A Multifunctional Isolated and Non-Isolated Dual Mode Converter for Renewable Energy Conversion Applications

Abstract

In this paper, a multifunctional isolated and non-isolated dual-mode low-power converter was designed for renewable energy conversion applications such as photovoltaic power generation to achieve different operating modes under bi-directional electrical conversion. The proposed topology consists of a bidirectional non-isolated DC/DC circuit and an isolated converter with a high-frequency transformer, which merge the advantages of both the conventional isolated converter and non-isolated converter with the combination of the two converter technologies. Compared with traditional converters, the multifunctional converter can not only realize conventional bi-directional functions, but can also be applied for many different operation modes and meet the high output/input ratio demands with the two converter circuits operating together. A novel control algorithm was proposed to achieve the various functions of the proposed converter. An experimental platform based on the proposed circuit was established. Both the simulation and experimental results indicated that the proposed converter could provide isolated and non-isolated modes in different applications, which could meet different practical engineering requirements.

1. Introduction

With the development of power electronics, solid state converters have been widely used in several renewable energy applications [1,2,3,4,5,6]. Different applications require different converter topologies, which can be divided into isolated and non-isolated converters. In recent years, with further renewable energy systems, new requirements for power electronic conversion have been proposed.

Currently, there are some converter topologies for solar power and other renewable energy applications [6,7,8,9]. Generally, the grid-connected systems use non-isolated converters in most conditions. In some distributed power generators, a single-phase two-stage transformerless circuit is usually used, and the converter can be used to step up the DC voltage [10] in the similar way to boost and buck-boost converters [11]. These topologies are suitable for grid-connected applications with high efficiency, low cost, and compact structures [12,13]. However, in stand-alone system applications, isolated converters or inverters are mostly employed in the power conversion circuits [14,15,16,17,18,19].

In [20], a power system using a high step-up converter for dc load applications in a photovoltaic energy conversion system was presented. The developed converter adopted a coupled inductor and a boost converter working in interleaved mode. A novel high step-up DC-DC converter was successfully integrated using a coupled inductor and the switched capacitor techniques described in [21]. Nevertheless, these converters only control the power flow from the input array to loads in a unique direction or could not achieve galvanic isolation applications. With the emergence of new energy applications, compared with the single-function of a grid-connected or off-grid (stand-alone) system, new dual-mode [22,23] or multifunctional applications [24,25] have become more popular, which are a new trend in the future for renewable energy applications [19]. As different conversion modes are required in different applications, more flexible and reliable conversion circuits are needed. This paper proposes a new multifunctional isolated and non-isolated dual mode low-power converter, and shows a block diagram of the proposed converter system is shown in Figure 1.

Compared to traditional converters, with the addition of the realization of conventional bi-directional function, the multifunctional converter can also be applied for both the isolated and non-isolated operation modes. Switching between the isolated and non-isolated operation modes is presented according to different applications by adjusting the control switch signals, which are flexible to change in the operation mode for renewable energy conversion applications. The simulation and experimental results indicated that the proposed topology could provide an efficient isolated or non-isolated dual mode converter under different applications and meet the different practical engineering requirements.

The paper is organized as follows: Section 2 describes the topology of the proposed multifunctional converter. Different operation modes and control strategies are explained in Section 3. Section 4 introduces the design of key parameters for the proposed converter. In Section 5, the simulation tests and experimental results are displayed and discussed. Finally, the conclusions are presented in Section 6.

