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

Authors: Sheng-Yu Tseng; Chien-Chih Chen; Hung-Yuan Wang
Journal: International Journal of Photoenergy
 (2014)

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## Abstract

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.


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## Body

## 1. 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, 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 Figure1. 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 VPV, battery voltage VB, and output voltage VO, 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 Tf, 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 S1 to control their operational states, inductor L1 of buck-boost converter and magnetizing inductor Lm of transformer Tf can be merged. Therefore, switches of the bidirectional buck-boost converter and active clamp forward 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.Figure 1
Block diagram of the proposed hybrid converter for electronic sign applications.Figure 2
Schematic diagram of the hybrid converter for electronic sign applications.Figure 3
Schematic diagram of the proposed hybrid converter for electronic sign applications.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 [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.


## 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 Figure2. Due to complementary operation between two converters, two switch pairs of (M1, M4) and (M2, M3) can be operated in synchronous. It will do not affect the operation of the proposed original converter. Since switch pairs of (M2, M3) has a common node, they meet the requirements of switch integration technique [11]. According to principle of switch integration technique, switches M2 and M3 can be merged, as shown in Figure 4(a). Since charger and LED driving circuit (discharger) are operated in complementary, switch M2 and M3 are also regarded as an independent operation. Therefore, voltages across switches M2 and M3 are the same value in each operation state. Diode DF231 and DF232 can be removed, while diode DB231 and DB232 can be shorted, as shown in Figure 4(b). In Figure 4(b), since LK ≪ Lm, LK can be neglected. The inductor L1 and magnetizing Lm are connected in parallel. Although features of inductors L1 and Lm are different, their design rules are to avoid them to operate in saturation condition. Therefore, they can be merged as inductor L1m, as shown in Figure 4(c).Derivation of the proposed hybrid converter for battery charger.
(a)
(b)
(c)
(d)
(e)
(f)From Figure4(c), it can be seen that switch M1 and M4 have a common node. They can use switch integration technique to combine them, as shown in Figure 4(d). Since voltages across M1 and M4 are the same values, diodes DB141 and DB142 are shorted and diodes DF141 and DF142 can be removed, as shown in Figure 4(e). From Figure 4(e), it can be found that capacitors C1 and C2 are connected in parallel. They can be integrated as capacitor CC, 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 S1 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 [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 Tf and achieves zero-voltage switch (ZVS) at turn-on transition for switches M1 and M2. 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.Figure 5
Key waveforms of forward converter with active clamp circuit over one switching cycle.


## 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 [23], 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 ratioD11 and inductor Lm are analyzed in the following.


#### 3.1.1. Duty RatioD11

Within charging mode, since battery voltageVB is regarded as a constant voltage during a switching cycle of the proposed hybrid converter, maximum duty ratio D11(max⁡) of the proposed one can be determined by volt-second balance of inductor Lm. Its relationship is expressed as
(1)VPV(min⁡)D11(max⁡)Ts+(-VB(max⁡))(1-D11(max⁡))Ts=0,
where VPV(min⁡) is the minimum output voltage of PV arrays, VB(max⁡) is the maximum voltage across battery, and Ts represents the period of the proposed hybrid converter. From (1), it can be found that D11(max⁡) can be illustrated by
(2)D11(max⁡)=VB(max⁡)VPV(min⁡)+VB(max⁡).
Moreover, transfer ratio M11(max⁡) can be also determined as follows:
(3)M11(max⁡)=D11(max⁡)1-D11(max⁡).When type of battery is chosen, its maximum charging currentIB(max⁡) is also determined. The charging current IB can be changed from its maximum charging current IB(max⁡) to 0 by variation duty ratio D11 of switch M1.


#### 3.1.2. InductorLm

Since the proposed hybrid converter is operated in CCM to obtain the maximum charging currentIB(max⁡), its conceptual waveforms of inductor current ILm and charging current IB are illustrated in Figure 6. If the proposed one is operated in the boundary of discontinuous conduction mode (DCM) and CCM, the charging current IB is expressed by
(4)IB(av)=(1-D11)2VBTs2LmB,
where LmB is the inductance Lm at the boundary condition. According to (4), variation of duty ratio D11 can obtain a different charging current IB. In general, the maximum charge current IB(av)max⁡ occurs at the maximum battery voltage VB(max⁡) and the maximum output voltage VPV(max⁡) of PV arrays. Therefore, the boundary inductor LmB can be determined by
(5)IB(av)max⁡=(1-D11B)2VB(max⁡)Ts2LmB,
where D11B is duty ratio of switch M1 under VB(max⁡) and VPV(max⁡). From (5), it can be found that the maximum boundary inductor LmB(max⁡) can be expressed as
(6)LmB(max⁡)=(1-D11B)2VB(max⁡)TsIB(av)max⁡.Figure 6
Conceptual waveforms of inductor currentILm and charging current IB in buck-boost converter.Since the proposed hybrid converter is operated in CCM, inductorLm must be greater than LmB(max⁡). Therefore, when VPV(max⁡), VB(max⁡), IB(av)max⁡, and Ts are specified, the minimum inductance Lm(min⁡) (=LmB(max⁡)) can be determined.In order to avoid the core of transformerTf operated in saturation state, the working flux density Bmax⁡ must be less than the saturation flux density Bsat⁡ of core. Since Bmax⁡ is proportional to the maximum inductor current ILm(peak), ILm(peak) must be first determined. In Figure 6, ILm(peak) can be expressed as
(7)ILm(peak)=ILm(0)+ΔILm(max⁡),
where ILm(0) is the initial value of inductor current ILm operated in CCM and ΔILm(max⁡) represents its maximum variation value. In general, its maximum value ΔILm(max⁡) can be determined by
(8)ΔILm(max⁡)=VPV(max⁡)D11RTsLm,
where D11R represents the duty ratio of switch M1 under VPV(max⁡) and VB(max⁡). Furthermore, the maximum charging current IB(av)max⁡ can be expressed as
(9)IB(av)max⁡=(1-D11R)2(ILm(peak)+ILm(0))=(1-D11R)2(2ILm(0)+VPV(max⁡)D11RTsLm).
From (9), the initial value ILm(0) can be determined as follows:
(10)ILm(0)=IB(av)max⁡1-D11R-VPV(max⁡)D11RTs2Lm.
From (7), (8), and (10), ILm(peak) can be denoted as
(11)ILm(peak)=IB(av)max⁡1-D11R+VPV(max⁡)D11RTs2Lm.
According to datasheet of core which is supplied by core manufacturer, the number of turns N1 on the primary side of transformer Tf can be obtained by
(12)N1=LmAL,
where AL represents nH per turns2. That is, Lm=N12AL. By applying Faraday’s law, Bmax⁡ can be determined as
(13)Bmax⁡=LmILm(peak)×104N1Ac,
where Ac is the effective cross-section area of the transformer core. In order to avoid saturation condition of core, Bmax⁡ must be less than saturation flux density Bsat⁡ of core.


