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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 lead-acid 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 10 V and output power of 20 W has been implemented to verify its feasibility. It is suitable for the electronic sign indicator applications.
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 , 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, zero-emission 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 [4–12]. In these circuit structures, according to the relationships among PV output voltage , 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 to control their operational states, inductor of buck-boost converter and magnetizing inductor of transformer can be merged. Therefore, switches of the bidirectional buck-boost converter and active clamp forward converter are integrated with synchronous switch technique  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 [14–21]. 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 . 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.
2. 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 (, ) and (, ) can be operated in synchronous. It will do not affect the operation of the proposed original converter. Since switch pairs of (, ) has a common node, they meet the requirements of switch integration technique . According to principle of switch integration technique, switches and can be merged, as shown in Figure 4(a). Since charger and LED driving circuit (discharger) are operated in complementary, switch and are also regarded as an independent operation. Therefore, voltages across switches and are the same value in each operation state. Diode and can be removed, while diode and can be shorted, as shown in Figure 4(b). In Figure 4(b), since ≪ , can be neglected. The inductor and magnetizing are connected in parallel. Although features of inductors and are different, their design rules are to avoid them to operate in saturation condition. Therefore, they can be merged as inductor , as shown in Figure 4(c).
From Figure 4(c), it can be seen that switch and have a common node. They can use switch integration technique to combine them, as shown in Figure 4(d). Since voltages across and are the same values, diodes and are shorted and diodes and can be removed, as shown in Figure 4(e). From Figure 4(e), it can be found that capacitors and 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 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 buck-boost converter is the same as the conventional buck-boot converter, its operational principle is described in . 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 and . 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 . It is also not described in this paper.
3. 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 , buck-boost converter is only analyzed briefly in the following.
3.1. 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 and inductor are analyzed in the following.
3.1.1. Duty Ratio
Within charging mode, since battery voltage is regarded as a constant voltage during a switching cycle of the proposed hybrid converter, maximum duty ratio of the proposed one can be determined by volt-second balance of inductor . Its relationship is expressed as where is the minimum output voltage of PV arrays, is the maximum voltage across battery, and represents the period of the proposed hybrid converter. From (1), it can be found that can be illustrated by Moreover, transfer ratio can be also determined as follows:
When type of battery is chosen, its maximum charging current is also determined. The charging current can be changed from its maximum charging current to 0 by variation duty ratio of switch .
Since the proposed hybrid converter is operated in CCM to obtain the maximum charging current , 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 can obtain a different charging current . In general, the maximum charge current occurs at the maximum battery voltage and the maximum output voltage of PV arrays. Therefore, the boundary inductor can be determined by where is duty ratio of switch under and . From (5), it can be found that the maximum boundary inductor can be expressed as
Since the proposed hybrid converter is operated in CCM, inductor must be greater than . Therefore, when , , , and are specified, the minimum inductance (=) can be determined.
In order to avoid the core of transformer operated in saturation state, the working flux density must be less than the saturation flux density of core. Since is proportional to the maximum inductor current , must be first determined. In Figure 6, can be expressed as where is the initial value of inductor current operated in CCM and represents its maximum variation value. In general, its maximum value can be determined by where represents the duty ratio of switch under and . Furthermore, the maximum charging current can be expressed as From (9), the initial value can be determined as follows: From (7), (8), and (10), can be denoted as According to datasheet of core which is supplied by core manufacturer, the number of turns on the primary side of transformer can be obtained by where represents per turns2. That is, . By applying Faraday’s law, can be determined as where is the effective cross-section area of the transformer core. In order to avoid saturation condition of core, must be less than saturation flux density of core.
4. 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.
4.1. 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 and current 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.
4.2. 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.
4.2.1. 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 where is the most insolation of sun, while is the one at the least insolation. Three maximum power point 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 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 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 , , 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 to . When , the proposed power system begins to charge battery and battery voltage is at the minimum value . When , battery is charged to its maximum voltage . According to limitation of the maximum battery charging current , power limitation curve of battery charging can be determined from 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, (= ) and (= ). According to relationship among , , and , they can be divided into three operational states: , , and , as shown in Figure 11. When , power curve of battery charging follows . When , power limitation curve and intersects at A point where its intersecting time is . Power curve tracks power limitation curve before , while it traces after , as shown in Figure 11(b). If operational state of , 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 and then is to read control signals. The control signals include , , , , , , , , , and . When control signals are obtained by microchip, next step is to calculate (=) and (=). Since , which is attained by MPPT control method, is the maximum output power of PV arrays, when is confirmed, is set to equal . If is denied, . The is the power command of battery charging. Therefore, power error value can be determined. It is equal to (). When is determined, current command can be obtained. It is equal to (/). The current command is sent to PWM IC to determine gate signals and . Next step is to judge next current command.