2. Proposed Multifunctional Isolated and Non-Isolated Dual Mode Converter

The single bidirectional buck/boost non-isolated converter or flyback isolated converter is a DC/DC electrical power conversion solution for grid-connected or stand-alone energy system projects due to their technical maturity. The flyback converter has been widely used in a large number of power conversions due to its relatively simple structure, good performance, and galvanic isolation [26,27,28]. A bidirectional buck/boost converter is a mature topology in power electronics systems due to its simplicity and high efficiency [29]; however, the conventional buck/boost converter cannot achieve input and output galvanic isolation. To simultaneously implement the functions of isolated and non-isolated converters and benefit from the advantages of the two converter circuits, this paper proposes a novel multifunctional isolated and non-isolated dual mode converter by increasing the control transfer units. The design methodology of the proposed new converter is depicted in Figure 2. Since the conventional bidirectional buck/boost converter and flyback converters are complementary in galvanic and other technical characteristics, it is feasible and reasonable to combine them to create a new multifunctional isolated/non-isolated dual mode converter where the buck/boost converter operates in the non-isolated operation mode and the flyback converter operates in the isolated operation mode. The new converter contains a non-isolated bidirectional step-up and step-down front-stage circuit, an isolated converter, and a conversion control switching circuit. By adjusting the controllable devices and transfer switches, the different operation modes can be interchanged.

The schematic of the proposed converter is illustrated in Figure 2. It consists of a bidirectional non-isolated conventional DC/DC circuit and an isolated converter with a high-frequency transformer, and these two parts are connected by the transfer switches. A common buck/boost bi-directional converter is employed as the non-isolated stage converter including a conventional non-isolated bidirectional step-up and step-down circuit in the proposed topology. The isolated stage converter is composed of a bidirectional flyback converter.

The proposed circuit schematic is shown in Figure 3, where U₁ and U₂ denote the port1 (DC bus or U₁ side) voltage and the port2 (storage system or U₂ side) voltage, respectively. S₁ (D₁), S₂ (D₂), S_i1 (D_i1), and S_i2 (D₂) are fully-controlled power electronic switching devices in the converter; S_i1 and S_i2 are the switches in the isolated converter; C₁ and C₂ are the non-isolated converter capacitors; C₃ represents the filter capacitor; L is the inductor of a non-isolated converter; and T is the isolated stage converter flyback transformer.

The proposed new dual-mode converter merges the advantages of both the conventional isolated converter and non-isolated converter to create a blend of two types of converter technologies. The new converter not only copes with the common function requirements, but also meets the high output/input ratio demands when the two converter circuits operate together.

As previously depicted, the proposed new converter topology is an integrated conventional bidirectional buck/boost circuit and a bidirectional flyback circuit. The novel converter works in two different modes.

2.1. Isolated Mode

In this mode, the bidirectional flyback converter is working. The equivalent circuit of the isolated operating mode is shown in Figure 4. In this operation mode, the trigger switch relay K₁ is switched to contact node B, and the switch relay K₂ is closed.

There are three working states in this operation mode, including the forward conversion state, reverse conversion state, and the fault protection state. In the forward conversion state, the proposed converter transfers the power from the U₂-side to the U₁-side. Since the converter works as a storage system discharge, this state is called the discharge mode.

The equivalent circuit of the forward conversion state is presented in Figure 5a. The switch S_i2 operates in the Pulse Width Modulation (PWM) mode, and the power flow through the flyback and the equivalent boost converter into the output loads. In this state, when the system requires a high conversion ratio, the converter can operate in the combination conversion state, which means that S₂ is controlled by a high frequency PWM signal, otherwise S₂ is turned off.

The equivalent circuit of the reverse conversion state is shown in Figure 5b. The operation control is similar to that in the forward conversion state, while the difference is in the direction of power flow. When switch S_i1 operates in the PWM mode, the topology is an equivalent circuit of a buck converter (high conversion ratio when PWM duty cycle is less than one DSiPWM < 1) or filter circuit (when the PWM duty cycle is equal to one DSiPWM = 1) with the flyback converter charging the storage system from the U₁-side (such as the DC bus).

As displayed in Figure 5c, if any external fault occurs in the system, the converter enters the fault-protection stage. To achieve electrical isolation between the critical equipment and components, all PWM control signals are turned off at this stage, then the fully-controlled switches S₁\S₂\S_i1 and S_i2 are turned off. The trigger switch K₁ is switched onto contact B and switch K₂ is open to realize the electrical isolation between the U₁-side and U₂-side, which can protect the critical equipment and reduce the impact of fault.

2.2. Non-Isolated Mode

The proposed converter can also operate in the non-isolated operation mode as per a conventional converter. Similarly, there are two operating states in this mode consisting of the forward conversion stage and the reverse conversion stage. For the non-isolated mode, the trigger switch relay K₁ is switched to contact node “A”, and switch K₂ is closed during this mode, as shown in Figure 6.

