A Reflex Charger with Zvs and Non-dissipative Cells for Photovoltaic Energy Conversion

In this paper, a reflex charger with zero-voltage-switching (ZVS) and non-dissipative cells for photovoltaic (PV) energy conversion is presented. The proposed reflex charger has the following advantages: (1) A ZVS cell is incorporated to reduce the switching losses of the main and auxiliary switches. Therefore, the conversion efficiency of the proposed reflex charger can be improved significantly; (2) A non-dissipative charging/discharging reflex cell is used to increase charging efficiency and extend lifecycle of battery. Finally, a prototype reflex charger with ZVS and non-dissipative cells is built and implemented. Experimental results are presented to verify the performance and the feasibility of the proposed reflex charger for PV energy conversion.


Introduction
Due to the rapid growth of global energy demand for developing industry, the overuse of fossil fuels results in environmental pollution, greenhouse effects and ecological damage.Adopting renewable and clean energy resources to replace fossil fuels is imperative.To reduce greenhouse effects, the demand for renewable energies has increased significantly.Among renewable energies, PV energy attracts more interest owing to its noiseless, pollution-free, non-radioactive, and inexhaustible characteristics [1,2].

OPEN ACCESS
Currently, PV energy is converted into electric power through PV panels.The output voltage and current of PV panels vary nonlinearly with irradiation, panel temperatures and loading conditions.Due to their unstable and intermittent characteristics, PV panels cannot provide a constant or stable power output.Thus, to achieve PV energy conversion, PV panels often adopt chargers and batteries [3][4][5][6][7][8][9][10].
PV panels usually need battery charger charging strategies such as the constant current (CC), constant voltage (CV), hybrid CC/CV and reflex charging strategies to increase the utility rate of energy storage.The CC, CV and hybrid CC/CV are simple charging methods.Figure 1 shows the charge methods of these strategies.However, their disadvantages are overcharging and temperature rise, resulting in the degradation of battery life.In order to reduce charging time and extend lifecycle of battery, the charging/discharging reflex method is often adopted [11][12][13][14].The charging/discharging waveform of the reflex charger is shown in Figure 2. The method consists of a charging period, a discharging period and a rest period.The charging period function the can reduce battery's charging time, the discharging period function can reduce battery's internal temperature and concentration polarization, and the rest period function can provide the battery's rest time.Therefore, the increased charging efficiency and extended lifecycle of battery are clear [15][16][17][18][19][20][21][22].

Figure 2. Charging/discharging reflex cycle.
A number of single-stage chargers have been developed and proposed.For example, Buck, Cuk, Zeta and SEPIC converters are adopted, which have simple constructions.However, the active switches of these converters are operated under hard-switching conditions resulting in one of the following t 0 disadvantages: (1) high switching losses; (2) high electromagnetic interference (EMI); and (3) low conversion efficiency.To address these disadvantages, a soft-switching technique called ZVS is used [23,24].
In this paper, a reflex charger with ZVS and non-dissipative cells for PV energy conversion is presented, as shown in Figure 3.The proposed reflex charger incorporates a ZVS cell to reduce the switching losses of the active switches and incorporates a non-dissipative charging/discharging reflex cell to extend lifecycle of battery.To obtain a fast dynamic response algorithm and a protected scheme, a microcontroller is added and presented to implement the optimum control of the proposed reflex charger.The operational principles of the proposed reflex charger are described in Section 2. The control scheme of the proposed reflex charger is described in Section 3. Experimental results obtained from a prototype with the proposed reflex charger for PV energy conversion are presented in Section 4. Finally, a conclusion is given in Section 5.

Figure 3.
Circuit structure of the proposed reflex charger with ZVS and non-dissipative cells for PV energy conversion.