## 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 ratioD11 and inductor Lm are analyzed in the following.


### 3.1.1. Duty RatioD11

Within charging mode, since battery voltageVB is regarded as a constant voltage during a switching cycle of the proposed hybrid converter, maximum duty ratio D11(max⁡) of the proposed one can be determined by volt-second balance of inductor Lm. Its relationship is expressed as
(1)VPV(min⁡)D11(max⁡)Ts+(-VB(max⁡))(1-D11(max⁡))Ts=0,
where VPV(min⁡) is the minimum output voltage of PV arrays, VB(max⁡) is the maximum voltage across battery, and Ts represents the period of the proposed hybrid converter. From (1), it can be found that D11(max⁡) can be illustrated by
(2)D11(max⁡)=VB(max⁡)VPV(min⁡)+VB(max⁡).
Moreover, transfer ratio M11(max⁡) can be also determined as follows:
(3)M11(max⁡)=D11(max⁡)1-D11(max⁡).When type of battery is chosen, its maximum charging currentIB(max⁡) is also determined. The charging current IB can be changed from its maximum charging current IB(max⁡) to 0 by variation duty ratio D11 of switch M1.


### 3.1.2. InductorLm

Since the proposed hybrid converter is operated in CCM to obtain the maximum charging currentIB(max⁡), its conceptual waveforms of inductor current ILm and charging current IB are illustrated in Figure 6. If the proposed one is operated in the boundary of discontinuous conduction mode (DCM) and CCM, the charging current IB is expressed by
(4)IB(av)=(1-D11)2VBTs2LmB,
where LmB is the inductance Lm at the boundary condition. According to (4), variation of duty ratio D11 can obtain a different charging current IB. In general, the maximum charge current IB(av)max⁡ occurs at the maximum battery voltage VB(max⁡) and the maximum output voltage VPV(max⁡) of PV arrays. Therefore, the boundary inductor LmB can be determined by
(5)IB(av)max⁡=(1-D11B)2VB(max⁡)Ts2LmB,
where D11B is duty ratio of switch M1 under VB(max⁡) and VPV(max⁡). From (5), it can be found that the maximum boundary inductor LmB(max⁡) can be expressed as
(6)LmB(max⁡)=(1-D11B)2VB(max⁡)TsIB(av)max⁡.Figure 6
Conceptual waveforms of inductor currentILm and charging current IB in buck-boost converter.Since the proposed hybrid converter is operated in CCM, inductorLm must be greater than LmB(max⁡). Therefore, when VPV(max⁡), VB(max⁡), IB(av)max⁡, and Ts are specified, the minimum inductance Lm(min⁡) (=LmB(max⁡)) can be determined.In order to avoid the core of transformerTf operated in saturation state, the working flux density Bmax⁡ must be less than the saturation flux density Bsat⁡ of core. Since Bmax⁡ is proportional to the maximum inductor current ILm(peak), ILm(peak) must be first determined. In Figure 6, ILm(peak) can be expressed as
(7)ILm(peak)=ILm(0)+ΔILm(max⁡),
where ILm(0) is the initial value of inductor current ILm operated in CCM and ΔILm(max⁡) represents its maximum variation value. In general, its maximum value ΔILm(max⁡) can be determined by
(8)ΔILm(max⁡)=VPV(max⁡)D11RTsLm,
where D11R represents the duty ratio of switch M1 under VPV(max⁡) and VB(max⁡). Furthermore, the maximum charging current IB(av)max⁡ can be expressed as
(9)IB(av)max⁡=(1-D11R)2(ILm(peak)+ILm(0))=(1-D11R)2(2ILm(0)+VPV(max⁡)D11RTsLm).
From (9), the initial value ILm(0) can be determined as follows:
(10)ILm(0)=IB(av)max⁡1-D11R-VPV(max⁡)D11RTs2Lm.
From (7), (8), and (10), ILm(peak) can be denoted as
(11)ILm(peak)=IB(av)max⁡1-D11R+VPV(max⁡)D11RTs2Lm.
According to datasheet of core which is supplied by core manufacturer, the number of turns N1 on the primary side of transformer Tf can be obtained by
(12)N1=LmAL,
where AL represents nH per turns2. That is, Lm=N12AL. By applying Faraday’s law, Bmax⁡ can be determined as
(13)Bmax⁡=LmILm(peak)×104N1Ac,
where Ac is the effective cross-section area of the transformer core. In order to avoid saturation condition of core, Bmax⁡ must be less than saturation flux density Bsat⁡ of core.


