Step-Up DC-DC Converter with High Voltage Gain for PV Power Application

Design of a New Step-Up DC-DC Converter with High Voltage Gain for PV Power Application

Abstract— This paper presents a single switch, high step-up, non-isolated dc-dc converter suitable for PV power application. The proposed converter is composed of a coupled inductor, a passive clamp circuit, a voltage multiplier cell and voltage lift circuit. The passive clamp recovers the leakage inductance energy of the coupled inductor and limits the voltage spike on the switch. The configuration of the passive clamp and voltage multiplier circuit increases the voltage gain. A high voltage gain without a large duty cycle, a low turn ratio for the coupled inductor, low voltage stress on the switch, low voltage stress on the diodes, leakage inductance energy recovery and high efficiency are the benefits of this converter. The steady state operation of the converter in the continuous conduction mode (CCM) is discussed and analyzed. Finally the validity of the proposed converter is confirmed by the simulation results via PSIM software.

Keywords—DC-DC converter; Step-up; High voltage gain; PV power application

I.          Introduction

Nowadays reducing of fossil fuels resources and their environmental consequences have made it necessary to benefit from clean energy sources such as wind and solar. Therefore, distributed generation (DG) systems based on renewable energy sources have attracted the researchers attention. The DG systems include photovoltaic (PV) cells, fuel cells and wind power [1-3]. However, the output voltages of these sources are not large enough for connecting to ac utility voltage. PV cells can be connected in series in order to obtain a large dc voltage. However, it is difficult to ignore the shadow effect in PV panels [4]. High step-up converters are a suitable solution for the aforementioned problem. DC-DC converters can boost and regulate the output voltage of PV panels [5].

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In renewable energy systems, when galvanic isolation is not required, the conventional boost converter appears as the first choice. However, extreme duty ratio and consequently low efficiency are the main drawbacks at high output voltage gain [6]. Recently, several DC/DC converters have been proposed to increase the output voltage gain [7].

Switched inductors and switched capacitors structures are common as well [8]. The current stress on the main power devices is high in switched inductor structures while the high voltage stress is the main problem in switched capacitor based circuits [9]. Coupled inductor based boost converters are proposed to solve the aforementioned problems [10-16]. The leakage current of the coupled inductor is restored via active or passive clamps. Single switch coupled inductor based converters are also proposed in the literature. Since the leakage inductor of coupled inductors is very low, the input current is pulsating with an undesirable current ripple. The peak current of the leakage inductor passes through the main power switch which deteriorates the overall efficiency. By cascading boost converters, high output voltage gain can be achieved. However, too many components will be used in the circuit structures which increase the complexity of the circuit and decrease the total efficiency [16]. Therefore, they are not very efficient in processing energy. Some novel structures of high step up converters are also presented in [10] and [17].

This paper proposes a new high step-up DC–DC  boost converter topology integrating coupled inductor, voltage multiplier cell and the voltage lift capacitor to further increase the voltage gain. The coupled inductor can be designed to extend the static gain and the voltage multiplier cell with voltage lift capacitor offers additional voltage gain. The energy stored in the leakage inductor of the coupled inductor can be recycled through the passive voltage clamp circuit. This causes the spike on power switch to be decreased and conduction loss is decreased, hence the efficiency is increased. The presented converter benefits from numerous advantages such as high output voltage gain without a large duty cycle, a high conversion ratio, low voltage stress on the switch, low voltage stress on the output diode, a wide CCM operation, leakage inductance energy recovery and high efficiency.

Fig. 1.    Circuit configuration of proposed converter.

The reminder of the paper is organized as follows. Section II introduces new high step-up DC–DC boost converter topology. In this section the steady-state analysis of the presented converter are discussed in details. Design procedure of the proposed converted is presented in sections III. Section IV is about comparison study and Simulation results are provided in section V. Finally, section VI concludes this paper.

II.         Operating Principle of The Proposed Converter

Fig. 1 shows the circuit configuration of the proposed
converter. The coupled inductor is shown with its equivalent
circuit including an ideal transformer, the magnetizing
inductance L_m and the leakage inductance L_lk. N_P and N_S are
the number of primary and secondary winding turns of an
ideal transformer, respectively. C₁ is a clamp capacitor, C₃,
C₄ are switched capacitors, C₂ is a voltage lift capacitor,
and C_O is an output capacitor. D₁ is a clamp diode, D_O is the
output diode and D₂, D₃, D₄ are blocking diodes. The proposed converter is suitable for PV power system applications where each PV module can be independently connected to the presented converter.

In order to simplify the circuit analysis of the proposed converter, the following assumptions are considered.

