Buck-Boost / Forward Hybrid Converter for PV Energy Conversion Applications

This paper presents a charger and LED lighting (discharger) hybrid system with a PV array as its power source for electronic sign indicator applications. The charger adopts buck-boost converter which is operated in constant current mode to charge leadacid battery and with the perturb and observe method to extract maximum power of PV arrays. Their control algorithms are implemented by microcontroller. Moreover, forward converter with active clamp circuit is operated in voltage regulation condition to drive LED for electronic sign applications. To simplify the circuit structure of the proposed hybrid converter, switches of two converters are integrated with the switch integration technique. With this approach, the proposed hybrid converter has several merits, which are less component counts, lighter weight, smaller size, and higher conversion efficiency. Finally, a prototype of LED driving system under output voltage of 10V and output power of 20W has been implemented to verify its feasibility. It is suitable for the electronic sign indicator applications.


Introduction
In recent years, light emitting diodes (LEDs) are becoming more prevalent in a wide application.Due to material advances over the past few decades, efficiencies of LEDs have increased many times [1], and their applications have rapidly grown for automotive taillights, LCD back lights, traffic signals, and electronic signs [2,3].Moreover, serious greenhouse effect and environmental pollution caused by overusing fossil fuels have disturbed the balance of global climate.In order to reduce emission of exhausted gases, zeroemission renewable energy sources have been rapidly developed.One of these sources is photovoltaic (PV) arrays, which is clean and quiet and an efficient method for generating electricity.As mentioned above, this paper proposes an LED driving system, which adopts the PV arrays for electronic sign applications.
In electronic sign applications using PV arrays, the power system will inevitably need batteries for storing energy during the day and for releasing energy to LED lighting during the night.Therefore, it needs a charger and discharger (LED driving circuit), as shown in Figure 1.Since the proposed power system belongs to the low power level applications, buck, boost, buck-boost, flyback, or forward converter is more applied to the proposed one [4][5][6][7][8][9][10][11][12].In these circuit structures, according to the relationships among PV output voltage  PV , battery voltage   , and output voltage   , the proposed hybrid converter can choose functions, which are of step-up and -down simultaneously as the charger or discharger for a wider application.Due to the previously described reasons, the charger of the proposed one adopts buck-boost converter and the discharger uses forward converter.Moreover, since forward converter exits two problems, which are the energies trapped in leakage inductor and magnetizing inductor of transformer   , it needs a snubber or circuit to recover these energies.Therefore, forward converter can use an active clamp circuit to solve these problems.In order to simplify the proposed hybrid converter and increase its conversion efficiency, a bidirectional buck-boost converter and active clamp forward converter are used, as shown in Figure 2. Since charger and discharger (LED driving circuit) of the proposed hybrid converter are operated in complement and they use switch  1 to control their operational states, inductor  1 of buck-boost converter and magnetizing inductor   of transformer   can be merged.Therefore, switches of the bidirectional buck-boost converter and active clamp forward

Buck-boost converter
Active clamp forward converter converter are integrated with synchronous switch technique [13] to reduce their component counts, as shown in Figure 3.With this circuit structure, the proposed one can yield higher efficiency, reduce weight, size, and volume and increase the discharging time of battery under the same storing energy, significantly.
The proposed hybrid converter using PV arrays supplies power to LED lighting for electronic sign applications.The proposed one includes charger and discharger.Since the proposed one uses PV arrays as its power source, it must be operated at the maximum power point (MPP) of PV arrays to extract its maximum power.Many maximum power point tracking (MPPT) methods of PV arrays have been proposed [14][15][16][17][18][19][20][21].They are, respectively, power matching [14,15], curve-fitting [16,17], perturb-and-observe [18,19], and incremental conductance [20,21] methods.Since power matching method requires a specific insolation condition or load, it will limit its applications.MPPT using curve-fitting technique needs prior establishment characteristic curve of PV arrays.It cannot predict the characteristics including other factors, such as aging, temperature, and a possible breakdown of individual cell.The incremental conductance technique requires an accurate mathematical operation.Its controller is more complex and higher cost.Due to a simpler control and lower cost of perturb and observe method, the proposed hybrid converter adopts the perturb and observe method to implement MPPT.
For electronic sign applications using LED, the power system needs battery to store energy during day and to discharge energy for driving LED during night.In order to generate better performances of battery charging, many battery charging methods have been proposed.They are constant trickle current (CTC), constant current (CC), and CC and constant-voltage (CC-CV) hybrid charge methods [22].Among these methods, the CTC charging method needs a larger charging time.Battery charging using CC-CV method requires to sense battery current and voltage, resulting in a more complex operation and higher cost.Due to a simpler controller of battery charger using CC charging method, it is adopted in the proposed hybrid converter.According to description above, the proposed hybrid converter uses the perturb and observe method to track MPP of PV arrays and adopts the CC charging method to simplify battery charging.All overall power system can achieve battery charging and LED driving.

