Design and Implementation Procedure of a High-Gain Three-Input Step-Up 1 kW Converter

Round 1

Reviewer 1 Report

The manuscript proposes a new variant of high-gain three-input step-up converter capable of providing a significant 1 kW output power. The paper is well organized and the scientific investigation is properly conducted, in general, both theoretically and practically. The following aspects and concerns may nevertheless be mentioned:

1. All the investigations are performed for a single charge resistance value, namely 33 Ω, whereas in practice a wider range interval is expected to occur. Are the obtained results valid for different resistance values? In other words, how does the charge resistance value influence the operation of the convertor?
2. Is the ideal waveform shown in Figure 3 the result of a simulation or some calculation, or are just qualitative, to illustrate the operation of the proposed converter?
3. Are the state variable equations further exploited in the investigation? Apparently, this part of the manuscript looks quite decoupled from the rest of the approach, which is mainly based on SaberTM simulation and experimental measurement.
4. It would be useful to mention how the voltage ripple existing at terminals of capacitors C1 and C2 are eliminated, attenuated or not transmitted, as claimed at page 6, lines 130 and 131.
5. The spikes and high-amplitude ripple affecting the captured waveform shown in Figures 11 and 12 are not consistent with the theoretical and simulation-based waveforms, in which these sharp peaks are absent. That proves the existence of some parasitic influences not taken into account during simulation (maybe some parasitic capacitances, high-harmonic resonances, etc.). Certainly, as mentioned by the authors, a cursory check of the two sets of results (simulated and measured) may result in a quite satisfactory similarity, but further explanations/comments in that sense may prove useful.

In what grammar and style are concerned, a few corrections are to be made, as follows:

1. Repetition in the same phrase of the word “proposed” (page 1, line 19) might be avoided.
2. At page 1, line 27, there is subject-predicate mismatch: “Techniques […] has been proven”.
3. At Figure 10 caption the logical word to be used is “measurement” not “measures”.
4. At page 10, line 182, the word “show” must be corrected to “shown”.

Author Response

Dear Editor:

We are thankful to reviewers for their constructive suggestions and comments. Their comments were very helpful to improve the manuscript. We have addressed all the reviewers’ comments and we consider the manuscript is now improved. Our responses to reviewer’s comments are given point-by-point below.

Reviewer 1.

The manuscript proposes a new variant of high-gain three-input step-up converter capable of providing a significant 1 kW output power. The paper is well organized, and the scientific investigation is properly conducted, in general, both theoretically and practically. The following aspects and concerns may nevertheless be mentioned:

1. All the investigations are performed for a single charge resistance value, namely 33 Ω, whereas in practice a wider range interval is expected to occur. Are the obtained results valid for different resistance values? In other words, how does the charge resistance value influence the operation of the convertor?

A closed loop control is necessary in case of load variation. To this end, the controller described in [12] could be used. However, there is a real difficulty in the converter design procedure for high power and more than two inputs. That is why the manuscript is focused on the converter design procedure for a relatively high power (1Kw) and more than two different inputs. To keep the analysis simple, the proposed design procedure only considers an open loop conditions without load changes during the converter operation. To clarify this point, the following texts have been added to Introduction and Section 3 respectively

“..a 1 kW prototype for three input voltages is implemented in this work, considering an open loop control.”

“The duty cycle is determined according to an open loop control, where the output power or the voltage gain, M_VDC, are specified”

2. 2. Is the ideal waveform shown in Figure 3 the result of a simulation or some calculation, or are just qualitative, to illustrate the operation of the proposed converter?

R: The waveforms shown in Figure 3 are qualitative ideal form to illustrate the converter operation. This have been clarified the first time the figure is referenced.

3. Are the state variable equations further exploited in the investigation? Apparently, this part of the manuscript looks quite decoupled from the rest of the approach, which is mainly based on SaberTM simulation and experimental measurement.

R: The state variable equations are provided in case an interested reader want to reproduce the simulations, it can be done easily. To better integrate state equations to the rest of the document a new subsection was added: “2.1 State variable equations model”.