## 3.1.1. Duty RatioD11

Within charging mode, since battery voltageVB is regarded as a constant voltage during a switching cycle of the proposed hybrid converter, maximum duty ratio D11(max⁡) of the proposed one can be determined by volt-second balance of inductor Lm. Its relationship is expressed as
(1)VPV(min⁡)D11(max⁡)Ts+(-VB(max⁡))(1-D11(max⁡))Ts=0,
where VPV(min⁡) is the minimum output voltage of PV arrays, VB(max⁡) is the maximum voltage across battery, and Ts represents the period of the proposed hybrid converter. From (1), it can be found that D11(max⁡) can be illustrated by
(2)D11(max⁡)=VB(max⁡)VPV(min⁡)+VB(max⁡).
Moreover, transfer ratio M11(max⁡) can be also determined as follows:
(3)M11(max⁡)=D11(max⁡)1-D11(max⁡).When type of battery is chosen, its maximum charging currentIB(max⁡) is also determined. The charging current IB can be changed from its maximum charging current IB(max⁡) to 0 by variation duty ratio D11 of switch M1.


## 3.1.2. InductorLm

Since the proposed hybrid converter is operated in CCM to obtain the maximum charging currentIB(max⁡), its conceptual waveforms of inductor current ILm and charging current IB are illustrated in Figure 6. If the proposed one is operated in the boundary of discontinuous conduction mode (DCM) and CCM, the charging current IB is expressed by
(4)IB(av)=(1-D11)2VBTs2LmB,
where LmB is the inductance Lm at the boundary condition. According to (4), variation of duty ratio D11 can obtain a different charging current IB. In general, the maximum charge current IB(av)max⁡ occurs at the maximum battery voltage VB(max⁡) and the maximum output voltage VPV(max⁡) of PV arrays. Therefore, the boundary inductor LmB can be determined by
(5)IB(av)max⁡=(1-D11B)2VB(max⁡)Ts2LmB,
where D11B is duty ratio of switch M1 under VB(max⁡) and VPV(max⁡). From (5), it can be found that the maximum boundary inductor LmB(max⁡) can be expressed as
(6)LmB(max⁡)=(1-D11B)2VB(max⁡)TsIB(av)max⁡.Figure 6
Conceptual waveforms of inductor currentILm and charging current IB in buck-boost converter.Since the proposed hybrid converter is operated in CCM, inductorLm must be greater than LmB(max⁡). Therefore, when VPV(max⁡), VB(max⁡), IB(av)max⁡, and Ts are specified, the minimum inductance Lm(min⁡) (=LmB(max⁡)) can be determined.In order to avoid the core of transformerTf operated in saturation state, the working flux density Bmax⁡ must be less than the saturation flux density Bsat⁡ of core. Since Bmax⁡ is proportional to the maximum inductor current ILm(peak), ILm(peak) must be first determined. In Figure 6, ILm(peak) can be expressed as
(7)ILm(peak)=ILm(0)+ΔILm(max⁡),
where ILm(0) is the initial value of inductor current ILm operated in CCM and ΔILm(max⁡) represents its maximum variation value. In general, its maximum value ΔILm(max⁡) can be determined by
(8)ΔILm(max⁡)=VPV(max⁡)D11RTsLm,
where D11R represents the duty ratio of switch M1 under VPV(max⁡) and VB(max⁡). Furthermore, the maximum charging current IB(av)max⁡ can be expressed as
(9)IB(av)max⁡=(1-D11R)2(ILm(peak)+ILm(0))=(1-D11R)2(2ILm(0)+VPV(max⁡)D11RTsLm).
From (9), the initial value ILm(0) can be determined as follows:
(10)ILm(0)=IB(av)max⁡1-D11R-VPV(max⁡)D11RTs2Lm.
From (7), (8), and (10), ILm(peak) can be denoted as
(11)ILm(peak)=IB(av)max⁡1-D11R+VPV(max⁡)D11RTs2Lm.
According to datasheet of core which is supplied by core manufacturer, the number of turns N1 on the primary side of transformer Tf can be obtained by
(12)N1=LmAL,
where AL represents nH per turns2. That is, Lm=N12AL. By applying Faraday’s law, Bmax⁡ can be determined as
(13)Bmax⁡=LmILm(peak)×104N1Ac,
where Ac is the effective cross-section area of the transformer core. In order to avoid saturation condition of core, Bmax⁡ must be less than saturation flux density Bsat⁡ 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 Figure1. 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 VPV and current IPV to achieve MPPT, which adopts perturb and observe method. The battery management unit acquires battery voltage VB and current IB 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 Figure1, 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.Table 1
Parameter definitions of control signals shown in Figure4.
Symbol
Definition
V
PV
Output voltage of PV arrays
I
PV
Output current of PV arrays
P
P
Maximum power of PV arrays
P
B
Charging power of battery(PB=VBIB)
P
B
(
max
⁡
)
Maximum charging power of battery(PB(max⁡)=VBIB(max⁡))
I
B
Charging current of battery
V
B
Battery voltage
V
O
Output voltage of forward converter
I
O
Output current of forward converter
V
B
(
max
⁡
)
Maximum voltage of battery
V
B
(
min
⁡
)
Minimum voltage of battery
V
O
(
max
⁡
)
Maximum output voltage of forward converter
I
O
(
max
⁡
)
Maximum output current of forward converter
I
B
(
max
⁡
)
Maximum current of battery
V
ref
Reference voltage for obtaining the desired output voltageVO
I
C
Current command for obtaining the desired charging currentIB
S
M
Control signal of operational mode (SM = 0, battery charging; SM = 1, LED driving)
S
n
Insolation level judgment (Sn = 1, low insolation level; Sn = 0, high insolation level)
V
f
Feedback signal of PWM IC
V
e
Error value
G
1, G2
pwM signals
M
1, M2
Gate signals of switchesM1 and M2Figure 7
Block diagram of the proposed hybrid converter.