1) All components are ideal, and all the parasitic

 parameters are ignored.

2) The capacitors C₁ ~ C₄ are large enough that the voltages across of them are assumed as constant in each switching period.

3) The turn ratio n of the coupled inductor is equal to N_S/N_P.

The following discussion is confined to continuous conduction mode (CCM) operation in one switching period. Some typical steady state waveform of the proposed converter in CCM operation is shown in Fig. 2, and the corresponding equivalent circuits are shown in Fig. 3. The operating modes are described as follows:

Mode 1 [t₀,t₁]: In this mode, the switch S is turned on, and diodes D₂ and D₃ are on. The current-flow path is shown in Fig. 3(a). The current of magnetizing inductor Lm is increased and leakage inductor current L_k is increased both linearly. The capacitor C₂, C₃ and C₄ are charged and the output capacitor C_O is discharged to the load. This mode finishes when the inductor currents i_lk and i_lm becomes equal.

Mode 2 [t₁,t₂]: During this time interval, the switch S is still on and the diodes D₁, D₄ and D_O are off and the diodes of D₂ and D₃ are turned on as shown in Fig. 3(b). The magnetizing and leakage inductors are charged by the input source. Capacitor C₂ and C₃ is charged by the secondary side current of the coupled inductor and the energy stored in capacitor C₁. The output capacitor provides energy to the load. This mode is finished when the switch S is turned off.

Mode 3 [t₂,t₃]: As it is shown in the Fig. 3(c), the switch S is turned off, and diodes D₁, D₂, D₃ and D_O are conducting. The leakage inductor current is decreased linearly. Capacitors

Fig. 2.    Some typical waveforms of the proposed converter at CCM operation.

of C₁ and C₃ are charged through diodes D₁ and D₃ respectively. Capacitors C₂ and C₄ are discharged to the load through diode D_O. This mode ends when the magnetizing and leakage inductor currents become equal.

Mode 4 [t₃,t₄]: During this time interval, the switch S is off, and diodes D₁, D₄ and D_O are conducting and diodes D₂ and D₃ are off. In this mode, the current of the leakage inductance i_lk is less than the current of the magnetizing inductance i_Lm. The current flow path is shown in Fig. 3(d). The leakage inductor is demagnetized to capacitor C₁ through diode D₁. Capacitors C₂, C₃ and C₄ are discharged to the load through diodes D₄ and D_O. This mode ends when diode D₁ is turned off.

Mode 5 [t₄,t₅]:As shown in Fig. 3(e), in this mode diodes D₄ and D_O are turned on and the switch S is still off. The magnetizing and leakage inductor currents are decreased linearly and the energy of the magnetizing inductance, leakage inductance and input source Vin along with C₂ , C₃ and C₄ is delivered to the load. Also the capacitor C₁ is discharged. This mode ends by turning switch S on again and the next switching period starts.

In order to simplify the analysis, the leakage inductance of the coupled inductor is neglected in the steady-state analysis. Also, the losses of the power devices are not considered. Only stages 2, 4 and 5 are considered for CCM operation because the time durations of stages 1 and 3 are short significantly. At stage2, the main switch S is turned ON, the magnetizing inductance L_m is charged by the input dc source V_in. The following equations for Lm can be written from Fig. 3(b):

      (1)

    (2)              

And the voltage of V_C3 can be expressed by:

     (3)

During stage 4 and 5, the main switch S is in the OFF state and magnetizing inductance is discharged. The voltages across the L_m can be obtained by:

     (4)

    (5)

In mode 4, the following equations for the capacitor voltages can be expressed:

    (6)

 In mode 5, the following equation, according to Fig. 3 (e), can be derived:

   (7)

Using the inductor volt-second balance principle to the magnetizing inductor L_m, the following equations can be expressed as:

  (8)

From (8), the voltage across capacitor C₁ is derived as follows:

     (9)

Fig. 3.    Current-flow path of operating modes during one switching period at CCM operation. (a) Mode 1. (b) Mode 2. (c) Mode 3. (d) Mode 4. (e) Mode 5

Fig. 4.    Voltage gain as a function of D and n values

Fig. 5.    Voltage gains as a function of the duty ratio of the proposed converter, and the other converters under turn ratio: n=2.

According to (9) and (1), the voltage of the capacitor C₂ will be written:

    (10)

By substituting (9) and (10) into (3), the voltage of the capacitor C₃ is obtained as:

   (11)

According to (4), (5) and (11), and simplifying the equations, the voltage of the capacitor C₄ can be expressed:

     (12)

From (6), (9), (10) and (12), the DC voltage gain M_CCM can be achieved as follows:

     (13)

The coupled inductor turns ratio n can be adjusted to achieve the desired voltage conversion ratio without resorting to extreme duty cycle operation. The relationship between the static gain, duty cycle and turns ratio is plotted in Fig. 4.

