meet the requirement of DC-DC micro-converter[6]. These kinds of approaches are the isolated
DC-DC converter type, however they have several weaknesses and suffer some inability to meet desired
outcome. Besides, the high efficiency has to be achieved by meeting the wide input voltage requirement, zero
voltage switching (ZVS) on low circulated currents and zero current switching (ZCS). ZVS and ZCS are the
process of switching turn-on and turn-off of converter’s switch during zero voltage and zero current input.
The importance of the converter achieving ZVS and ZCS are to reduce losses during switching operation and
provide soft switching method to reduce the electromagnetic interference [7-9].
Based on the previous studis of DC-DC isolated resonant converter topologies, it can be described
that all the converters suffer the drawbacks in order to achieve the desired output. Hence, in order to obtain
high efficiency over wide input range, soft switching in Zero Current Switching (ZCS) and Zero Voltage
Switching (ZVS) condition, maximum direct power transfer from source to load and good voltage regulation
through fixed PWM control, the development of new isolated resonant converter is considered[8]. In
developing the new isolated resonant converter, several processes has been planned properly in order to avoid
the drawbacks and maintaining the soft switching and good voltage regulation along the operation. The
processes consist of the selection of transformer turns ratio, the designation of resonant tank, transformer
design and dead time selection and the most important thing, the selection of semiconductor devices[7].
Besides, the study of previous works on isolated DC-DC resonant converters was really much help in order to
develop the new isolated resonant converter. This is because the research can benefits the strengths and
relates the certain important points of the previous studies in terms of the circuit designation and modes of
operations. Thus, the new developed resonant converter operates much like conventional series resonant
converter and PWM boost converter [3]. The newly proposed design has several modifications to the
conventional series resonant converter, LLC converter and PWM boost converter, in order to obtain the high
efficiency over wide input range under fixed-frequency control [10-12]. One of the significant changes on the
proposed circuit is adding bidirectional AC switch on secondary winding of circuit. The bidirectional AC
switch is MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The purpose of adding
bidirectional AC switch on secondary winding is to utilizing its high frequency for regulation capability of
the output, either the output has load or no load by using PWM boost converter design and operation concept
[4, 13, 14]. Besides, it helps to maximize the direct power transfer from source to load and providing good
wide range of regulation in its operation, thus increasing the converter’s efficiency [15-18]. The paper
suggests the proposed converter shall provide the high DC-DC voltage conversion ratio, regulates the output
transformer voltage and reduce the switching losses when MOSFET switches on primary winding of
converter’s transformer achieve ZVS. The new proposed resonant converter has been designed, with the
modification of series resonant converter and PWM boost converter that utilizizes the high frequency of AC
bidirectional switch to overcome all weaknesses of previous types of converters. The topology of proposed
converter includes the mode of operations, designing procedure and components selection of the new
proposed resonant converter.
2. CIRCUIT CONFIGRATON AND OPERATION MODES
The proposed isolated resonant converter has 8 modes of operations. The converter consists of 4
MOSFETs N-Channel on the primary winding of transformer to mitigate the switch count as they form a full
bridge switch network, S1, S2, S3 and S4. The additional of two drain-connected of MOSFETs at the
secondary winding of transformer as bidirectional switch, S5 allows the converter to operate in conventional
series resonant converter mode during OFF condition and PWM boost converter mode during ON condition.
The inductor Lr at the secondary winding transformer either can be a resonant inductor or a leakage
inductance. The n and Lm from the circuit represent the turns ratio and magnetizing inductance. The resonant
inductor, Lr along with output resonant capacitors, Cr1 and Cr2 determining the series resonant frequency, fr
for the switch network operation. The Lm value is larger than Lr, while the output capacitor, Co value is
larger than Cr1 and Cr2 combined, which also one of the factor that allows the converter to operate in series
resonant mode during S5 is OFF. The utilization of PCS in PV application steps up the low voltage into high
voltage. Besides, the isolated DC – DC converter type provides the isolation between source and loads which
is necessary. Other than that, it also tracks the maximum power point of PV module effectively.

Int J Pow Elec & Dri Syst ISSN: 2088-8694 
A bidirectional resonant converter based on wide input range and … (Ibrahim Alhamrouni)
1471
Figure 1. The circuit configtation of the proposed converter
2.1. Mode 1 and mode 5
The circuit operates in PWM boost mode. The positive voltage is generated when switch S1 and S4
are ON during Mode 1. On the other hand, the switch S2 and S3 generates the negative voltage during Mode
5 when both are ON. Bidirectional switch, S5 is ON. The secondary winding of transformer is shorted due to
nearly zero current and voltage across leakage inductance, Lr and capacitor Cr1. During the Mode 5, the S2
and S3 are ON and generate the negative voltage.
(a) (b)
Figure 2. Operation modes; (a) mode 1, and (b) mode 5
The circuit operates as conventional series resonant converter as the bidirectional switch, S5 is OFF.
The resonant inductor, Lr has been charged completely during previous stages (Mode 1 and Mode 5) and
discharged after entering these stages through the capacitor duo, Cr1 and Cr2. When the switch S1 and S4 are
ON during Mode 2, the converter operates in positive duty cycle, while vice versa when S2 and S3 are ON
during Mode 6.
The converter operates in idle mode. During these stages, the current of inductor Lr declines to zero.
Besides, there is no direct power transfer between source and loads as the converter works with fixed
frequency. Also, the zero voltage switching (ZVS) is achieved on the primary winding and zero current
switching (ZCS) is achieved on the secondary winding. The utilized small current allows the converter to
operate close to series resonant frequency.
These stages are the dead time period of the converter. The magnetizing inductor, Lm has the
maximum current value to supplies the converter during Mode 4. The S2 and S3 are turn on under ZVS
condition. During Mode 8, the S2 and S3 are initially off. The current in Lm reaches its minimum value,
charges the parasitic capacitance of S2 and S3 and discharging those of S1 and S4.