The details of the procedure to obtain them is not included, due to it was described in reference [12], for the two input voltage case. To clarify it, the following text was added:

“According to the procedure described in [12],…”

4. It would be useful to mention how the voltage ripple existing at terminals of capacitors C1 and C2 are eliminated, attenuated or not transmitted, as claimed at page 6, lines 130 and 131.

R: We appreciate your suggestion. To clarify the point, the following text was added in these lines

“Note that voltage on capacitors C1 and C2 affect the output voltage only trough inductor currents. This can be observed from state equations, particularly, from the output voltage (x6) equation; the output voltage derivative only depends on the output voltage itself, the current x1, the load and the output capacitor. Hence …”

5. The spikes and high-amplitude ripple affecting the captured waveform shown in Figures 11 and 12 are not consistent with the theoretical and simulation-based waveforms, in which these sharp peaks are absent. That proves the existence of some parasitic influences not taken into account during simulation (maybe some parasitic capacitances, high-harmonic resonances, etc.). Certainly, as mentioned by the authors, a cursory check of the two sets of results (simulated and measured) may result in a quite satisfactory similarity, but further explanations/comments in that sense may prove useful.

R: Thanks for your comment. As you mentioned, in the simulation we only considered ideal components; obtaining waveforms without spikes and high-amplitude ripples, the following explanation was included in the section 4:

“A comparison of these waveforms with the ideal waveforms of Figure 3 and Figures 5 to 8 shows its similarity, validating the experimental results. Spikes and high-amplitude ripples observed in the experimental waveforms show the existence of parasitic components that are not considered in simulation.”

In what grammar and style are concerned, a few corrections are to be made, as follows:

1. Repetition in the same phrase of the word “proposed” (page 1, line 19) might be avoided.

R: Thanks for your comment. It have been corrected, one instance of the word “proposed” was replaced by “designed”

2. 2. At page 1, line 27, there is subject-predicate mismatch: “Techniques […] has been proven”.

R:  We are sorry for the mistake. It has been corrected, the word “has” was replaced by “have”.

3. At Figure 10 caption the logical word to be used is “measurement” not “measures”.

R: Thanks you very much. The mistake has been corrected.

4. At page 10, line 182, the word “show” must be corrected to “shown”.

R: Thank you for pointing it out. The mistake has been corrected.

Reviewer 2 Report

The comparison of the converters would be much more valuable with the same output power level and with the same power elements. The level of the output power and the type of components used affect the overall efficiency of the converter. Additionally, it would be best to compare with the same switching frequency.

In Figure 12, the switching frequency is much lower than the declared 100 kHz? Figure 10 shows an assembled circuit with inductances that do not appear to work effectively at 100 kHz. There is no information in the work about the selection and construction of inductances and cores.

Author Response

Reviewer 2.

1. The comparison of the converters would be much more valuable with the same output power level and with the same power elements. The level of the output power and the type of components used affect the overall efficiency of the converter. Additionally, it would be best to compare with the same switching frequency.

R: Thanks for your comment. Table 3 is included to contrast the advantages and disadvantages of the proposed converter with similar topologies.  However, each converter employs a different approach, hence the operating parameters must be different; for example, the proposed converter has an efficiency of 90.5%, which is lower than references [10], [12] and [14], but its output power is higher.   We consider that the information of this table may help interested readers to take better decision about what converter is convenient to interconnect multiple sources in a concrete situation.

2. In Figure 12, the switching frequency is much lower than the declared 100 kHz? Figure 10 shows an assembled circuit with inductances that do not appear to work effectively at 100 kHz. There is no information in the work about the selection and construction of inductances and cores.

R: Thanks for your comment. According to Figure 12 the time scale is 2.000 µs (as is shown at the top of the image), to obtain the switching period and frequency can be used (1) and (2):

     (1)

   (2)

We hope this clarifies the point. 

With respect to inductors design, we consider that core design has been widely analysed in several excellent references. Particularly, we follow the methodology presented in “Transformer and Inductor design handbook” by Colonel T. McLyman.  This is now mentioned in the first paragraph of the Section “Experimental Results” and the book has been included in the references.