Operational Principles
In Figure 3, the proposed reflex charger includes a ZVS and a non-dissipative charging/discharging reflex cell.The ZVS cell consists of a main switch (M1), an auxiliary switch (M2), a resonant inductor (Lr) and a clamp capacitor (Cclamp).The non-dissipative charging/discharging reflex cell uses only passive elements (diodes, capacitors and inductors) to implement charging efficiency and extended lifecycle of battery.For convenience of illustration and analysis, the PV panels of Figure 3 can be seen as a constant voltage source.This is simplified and redrawn in Figure 4. Figure 5 shows conceptual current and voltage waveforms of the key components and the driving signal switches (M1, M2 and M3) to facilitate the operational principle.Figure 6 shows the topological modes of the proposed reflex charger during a switching cycle.To simplify the description of the operational modes, the following assumptions are made: (1) To analyze the ZVS feature, the body diodes (D1 and D2) and parasitic capacitors (C1 and C2) of the active switches (M1 and M2) will be considered at the steady-state operation of the circuit.(2) All of components are ideal.
The proposed charger for the reflex charging/discharging mode can be divided into eight modes and explained mode by mode as follows: At time t0, the driving signals of the active switches M1 and M3 are turned on, and the current iLa and iLb are flowing through resonant inductor Lr and the active switch M1.Simultaneously, the battery is discharged through the components (C4, D5, L1, C3 and M3) of the non-dissipative charging/discharging reflex cell, as shown in Figure 6a.During this interval, the current of resonant inductor iLr and discharging current IB of the battery can be expressed as: where C = C3 = C4, and 0 The voltage on the capacitor (C3 and C4) can be expressed as: Mode 2 [Figure 6b, t1 < t < t2]: When the charging voltage of capacitors (C3 and C4) is equal to the battery voltage (VB), the battery will stop its discharging action and stay in a rest interval.During this interval, the current of resonant inductor iLr and discharging current IB of the battery can be expressed as: The voltage on the capacitor (C3 and C4) can be expressed as: The equivalent circuit is shown in Figure 6b.
Mode 3 [Figure 6c, t2 < t < t3]: The driving signals of the switches M1 and M3 are turned off at time t2, and resonant inductor Lr resonates with the parasitic capacitor C1 of the switch M1 and the parasitic capacitor C2 of the switch M2.Due to the charging time of the parasitic capacitor C1 of the switch M1 is very short, the voltage of the switch M1 will raise steeply.During this interval, the current of the resonant inductor iLr and discharging current IB of the battery can be expressed as: The equivalent circuit is shown in Figure 6c.
Mode 4 [Figure 6d, t3 < t < t4]: At time t3, the parasitic capacitor C1 of the switch M1 is charged to Vclamp and the parasitic capacitor C2 of the switch M2 is discharged down to zero.The switch M1 is completely turned off and capacitors C3 and C4 of non-dissipative reflex cell will be discharged.Simultaneously, the inductor current iLr forces the body diode D2 of M2 to be conducting and diverts to clamping capacitor Cclamp continuously.When the body diode D2 of the switch M2 is conducted, a ZVS feature will be created for the switch M2.During this interval, the current of resonant inductor iLr, the clamping voltage of clamping capacitor Cclamp and charging current of battery can be expressed as: The equivalent circuit of mode 4 is shown in Figure 6d.
Mode 5 [Figure 6e, t4 < t < t5]: At time t4, the switch M2 is turned on under ZVS conditions.During this interval, the capacitors C3 as well as C4 are discharged to zero, and diode D6 is conducted.The currents iLa and iLb begin flowing through the diode D6, and charge to the battery.During this interval, the current of resonant inductor iLr, the clamping voltage of clamping capacitor Cclamp and charging current of battery can be expressed as: ( ) ( )sin ω( ) ( ) cos ω() ( ) ( ) The equivalent circuit of mode 5 is shown in Figure 6e.
Mode 6 [Figure 6f, t5 < t < t6]: When the resonant inductor current iLr reaches zero at time t5, it will be reversed and flow through the resonant inductor Lr.At this interval, the battery is continuously charged.During this interval, the current of resonant inductor iLr, the clamping voltage of clamping capacitor Cclamp and charging current of battery can be expressed as: ( ) ( ) cos ω() ( ) ( ) The equivalent circuit of mode 6 is shown in Figure 6f.
Mode 7 [Figure 6g, t6 < t < t7]: The switch M2 is turned off at time t6, and resonant inductor Lr resonates with the parasitic capacitor C1 of the switch M1 and the parasitic capacitor C2 of the switch M2.Due to the charging time of the parasitic capacitor C2 is very short, the voltage of the switch M2 will be steeply raised.During this interval, the currents iLa and iLb will flow through the diode D6 and the battery continuously.The current of resonant inductor iLr, the clamping voltage of clamping capacitor Cclamp and charging current of battery can be expressed as: ( ) ( ) sin ω() ( ) ( ) cos ω() ( ) ( ) where The equivalent circuit of mode 7 is shown in Figure 6g.
Mode 8 [Figure 6h, t7 < t < t8]: At time t7, the parasitic capacitor C2 of the switch M1 is charged to Vclamp and the parasitic capacitor C1 of the switch M1 is discharged down to zero.The inductor current iLr forces the body diode D1 of M1 conducting.When the body diode D1 of the switch M1 is conducted, a ZVS feature will be created for the switch M1.The equivalent circuit of mode 8 is shown in Figure 6h.
At time t8, the switch M1 is turned on under ZVS condition.When the resonant inductor current iLr reaches zero, it will be forwarded and flow through the resonant inductor.Simultaneously, the battery is discharged again through the components (C4, D5, L1, C3 and M3) of the non-dissipative reflex cell.The operation of the reflex charger over one switching cycle is completed.

