https://doi.org/10.6113/JPE.2019.19.4.1011

ISSN(Print): 1598-2092 / ISSN(Online): 2093-4718

Buck-Flyback (fly-buck) Stand-Alone Photovoltaic System for Charge Balancing with Differential Power Processor Circuit

Chun-Gu Lee^*, Jung-Hyun Park^*, and Joung-Hu Park^†,*

^†,*Department of Electrical Engineering, Soongsil University, Seoul, Korea

Abstract

In this paper, a buck-flyback (fly-buck) stand-alone photovoltaic (PV) system for charge balancing with a differential power processor (DPP) circuit is proposed. Conventional feed-back DPP converters draw differential feed-back power from the output of a string converter. Therefore, the power is always through the switches and diodes of the string converter. Because of the returning conduction path, there are always power losses due to the resistance of the switch and the forward voltage of the diode. Meanwhile, the proposed feed-back DPP converter draws power from the magnetically-coupled inductor in a string converter. This shortens the power path of the DPP converter, which reduces the power losses. In addition, the extra winding in the magnetically-coupled inductor works as a charge balancer for battery-stacked stand-alone PV systems. The proposed system, which uses a single magnetically-coupled inductor, can control each of the PV modules independently to track the maximum power point. Thus, it can overcome the power loss due to the power path. It can also achieve charge balancing for each of the battery modules. The proposed topology is analyzed and verified using 120W hardware experiments.

Key words: Buck-flyback, Charge balancing, Differential power processor, Fly-buck converter, Forward converter, Photovoltaic applications

Manuscript received Aug. 10, 2018; accepted Mar. 18, 2019

Recommended for publication by Associate Editor Seongjun Lee.

^†Corresponding Author: wait4u@ssu.ac.kr, Tel: +82-2-820-0645, Fax: +82-827-7961, Soongsil University, Korea

^*Dept. of Electronic Eng., Soongsil University, Korea

Ⅰ. INTRODUCTION

Among renewable energy resources, solar energy has been put in the spot light. Systems using solar energy have some advantages since the generators are small in size, low in weight and easy to expand. Due to these advantages, solar energy applications such as photovoltaic systems can be used for a wide power range from small power residential applications up to large mega-watt power solar farms [1]-[3]. Because solar panels have low voltage high current output characteristics, they are connected in series the early stage. However, in the case of a simple serial structure, the overall system efficiency is low due to partial shading, and the entire system cannot operate even if one panel is in trouble [4]-[6].

To solve this problem, the module integrated converter (MIC) was proposed. In the MIC, while the whole manufacturing cost is more expensive than that of conventional centralized photovoltaic structures, it can control each PV panel independently which solves the problem of poor efficiency due to partial shading and the system operation problem due to partial failures [7]-[13].

The differential power processor (DPP) was recently proposed to overcome the disadvantages of the MIC while maintaining its advantages. Fig. 1 shows the DPP converter structure. It is different from the MIC since a converter is connected with each solar panel in parallel while the PV panels are connected in series. The DPP converter topology is divided into two types depending on the DPP power flow. One is the feed- forward DPP converter and the other is the feed-back DPP converter. Fig. 2 shows a feed-forward type DPP converter, and Fig. 3 shows a feed-back type DPP converter. In the feed-forward type, when the output power of a certain panel is higher than that of the other panels, the power difference between the high output panel and the other panels is transferred directly to the output of string converter through a DPP converter. As shown in Fig. 2, when the PV1 output power is 30W and the other PV panels are all 20W under the same voltage, 10W of the PV1 power is transferred to the output of a string converter through a DPP converter. In the feed-back type, when the output power of a certain panel is lower than the other panels, the power difference between the low power panel and the other panels is transferred back to the low power panel to compensate its low power through a DPP converter. As shown in Fig. 3, when the PV1 output power is 10W and the other PV panels are all 20W, 10W of the string converter power is transferred to the output of PV1 through a DPP converter with the same MPP voltage. The feed-forward and feed-back DPP converters are totally different from the MICs because they operate only when there is a power difference between PV panels. Thus, there are no power losses under full radiations, which overcomes the loss and cost problems of the MIC. In addition, it relieves an aging problem.

Fig. 1. Differential power processing (DPP) converter structure.

Fig. 2. Feed-forward DPP converter structure.

Fig. 3. Feed-back DPP converter structure.