Author Response File: Author Response.pdf

Reviewer 3 Report

The article presents a design and implementation procedure of a high-gain three input step-up converter. Open-loop three input converter operation design is presented. A design procedure was validated through experimental results for a 1kW power converter. As it is clearly presented to use the harvested energy, the systems must be adopted to the energy and voltage waveform of the specific energy harvesting technology. The paper is clearly written but lacks some details.

Limitations of such converter design procedure are missing in the article. The input/output voltage limitations should be considered. The design procedure presented is for the voltage gain Mvdc with 3 inputs of 12V, which is not the case (also presented in Table2). The presented simulation results are presented for different operational points than in experimental results. A comparison between the similar topologies should not be focused on the number of components used and efficiency but also on input voltage and generated output voltage.

Other few comments and suggestions.

Has the voltage input value and position in the converter an impact on the converter operation? 

On page 7, Figure 5. a variable VL1 is presented, which is not explained or presented in the nomenclature. Same on other Figures with VL2, VL3.

On page 9 the input currents do not correspond to the simulation results in Figure 5,6,7.

References should be unified.

Author Response

Reviewer 3.

The article presents a design and implementation procedure of a high-gain three input step-up converter. Open-loop three input converter operation design is presented. A design procedure was validated through experimental results for a 1kW power converter. As it is clearly presented to use the harvested energy, the systems must be adopted to the energy and voltage waveform of the specific energy harvesting technology. The paper is clearly written but lacks some details.

1. Limitations of such converter design procedure are missing in the article. The input/output voltage limitations should be considered. The design procedure presented is for the voltage gain Mvdc with 3 inputs of 12V, which is not the case (also presented in Table2). The presented simulation results are presented for different operational points than in experimental results. A comparison between the similar topologies should not be focused on the number of components used and efficiency but also on input voltage and generated output voltage.

R: Thanks for your comments. a) The design procedure shows that the three voltage sources may be different. In the manuscript, three different voltages are considered: V_in1= 12V, V_in2=24V and V_in3 = 48V. However, inductors must be designed for the worst scenario, which happen when the three input sources have its minimum value. In this case equation (9) is used to determine the M_VDC.

The parameters of Table 2 are used in simulation and experimental results as well.  Differences between both situations, for example, the spikes and high-amplitude ripples observed in  experimental results, are due to the presence of parasitic components that do not exist in the ideal components considered in the simulation.  To clarify this point an explanation was added in section 4:

“A comparison of these waveforms with the ideal waveforms of Figure 3 and Figures 5 to 8 shows its similarity, validating the experimental results. Spikes and high-amplitude ripples observed in the experimental waveforms show the existence of parasitic components that are not considered in  simulation.”

With respect to comparison with other converters, In Table 3 were included the input and output voltages, the rest of the information was included to contrast the advantages and disadvantages of the proposed converter with similar topologies. However, each converter employs a different approach, hence the operating parameters must be different; for example, the proposed converter has an efficiency of 90.5%, which is lower than references [10], [12] and [14], but its output power is higher. We consider that the information of this table may help interested readers to take better decision about what converter is convenient to interconnect multiple sources in a concrete situation.

2. Other few comments and suggestions:
3. a) Has the voltage input value and position in the converter an impact on the converter operation?

R: As long as the two control conditions are satisfied (phase shift and minimum duty cycle),  voltage input value and its position in the converter of each source does not matter.

5. b) On page 7, Figure 5. a variable VL1 is presented, which is not explained or presented in the nomenclature. Same on other Figures with VL2, VL3.

R: Sorry for the confusion. Figures 5-7 has been corrected: VL1 to V_L1, VL2 to V_L2 and VL3 to V_L3,

7. c) On page 9 the input currents do not correspond to the simulation results in Figure 5,6,7.

R: We are sorry for the mistake. It has been corrected.

1. d) References should be unified.

R: We appreciate your comment. We have edited some references to make them unifor

Round 2

Reviewer 3 Report

All reviewer's comments have been addressed by the authors and inserted in the text. Thank you for the effort. The paper can be accepted for publication.