When the coupled inductor turns ratio is zero, the voltage gain of the proposed circuit is higher than that of the conventional boost converter. The voltage gain increases significantly at higher turns ratio.

Fig. 5 shows voltage gains comparison of the proposed converter and presented converters in [13-15]. As shown in this picture, the voltage gain of the proposed converter is more than the other converters.

III.        Design Procedure of The Presented Converter

A.      The Capacitors Design

Equations (9)-(12) are used to obtain the voltage of the capacitors C₁, C₂, C₃ and C₄. Since the charges absorbed or produced by the capacitors C₁, C₂, C₃ and C₄ are all equal, the size of these capacitors are obtained as:

    (14)

where I_O is the load current, T_S is the switching period, and r is the ripple factor of the capacitor voltage, which is equal to the ratio of the capacitor peak-to-peak ripple voltage to the capacitor average voltage. V_i is the average voltage of the capacitor.

B.      The Magnetizing Inductor current ripple

In terms of the operating principles, the current ripple on the magnetizing inductor is expressed as:

    (15)

    (16)

C.      Voltage Stress Analysis

In order to choose the appropriate semiconductors of the presented converter, their voltage stresses should be obtained. According to the operation principle of the proposed converter, the voltage stresses of the switches and diodes can be formulated as follows:

   (17)

   (18)

  (19)

    (20)

    (21)

    (22) 

IV.        Comparision Study

Some features of the proposed converter and other presented converters in [11], [13]-[15] are collected in Table I. As shown in this table, the voltage gain of the proposed converter is more than the others and the voltage stress on the main switch in the proposed converter under the same voltage gain is less than the converter in [15]. However the suggested converter has some advantages such as high voltage gain, low current ripple, recycled leakage inductor and high efficiency.

V.         Simulation Results

In order to verify the steady state analysis, simulation results of the proposed converter in CCM operation with PSIM software is prepared. The circuit parameters are given in Table II.

 The input and output voltage waveforms are shown in Fig. 6(a), 6(b). The output voltage is equal to 303 V. The capacitors C₁, C₂, C₃ and C₄ voltage waveforms are shown in Figs. 6(c), 6(d). The leakage current of the coupled-inductor is shown in Fig. 6 (e). The obtained waveform is similar to Fig. 2 and the analysis of the presented converter. The voltage and current stress of the power switch is also depicted in Fig. 6(f). The maximum voltage of the power switch is 67V. It is also shown that there are no voltage spikes on the power switch.

According to Fig. 6(g), 6(h), 6(i) and 6(j) the voltages and currents of the diodes D₁, D₂, D₃ and D₄ are also depicted. The

TABLE I.  The Comparison Between the Proposed Converter and Some other Converters.

TABLE II.  The Circuit Parameters of The Simulated Prototype.

voltage stresses of the diodes validate the analysis of the presented converter.Fig.7 shows the efficiency of the proposed converter for several output powers. It is considered that r_DS is , r_D1, r_D2, r_D3, r_D4, r_L1, r_C1, r_C2, r_C3 and r_C4 are. The threshold voltages of the diodes D₁, D₂, D₃, D₄ and D_O are also considered to be 0.7V, 1.2V, 0.8V, 0.8V, 1.2V respectively. The maximum efficiency of the proposed converter under the output power of 75W is 95% and for the output power of 200W, the efficiency is approximately 92.3%.

According to the simulation results, the analysis and the feasibility of the proposed converter are validated. Various advantages of the presented converter make it suitable for several applications such as PV panels.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

Fig. 6.    Simulation results of the proposed converter under the output power of 200W

Fig. 7.    Simulation efficiency of the proposed converter

VI.        Conclusion

This paper proposed a new high step up DC-DC converter. By using a coupled inductor and utilizing the voltage multiplier cell and voltage lift techniques, a high voltage gain is achieved. Clamp circuit in this converter is employed to recycle the stored energy in the leakage inductor. Therefore the voltage stress on the main switch is reduced and the efficiency of the proposed converter is increased. Continues conduction mode operation (CCM), voltage gain, voltage stress of the semiconductors in steady state analysis are discussed in this paper. Finally voltage gain of this converter is compared with some other converters. The proposed converter is simulated under the output power of 200W in PSIM to validate the analysis.