However, there is a significant difference between the feed-forward DPP converter and the feed-back converter. From the perspective of system efficiency optimization, it seems advantageous to use a feed-forward DPP converter. The feed-forward DPP converter delivers differential power directly to the output through the DPP converter, while the feed-back DPP converter delivers the necessary power back to the PV input from the string converter through the DPP converter. This means that the power flow path of the feed- forward is shorter than that of the feed-back DPP converter. However, since PV systems are usually installed in environments where very few PV panels are shaded, and shading occurs rarely to produce power maximally, all of the feed-forward DPP converters, except for a DPP with shaded panels, transfer the extra power. Meanwhile, feed-back DPP converters in this situation have just one or two in operation. Considering the environment, it is reasonable to use feed-back type DPP converters to optimize system efficiency.

A buck-flyback (fly-buck) stand-alone PV system for charge-balancing with DPP circuit is proposed in this paper. This stand-alone system uses the feed-back DPP converter structure to track the maximum power point of a PV panel. However, the proposed feed-back DPP converter uses a magnetically-coupled inductor to overcome the long power path problem of the conventional feed-back DPP converter structure. In the proposed feed-back DPP, the DPP power for maximum power generation is not transferred through the output of the string converter (battery side). Instead it is transferred through the magnetically-coupled inductor in the middle of the string converter. Furthermore, a typical stand- alone PV-battery charger uses additional DC-DC converters controlled for charge balancing. Meanwhile, the proposed system uses a coupled inductor for the balancing instead of an additional DC-DC converter. The proposed string converter has some extra windings in the magnetically-coupled inductor for charge balancing. This works as a secondary equally- multi-winding flyback for equalizing the secondary battery module voltages. Since an additional DC-DC converter is not used, the topology is very cost-effective and easy to use without a controller [14], [15].

The proposed topology is verified by 120W prototype hardware experiments. The paper is organized as follows. Section II explains the structure of the proposed topology along with a functional classification and explanation. Section III presents design guidelines along with formulas. Section IV shows the experimental specification and experimental results. Finally, section V presents some conclusions.

Ⅱ. OPERATING PRINCIPLE

A. Structure of the Proposed Converter

Fig. 4 shows a circuit diagram of the proposed power conditioning system topology. The proposed power conditioning system consists of a string converter, DPP converters and charge-balancing circuitry. The proposed converter operates through the main switch Q_b and the switches Q_f1 and Q_f2 of the DPP converter.

V_O1 and V_O2 are the output battery voltage, which is almost fixed. Thus, the total PV voltage V_PV is regulated by the main switch Q_b in the string converter.

Fig. 4. Equivalent circuit model of the proposed converter in a single-core 5-winding coupled-inductor.

When a power deviation occurs between PV₁ and PV₂, one of the switches Q_f1 or Q_f2 is in the rest state, and the other is switched to regulate the corresponding PV voltage through the inductor voltage V_L, which is determined by N_f1 or N_f2. In the operation of the above DPP converter, the critical principle is that the DPP converter input voltage is induced through the coupled inductor while the string switch Q_b turns on. Thus, Q_b should stay turned on until Q_f1 and Q_f2 are turned off. Therefore, the duty ratio of Q_b in the buck-flyback (fly-buck) converter must be greater than that of the DPP (Q_f1 or Q_f2) under synchronization [16].

The anode voltages of the diodes D_O1 and D_O2 are half of the output voltage determined by N_O1 or N_O2. When there is an imbalance between the battery voltages, D_O1 or D_O2 turns on because of the cathode voltage difference between the diodes sharing the anode voltages through the winding N_O1 and N_O2. The secondary balancing circuit shares a switch with the string converter. The string converter charges the batteries when the switch Q_b is on. In addition, the balancing circuit charges the minimally charged battery unit when Q_b is off.

B. Operating Principles

There are three operating modes when the proposed converter is operated under the continuous conduction mode (CCM). Assume the PV modules are series connected, where the maximum power of PV₁ is greater than that of PV₂. The solid black line in Figure 5 means the conduction path.

Fig. 5. Operating modes of the proposed converter. (a) Q_b on, Q_f2 on [0~D_PT]. (b) Q_b on, Q_f2 off [D_PT~D_bT]. (c) Qb on, Qf2 off [D_bT~Ts].