The equivalent circuits for the two operation states are shown in Figure 7. In the forward conversion state, diode D₁ along with switch S₁ are used to boost the voltage, the converter transfers the power from the U₂-side to the U₁ side, and the grid-connected power generation is a typical example of this state. Similarly, in the reverse conversion state, switch S₁ along with D₂ is used for bucking the voltage from the U₁-side to the U₂-side, which can draw power from the DC bus to charge the storage system.

3. Converter Analysis and Control Strategies

The proposed original converter shown in Figure 3 is analyzed. In different operating modes, the working status of each switch is different.

3.1. Converter Analyzation during Isolated Operation Mode

According to the direction of the power flow, the isolated operation mode has two operating states: the forward conversion state and the reverse conversion state as illustrated in Figure 5a,b, respectively. It was assumed that the proposed converter operated in the continuous conduction mode (CCM), and all switches and components were ideal at the beginning. The theoretical key waveforms of the proposed converter in the isolated operation mode are shown in Figure 8.

The steady-state key conversion waveforms of the isolated mode are shown in Figure 8, where V_gs are the gate signals of the main switches S₁, S₂, S_i1, and S_i2; K₁ and K₂ are the states of trigger switches; i_S1, i_S2 and i_Si1, i_Si2 are the currents flowing through the main switches S₁, S₂, S_i1, and S_i2, respectively; i_D1, i_D2, i_Di1, and i_Di1 are the currents flowing through D₁, D₂, D_i1, and D_i2, respectively; and i₁ and i₂ are the currents flowing through U₁-side and U₂-side, respectively.

As presented in Figure 8a, the analyzation of the forward conversion stage in one switching period was carried out according to Figure 5a as follows: In this stage, the main switches S₁, S₂, and S_i1 were turned off, and the current flows through D_i1 and D₁; S_i1 was turned on with the high frequency PWM signal where the duty ratio was adjusted. During the interval [t₁, t₂], the switch S_i2 was turned on, and the U₂-side current path was U₂-K₁(B)-T-S_i2; during the interval [t₃, t₄], the switch S_i2 was turned off, and the U₁-side current path was T-L-D₁-U₁-D_i1. The voltage conversion ratio between the input U₂-side and output U₁-side is expressed as:

$$\frac{U_2}{U_1}=\frac{n_2}{n_1}\frac{D_{S_{i2}}}{1-D_{S_{i2}}}$$

(1)

where n₂/n₁ is the turns ratio of the high frequency transformer; and D_si1 is the duty cycle of the PWM control signal. Based on Figure 5b and Figure 8b, the reverse conversion stage analysis in the switching period was similar to that in the forward conversion stage. However, there was still a difference between the power flow direction and the S₁ working state. In the reverse conversion stage, the switch S₁ was OFF and the energy was transferred from the U₁-side to U₂-side, whose operation and voltage ratio were similar to those in the forward stage.

From the above analysis, it can be seen that the output voltage can be higher or lower than the input voltage, and the duty cycle can be adjusted according to the application demands. However, if there are more high-conversion-ratio application requirements, the converter will be in a combined operation state where a high-conversion-ratio forward conversion or reverse conversion converter is obtained. In these cases, the related equivalent circuits are demonstrated in Figure 5a,b. Switches S₁ and S₂ are controlled by PWM signals, and the key waveforms for the high-conversion-ratio equivalent are also shown in Figure 5c,d, respectively.

Figure 5a shows the equivalent circuit for the high-converter-ratio forward conversion, and Figure 8c shows the control signals and key waveforms across the related components. At this stage, the converter transfers power from the U₂-side to the U₁-side, which is also called the forward converter stage. The switch control methods at this stage are similar to the conventional isolation forward state, except that S₂ is switched by a high frequency adjustable PWM control technique and S₁ is ON in the conventional isolation forward state. When combining the integrated control strategies, the circuit can achieve a higher and more flexible output and input ratio, which is expressed as:

$$U_1=(k_{flyback}\cdot k_{boost})\cdot U_2$$

(2)

where k_flyback is the equivalent isolate flyback converter ratio; and k_boost is the equivalent non-isolate boost converter ratio. The combination ratio is k_flyback × k_boost, which can obtain a wider range of conversion ratio when compared to the conventional isolation forward state.

