# Efficient, Load Independent and Self-Regulated Wireless Power Transfer with Multiple Loads for Long Distance IoT Applications

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Description of the Proposed Multi-Load WPT Repeater System

_{TX}) and the N-number of power relays (L

_{Ri}, i = 1,2,…,N). Where R’

_{Li}and R

_{Ri}(i = 1,2,…,N) are the equivalent AC load and parasitic resistances of the repeater coils, respectively. R

_{TX}is the transmitter parasitic resistance and source resistance is represented by Rs resistor. Two MOSFETs are used in a full-wave rectifier instead of diodes for each repeater, to realize the switches (S

_{i}, i = 1,2,…,N).

_{Li}) and the reflected load (R

_{refl_i}) (i = 1,2,…N) from the adjacent repeater is shown in Figure 4a. Figure 4b is the switch turned off and on, the equivalent circuit of the last N

^{th}receiver.

#### 2.1. Circuit Structure

_{S}. L

_{TX}and the capacitor C

_{TX}constitute the series resonant tank with the parasitic resistance of R

_{TX}. At the repeater side, the coil L

_{R1}and the capacitor C

_{RX}formed a series resonant tank. The full-bridge rectifier consists of two diodes, D

_{1}, D

_{2}, and two MOSFETs, M

_{1}, M

_{2}, to provide DC voltage to the load R

_{L1}. In this implementation, two NMOSs are used instead of two rectifying diodes of the conventional full-wave rectifier. M

_{1}and M

_{2}are not only used for rectification but also regulate the output power of the repeater. When M

_{1}and M

_{2}are turned off, power is delivered to the load R

_{L1}and current is passed through the body diodes of M

_{1}and M

_{2}. Power is not delivered to the load R

_{L1}, upon turning on of M

_{1}and M

_{2}and the repeater worked as a power relay to deliver power to the adjacent repeater.

#### 2.2. Power Regulation

^{th}receiver are not fully charged; therefore, the switches are turned off. First, the capacitor of the first repeater is charged to the targeted voltage, and then the switch of the first repeater is turned on. Then, the capacitor of the second repeater is charged to the targeted voltage, followed by the third repeater capacitor, so on, up to the last receiver. Figure 6 shows output voltages and PWM signals of the MOSFETs gates for four multiple loads WPT system, that is, three repeaters, and the fourth one is the receiver.

_{L}

_{1}of the repeater #1 reaches the targeted voltage first and its switches are turned on, then repeater #2’s switches are turned on when output voltage V

_{L2}reaches the targeted voltage followed by repeater #3 and the last receiver switches are turned on upon output voltages, V

_{L3}and V

_{L4}, reaching their targeted voltages.

## 3. System Analysis and Coil Design

_{refl_3}is the reflected impedance from the repeater #3 to repeater #2 and R

_{L2}is the equivalent AC load of the repeater #2. Therefore, depending on these two cases, the reflected impedance at repeater # 1 can be calculated as follows.

- Case 1: When repeater #2 MOSFETs are turned off, then reflected impedance at repeater #1, assuming full resonance at the resonance frequency fo, can be calculated by Equation (1):$${R}_{ref{l}_{\_2}}={k}^{2}{w}^{2}{L}_{R1}\frac{{L}_{R2}}{{R}_{ref{l}_{3}}+{R}_{L2}+{R}_{R2}}$$
- Case 2: When repeater #2 MOSFETs are turned on, then reflected impedance at repeater #1, can be calculated as:$${R}_{Refl\_2}={k}^{2}{w}^{2}{L}_{R1}\frac{{L}_{R2}}{{R}_{refl\_3}+{R}_{R2}}$$

_{refl_}

_{2}at the repeater #1 is larger when the switch (MOSFETs) of the repeater #2 is turned on as compare to turned off because the AC load R

_{L2}of the repeater 2 is omitted for the turned-on case. Similarly, reflected impedance R

_{refl_}

_{1}at the transmitter can be calculated as

_{refl_}

_{1}at the TX side, for the repeater #1 switch turned off case is given by

_{refl_}

_{3}seen by repeater #3 from the last receiver is given by Equation (5)

_{refl_}

_{3}, when the receiver is turned on is given by

## 4. Experimental Results

_{R1}is changed while all other loads are constant) showed a small decrease of efficiency at large load resistance. For repeater #2, the increase of load resistance from its nominal value did not have a significant change of efficiency and a small increase of efficiency if the load value was decreased from the nominal value. The load variation at repeater #3 or the last receiver had a significant increase in efficiency when the load value was increased but for the last receiver, if the load value was decreased from its value of 11 Ω, then the overall system efficiency dropped to 49.8% at 8 Ω.