(a)

(b)

(c)

Mode 1: In Fig. 5(a), the DPP converter with PV₂ is operated while the other DPP is off because of the difference between the MPP current of PV₁ and that of PV₂. The sum of the voltages of PV₁ and PV₂ is applied to the primary of the coupled inductor when Q_b is turned on. Then the inductor primary voltage is applied to the DPP converter input with the turn ratio . Then the switch Q_f2 is turned on, which supplies current to PV₂ in order to compensate the small MPP current to match the string current. All of the power from the PVs instantaneously charged the battery units.

Mode 2: In Fig. 5(b), Q_f2 is turned off and Q_b stays on. The diode D_f2 is turned on and the L_f2 current freewheels through the diode.

Mode 3: In Fig. 5(c), Q_b is turned off and the diode (D_O1 or D_O2) with the lower voltage battery is turned on. Magnetizing inductor current flows through the diode and charges the battery unit. The off-time flyback operation only compensates the lower-charged battery to equalize the battery voltages.

C. DPP Converter Feedback Power Path

Fig. 6 shows a conventional feedback-type DPP flyback converter circuit. The input of the conventional DPP circuit is the battery stack for stand-alone PV operation, and the output of the conventional DPP flyback is the PV module. The DPP converters can operate independently. Therefore, when a PV partial-shading or a failure occurs, the DPPs allow each of the modules to continue to operate at the maximum power points (MPPs). However, since the input of the conventional DPP topology is the output of the string converter, the DPP must be designed for a high step-up gain. This gain results in large losses and a high cost. The high step-up converter requires a high on-resistance transistor for Q_f, which results in a decrease in efficiency. Fig. 7 has an arrow showing the power flow in the system. The conventional DPP circuit transfers power through the output of the string converter, which results in some power losses due to the parasitic resistance of Q_b, L_m, G_f1, N_p1 and so on. In order to overcome the loss problem, a new DPP topology is proposed. Like the previously mentioned DPP in the feedback mode, the proposed DPP transfers power to the PV module only when the PV current is less than that of the string. Fig. 8 has an arrow to show the power flow in the proposed DPP converter. In this topology, the DPP circuit input is not the output of the string converter. It is the tertiary windings of the magnetically-coupled string inductor. Since the power for the DPP bypasses the string converter, there are fewer power losses due to the parasitic resistances of the string. In addtion, the on-resistances of the DPP switches (Q_f1 and Q_f2) are small due to the low voltage stress, which contributes to the high efficiency. Furthermore, by the extended version of the coupled inductor, the tertiary winding (N_O1 and N_O2) can do the charge-balance among the battery units without extra charge-balancing circuits since the balancing circuit operation is totally decoupled.

Fig. 6. Conventional flyback DPP converters with a buck-type string converter.

Fig. 7. DPP power flow of the conventional converter topology.

Fig. 8. DPP Power flow of the proposed converter.

D. Charge Balance for a Battery Stack

If there is an imbalance between battery voltages, one of the tertiary diodes starts to conduct. When the switch Q_b is turned off, the diodes D_O1 and D_O2 are turned on and off repeatedly according to the battery voltages for charge balancing [17]. When the switch Q_b is turned off, the diode current I_DO1 is derived as equation (1), under an assumption that the voltage of battery-2 (V_O2) is higher than that of battery-1 (V_O1).

     (1)

where D_P is the main duty-ratio plus the conduction ratio of the diode current I_Do1 during the off time, and i_Lm is the magnetizing current. N_P is the turn ratio of the primary side of the magnetically-coupled inductor, and N_O1 is the tertiary winding.

I_DO1 depends on the load imbalance as follows:

     (2)

Equation (3) is derived from equations (1) and (2) as follows:

     (3)

where I_Lm is the average current of the magnetically-coupled inductor.

In equation (3), I_Lm can be replaced by I_PV1 / D_b, where I_PV1 is the average current of PV₁. Therefore, equation (4) is derived from equation (3) as follows:

     (4)

Then D_P is derived as equation (5) from equation (4). Since the duty cycle should exist between 0 ≤ D_P ≤ 1- D_b, equation (6) is derived from equation (5).

          (5)

     (6)

From equation (6), the limit of the imbalance load current (I_DO1 = I_batt2 - I_batt1) that this balancer can compensate maximally is derived by equation (7).

     (7)

In equation (7), the left term is negative, which is meaningless. Finally, equation (8) is derived as shown below.

     (8)

From equation (8), the performance limit of the battery- voltage balancer is derived. Then it is used for the design constraint of the balancing circuit.