Control Scheme Description
In order to achieve a fast dynamic response and high efficiency conversion, an optimal control scheme must be integrated into the proposed reflex charger.The optimal control scheme includes the maximum-power-point-tracking (MPPT) control algorithm of the PV panels and the ZVS control signals of the proposed reflex charger.In this section, the control scheme of the proposed reflex charger will be described in detail.

Description of the MPPT Algorithm
The PV cells are combined into PV panels.The equivalent circuit of a typical PV cell is shown in Figure 7. Since a PV cell produces less than 3 watts at approximately 0.5 Vdc, PV cell must be connected in series-parallel configurations to produce enough power for high-power applications.In the actual case, the Ipv-Vpv and Ppv-Vpv characteristic equations are expressed as follows: or: and: exp 1 where np is the parallel number of the PV cells, ns is the series number of the PV cells, Isat is the reverse-saturation-current, Iph is the output current of the PV panels, q is the electronic charge, k is Boltzmann's gas constant, T is the cell temperature of the PV panels and A is the ideality factor of the PV panels.From Equations ( 22) and ( 23), one can obtain Ipv-Vpv and Ppv-Vpv curves as shown in Figure 8.In Figure 8, it can be seen that the PV panels produce power only on illuminated isolation.There exists one operating point where the PV panels can generate their maximum output power Pmax.From Figure 8, it can be seen that a common inherent drawback of the PV panels is the intermittent nature of their power.These drawbacks tend to make the PV panels inefficient.However, by incorporating MPPT algorithms, the power transfer efficiency of the PV panels can be improved significantly.In order to achieve the best energy utilization of the PV panels, MPPT algorithms and energy storage circuits must be integrated into battery charger.Figure 9 shows the conceptual diagram of the battery charger with an MPPT controller.The maximum power point Pmax of PV panels can be obtained, as shown in Figure 10.In Figure 10, except for the maximum power point Pmax, two possible operating regions A and B can be defined for a given Ppv.The current-operating-point location can be determined by a perturbation in the maximum power Pmax of PV.Therefore, to achieve the best energy utilization of the PV panels, an MPPT algorithm with perturbation-and-observation method is usually adopted.The advantage of the perturbation-and-observation method is that MPPT controller only requires a few parameters to easily measure and control the maximum power point.Therefore, it is often applied to PV panels for enhancing the power capacity [5][6][7][8][9].

ZVS Cell Description
To reduce switching losses and increase conversion efficiency, a dc-dc converter with ZVS cells is usually adopted.The ZVS cell requires four main components, including an active switch, a passive diode, an inductor and a capacitor, as shown in Figure 11a.In general, an active switch is used to replace its passive diode, and then the ZVS cell can be modified as shown in Figure 11b.From Figure 11b, Energies 2015, 8 1383 a basic ZVS cell consists of two bidirectional active switches M1 and M2, two resonant capacitors C1 and C2, a resonant inductor Lr and a clamping capacitor Cclamp.The ZVS cell allows its inductor current iLr flowing in bidirectional manner, which can achieve ZVS features for active switches.Based on this ZVS feature, the topology of the proposed reflex charger with ZVS cell is composed, as shown in Figure 3.In Figure 3, to reduce the switching losses of the active switches (M1 and M2), the proposed reflex charger is necessary to store enough energy in the resonant inductor Lr to achieve ZVS at the turn-on transition of the switches.The ZVS condition of active switches can be obtained as the following inequality: When the value of the parasitic capacitors is equal (C1 = C2 = C), the inequality can be expressed as follows: Figure 11.Illustration of a ZVS cell.