Similarly, Figure 5b shows the equivalent circuit of a high-converter-ratio reverse conversion, and Figure 8d shows the control pulses and key waveforms across the related components. The analysis of the reverse conversion stage can be duplicated from the forward conversion stage. S₁ is switched by a high frequency adjustable PWM control scheme. The power transfer direction at this stage is reversed to that in the forward conversion stage when power is transferred from the U₁-side to the U₂-side, as is illustrated in Figure 5b. The relationship between U₁ and U₂ can be expressed as:

$$U_2=(k_{buck}\cdot k_{flyback}^{\prime })\cdot U_1$$

(3)

where k_fbuck is the non-isolated buck converter ratio; and k’_flyback is the isolated flyback converter ratio. The combination ratio is k_buck × k’_flyback, where a wider range of conversion ratio compared to that in the conventional isolation reverse state is derived.

According to Equations (2) and (3), it is clear that the proposed converter operating in the combination state can obtain a high conversion ratio. Therefore, compared with other types of converter topologies, the proposed converter has a wider range of applications.

3.2. Converter Analysis during Non-Isolated Operation Mode

The waveforms of the two stages under the non-isolated mode are shown in Figure 9.

Figure 9a shows the step-up or boost stage. According to Figure 7a, the analysis of the step-up stage is the same as that for a conventional boost converter. During the entire switching cycle, all three switches (S₁, S_i1, and S_i2) are turned off. Switch S₂ is controlled by an adjustable high frequency PWM signal. The trigger switch K₁ is connected to A, and K₂ is activated. At t₁, S₂ is turned on and i_S1 is increased. At t₂, S₂ is turned off and the current I_s2 drops to zero with the power transferred to inductor L. During the step-up converter stage, the relationship between U₂ and U₁ can be expressed as:

$$U_1=\frac{1}{1-D_{S2}}{U}_2$$

(4)

where D_S2 is the duty cycle of switch S₂.

Figure 9b shows the step-down or buck stage. According to Figure 7b, the analysis of this stage is the same as that for a conventional buck converter. Similar to the case in the forward stage, all three switches (S₂, S_i1, and S_i2) are turned off during the entire switching cycle. Switch S₁ is controlled by an adjustable high frequency PWM signal. The trigger switch K₁ is connected to node A, and switch K₂ is on. At time t₁, S₁ is turned on with i_S1 increasing, and the energy transfers to the inductor until t₂, when S₁ is turned off and the current i_S1 drops to zero. During this converter stage, the relationship between U₂ and U₁ is expressed as:

$$U_1=\frac{1}{1-D_{S1}}{U}_2$$

(5)

where D_S1 is the duty of switch S₁.

3.3. Control Strategy

There are several different operation states or modes in the proposed new converter, so the control strategy is more complex when compared to that of a conventional single-function converter. As conventional bidirectional isolated or non-isolated converter control methods and strategies have been widely presented in the literature, details on the related control analyses and design are provided in [26,27,28,29,30,31,32,33,34,35,36,37,38]. Given that the proposed new converter has many different transition control strategies across different states and modes, the switching control is mainly used during the transition process between the switching devices depending on the application requirements. The state or mode transition control logic diagram is shown in Figure 10. According to our previous analysis and the illustration in Figure 10, the operation mode mainly includes the isolated and non-isolated dual modes, which are achieved by the switching control of switches K₁ and K₂. There are many different working states in the two operation modes achieved by controlling the switches S₁, S₂, S_i1, and S_i2.

According to the modes described and analyzed in Section 2, the control logics of the switches are given in

Table 1. The control logics of switches states.