Ⅲ. DESIGN GUIDELINES

A. Design of the Fly-Buck Converter

In the CCM, the input voltage of the buck-flyback (fly- buck) string converter is determined by the duty ratio D_b of Q_b and the battery stack voltage [15]. The duty ratio D_b of an arbitrary n-module system is derived as equation (9) by the voltage-second balance of the magnetizing inductor. In addition, the n^th PV module output voltage is the input voltage of the n^th DPP converter. Under the CCM, the n^th DPP converter duty ratio D_f(n) is derived by equation (10).

     (9)

where V_PV(n) is the voltage of each panel, V_O(n) is voltage of each battery, N_O is tertiary winding, and n is the number of outputs.

     (10)

Another constraint, the duty cycle D_b must be greater than D_f(n). Therefore, design equation (11) is derived from equations (10) and (9).

     (11)

B. Inductor Design of the Proposed Converter

In the string converter, Δi_Lm is defined as equation (12), where N_O(n)/N_P=n, n is the number of outputs. In addition, the output voltage is balanced.

     (12)

The condition for the string converter to operate in the CCM is equation (13).

     (13)

Equation (14) is derived from equations (12) and (13).

     (14)

Equation (15), derived from equation (14), can be used to design the magnetizing inductance of the string converter (fly-buck), which is derived from the CCM condition, where PO is the output power of the converter, and f is the switching frequency of Q_b.

     (15)

In the DPP converter, Δi_Lf(n) is defined as equation (16).

     (16)

The condition for the DPP converter to operate in the CCM is equation (17).

     (17)

Equation (18) is derived from equations (16) and (17).

     (18)

In the proposed converter, I_Lf(n) is the current difference between I_string and I_PV(n). Therefore, equation (19) is derived for the filter inductance design of the n^th DPP buck converter, where I_Lf(n) is the inductor average current of the DPP, I_PV(n) is the average current of the PV_(n), f is the switching frequency, and I_string is the average current of the string converter.

     (19)

Ⅳ. EXPERIMENTAL RESULTS

A 120W hardware prototype is designed and built in the laboratory. Fig. 9 shows a picture of the prototype and Fig. 10 shows a photograph of the experiment setup. In addtion, Table I presents the design parameters of the hardware and Table II presents the hardware component part numbers. Fig. 11 shows the charge balancer output voltages, where V_O1 is the yellow line and V_O2 is the red line. In this figure, when there is an imbalance between V_O1 and V_O2, the tertiary diodes connected to V_O2 start to conduct to charge the battery. Therefore, V_O1 and V_O2 equally maintain an average voltage of around 12 V. Fig. 12 shows voltage and current waveforms of the hardware experiment. In Fig. 12(a), it is shown that the PV voltage and current waveforms change from a steady- state to another when an MPP step occurs. In this figure, on the left, the steady state of PV₁ is 20V, 3A and that of PV₂ is 19.5V, 2.8A. In this situation, the DPP converters do not operate since there is no need to compensate the PV current among the panels. After the step, on the right, PV₁ is 20V, 3.3A and PV₂ is 20V, 2.7A. Thus, the DPP converter start to operate to compensate the current difference, which makes the PV currents maintain the difference even though the voltages are kept similar. In addition, Fig. 12(b) shows drain- source voltage waveforms of Q_b (V_{ds, string}) and Q_f (V_{ds, dpp}). In Fig. 12(b), it can be seen that Q_f2 turns on after Q_b turns on. It can also be seen that Q_f2 turns off before Q_b turns off. Therefore, both of the switching operation are well matched when the synchronization of the coupled inductor was realized by a multi-inner winding coaxial-cabled single-core transformer, which contributes to the small spikes on the drain-source voltages through the small leakage inductance, as shown in Fig. 12(b). Fig. 12(c) shows an input voltage waveform of the DPP converter. In addition, Fig. 12(d) shows a current waveform of the inductor of the DPP converter and a voltage waveform of the inductor of the DPP converter.

Fig. 9. Hardware prototype.

Fig. 10. Experimental setup.

Fig. 11. Charge balancer output voltages.

Fig. 12. Key waveforms of the hardware prototype. (a) PV_{1, 2} voltage and PV_1,2 current at an MPP step. (b) Switch drain- source voltage. (c) DPP input voltage; (d) DPP inductor voltage and DPP inductor current.