Circuit Structure Derivation of the Proposed Hybrid Converter
The hybrid converter consists of a bidirectional buck-boost and active clamp forward converter, as shown in Figure 2. Due to complementary operation between two converters, two switch pairs of ( 1 ,  4 ) and ( 2 ,  3 ) can be operated in synchronous.It will do not affect the operation of the proposed original converter.Since switch pairs of ( 2 ,  3 ) has a common node, they meet the requirements of switch integration technique [11].According to principle of switch integration technique, switches  2 and  3 can be merged, as shown in Figure 4(a).Since charger and LED driving circuit (discharger) are operated in complementary, switch  2 and  3 are also regarded as an independent operation.Therefore, voltages across switches  2 and  3 are the same value in each operation state.Diode  231 and  232 can be removed, while diode  231 and  232 can be shorted, as shown in Figure 4(b).In Figure 4(b), since   ≪   ,   can be neglected.The inductor  1 and magnetizing   are connected in parallel.Although features of inductors  1 and   are different, their design rules are to avoid them to operate in saturation condition.Therefore, they can be merged as inductor  1 , as shown in Figure 4(c).
From Figure 4(c), it can be seen that switch  1 and  4 have a common node.They can use switch integration technique to combine them, as shown in Figure 4(d).Since voltages across  1 and  4 are the same values, diodes  141 and  142 are shorted and diodes  141 and  142 can be removed, as shown in Figure 4(e).From Figure 4(e), it can be found that capacitors  1 and  2 are connected in parallel.They can be integrated as capacitor   , as illustrated in Figure 4(f).To simplify symbols of components illustrated in Figure 4(f), component symbols will be renamed, as shown in Figure 3.Note that switch  1 can be operated by manual or automatic method to control the operational states of the proposed hybrid converter.
Buck-boost and forward converters are combined to form the proposed hybrid converter.Since operation of buckboost converter is the same as the conventional buck-boot converter, its operational principle is described in [8].It will not be described in this paper.The forward converter with the active clamp circuit recovers the energies stored in magnetizing and leakage inductors of transformer   and achieves zero-voltage switch (ZVS) at turn-on transition for switches  1 and  2 .It operational mode can be divided into 9 modes and their Key waveforms are illustrated in Figure 5, since their operational modes are similar to those modes of the conventional converter illustrated in [23].It is also not described in this paper.

Design of the Proposed Hybrid Converter
The proposed hybrid converter consists of buck-boost converter and active clamp forward converter.Since switches and inductors in two converters are integrated with the synchronous switch technique, design of the proposed one must satisfy requirements of each converter.Since design of the active clamp forward converter is illustrated in [23], buckboost converter is only analyzed briefly in the following.

Buck-Boost
Converter.Since buck-boost converter is regarded as the battery charger under constant current charging.Its design consideration is to avoid a completely saturation of inductor.Therefore, duty ratio  11 and inductor   are analyzed in the following.