#### 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 illustratesP-V curve of PV arrays at different insolation of sun, from which it can be seen that each insolation level has a maximum power Pmax⁡ where Pmax⁡1 is the most insolation of sun, while Pmax⁡3 is the one at the least insolation. Three maximum power point Pmax⁡1~Pmax⁡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 IPV 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 IPV will be increased to operate PV arrays at its MPP.Figure 8
Plot ofP-V curve for PV arrays at different isolation of sun.The proposed power system adopts perturb and observe method to implement MPPT. Its flowchart is shown in Figure9. In Figure 9, Vn and Pp separately represent their old voltage and power, and Pn (=VnIn) is its new power. According to flowchart procedures of MPPT using perturb and observe method, first step is to read new voltage Vn and current In of PV arrays and then to calculate new PV power Pn. Next step is to judge relationship of Pn and Pp. Since the relationship of Pn and Pp has three different relationships, they are separately Pn>Pp, Pn=Pp, and Pn<Pp. Each relationship can be corresponded to the different relationship of Vn and Vp. Therefore, when the relationship of Pn and Pp is decided, next step is to find the relationship of Vn and Vp. According to the relationship ofP-V curve of PV arrays, when the relationships of Pn and Pp and Vn an Vp 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, Vp is replaced by Vn and Pp is also substituted by Pn. The procedure of flowchart returns first step to judge next working point of PV arrays. Moreover, when Pn=PP and Vn=VP, working point of PV arrays set in the MPP of PV arrays. The maximum power PP of PV arrays is transferred to power management unit for regulating power of battery charging.Figure 9
Flowchart of MPPT using perturb and observe method for PV arrays system.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 T0 to Tc. When t=T0, the proposed power system begins to charge battery and battery voltage VB is at the minimum value VB(min⁡). When t=Tc, battery is charged to its maximum voltage VB(max⁡). According to limitation of the maximum battery charging current IB(max⁡), power limitation curve of battery charging can be determined from T0 to Tc. The charging power of battery follows the power limitation curve for extending its life cycle.Figure 10
Conceptual waveforms of charging current, voltage, and power for battery charger with CC charging method.Since power limitation curve of battery has upper and lower values, they are, respectively,PB(min⁡) (=VB(min⁡)IB(max⁡)) and PB(max⁡) (=VB(max⁡)IB(max⁡)). According to relationship among PPV(max⁡), PB(min⁡), and PB(max⁡), they can be divided into three operational states: PPV(max⁡)<PB(min⁡), PB(min⁡)≤PPV(max⁡)<PB(max⁡), and PPV(max⁡)>PB(max⁡), as shown in Figure 11. When PPV(max⁡)<PB(min⁡), power curve of battery charging follows PPV(max⁡). When PB(min⁡)≤PPV(max⁡)<PB(max⁡), power limitation curve and PPV(max⁡) intersects at A point where its intersecting time is TA. Power curve tracks power limitation curve before t=tA, while it traces PPV(max⁡) after t=tA, as shown in Figure 11(b). If operational state of PPV(max⁡)>PB(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.Conceptual waveforms of maximum output power of PV arrays and power limitation and power curves of battery fromTO to TC: (a) PPV(max⁡)<PB(min⁡), (b)PB(min⁡)≤PPV(max⁡)<PB(max⁡), and (c) PPV(max⁡)>PB(max⁡).
(a)
P
PV
(
max
⁡
)
<
P
B
(
min
⁡
)
(b)
P
B
(
min
⁡
)
≤
P
PV
(
max
⁡
)
<
P
B
(
max
⁡
)
(c)
P
PV
(
max
⁡
)
>
P
B
(
max
⁡
)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 SP=0 and then is to read control signals. The control signals include VO(max⁡), VB(max⁡), VB(min⁡), VB, IB, IB(max⁡), IO, IO(max⁡), Vref, and PP. When control signals are obtained by microchip, next step is to calculate PB(max⁡) (=VBIB(max⁡)) and PB (=VBIB). Since PP, which is attained by MPPT control method, is the maximum output power of PV arrays, when PP≥PB(max⁡) is confirmed, Pset is set to equal PB(max⁡). If PP≥PB(max⁡) is denied, Pset=PP. The PP is the power command of battery charging. Therefore, power error value ΔP can be determined. It is equal to (Pset-PB). When ΔP is determined, current command IC can be obtained. It is equal to (ΔP/VB). The current command IC is sent to PWM IC to determine gate signals G1 and G2. Next step is to judge next current command.Figure 12
Flowchart of power and battery managements of the proposed hybrid converter.(
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 IO≥IO(max⁡) is confirmed, overcurrent condition of the proposed hybrid converter occurred. When overcurrent condition occurred, signal SP is set to 1. The SP 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 VO≥VO(max⁡) (overcharge condition), VB≤VB(min⁡) (undercharge condition) and VB≥VB(max⁡) (overcharge condition), the control procedure enters to set SP=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, Sn=0. If insolation is a low level, Sn=1. The signal Sn is sent to operational mode judgment to obtain mode control signal SM. When SM=0, the hybrid converter enters battery charging mode. That is, the insolation of sun is at a high level and Sn=0. If SM=1, the one is in LED driving mode. The signal Sn=1 and insolation is at a low level. The mode control signal SM is sent to mode selector, switch selector, and switch S1. When mode selector receives SM=0, the feedback signal Vf is set to equal IC. The feedback signal Vf and reference single Vref are sent to error amplifier to obtain error value Ve. When error value Ve is attained, PWM generator can depend on Ve to determine duty ratios of PWM signals G1 and G2. When G1 and G2 are specified and SM=0, switch selector can set that M1=G1 and M2=G2 to control the charging current IB 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,SM=1. The mode selector can be operated to set Vf=VO. The Vf and Vref are sent to error amplifier to attain Ve. The Ve is through PWM generator to generate signals G1 and G2. Since SM=1, switch selector is controlled by SM to set M1=G2 and M2=G1. Therefore, the proposed hybrid converter can depend on duty ratios of gate signals M1 and M2 to supply power to LED until battery voltage VB is equal to or less than VB(min⁡). When VB≤VB(min⁡), the proposed hybrid converter is shutdown.