(a)

(b)

(c)

(d)

TABLE I  HARDWARE PARAMETERS

Parameters	Value	Parameters	Value
MPP of VPV1, VPV2	20 [VDC]	Lf1, Lf2	450 [uH]
IPV1	3.3 A	IPV2	2.7 A
Vbatt1, Vbatt2	12 [VDC]	CPV1, CPV2	200 [uF]
PO	120 [W]	Cbatt1, Cbatt2	200 [uF]
F	50 [kHz]	NP : Nf1 : Nf2	1 : 4 : 4
Lm	150 [uH]	NP : NO1 : N O2	1 : 1 : 1

TABLE II  HARDWARE COMPONENT LIST

Components	Part
Qb	IRFP4568
Qf1, Qf2	IRFP4568
Gate driver	TLP250
Df1, Df2, Df3, Df4	B40250
DO1, DO2	B40250
PV simulator	Elgar TerraSAS

Fig. 13 shows the MPP tracking (MPPT) operation of the proposed converter. The PV₁ and PV₂ MPPs are 20V. In Fig. 13, each PV voltage is sustained around 20V due to the DPP converter regulation and the P&O (perturb-and-observe) algorithm.

Fig. 13. Waveforms of the DPP converter output voltages (PV voltages) under MPP tracking operation.

Equation (20) can be used to estimate the total system efficiencies of the proposed and conventional converters.

     (20)

where P_PV1 and P_PV2 are the PV output powers, and h_DPP and h_string are the efficiencies of each converter. Fig. 14 shows theoretically estimated and hardware measured efficiency graphs of both the proposed converter and the conventional converter. In Fig. 14, it can be seen that the estimated and experimental efficiencies agree well each other. This means that equation (20) is available for the efficiency estimation. In addition, it can be seen that when the DPP power increases, both the conventional converter and the proposed converter efficiencies decrease since the DPP power circulates from the output to the input of the power conditioning systems, which results in some conduction losses. However, the efficiency reduction of the conventional converter is greater than that of the proposed converter. This means that the proposed magnetically-coupled inductor reduces the power-loss increase when the power is transferred through the DPP converter since the circulating route is far shorter than that of the conventional case. Fig. 15 shows the temperature distribution of the main power devices when the proposed converter operates in the thermal steady-state. S1 shows the location of D_O2, S2 is the magnetically-coupled inductor, S3 is Q_b, and S4 is Q_f2. Fig. 15 shows that all the main power devices operate in the normal temperature range without any heat sinks which makes it feasible for real applications.

Fig. 14. Entire-system efficiency comparison between the proposed and conventional topologies according to the DPP power (power difference among the panels) variation.

Fig. 15. Thermal camera image at a DPP power of 15W.

Ⅴ. CONCLUSIONS

In this paper, a buck-flyback (fly-buck) DPP circuit stand- alone PV system with battery charge balancing is proposed. Because of the use of a single-core magnetically-coupled multi-winding inductor, the converter avoids a high step-up DPP converter design and the long-circulating path of conventional DPP topologies. Hence, the proposed converter is more power-efficient and cost-effective when compared to conventional converters. In addition, the proposed converter has a simple charge-balancing circuit composed of tertiary windings for the battery stacks in stand-alone systems. This balancing circuit operates in a passive manner like flyback multi-winding balancers. As a result, no additional control is necessary. In addtion, due to the DPP converters, this topology can operate independent MPPT control even under partial shading conditions. The proposed topology was analyzed and a design guideline was presented. In addition, they were validated using a 120W hardware prototype.

ACKNOWLEDGMENT

This research was supported by Korea Electric Power Corporation (Grant number: R17XA05-54).

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Chun-Gu Lee received his B.S. degree from the Department of Electrical Engineering of Soongsil University, Seoul, Korea, in 2015, where he is presently working towards his combined Master-Ph.D. degree. His current research interests include the analysis and design of differential power processing converters and renewable energy applications.

Jung-Hyun Park received his B.S. and M.S. degrees from the Department of Electrical Engineering of Soongsil University, Seoul, Korea, in 2015 and 2017, respectively. His current research interests include the analysis and design of differential power processing converters and renewable energy applications.

Joung-Hu Park received his B.S., M.S. and Ph.D. degrees from the Department of Electrical Engineering and Computer Science of Seoul National University, Seoul, Korea, in 1999, 2001 and 2006, respectively. He was a Visiting Scholar at the Center for Power Electronics Systems, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, from 2004 to 2005. He was a Visiting Professor at the University of British Columbia, Vancouver, BC, Canada, from 2015 to 2016. He is presently working as an Associate Professor at Soongsil University, Seoul, Korea.