3.1.1.
Duty Ratio  11 .Within charging mode, since battery voltage   is regarded as a constant voltage during a switching cycle of the proposed hybrid converter, maximum duty ratio  11(max) of the proposed one can be determined by voltsecond balance of inductor   .Its relationship is expressed as where  PV(min) is the minimum output voltage of PV arrays,  (max) is the maximum voltage across battery, and   represents the period of the proposed hybrid converter.From (1), it can be found that  11(max) can be illustrated by .
(2) Moreover, transfer ratio  11(max) can be also determined as follows: When type of battery is chosen, its maximum charging current  (max) is also determined.The charging current   can be changed from its maximum charging current  (max) to 0 by variation duty ratio  11 of switch  1 .
3.1.2.Inductor   .Since the proposed hybrid converter is operated in CCM to obtain the maximum charging current  (max) , its conceptual waveforms of inductor current   and charging current   are illustrated in Figure 6.If the proposed one is operated in the boundary of discontinuous conduction mode (DCM) and CCM, the charging current   is expressed by where   is the inductance   at the boundary condition.
According to (4), variation of duty ratio  11 can obtain a different charging current   .In general, the maximum charge current  (av) max occurs at the maximum battery voltage  (max) and the maximum output voltage  PV(max) of PV arrays.Therefore, the boundary inductor   can be determined by where  11 is duty ratio of switch  1 under  (max) and  PV(max) .From (5), it can be found that the maximum boundary inductor  (max) can be expressed as Since the proposed hybrid converter is operated in CCM, inductor   must be greater than  (max) .Therefore, when  PV(max) ,  (max) ,  (av) max , and   are specified, the minimum inductance  (min) (= (max) ) can be determined.
In order to avoid the core of transformer   operated in saturation state, the working flux density  max must be less than the saturation flux density  sat of core.Since  max is proportional to the maximum inductor current  (peak) ,  (peak) must be first determined.In Figure 6,  (peak) can be expressed as where  (0) is the initial value of inductor current   operated in CCM and Δ (max) represents its maximum variation value.In general, its maximum value Δ (max) can be determined by where  11 represents the duty ratio of switch  1 under  PV(max) and  (max) .Furthermore, the maximum charging current  (av) max can be expressed as From ( 9), the initial value  (0) can be determined as follows: From ( 7), (8), and (10),  (peak) can be denoted as According to datasheet of core which is supplied by core manufacturer, the number of turns  1 on the primary side of transformer   can be obtained by where   represents nH per turns 2 .That is,   =  2 1   .By applying Faraday's law,  max can be determined as where   is the effective cross-section area of the transformer core.In order to avoid saturation condition of core,  max must be less than saturation flux density  sat of core.

Configuration of the Proposed PV Hybrid Converter
Since the proposed PV power system includes charger and discharger and adopts PV arrays as its power source, its circuit structure and control algorithm are described in the following.

Circuit Structure of the Proposed PV Power
System.The proposed PV power system consists of battery charger, LED driving circuit (discharger), and controller, as shown in Figure 1.The battery charger and LED driving circuit using buck-boost and active clamp forward hybrid converter are shown in Figure 3.In addition, controller adopts microchip and PWM IC for managing battery charging and LED driving circuit.The microchip is divided into 3 units (MPPT, power management, and battery management units) to implement MPPT of PV arrays and battery charging.The PWM IC is used to regulate output voltage of LED driving circuit.In the microchip of the controller, the MPPT unit senses PV voltage  PV and current  PV to achieve MPPT, which adopts perturb and observe method.The battery management unit acquires battery voltage   and current   for implementing CC charging of battery.Since the proposed hybrid converter is required to match MPPT of PV arrays and CC charging mode, the power management unit can manage the power flow between PV arrays and battery, depending on the relationship of the generated power of PV arrays and the required power of battery charging.All of protections are implemented by microchip.The protections include overcurrent and overvoltage protections of the proposed hybrid converter and undercharge and overcharge of battery.Therefore, the proposed one can achieve the optimal utility rate of PV arrays and a better performance of battery charging.

Control Algorithm of the Proposed Hybrid Converter.
In Figure 1, the controller of the proposed hybrid converter includes microchip and PWM IC to achieve battery charging and LED driving.In order to implement battery charging and LED driving, block diagram of the hybrid converter is shown in Figure 7.In Figure 7, control signals are defined in Table 1.In the following, control algorithms for battery charging and LED driving are briefly described.