## 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 Figure1. 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 VPV and current IPV to achieve MPPT, which adopts perturb and observe method. The battery management unit acquires battery voltage VB and current IB 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 Figure1, 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.Table 1
Parameter definitions of control signals shown in Figure4.
Symbol
Definition
V
PV
Output voltage of PV arrays
I
PV
Output current of PV arrays
P
P
Maximum power of PV arrays
P
B
Charging power of battery(PB=VBIB)
P
B
(
max
⁡
)
Maximum charging power of battery(PB(max⁡)=VBIB(max⁡))
I
B
Charging current of battery
V
B
Battery voltage
V
O
Output voltage of forward converter
I
O
Output current of forward converter
V
B
(
max
⁡
)
Maximum voltage of battery
V
B
(
min
⁡
)
Minimum voltage of battery
V
O
(
max
⁡
)
Maximum output voltage of forward converter
I
O
(
max
⁡
)
Maximum output current of forward converter
I
B
(
max
⁡
)
Maximum current of battery
V
ref
Reference voltage for obtaining the desired output voltageVO
I
C
Current command for obtaining the desired charging currentIB
S
M
Control signal of operational mode (SM = 0, battery charging; SM = 1, LED driving)
S
n
Insolation level judgment (Sn = 1, low insolation level; Sn = 0, high insolation level)
V
f
Feedback signal of PWM IC
V
e
Error value
G
1, G2
pwM signals
M
1, M2
Gate signals of switchesM1 and M2Figure 7
Block diagram of the proposed hybrid converter.