References

[1]      F. Sedaghati, S. H. Hosseini, M. Sabahi, and G. B. Gharepetian “Extended configuration of dual active bridge DC–DC converter with reduced number of switches,” IET Power Electronics, Vol. 8, Iss. 3, pp. 401–416, 2015

[2]      F. Sedaghati, S. H. Hosseini, M. Sabahi, and G. B. Gharepetian “Analysis and implementation of a modular isolated zero-voltage switching bidirectional dc–dc converter,” IET Power Electronics, Vol. 7, Iss. 8, pp. 2035–2049, 2014

[3]      M. Muhannad, M. Armsrong, M. A. Elgendy, “Analysis and implementation of high-gain non-isolated DC–DC boost converter,” IET Power Electron., Vol. 10 Iss. 11, pp. 1241-1249, 2017

[4]      C. W. Chen, K. H. Chen, and Y. M. Chen “Modeling and contoroller Design of an Autonomous PV module for DMPPT PV Systems ,” IEEE Trans. Power Electron., vol. 29, no. 9, pp. 4723 – 4732, Sept. 2014.

[5]      M. Forouzesh, Y. P. Siwakoti, S.A. Gorji, F. Blaabjerg, B. Lehman, “ Step-Up DC–DC Converters: A Comprehensive Review of Voltage-Boosting Techniques, Topologies, and Applications,” IEEE Trans. Power Electron, vol. 32, no. 12, december 2017

[6]      Y. P. Hsieh, J. F. Chen, T. J. Liang, and L. S. Yang, “Novel high step-up DC–DC converter with coupled-inductor and switched-capacitor techniques,” IEEE Trans. Ind. Electron., vol. 59, no. 2, pp. 998–1007, Feb. 2012.

[7]      W. Li and X. He, “Review of non isolated high-step-up DC/DC converters in photovoltaic grid-connected applications,” IEEE Trans. Ind. Electron., Vol. 58, No. 4, pp. 1239-1250, April 2011.

[8]      S. M. Chen, T. J. Liang, L. S. Yang, and J. F. Chen, “A Boost Converter with Capacitor Multiplier and Coupled Inductor for AC Module Applications,” IEEE Trans. Ind. Electron., vol. 60, no. 4, pp.1503-1511, Apr. 2013.

[9]      R. Moradpour, H. Ardi, A. Tavakoli, “Design and Implementation of a New SEPIC-Based High Step-Up DC/DC Converter for Renewable Energy Applications,” IEEE Trans. Ind. Electron.,early access, Jul, 2017.

[10]   H. Ardi, A. Ajami, “Study on A High Voltage Gain SEPICBased DC-DC Converter with Continuous Input Current for Sustainable Energy Applications,” IEEE Trans. Ind. Electron,  Feb, 2018.

[11]   H. Ardi, A. Ajami, M. Sabahi, “A Novel High Step-upDC-DC converter with Continuous Input Current Integrating Coupled Inductor for Renewable Energy Application,” IEEE Trans. Ind. Electron., early access, Jul, 2017.

[12]   A Ajami, H. Ardi, and A Farakhor, “A novel high step-up DC-DC converter based on integrating coupled inductor and switched-capacitor techniques for renewable energy applications,” IEEE Trans. Power Electron., vol. 30, no. 8, pp. 4255-4263, Aug. 2015.

[13]   A. Farakhor, H. Ardi, M. Abapour, “Analysis and design procedure of a novel high voltage gain DC/DC boost converter,” Power Electronics, Drive Systems & Technologies Conference (PEDSTC), 2017 8th. IEEE, 2017.

[14]   S. M. Chen, T. 1. Liang, L. S. Yang, and 1. F. Chen, “A Boost Converter With Capacitor Multiplier and Coupled Inductor for AC Module Applications,” IEEE Trans. Ind. Electron., vol. 60, no. 4, pp.1503-1511, Apr. 2013.

[15]   Y. P. Hsieh, J. F. Chen, T. J. Liang, and L. S. Yang, “A novel high step-up DC–DC converter for a microgrid system,” IEEE Trans. Power Electron., Vol. 26, No. 4, pp. 1127-1136, Apr. 2011.

[16]   G. R. Walker; P. C. Sernia,: ” Cascaded DC-DC Converter Connection of Photovoltaic Modules”, IEEE Transactions on Power Electronics,   Volume: 19, Issue: 4, pp. 1130 – 1139, 2004

[17]   B. Honarjoo, S, M. Madani, M. Niroomand, and E. Adib “Analysis and Implementation of a New Single Switch, High Voltage Gain DC-DC Converter with a Wide CCM Operation Range and Reduced Components Voltage Stress,” Journal of Power Electronics, Vol. 18, No. 1, pp. 11-22, January 2018