Battery Charging.
Since the proposed hybrid converter supplies power to load from PV arrays, the proposed one must perform MPPT for PV arrays and battery charging for battery.The MPPT control method and battery charging method are described as follows.
MPPT Algorithm.Since solar cell has a lower output voltage and current, a number of solar cells are connected in series and parallel to form PV arrays for attaining the desired PV voltage and current.Their output characteristic variations depend on ambient temperature and insolation of sun. Figure 8 illustrates P-V curve of PV arrays at different insolation of sun, from which it can be seen that each insolation level has a maximum power  max where  max 1 is the most insolation of sun, while  max 3 is the one at the least insolation.Three maximum power point  max 1 ∼  max 3 can be connect by a straight line.The operational area is divided into two areas: A area and B area.When operational point of PV arrays locates in A area, output current  PV of PV arrays is decreased to make the operational point close to its maximum power point (MPP).If operational point is set in B area, current  PV will be increased to operate PV arrays at its MPP.The proposed power system adopts perturb and observe method to implement MPPT.Its flowchart is shown in Figure 9.In Figure 9,   and   separately represent their old voltage and power, and   (=    ) is its new power.According to flowchart procedures of MPPT using perturb and observe method, first step is to read new voltage   and current   of PV arrays and then to calculate new PV power   .Next step is to judge relationship of   and   .Since the relationship of   and   has three different relationships, they are separately International Journal of Photoenergy Increase load P max1 P max2 P max3   >   ,   =   , and   <   .Each relationship can be corresponded to the different relationship of   and   .Therefore, when the relationship of   and   is decided, next step is to find the relationship of   and   .According to the relationship of P-V curve of PV arrays, when the relationships of   and   and   an   are decided, working point of PV arrays can be specified.When working point of PV arrays locates in A area, power system connected in PV arrays to supply load power must decrease output power to close the distance between working point and MPP of PV arrays.On the other hand, when working point sets in B area, power system must increase output power to make working point to approach MPP of PV arrays.Finally,   is replaced by   and   is also substituted by   .The procedure of flowchart returns first step to judge next working point of PV arrays.Moreover, when   =   and   =   , working point of PV arrays set in the MPP of PV arrays.The maximum power   of PV arrays is transferred to power management unit for regulating power of battery charging.Battery Charging Method.The proposed hybrid converter uses CC charging method to charge battery.According to battery specifications, charging voltage and current are limited for extending its life cycle.Therefore, the power limitation curve for battery charger will be limited.Figure 10 depicts conceptual waveforms of charging current, voltage, and power for battery charger with CC charging method.The battery charging time is from  0 to   .When  =  0 , the proposed power system begins to charge battery and battery voltage   is at the minimum value  (min) .When  =   , battery is charged to its maximum voltage  (max) .According to limitation of the maximum battery charging current  (max) , power limitation curve of battery charging can be determined from  0 to   .The charging power of battery follows the power limitation curve for extending its life cycle.
Since power limitation curve of battery has upper and lower values, they are, respectively,  (min) (= (min)  (max) ) and  (max) (= (max)  (max) ).According to relationship among  PV(max) ,  (min) , and  (max) , they can be divided into three operational states:  PV(max) <  (min) ,  (min) ≤  PV(max) <  (max) , and  PV(max) >  (max) , as shown in Figure 11.When  PV(max) <  (min) , power curve of battery charging follows  PV(max) .When  (min) ≤  PV(max) <  (max) , power limitation curve and  PV(max) intersects at A point where its intersecting time is   .Power curve tracks power limitation curve before  =   , while it traces  PV(max) after  =   , as shown in Figure 11(b).If operational state of  PV(max) >  (max) , power curve is regulated by power limitation curve, as shown in Figure 11(c).As mentioned above, battery charging can be operated in a better charging mode.
In order to implement a better battery charging, power management and battery management units are adopted and they are implemented by microchip.In the following, power management and battery management are briefly described.
(1) Power Management.In Figure 7, the controller includes microchip and PWM IC.When the microchip is used to execute power management, its control procedures are depicted in Figure 12.First step is to set   = 0 and then is to read control signals.(2) Battery Management.In Figure 12, the right hand side of flowchart shows procedures of battery management.When the microchip reads control signals, the procedure of battery management is to judge overcurrent condition.When   ≥  (max) is confirmed, overcurrent condition of the proposed hybrid converter occurred.When overcurrent condition occurred, signal   is set to 1.The   is sent to PWM IC to shutdown PWM generator and the proposed hybrid converter is also shutdown.Next step is to judge next current command.Moreover, when   ≥  (max) (overcharge condition),   ≤  (min) (undercharge condition) and   ≥  (max) (overcharge condition), the control procedure enters to set   = 1 and to shutdown the proposed hybrid converter.According to previously describing procedures, battery can be properly controlled to complete a better charging condition.
(3) PWM IC.In the battery charging mode, PWM IC is used to control charging current with CC method.First, photosensor is used to detect insolation level of sun.When insolation is a high level,   = 0.If insolation is a low level,   = 1.The signal   is sent to operational mode judgment to obtain mode control signal   .When   = 0, the hybrid converter enters battery charging mode.That is, the insolation of sun is at a high level and   = 0.If   = 1, the one is in LED driving mode.The signal   = 1 and insolation is at a low level.The mode control signal   is sent to mode selector, switch selector, and switch  1 .When mode selector International Journal of Photoenergy According to previously specifications and design of the hybrid converter, inductors  2 and   and capacitor   can be determined.In (17) illustrated in [23], since inductor   must be greater than 7.09 uH under   = 10 V,   = 7 V, and  PV = 17 V,  2 is chosen by 40 uH.According to ( 6) and ( 22) illustrated in [23], the magnetizing inductor   must be greater than 6.8 uH under  = 5,   = 7 V, and  2 = 40 uH.Therefore, magnetizing inductor   is determined by 40 uH, while its leakage inductor   is obtained by 0.2 uH.Moreover, capacitor   can be attained by (29) illustrated in [23].Its capacitance   is 0.22 nF under  = 5 and   = 7 V.Therefore,   is chosen by 0.24 uF.The components of power stage in the proposed hybrid converter was determined as follows: (i) switches  1 ,  2 : PSMN005-75B, (ii) diodes  1 ,  2 : UF601, (iii) transformer   : EE-25 core, (iv) inductor L 2 : EE-22 core,  current   is limited at 1.5 A under battery voltage   of 6.5 V due to control of power management.
When the proposed hybrid converter is operated in the discharging state (LED driving state), active clamp forward is in working.Measured voltage   and current   waveforms of switched  1 and  2 are, respectively, illustrated in Figures 16 and 17.Figures 16(a  at turn-on transition.Comparison of conversion efficiency between forward converter with hard-switching circuit and with the proposed active clamp circuit from light load to heavy load is depicted in Figure 18, from which it can be found that the efficiency of the proposed converter is higher than that of hard-switching one.Its maximum efficiency is 90% under 80% of full load and its efficiency is 83% under full load.Figure 19 illustrates step-load change between 20% of full load and full load, illustrating that the voltage regulation   has been limited within ±2%.From experimental results, it can be found that the proposed hybrid converter is suitable for electronic sign applications.