### 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 illustratesP-V curve of PV arrays at different insolation of sun, from which it can be seen that each insolation level has a maximum power Pmax⁡ where Pmax⁡1 is the most insolation of sun, while Pmax⁡3 is the one at the least insolation. Three maximum power point Pmax⁡1~Pmax⁡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 IPV 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 IPV will be increased to operate PV arrays at its MPP.Figure 8
Plot ofP-V curve for PV arrays at different isolation of sun.The proposed power system adopts perturb and observe method to implement MPPT. Its flowchart is shown in Figure9. In Figure 9, Vn and Pp separately represent their old voltage and power, and Pn (=VnIn) is its new power. According to flowchart procedures of MPPT using perturb and observe method, first step is to read new voltage Vn and current In of PV arrays and then to calculate new PV power Pn. Next step is to judge relationship of Pn and Pp. Since the relationship of Pn and Pp has three different relationships, they are separately Pn>Pp, Pn=Pp, and Pn<Pp. Each relationship can be corresponded to the different relationship of Vn and Vp. Therefore, when the relationship of Pn and Pp is decided, next step is to find the relationship of Vn and Vp. According to the relationship ofP-V curve of PV arrays, when the relationships of Pn and Pp and Vn an Vp 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, Vp is replaced by Vn and Pp is also substituted by Pn. The procedure of flowchart returns first step to judge next working point of PV arrays. Moreover, when Pn=PP and Vn=VP, working point of PV arrays set in the MPP of PV arrays. The maximum power PP of PV arrays is transferred to power management unit for regulating power of battery charging.Figure 9
Flowchart of MPPT using perturb and observe method for PV arrays system.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 T0 to Tc. When t=T0, the proposed power system begins to charge battery and battery voltage VB is at the minimum value VB(min⁡). When t=Tc, battery is charged to its maximum voltage VB(max⁡). According to limitation of the maximum battery charging current IB(max⁡), power limitation curve of battery charging can be determined from T0 to Tc. The charging power of battery follows the power limitation curve for extending its life cycle.Figure 10
Conceptual waveforms of charging current, voltage, and power for battery charger with CC charging method.Since power limitation curve of battery has upper and lower values, they are, respectively,PB(min⁡) (=VB(min⁡)IB(max⁡)) and PB(max⁡) (=VB(max⁡)IB(max⁡)). According to relationship among PPV(max⁡), PB(min⁡), and PB(max⁡), they can be divided into three operational states: PPV(max⁡)<PB(min⁡), PB(min⁡)≤PPV(max⁡)<PB(max⁡), and PPV(max⁡)>PB(max⁡), as shown in Figure 11. When PPV(max⁡)<PB(min⁡), power curve of battery charging follows PPV(max⁡). When PB(min⁡)≤PPV(max⁡)<PB(max⁡), power limitation curve and PPV(max⁡) intersects at A point where its intersecting time is TA. Power curve tracks power limitation curve before t=tA, while it traces PPV(max⁡) after t=tA, as shown in Figure 11(b). If operational state of PPV(max⁡)>PB(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.Conceptual waveforms of maximum output power of PV arrays and power limitation and power curves of battery fromTO to TC: (a) PPV(max⁡)<PB(min⁡), (b)PB(min⁡)≤PPV(max⁡)<PB(max⁡), and (c) PPV(max⁡)>PB(max⁡).
(a)
P
PV
(
max
⁡
)
<
P
B
(
min
⁡
)
(b)
P
B
(
min
⁡
)
≤
P
PV
(
max
⁡
)
<
P
B
(
max
⁡
)
(c)
P
PV
(
max
⁡
)
>
P
B
(
max
⁡
)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 SP=0 and then is to read control signals. The control signals include VO(max⁡), VB(max⁡), VB(min⁡), VB, IB, IB(max⁡), IO, IO(max⁡), Vref, and PP. When control signals are obtained by microchip, next step is to calculate PB(max⁡) (=VBIB(max⁡)) and PB (=VBIB). Since PP, which is attained by MPPT control method, is the maximum output power of PV arrays, when PP≥PB(max⁡) is confirmed, Pset is set to equal PB(max⁡). If PP≥PB(max⁡) is denied, Pset=PP. The PP is the power command of battery charging. Therefore, power error value ΔP can be determined. It is equal to (Pset-PB). When ΔP is determined, current command IC can be obtained. It is equal to (ΔP/VB). The current command IC is sent to PWM IC to determine gate signals G1 and G2. Next step is to judge next current command.Figure 12
Flowchart of power and battery managements of the proposed hybrid converter.(
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 IO≥IO(max⁡) is confirmed, overcurrent condition of the proposed hybrid converter occurred. When overcurrent condition occurred, signal SP is set to 1. The SP 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 VO≥VO(max⁡) (overcharge condition), VB≤VB(min⁡) (undercharge condition) and VB≥VB(max⁡) (overcharge condition), the control procedure enters to set SP=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, Sn=0. If insolation is a low level, Sn=1. The signal Sn is sent to operational mode judgment to obtain mode control signal SM. When SM=0, the hybrid converter enters battery charging mode. That is, the insolation of sun is at a high level and Sn=0. If SM=1, the one is in LED driving mode. The signal Sn=1 and insolation is at a low level. The mode control signal SM is sent to mode selector, switch selector, and switch S1. When mode selector receives SM=0, the feedback signal Vf is set to equal IC. The feedback signal Vf and reference single Vref are sent to error amplifier to obtain error value Ve. When error value Ve is attained, PWM generator can depend on Ve to determine duty ratios of PWM signals G1 and G2. When G1 and G2 are specified and SM=0, switch selector can set that M1=G1 and M2=G2 to control the charging current IB 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,SM=1. The mode selector can be operated to set Vf=VO. The Vf and Vref are sent to error amplifier to attain Ve. The Ve is through PWM generator to generate signals G1 and G2. Since SM=1, switch selector is controlled by SM to set M1=G2 and M2=G1. Therefore, the proposed hybrid converter can depend on duty ratios of gate signals M1 and M2 to supply power to LED until battery voltage VB is equal to or less than VB(min⁡). When VB≤VB(min⁡), the proposed hybrid converter is shutdown.