Description of a Non-Dissipative Charging/Discharging Reflex Cell
A non-dissipative charging/discharging reflex cell consists of passive switches, inductors and capacitors.Figure 12a shows the original concept of a bidirectional charging cell.In Figure 12a, the capacitors C1 and C2 are treated as energy buffers, the current tank I is treated as a charging current for the energy buffers, and the switches S1 and S2 are treated as charging switches of the energy buffers.In Figure 12a, the switches S1 and S2 can be identified as passive diode D1 and D2, and the current tank can be identified as an inductor L1, as shown in Figure 12b.To obtain the charging/discharging function Figure 12b can be modified as shown in Figure 12c.In Figure 12c, to prevent from charging current following through inductor L1, the extra passive diode D3 is in series with the inductor L1.Based on this illustration, the topology of the proposed charger with a non-dissipative charging/discharging reflex cell is composed, as shown in Figure 13.To achieve the optimal stability and safety of the proposed reflex charger, the functions of under-voltage, over-voltage, over-current, and over-temperature protection circuits are required.For cost considerations and best control, the MPPT algorithm and all the protection signals of the proposed reflex charger are implemented on the TMS320F240 microcontroller along with auxiliary analog circuits.The conceptual control block diagram of the proposed reflex charger for PV energy conversion is shown in Figure 13.

Experimental Results
In Figure 13, to verify the performance of the proposed reflex charger with ZVS and non-dissipative cells for the PV energy conversion, its specifications are listed as follows:  Figure 14 shows measured output currents, voltages and their corresponding powers from start-up to the steady state for PV panels with the perturbation-and-observation MPPT method.In Figure 14, it can be observed that the maximum power point can be always obtained from PV panels.Figures 15  and 16 show the voltage and current waveforms of the main switch M1 and auxiliary switch M2.In Figures 15 and 16, it can be seen that the main switch M1 and auxiliary switch M2 have ZVS features during turn-on transition.Therefore, the switching losses of main switch M1 and auxiliary switch M2 can be eliminated during turn-on transition.Figure 17 shows measured charge/discharge current waveforms of battery under charging/discharging reflex cycle.Figure 18 shows measured charging voltage curve of battery, in which it can be seen that the battery can be improved by the proposed reflex charger in a short time.Figure 19 shows efficiency measurements of the proposed reflex charger with ZVS and non-dissipative reflex cells, in which it can be seen that the maximum efficiency can be reached as high as 92% under ZVS condition.

Conclusions
In this paper, a reflex charger with ZVS and non-dissipative cells for PV energy conversion is proposed.The active switches of the proposed reflex charger have a ZVS feature at turn-on transition.Hence, the switching losses of the active switches can be ignored.The non-dissipative reflex cell uses only passive elements, which can increase charging efficiency and extend the battery lifecycle.In order to draw maximum power from PV arrays, a simple perturbation-and-observation method is incorporated to realize maximum power conversion.To achieve the optimal stability, safety and cost effective of the proposed reflex charger, the protected circuits and MPPT algorithms consist of a microcontroller (TMS320F240, Texas Instruments, Dallas, TX, USA) to implement maximum power conversion and protect the battery charger.Experimental results have been verified that the proposed reflex charger with ZVS and non-dissipative cells is relatively suitable for PV energy conversion.

Figure 1 .
Figure 1.Simple charging strategies: (a) charging curve of CC; (b) charging curve of CV; and (c) charging curve of hybrid CC/CV.

3 Figure 4 .
Figure 4. Simplified circuit diagram of the proposed reflex charger.

Figure 5 .
Figure 5. Driving signals of active switches and conceptual current and voltage waveforms of key components.

Figure 9 .
Figure 9. Conception diagram of battery charger for PV energy conversion.

Figure 10 .
Figure 10.Maximum power point curve of PV panels with respect to Vpv.

Figure 15 .
Figure 15.Voltage and current waveforms of main switch M1: (a) simulated results; (b) experimental results.

Figure 17 .
Figure 17.Charge and discharge current waveforms of battery: (a) simulated results; (b) experimental results.

Figure 18 .
Figure 18.Measured voltage curve of battery under charge time.

Figure 19 .
Figure 19.Efficiency of the proposed reflex charger with ZVS and non-dissipative cells.