_{L3}of repeater #3 was changed while load resistance of the first and second repeater and the last receiver were kept the same and constant. When the load resistance R

_{L3}is changed from 30 Ω to 50 Ω and vice versa, the power regulation is achieved well, as shown by the load transient waveforms of Figure 16. Output voltage V

_{L1}of the first repeater is not shown due to the limitation of oscilloscope channels. When the R

_{L3}is changed from 30 Ω to 50 Ω the output current is decreased to regulate the output voltage V

_{L3}of the third repeater. The load transition at the third repeater does not have any effect on the voltage or output power of the first and second repeaters, and the last receiver.

## 5. Comparison of the Proposed Multi-Load WPT Relay System with the Previously Reported Systems

## 6. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Circuit topology of a power relay system with a transmitter and N receivers [19].

**Figure 4.**Switch turned off and on, equivalent circuit (

**a**) of a repeater and (

**b**) the last Nth receiver (

**b**).

**Figure 6.**Sequential turn on of repeaters and last receiver switches at respective output voltages (

**a**) power regulation (

**b**).

**Figure 7.**(

**a**) Equivalent circuit of repeater #2 (

**a**) when its MOSFETs are turned off or on (

**b**) Equivalent circuit of the last receiver when its MOSFETs are turned off or on (

**b**).

**Figure 15.**Sequence of repeater regulation for proposed multi-load WPT relay system when TX power (

**a**) increases and (

**b**) decreases.

TPE 2019 [5] | JESTPE 2020 [19] | MWC Lett. 2015 [21] | This work | |
---|---|---|---|---|

No.# of coils/repeater | 2 | 1 | 1 | 1 |

Additional inductor | Yes | No | No | No |

Load-independent power | Yes | No | No | Yes |

Efficiency with load variations | High | Low | Low | High |

Carrier Freq. | 200 KHz | 1 MHz | 193 KHz | 1 MHz |

Efficiency | 83.9% ^{1} @ k = 0.25 | 70.0% ^{2} @ k = 0.035 | NA | 51.7% @ k = 0.035 |

Ferrite core | Yes | No | No | No |

Rectification | Not implemented | Not implemented | Not implemented | Implemented |

Power regulation | Not implemented | Not implemented | Not implemented | Implemented |

^{1}This is AC load efficiency without rectification and a large k is required for high efficiency.

^{2}This is AC load efficiency without rectification with optimized load and a DC/DC converter is needed at each repeater or receiver for load modulation.

Component | Parameter | Implementation |
---|---|---|

Tx coil, Repeaters & Receiver coil | Inductance | 34 μH |

Rectifier Diodes | D_{1}, D_{2} | MBRS340T3G |

MOSFETs (Switches) | M_{1}, M_{2} | IRLML6344TRPBF |

Compensation Capacitor | C_{R1} | 745 pF |

Output Capacitor | C_{1} | 10 μF |

Output Resistor | R_{L1} | 11 Ω |

Feedback Resistors | R_{1}, R_{2} | 3 K |

Reference Voltage | V_{ref} | 2.5 V (LT6657BHMS8) |

Op-amp | Op-amp | LT6220IS5 |

Comparator | PWM | TLC3702CPWR |

Triangular generator IC | PWM | Max9000ESA |

Component | Cost $ (USD) |
---|---|

Tx coil and a Repeater coil (Litz wire 0.06 mm/1000) | approx. 8 |

Rectifier Diodes | 0.2198 × 2 |

MOSFET (IRLML6344TRPBF) | 0.304 × 2 |

Tx MOSFET (IPD30N10S3L34ATMA1) | 0.812 |

Capacitors | 0.723 × 7 |

Resistors | 0.452 × 6 |

Op-amp (LT6220IS5#TRMPBF) | 2.19 |

Comparator (TLC3702CPWR) | 0.8876 |

Triangular generator IC (MAX9000ESA+) | 3.8696 |

Tx inductor | 3.99 |

Total Cost | approx. $28 |

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**MDPI and ACS Style**

Hassan, N.u.; Lee, W.; Lee, B. Efficient, Load Independent and Self-Regulated Wireless Power Transfer with Multiple Loads for Long Distance IoT Applications. *Energies* **2021**, *14*, 1035.
https://doi.org/10.3390/en14041035

**AMA Style**

Hassan Nu, Lee W, Lee B. Efficient, Load Independent and Self-Regulated Wireless Power Transfer with Multiple Loads for Long Distance IoT Applications. *Energies*. 2021; 14(4):1035.
https://doi.org/10.3390/en14041035

**Chicago/Turabian Style**

Hassan, Najam ul, Woochan Lee, and Byunghun Lee. 2021. "Efficient, Load Independent and Self-Regulated Wireless Power Transfer with Multiple Loads for Long Distance IoT Applications" *Energies* 14, no. 4: 1035.
https://doi.org/10.3390/en14041035