Conclusion
In this paper, the buck-boost converter combined with active clamp forward converter to form the proposed hybrid converter is used to implement battery charger and driving LED. Circuit derivation of the hybrid converter with switch integration technique is presented in this paper to reduce component counts.Operational principle, steadystate analysis, and design of the proposed hybrid converter have been described in detail.From efficiency comparison between forward converter with hard-switching circuit and with the proposed active clamp circuit, the proposed active clamp converter can yield higher efficiency.An experimental

2 InternationalFigure 1 :
Figure 1: Block diagram of the proposed hybrid converter for electronic sign applications.

Figure 2 :
Figure 2: Schematic diagram of the hybrid converter for electronic sign applications.

Figure 3 :
Figure 3: Schematic diagram of the proposed hybrid converter for electronic sign applications.

Figure 4 :Figure 5 :
Figure 4: Derivation of the proposed hybrid converter for battery charger.

Figure 6 :
Figure 6: Conceptual waveforms of inductor current   and charging current   in buck-boost converter. Photosensor

Figure 7 :
Figure 7: Block diagram of the proposed hybrid converter.

Figure 8 :
Figure 8: Plot of P-V curve for PV arrays at different isolation of sun.

Figure 10 :Figure 11 :
Figure 10: Conceptual waveforms of charging current, voltage, and power for battery charger with CC charging method.

Figure 13 :
Figure 13: Measured voltage   waveform of switch  1 and the charged current   waveform operated in duty ratio of (a) 0.31 and (b) 0.35 for working in the charging state.

Figure 14 :
Figure 14: Measured voltage  PV , current  PV , and power  PV waveforms of PV arrays using the perturb and observe method to track MPPT of arrays (a) under  PV(max) = 10W and (b) under  PV(max) = 20 W.
) and16(b)  show those waveforms under 20% of full load, while Figures17(a) and 17(b) depict those waveforms under full load.From Figures 16 and 17, it can be seen that switches  1 and  2 are operated with ZVS V B I B LeCroy (V B : 5 V/div, I B : 1 A/div, time: 10 ms/div)

Figure 15 :
Figure 15: Measured battery voltage V B and current I B waveforms under  PV(max) = 10 W.

Table 1 :
Parameter definitions of control signals shown in Figure 4. Gate signals of switches  1 and  2 The control signals include  (max) ,  (max) ,  (min) ,   ,   ,  (max) ,   ,  (max) ,  ref , and   .When control signals are obtained by microchip, next step is to calculate  (max) (=   (max) ) and   (=    ).Since   , which is attained by MPPT control method, is the maximum output power of PV arrays, when   ≥  (max) is confirmed,  set is set to equal  (max) .If   ≥  (max) is denied,  set =   .The   is the power command of battery charging.Therefore, power error value ΔP can be determined.It is equal to ( set −   ).When ΔP is determined, current command   can be obtained.It is equal to (ΔP/  ).The current command   is sent to PWM IC to determine gate signals  1 and  2 .Next step is to judge next current command.