# A High Power-Conversion-Efficiency Voltage Boost Converter with MPPT for Wireless Sensor Nodes

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## Abstract

**:**

## 1. Introduction

## 2. System Design and Analysis

#### 2.1. Modeling of the Photovoltaic Cell

_{ph}is the sunlight-generated current, which is proportional to the light intensity; I

_{sat}is the reverse saturation current, which is related to temperature; I

_{sh}is the short-circuit current; T is the temperature of the photovoltaic cell; q is the charge of an electron; A is the ideal factor of the PN junction; K is the Boltzmann constant; R

_{p}is the equivalent parallel resistance; R

_{s}is equivalent series resistance. I

_{ph}varies with light intensity as Equation (2).

_{sc}is the short circuit current; T

_{ref}is the absolute temperature; T represents the battery temperature; C

_{T}is the temperature coefficient.

#### 2.2. Analyze of MPPT Algorithm

_{d}is the power loss during the boosting process, P

_{sys_out}is the system’s output power, and P

_{pv_out}is the output power of the photovoltaic cell.

_{pv_out}. We design the charge pump modulated by PFM. Reducing the frequency P

_{d}can further reduce loss and P

_{sys_out}further increases, as shown in Figure 3. When Equation (7) is established, the positions of the two power points are different. As a result, we hope that when the output voltage reaches the maximum value in practical applications, the maximum power point will move to the right.

#### 2.3. Design of the VBC System with MPPT

_{out}of VBC will be given to LDO, and then the regulated voltage can be used in sensor application circuits. This power system management can apply in small WSNs powered by PV cells.

_{ref}of the nano-ampere bias circuit module is generated and maintained stable. The detection circuit detects the reasonable value of the feedback voltage and controls the data selector to start the second stage and stop the ring oscillator. The detection circuit is an ultra-power comparator.

## 3. Circuit Block Implementation Details

#### 3.1. High PCE Charge Pump

_{D}can improve output power. Nevertheless, at the same time, it is necessary to consider the dynamic loss caused by the parasitic capacitance charging and discharging, and the frequency or capacitance cannot be increased infinitely. The analysis in the previous section shows that the solution adopted in this paper is to pursue the maximum value of the output voltage to provide a stable voltage to the subsequent modules. The feedback MPPT control module tracks the maximum value of the voltage and gives the appropriate value of the clock frequency. Finally, we can maximize the efficiency of the entire system, and the output power of the PV cell may not be at the maximum power point at this time.

#### 3.2. Ultra-Low-Power P&O MPPT Algorithm and Control Block

_{out}in a period, and the current and previous time V

_{out}is stored in two input capacitors of the comparator, respectively. Secondly, the comparator in Figure 9, is a fast and dynamic comparator with ultra-low power consumption, is used to compare the voltage at the current time with the previous voltage to obtain the changing trend of the voltage. It can be observed that the changing trend of the output voltage and the direction of the voltage disturbance are in an XOR relationship from the logic block diagram of the P&O method. Then, the DFF can save the last disturbance direction, judge the next disturbance direction, perform the XOR operation to get the next disturbance direction, and then give the result to the subsequent logic gate. Finally, a structure based on the charge pump changes the voltage of the output node by charging and discharging the node capacitance to control the oscillation frequency of the VCO and then changes the system’s impedance. To minimize the power loss, we set up I

_{ref}= 10 nA, and the MPPT control accuracy is set to ∆V

_{vco}= 10 mV per cycle. The capacitance of MPPT can be known by Formula (9) and (10). The dissipation power of the MPPT module is lower than 1 uW.

#### 3.3. Nano-Ampere Current Reference and Current Starvation VCO

_{ref}and the control voltage of VCO.

_{n}and I

_{p}current-source circuit. To reduce the power consumption to the nano-ampere level, we all use MOS devices, and all MOS transistors work in the sub-threshold region (except MR working in a deep linear region).

_{p}with holes as carriers can be designed. We adjust the size of the MOS. I

_{n}and I

_{p}are both 10nA, and the error is less than 150 pA.

_{ref}are 653.89 mV and 16.67 mV, respectively, and the coefficient of variation (σ/μ) is 2.549%.

_{D}is a single inverter delay time.