## 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 illustratesP-V curve of PV arrays at different insolation of sun, from which it can be seen that each insolation level has a maximum power Pmax⁡ where Pmax⁡1 is the most insolation of sun, while Pmax⁡3 is the one at the least insolation. Three maximum power point Pmax⁡1~Pmax⁡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 IPV 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 IPV will be increased to operate PV arrays at its MPP.Figure 8
Plot ofP-V curve for PV arrays at different isolation of sun.The proposed power system adopts perturb and observe method to implement MPPT. Its flowchart is shown in Figure9. In Figure 9, Vn and Pp separately represent their old voltage and power, and Pn (=VnIn) is its new power. According to flowchart procedures of MPPT using perturb and observe method, first step is to read new voltage Vn and current In of PV arrays and then to calculate new PV power Pn. Next step is to judge relationship of Pn and Pp. Since the relationship of Pn and Pp has three different relationships, they are separately Pn>Pp, Pn=Pp, and Pn<Pp. Each relationship can be corresponded to the different relationship of Vn and Vp. Therefore, when the relationship of Pn and Pp is decided, next step is to find the relationship of Vn and Vp. According to the relationship ofP-V curve of PV arrays, when the relationships of Pn and Pp and Vn an Vp 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, Vp is replaced by Vn and Pp is also substituted by Pn. The procedure of flowchart returns first step to judge next working point of PV arrays. Moreover, when Pn=PP and Vn=VP, working point of PV arrays set in the MPP of PV arrays. The maximum power PP of PV arrays is transferred to power management unit for regulating power of battery charging.Figure 9
Flowchart of MPPT using perturb and observe method for PV arrays system.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 T0 to Tc. When t=T0, the proposed power system begins to charge battery and battery voltage VB is at the minimum value VB(min⁡). When t=Tc, battery is charged to its maximum voltage VB(max⁡). According to limitation of the maximum battery charging current IB(max⁡), power limitation curve of battery charging can be determined from T0 to Tc. The charging power of battery follows the power limitation curve for extending its life cycle.Figure 10
Conceptual waveforms of charging current, voltage, and power for battery charger with CC charging method.Since power limitation curve of battery has upper and lower values, they are, respectively,PB(min⁡) (=VB(min⁡)IB(max⁡)) and PB(max⁡) (=VB(max⁡)IB(max⁡)). According to relationship among PPV(max⁡), PB(min⁡), and PB(max⁡), they can be divided into three operational states: PPV(max⁡)<PB(min⁡), PB(min⁡)≤PPV(max⁡)<PB(max⁡), and PPV(max⁡)>PB(max⁡), as shown in Figure 11. When PPV(max⁡)<PB(min⁡), power curve of battery charging follows PPV(max⁡). When PB(min⁡)≤PPV(max⁡)<PB(max⁡), power limitation curve and PPV(max⁡) intersects at A point where its intersecting time is TA. Power curve tracks power limitation curve before t=tA, while it traces PPV(max⁡) after t=tA, as shown in Figure 11(b). If operational state of PPV(max⁡)>PB(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.Conceptual waveforms of maximum output power of PV arrays and power limitation and power curves of battery fromTO to TC: (a) PPV(max⁡)<PB(min⁡), (b)PB(min⁡)≤PPV(max⁡)<PB(max⁡), and (c) PPV(max⁡)>PB(max⁡).
(a)
P
PV
(
max
⁡
)
<
P
B
(
min
⁡
)
(b)
P
B
(
min
⁡
)
≤
P
PV
(
max
⁡
)
<
P
B
(
max
⁡
)
(c)
P
PV
(
max
⁡
)
>
P
B
(
max
⁡
)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 SP=0 and then is to read control signals. The control signals include VO(max⁡), VB(max⁡), VB(min⁡), VB, IB, IB(max⁡), IO, IO(max⁡), Vref, and PP. When control signals are obtained by microchip, next step is to calculate PB(max⁡) (=VBIB(max⁡)) and PB (=VBIB). Since PP, which is attained by MPPT control method, is the maximum output power of PV arrays, when PP≥PB(max⁡) is confirmed, Pset is set to equal PB(max⁡). If PP≥PB(max⁡) is denied, Pset=PP. The PP is the power command of battery charging. Therefore, power error value ΔP can be determined. It is equal to (Pset-PB). When ΔP is determined, current command IC can be obtained. It is equal to (ΔP/VB). The current command IC is sent to PWM IC to determine gate signals G1 and G2. Next step is to judge next current command.Figure 12
Flowchart of power and battery managements of the proposed hybrid converter.(
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 IO≥IO(max⁡) is confirmed, overcurrent condition of the proposed hybrid converter occurred. When overcurrent condition occurred, signal SP is set to 1. The SP 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 VO≥VO(max⁡) (overcharge condition), VB≤VB(min⁡) (undercharge condition) and VB≥VB(max⁡) (overcharge condition), the control procedure enters to set SP=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, Sn=0. If insolation is a low level, Sn=1. The signal Sn is sent to operational mode judgment to obtain mode control signal SM. When SM=0, the hybrid converter enters battery charging mode. That is, the insolation of sun is at a high level and Sn=0. If SM=1, the one is in LED driving mode. The signal Sn=1 and insolation is at a low level. The mode control signal SM is sent to mode selector, switch selector, and switch S1. When mode selector receives SM=0, the feedback signal Vf is set to equal IC. The feedback signal Vf and reference single Vref are sent to error amplifier to obtain error value Ve. When error value Ve is attained, PWM generator can depend on Ve to determine duty ratios of PWM signals G1 and G2. When G1 and G2 are specified and SM=0, switch selector can set that M1=G1 and M2=G2 to control the charging current IB 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,SM=1. The mode selector can be operated to set Vf=VO. The Vf and Vref are sent to error amplifier to attain Ve. The Ve is through PWM generator to generate signals G1 and G2. Since SM=1, switch selector is controlled by SM to set M1=G2 and M2=G1. Therefore, the proposed hybrid converter can depend on duty ratios of gate signals M1 and M2 to supply power to LED until battery voltage VB is equal to or less than VB(min⁡). When VB≤VB(min⁡), 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 voltageVPV:DC 17 V~21 V (PV arrays)
(ii)
Switching frequencyfs1:250KHz
(iii)
Output voltageVB:DC 5~7 V (lead-acid battery: 6 V/2.3 Ah)
(iv)
Maximum output currentIB(max⁡):2.3A.