Control Logics
Switches	Isolated					Non-Isolated
	Forward	Reverse	High-Converter-Ratio		Fault	Step-Up	Step-Down
			Forward	Reverse
S1	OFF	ON	OFF	PWM	OFF	OFF	PWM
S2	OFF	OFF	PWM	OFF	OFF	PWM	OFF
Si1	OFF	PWM	OFF	PWM	OFF	OFF	OFF
Si2	PWM	OFF	PWM	OFF	OFF	OFF	OFF
K1	B					A
K2	Disconnected					Connected

. The whole control logics of the proposed converter are relatively simple as only the states of toggle switches K₁ and K₂ need to be changed when the operation mode is required to change. Furthermore, the operating stage can also be changed by controlling one or two active switches. Additionally, all switches are OFF in the fault protection mode. Thus, the control hardware implementation is relatively easy to achieve and the system cost is reduced.

As described above, the control flow chart for the proposed new converter is shown in Figure 11. First, the control system reads the set value and determines the operation modes and states, then according to the different operation mode or working state, it selects and calls the corresponding control method and related subroutines; then, the control signal output is calculated. Finally, the converter implements the required conversion function.

4. Design Consideration of Main Circuit

Based on the circuit conversion characteristics, the inductor and the transformer are the most important components of the proposed converter. Therefore, the parameter design of the non-isolated converter inductor L, and the design of the isolation converter transformer T will be discussed in this section.

4.1. Designing Non-Isolated Converter Inductor L

The non-isolated converter modes include those for the step-up and step-down conversion. Therefore, the selection of inductor L should satisfy the parameter requirements for both operation states. The design calculation and analysis of the two related modes are illustrated in [39]. The inductor L of two states can be calculated as shown in Equations (6) and (7), respectively:

$$\{\begin{array}{l}{I}_{L(step\text{-}down)}=\frac{U_{C2}\cdot (1-D_{S1})\cdot T_{step\text{-}down}}{L_{step\text{-}down}}\\ I_{L(step\text{-}down)\max }=I_o-\frac{U_{C2}\cdot (1-D_{S1})\cdot T_{step\text{-}down}}{2L_{step\text{-}down}}\\ I_{L(step\text{-}down)\min }=I_o-\frac{U_{C2}\cdot (1-D_{S1})\cdot T_{step\text{-}down}}{2L_{\mathrm{step}\text{-}\mathrm{down}}}\\ L_{(step\text{-}down)\min }=\frac{U_{C2}\cdot (1-D_{(step\text{-}down)})\cdot T_{step\text{-}down}}{I_{L(step\text{-}down)\max }}\end{array}$$

(6)

$$\{\begin{array}{l}{I}_{L(step\text{-}up)}=\frac{U_{C2}\cdot D_{S2}\cdot T_{step\text{-}up}}{L_{step\text{-}up}}\\ I_{L(step\text{-}up)\max }=I_1+\frac{U_{C2}\cdot D_{S2}\cdot T_{step\text{-}up}}{2L_{step\text{-}up}}\\ I_{L(step\text{-}up)\min }=I_1-\frac{U_{C2}\cdot D_{S2}\cdot T_{step\text{-}up}}{2L_{step\text{-}up}}\\ L_{(step\text{-}up)\min }=\frac{U_{C2}\cdot D_{S2}\cdot T_{step\text{-}up}}{I_{L(step\text{-}down)\max }}\end{array}$$

(7)

According to Equations (6) and (7), the non-isolated converter inductor L can be selected as per Equation (8) by considering the cost and other factors, and the value can be as large as possible.

$$L=\max (L_{(step\text{-}down)\min },L_{(step\text{-}up)\min })$$

(8)

4.2. Designing the Isolated Converter Transformer

The design of the isolated converter transformer in the bidirectional flyback circuit is similar to that of the traditional flyback converter, which can be selected based on the experience in theoretical calculations and engineering applications. Due to the page limitations, the detailed design can be found in the related literature [40,41,42,43,44,45,46].

5. Simulation Test and Experimental Results

To validate the design scheme of the proposed multifunctional converter, both software simulation and experimental tests were conducted. The main parameters of the converter are summarized in

Table 2. Main parameters of the converter.

Parameter	Value
Rated power	150 W
Capacitance C1/C2/C3	220 μF/220 μF/1000 μF
Inductance L	40 μH
Input/output voltage range	12~100 V

.