## 4. Experimental Results and Discussion

_{in}and V

_{out}in Oscilloscope when the sunlight density is 400 W/m

^{2}. The output voltage of the photovoltaic cell is 1.09 V, and the load resistance is 5 K. The output voltage can reach 3.18 V, and the ripple is less than 15 mV.

_{ref}and the mode switch process of VBC. When the output voltage is established, V

_{ref}stabilizes at 680 mV. Furthermore, the waveform depicts that P&O MPPT algorithm makes output voltage improve 276 mV.

^{2}. The data points are shown as Figure 16. When the output current is less than 100 uA, the effect of MPPT is not significant because the structure based on RCO and CP has the function of tracking the max power point in linear mode.

^{2}, and the load impedance is 50 K, the output voltage of VBC is 5.051 V. Figure 17 also illustrates that an excessive output resistance can cause a failure to boost voltage.

^{2}to 1000 W/m

^{2}. During the change, the higher the light intensity, the higher the power corresponding value of the highest conversion efficiency, the greater the turning point. The highest conversion efficiency is relatively large when the light intensity is higher than a particular value. When the light intensity is constant, the PCE will rise and decline as the output current increases. Taking the effective light intensity of 300 W/m

^{2}as an example, it is about 85.1% when the output current is 297 μA. When the voltage is large, the current is small, and the efficiency is not high. Results are identical to the relationship among the PV characteristics of the photovoltaic cell, the loss of the charge pump, and the frequency.

## 5. Discussion and Conclusions

^{2}, the output voltage of the photovoltaic cell is 1.09 V, the output voltage can reach 3.18 V, and the ripple is less than 15 mV. Calculation results show that the maximum voltage conversion efficiency is 99.4%. MPPT algorithm can help PCE to improve 8.53%. Regarding PCE, when the output current is 297 uA, the output efficiency can reach 85.1% at the maximum.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 6.**(

**a**) A single charge pump voltage transfer schematic; (

**b**) As single charge pump charging schematic; (

**c**) Circuit diagram of Cross-coupled CMOS charge pump; (

**d**) Circuit diagram of Cross-coupled CMOS charge pump with drivers.

**Figure 15.**(

**a**) The micro-photograph of VBC IC; (

**b**) Measured waveform of output and input voltage; (

**c**) Transfer characteristic of VBC; (

**d**) Measured waveform of V

_{ref}and the output voltage in the switching process.

Device | Parameter |
---|---|

MN1/MN2 | 80/0.5 |

MP1/MP2 | 100/0.5 |

C1 | 5 pF |

Cfly | 200 pF |

Properties | [7] | [8] | [12] | [13] | This Work |
---|---|---|---|---|---|

CMOS process | 0.18 um | 0.13 um | 0.25 um | 0.18 um | 0.35 um |

Chip area | 1.6 × 1.1 | 0.066 | 3.4 × 3.4 | 1.6 × 1.7 | 3.15 × 2.43 |

Input voltage | 0.5–0.6 V | 0.15 | 0.5~2 V | <0.6 V | 0.5–1.8 V |

Output voltage | 1.8/4.2 V | 0.619 | 0~5 V | 1.8/3 V | 1.5–5.4 V |

PCE | 75.8%/49.1% | 72.5% | 87% | 50% | 85.1% |

MPPT | yes | no | yes | yes | yes |

Fully integrated | yes | yes | no | no | yes |

Boost device | cap | cap | ind | - | cap |

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**MDPI and ACS Style**

Zhu, X.; Fu, Q.; Yang, R.; Zhang, Y.
A High Power-Conversion-Efficiency Voltage Boost Converter with MPPT for Wireless Sensor Nodes. *Sensors* **2021**, *21*, 5447.
https://doi.org/10.3390/s21165447

**AMA Style**

Zhu X, Fu Q, Yang R, Zhang Y.
A High Power-Conversion-Efficiency Voltage Boost Converter with MPPT for Wireless Sensor Nodes. *Sensors*. 2021; 21(16):5447.
https://doi.org/10.3390/s21165447

**Chicago/Turabian Style**

Zhu, Xiwen, Qiang Fu, Ruimo Yang, and Yufeng Zhang.
2021. "A High Power-Conversion-Efficiency Voltage Boost Converter with MPPT for Wireless Sensor Nodes" *Sensors* 21, no. 16: 5447.
https://doi.org/10.3390/s21165447