### 5.2. Active Clamp Forward Converter (LED Driving Circuit)

(i)
Input voltageVB:DC5 V~7 V
(ii)
Switching frequencyfs2:250KHz
(iii)
Output voltageVO:DC10V
(iv)
Maximum output currentIO(max⁡):2A.According to previously specifications and design of the hybrid converter, inductorsL2 and Lm and capacitor CC can be determined. In (17) illustrated in [23], since inductor Lm must be greater than 7.09 uH under VO=10V, VB=7V, and VPV=17V, L2 is chosen by 40 uH. According to (6) and (22) illustrated in [23], the magnetizing inductor Lm must be greater than 6.8 uH under N=5, VB=7V, and L2=40uH. Therefore, magnetizing inductor Lm is determined by 40 uH, while its leakage inductor LK is obtained by 0.2 uH. Moreover, capacitor CC can be attained by (29) illustrated in [23]. Its capacitance CC is 0.22 nF under N=5 and VB=7V. Therefore, CC is chosen by 0.24 uF. The components of power stage in the proposed hybrid converter was determined as follows:(i)
switchesM1, M2: PSMN005-75B,(ii)
diodesD1, D2: UF601,(iii)
transformerTf: EE-25 core,(iv)
inductorL2: EE-22 core,(v)
capacitorCO: 47 μF/25 V, and(vi)
switchS1: IRFP540.Since the charging currentIB of battery can be varied by duty ratio of switch M1 in buck-boost converter, its value is proportional to duty ratio D11. Figures 13(a) and 13(b), respectively, depict the measured voltage VDS waveform of switch M1 and current IB waveform under duty ratio of 0.31 and 0.35, illustrating that the charging current IB can be increased by duty ratios increase. Measured waveforms of PV arrays current IPV and voltage VPV 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 VB and current IB under MPP of PV arrays at 10 W, from which it can be found that the maximum charging current IB is limited at 1.5 A under battery voltage VB of 6.5 V due to control of power management.Measured voltageVDS waveform of switch M1 and the charged current IB waveform operated in duty ratio of (a) 0.31 and (b) 0.35 for working in the charging state.
(a)
(b)Measured voltageVPV, current IPV, and power PPV waveforms of PV arrays using the perturb and observe method to track MPPT of arrays (a) under PPV(max⁡)=10W and (b) under PPV(max⁡)=20W.
(a)
(b)Figure 15
Measured battery voltageVB and currentIB waveforms under PPV(max⁡)=10W.When the proposed hybrid converter is operated in the discharging state (LED driving state), active clamp forward is in working. Measured voltageVDS and current IDS waveforms of switched M1and M2 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 M1 and M2 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 VO has been limited within ±2%. From experimental results, it can be found that the proposed hybrid converter is suitable for electronic sign applications.Measured voltageVDS and current IDS waveforms of (a) switch M1 and (b) switch M2 for working under 20% of full load.
(a)
(b)Measured voltageVDS and current IDS waveforms of (a) switch M1 and (b) switch M2 for working under full load.
(a)
(b)Figure 18
Comparison conversion efficiency between the conventional hard-switching forward converter and the proposed one from light load to heavy load for working in the discharging state.Figure 19
Output voltageVO and output current IO under step-load charges between 30% and 100% of full load of the proposed forward converter operated in the discharging state.


## 5.1. Buck-Boost Converter (Charger)

(i)
Input voltageVPV:DC 17 V~21 V (PV arrays)
(ii)
Switching frequencyfs1:250KHz
(iii)
Output voltageVB:DC 5~7 V (lead-acid battery: 6 V/2.3 Ah)
(iv)
Maximum output currentIB(max⁡):2.3A.


## 5.2. Active Clamp Forward Converter (LED Driving Circuit)

(i)
Input voltageVB:DC5 V~7 V
(ii)
Switching frequencyfs2:250KHz
(iii)
Output voltageVO:DC10V
(iv)
Maximum output currentIO(max⁡):2A.According to previously specifications and design of the hybrid converter, inductorsL2 and Lm and capacitor CC can be determined. In (17) illustrated in [23], since inductor Lm must be greater than 7.09 uH under VO=10V, VB=7V, and VPV=17V, L2 is chosen by 40 uH. According to (6) and (22) illustrated in [23], the magnetizing inductor Lm must be greater than 6.8 uH under N=5, VB=7V, and L2=40uH. Therefore, magnetizing inductor Lm is determined by 40 uH, while its leakage inductor LK is obtained by 0.2 uH. Moreover, capacitor CC can be attained by (29) illustrated in [23]. Its capacitance CC is 0.22 nF under N=5 and VB=7V. Therefore, CC is chosen by 0.24 uF. The components of power stage in the proposed hybrid converter was determined as follows:(i)
switchesM1, M2: PSMN005-75B,(ii)
diodesD1, D2: UF601,(iii)
transformerTf: EE-25 core,(iv)
inductorL2: EE-22 core,(v)
capacitorCO: 47 μF/25 V, and(vi)
switchS1: IRFP540.Since the charging currentIB of battery can be varied by duty ratio of switch M1 in buck-boost converter, its value is proportional to duty ratio D11. Figures 13(a) and 13(b), respectively, depict the measured voltage VDS waveform of switch M1 and current IB waveform under duty ratio of 0.31 and 0.35, illustrating that the charging current IB can be increased by duty ratios increase. Measured waveforms of PV arrays current IPV and voltage VPV 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 VB and current IB under MPP of PV arrays at 10 W, from which it can be found that the maximum charging current IB is limited at 1.5 A under battery voltage VB of 6.5 V due to control of power management.Measured voltageVDS waveform of switch M1 and the charged current IB waveform operated in duty ratio of (a) 0.31 and (b) 0.35 for working in the charging state.
(a)
(b)Measured voltageVPV, current IPV, and power PPV waveforms of PV arrays using the perturb and observe method to track MPPT of arrays (a) under PPV(max⁡)=10W and (b) under PPV(max⁡)=20W.
(a)
(b)Figure 15
Measured battery voltageVB and currentIB waveforms under PPV(max⁡)=10W.When the proposed hybrid converter is operated in the discharging state (LED driving state), active clamp forward is in working. Measured voltageVDS and current IDS waveforms of switched M1and M2 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 M1 and M2 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 VO has been limited within ±2%. From experimental results, it can be found that the proposed hybrid converter is suitable for electronic sign applications.Measured voltageVDS and current IDS waveforms of (a) switch M1 and (b) switch M2 for working under 20% of full load.
(a)
(b)Measured voltageVDS and current IDS waveforms of (a) switch M1 and (b) switch M2 for working under full load.
(a)
(b)Figure 18
Comparison conversion efficiency between the conventional hard-switching forward converter and the proposed one from light load to heavy load for working in the discharging state.Figure 19
Output voltageVO and output current IO under step-load charges between 30% and 100% of full load of the proposed forward converter operated in the discharging state.


## 6. 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, 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.



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*Source: 392394-2014-04-07.xml*