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(a) (b)
(a) (b)
(a) (b)
3. PROPOSED CONVERTER PARAMETRES DESIGN
The converter also designed to produce output power of 350W. As the inverter is about to produce
maximum output AC voltage of 230V, the converter also expected to produce DC voltage of 325V. The
switching time, Ts is set at 60µs and switching frequency is set at 17 kHz which based on LLC resonant
converter topology in [13]. In designing the resonant tank, first thing first is choosing the large value of
resonant inductor, Lr. The internal reactance of resonant inductor, XLr and resonant capacitor, XCr is set at
12.94Ω which also based on LLC resonant converter topology. The large Lr value is necessary for Mode 1
and Mode 2 to last longer than Mode 3. This is because there’s no power transfer from load to source in
Mode 3 besides by minimizing this operation mode period, it will increase the efficiency of power
conversion. The value of Lr can be determined as ;
Lr (1)
Meanwhile, the resonant capacitors, Cr1 and Cr2 are chosen based on the resonant frequency
needed. However, the ripple voltage across those capacitors cannot exceed their own DC voltage. The DC
voltage value of the capacitors is half of the output voltage value. The voltage stress across S5 and the
resonant capacitors can be reduced by selecting the lower Cr value. The Cr value can be determined as ;

1473
Cr (2)
On the other hand, the resonant frequency, Fr of the converter based on Cr is determined as ;
Fr (3)
It is more important to choose suitable value of magnetizing inductance, Lm to obtain the desired
outcome. Still, choosing the larger value of Lm over Lr is necessary to compensate the circulating current on
primary winding, thus maximizing the power transfer from source to load besides increasing the efficiency of
power conversion. The Lm is 20 times bigger than value of Lr. Last but not least, if the input voltage of the
converter is greater than the nominal voltage, the S5 duty cycle becomes zero and the PWM from the primary
side switching will provides the conversion ratio that less than the series resonant converter. If the input
voltage of the converter is less than the nominal voltage, the utilization of bidirectional switch will provide
the conversion ratio that greater than series resonant converter. The number of turns, n can be expressed as ;
n (4)
4. RESULTS AND DISCUSSION
The simulation of the converter is done by MATLAB 2016b version. The converter was designed as
suggested in previous csection. By using the mathematical formula shown in previous section, the converter
was designed to fulfill the requirement to produce DC output voltage to be inverted by the inverter for AC
output voltage range 120Vac - 230Vac. With the implementation of MOSFETs as bidirectional switch on
secondary winding, this hybrid converter yields greater conversion ratio than conventional step up DC-DC
converter and PWM boost converter with smaller transformer turns ratio and smaller resonant tank
parameters. The converter was designed to have the input voltage at range 15V to 50V. However, the
converter must have nominal input voltage of 30V and less to be operated under the normal AC switch
operation in order to be produced by inverter for AC output voltage of 120Vac – 230Vac. Thus the chosen
nominal input voltage is 30V. Figure 6 shows the measured gate voltage and collector voltage of all switches
(S1 – S4) at full load condition. The collector voltage VDS dropped to zero, before the switches were turned
on. Thus, all the switches were turned on at ZVS.
Figure 6. Measured waveforms of the gate voltage and drain voltage for each switch separately
As the 4 power MOSFETs sorted as full-bridge switch on primary winding of the converter, the
transformer output voltage produce voltage peak to peak AC 340.6V from the 30V input voltage. The
transformer output voltage peaked at maximum value 171.8V and minimum value -168.9V as shown in
Figure 8.

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Figure 7. Transformer output voltage Vto
Figure 8. Measured waveform of resonant inductor current iLr
Figure 9. Output voltage Vo
With another installment of MOSFETs on secondary winding of converter as bidirectional switch,
the transformer output voltage is regulated and boosted. Thus, produce the DC voltage output of 327.7V as
shown in Figure 9. The switching losses were caused by the element called parasitic capacitance. This
element is unavoidable when the transistors, diodes and other components that have internal capacitance are
draw close to each other in the circuit. The proposed converter also has switching losses but it was too small
to be compared with step up DC-DC converter. This is because HRC has the attachment of MOSFETs on
secondary winding to reduce the switching losses. In this contest, the proposed converter is more reliable
than step up DC-DC converter.
5. CONCLUSION
The proposed resonant converter, HRC has proven that it can yield higher conversion ratio of the
voltage than any other resonant converter, besides regulating the transformer output voltage to provide a
sufficient voltage supply for the inverter in the PV system. In addition, the designed DC-DC converter is able
to track the MPP of PV panels while operating under the different input voltages. Thus, HRC has fulfilled the
criteria and goals. The implementation of two MOSFETs as bidirectional switch on the secondary winding of
converter’s transformer has also helped reducing switching losses that occur after the full bridge MOSFETs
on the primary winding achieves ZVS. Although HRC has 8 different modes of operation, the suitable

1475
injection of switching frequency on the MOSFETs made the converter switching the mode after mode very
fast and smooth. After all, HRC is the best resonant DC-DC converter to be implemented on modular PCS of
PV. In the future, HRC and the inverter can be combined together as one part in modular PCS. It will be a
DC-AC converter, thus reducing the components and cost to be used. Further research and development of
the recommended project will be considered. With one condition, the converter must produce sufficient
output voltage to utility grid with good voltage regulation.
ACKNOWLEDGEMENTS
The authors would like to express their gratitude to Universiti Kuala Lumpur for supporting and
funding this research under grant No. str18005.
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