5.1. Simulation Test

A Matlab/Simulink simulation model of the proposed converter was developed to validate the design scheme. The simulation experimental results from the different operation modes and working states are shown in Figure 12 and Figure 13.

Figure 12a,b shows the key waveforms of the forward conversion under different conversion ratios. Figure 12a shows the key waveforms of the isolated flyback conventional forward operation mode, which includes the PWM control signal ν_gSi2, the main switch S_i2, currents i_Si2 and the secondary side current i_Di2, and the U₁-side voltage, respectively. Figure 12b presents the key waveforms of the isolated flyback high conversion-ratio operation mode including the control signal and currents through the main switch S_i2 currents i_Si2, and the inductor L current. From Figure 12, it can be seen that the proposed converter operating in the isolated mode performed well with a higher conversion ratio. The output voltage could be higher or lower than the conventional single flyback converter under the same operating condition.

The key waveforms of the non-isolated operation mode under different states are shown in Figure 13. Figure 13a presents the waveforms of the control signals, inductance current, switch and diode currents, and the output voltage in the step-up state. Figure 13b shows the waveforms of the step-down state. The control method and working waveforms are similar to those in conventional boost and buck converters.

5.2. Experimental Results

The performance and effectiveness of the proposed converter were verified in an experimental prototype. Experimental tests were conducted with a simulated photovoltaic storage application system under different operation modes and working conditions. Figure 14 shows the measured waveforms of the proposed converter in the isolated mode.

Figure 14a shows the high conversion ratio in the step-up mode. The key waveforms in the step-down converter mode are given in Figure 14b. Compared to the conventional converter, the proposed converter could simultaneously achieve isolation, and higher input and output ratios, which satisfied the requirements of the different applications.

Figure 15a shows the experimental waveforms of the non-isolated operation mode from step-up to step-down. In step-up mode, switch S₂ is controlled by the PWM signal and switch S₁ is OFF. When switching to the step-down mode, switch S₁ is controlled by the PWM signal and switch S₂ is OFF. The converter operates in the step-up/step-down state, which is similar to the operation of a conventional boost/buck converter. The proposed system can realize the transition between different operating modes by changing the switches and the control signals, and the proposed converter can be quickly changed from the isolated (reverse) mode to the non-isolated (step-up) mode as shown in Figure 15b. The experimental results showed good consistency with the theoretical analysis and simulation results.

Figure 16 depicts the efficiency curves of the proposed converter operating in different modes. As seen in the curves, the proposed converter had maximum efficiency at the non-isolated reverse mode, which was due to the single-stage power converter under high-efficiency step-down operation. The converter had relatively low efficiency at the high-conversion-ratio isolated modes due to the reduction of the entire efficiency when the converter works in cases of high ratio, which uses two-stage circuits in series.

Table 3. Comparison of the proposed converter with other bidirectional converters.

Aspect	Converter Studied in [47]	Converter Proposed in [48]	The Proposed Converter
Topology	non-isolated	Isolated	Non-isolated/isolated
Number of ports	2	3	2
Number of switches	4	4	4 + 2 *
Operating stages or modes	4	3	7
Efficiency	Higher	Higher	High **

* 2 relays; ** Specified mode.

presents a comparison between the proposed converter with another related bidirectional non-isolated converter studied in [47] and the isolated converter described in [48]. The proposed converter in this paper had better features including diversified operation modes, adjustable electrical isolation non-isolation, and wider voltage conversion ratios. For example, the converter mode can be changed depending on different applications. In addition, given that the relays are cheaper than switching devices, the proposed converter has almost the same cost as it has the same number of switching devices used in other related bidirectional converters.

6. Conclusions

This paper presented a multifunctional isolated and non-isolated dual-mode converter for renewable energy electrical conversion applications. The designed converter integrated the advantages of both the conventional bi-directional non-isolated converter and the isolated converter through a simple switch transition to realize multiple functions to meet different engineering application requirements. The circuit topology and composition were introduced and the working principle described in detail. Next, the different operation modes and states were analyzed. The simulation and experimental results indicated that the proposed converter could achieve dual-mode and operations under different application requirements.