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 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 (overcharge condition), (undercharge condition) and (overcharge condition), the control procedure enters to set 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, . If insolation is a low level, . The signal is sent to operational mode judgment to obtain mode control signal . When , the hybrid converter enters battery charging mode. That is, the insolation of sun is at a high level and . If , the one is in LED driving mode. The signal and insolation is at a low level. The mode control signal is sent to mode selector, switch selector, and switch . When mode selector receives , the feedback signal is set to equal IC. The feedback signal and reference single are sent to error amplifier to obtain error value . When error value is attained, PWM generator can depend on to determine duty ratios of PWM signals and . When and are specified and , switch selector can set that and to control the charging current of battery. As mentioned above, the proposed hybrid converter can use microchip and PWM IC to achieve battery charging.
4.2.2. LED Driving
The LED driving mode is regarded as battery discharging mode. When operational mode enters LED driving mode, . The mode selector can be operated to set . The and are sent to error amplifier to attain . The is through PWM generator to generate signals and . Since , switch selector is controlled by to set and . Therefore, the proposed hybrid converter can depend on duty ratios of gate signals and to supply power to LED until battery voltage is equal to or less than . When , the proposed hybrid converter is shutdown.
5. Experimental Results
In order to verify the circuit analysis and component design of the proposed power system, a prototype, which is composed of a charger and LED driving circuit (discharger), with the following specifications was implemented.
5.1. Buck-Boost Converter (Charger)
(i)Input voltage 17 V~21 (PV arrays)(ii)Switching frequency (iii)Output voltage 5~7 V (lead-acid battery: 6 V/2.3 Ah)(iv)Maximum output current .
5.2. Active Clamp Forward Converter (LED Driving Circuit)
(i)Input voltage V~7 (ii)Switching frequency (iii)Output voltage (iv)Maximum output current .
According to previously specifications and design of the hybrid converter, inductors and and capacitor can be determined. In (17) illustrated in , since inductor must be greater than 7.09 under , , and , is chosen by 40 . According to (6) and (22) illustrated in , the magnetizing inductor must be greater than 6.8 under , , and . Therefore, magnetizing inductor is determined by 40 , while its leakage inductor is obtained by 0.2 . Moreover, capacitor can be attained by (29) illustrated in . Its capacitance is 0.22 under and . Therefore, is chosen by 0.24 . The components of power stage in the proposed hybrid converter was determined as follows:(i)switches , : PSMN005-75B,(ii)diodes , : UF601,(iii)transformer : EE-25 core,(iv)inductor L2: EE-22 core,(v)capacitor : 47 μF/25 V, and(vi)switch : IRFP540.
Since the charging current of battery can be varied by duty ratio of switch in buck-boost converter, its value is proportional to duty ratio . Figures 13(a) and 13(b), respectively, depict the measured voltage waveform of switch and current waveform under duty ratio of 0.31 and 0.35, illustrating that the charging current can be increased by duty ratios increase. Measured waveforms of PV arrays current and voltage using the perturb and observe method are illustrated in Figure 14. Figure 14(a) illustrates the MPP of PV arrays at 10 W, while Figure 14(b) depicts that at 20 W. Figure 15 shows the measured battery voltage and current under MPP of PV arrays at 10 W, from which it can be found that the maximum charging 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 and are, respectively, illustrated in Figures 16 and 17. Figures 16(a) and 16(b) show those waveforms under 20% of full load, while Figures 17(a) and 17(b) depict those waveforms under full load. From Figures 16 and 17, it can be seen that switches and are operated with ZVS 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.
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, steady-state 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 prototype for a battery charger for lead-acid battery of 6 V/2.3 Ah and discharger for LED driving under 10 V/2 A has been built and evaluated, achieving the maximum efficiency of 90% under 80% of full load and verifying the feasibility of the proposed hybrid converter. Moreover, constant current charging method, MPPT with perturb and observe method, and power management have been implemented by microchip and PWM IC.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
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