A Power Management Circuit with Supercapacitors
for Magnetic Field Energy Harvesting System in
Power Transmission Lines
Yuan Gao
Power system wide area
measurement and control of the
Key Laboratory of Sichuan
Province
University of Electronic Science
and Technology
Chengdu, China
3221283970@qq.com
Libo Fan
The State Grid Zhejiang Electric
Power Company
Hangzhou, China
13516877799@139.com
Rongjie Han
The State Grid Zhejiang Electric
Power Company
Hangzhou, China
3806806@qq.com
Ming Liu
Power system wide area
measurement and control of the
Key Laboratory of Sichuan
Province
University of Electronic Science
and Technology
Chengdu, China
liuming@std.uestc.edu.cn
Min Sun*
Power system wide area
measurement and control of the
Key Laboratory of Sichuan
Province
University of Electronic Science
and Technology
Chengdu, China
minsun@uestc.edu.cn
Abstract- For magnetic energy harvesting systems, power
management circuits play a very important role in the system. This
paper introduces an energy management circuit for the energy
collection system, which can efficiently deposit the collected
energy into the load. The energy management circuit is mainly
composed of supercapacitor and MCU. The supercapacitor is used
as a start-up excitation and energy storage element. A low power
MCU is employed to control the synchronous rectifier circuit
Three operating modes based on the primary transmission line
current are proposed. The system can switch between the three
operating modes autonomously. The system is verified through
experiments and effectively achieves steady-state operation and
efficient energy management.
Keywords—magnetic field energy harvesting,
management circuits, supercapacitor, sensor node
energy
I. INTRODUCTION
The requirement of real-time state monitoring in AC
transmission and distribution lines makes magnetic field energy
prevalent in the power transmission and distribution network[1].
Magnetic field energy can be converted into electrical energy
by Faraday's law of electromagnetic induction. Therefore,
numerous research work has been published on magnetic field
energy harvesting systems [2-5]. However, most of the work
focused on the magnetic core while the power management unit
for the energy system is rarely mentioned.
The power management circuit should have the ability to
self-start when the harvested environment energy generates
enough voltage for the self-starting unit. Hence it is common
design to add a backup battery to the power management circuit.
The backup battery provides a high current for system self-start
This work is supported by Science and Technology Project from State
Grid (No. 5700-202305277A-1-1-ZN).
Jiabin Huang
The State Grid Zhejiang Electric
Power Company
Hangzhou, China
1334117037@qq.com
and simplifies the design of the system's startup circuit.
However, the battery has a limited lifetime and needs to be
replaced frequently, which again increases the maintenance
cost [6-7]. Meanwhile, how to perform autonomous startup and
maintain the robust operation is a key challenge that needs to
be solved for the power management circuit.
In this paper, the power management circuit is designed
without using a backup battery as the system start-up power
source. A supercapacitor is added to replace the backup battery
and serve as an energy storage element at the same time. The
use of a supercapacitor improves the serviceable life of the
system compared to the battery backup design while it also
brings design challenges in terms of cold-start energy
management, rectifier MOSFET drive and energy management
under various conditions.
In this paper, a power management circuit with
supercapacitors for magnetic field energy harvesting system is
introduced for wireless sensor nodes [8-10]. Section II focuses
on the management circuits. Section III describes the system's
three modes of operation and Section IV presents the
experimental validation of the designed system.
II. POWER MANAGEMENT CIRCUIT DESIGN
A. Overall System Structure Design
The energy harvesting system consists of three parts which
are magnetic core, energy management circuit and load. The
magnetic core forms a CT to generate AC current. The load of
the energy harvesting system usually is a battery to power the
sensor and communication unit. The power management circuit
contains an energy storage unit, a synchronous unit and a low
power MCU. The structure of the magnetic field energy
harvesting system is shown in Fig. 1.
Fig.1. System overview of the power management circuit for magnetic field energy harvesting systems.
𝑅1
)
𝑅2
4.096 × (𝑅1 + 𝑅2 ) − 5 × 𝑅1
𝑉L =
{
𝑅2
𝑉H = 4.096 × (1 +
B. Energy Storage Unit
R2
R1
VCAP
4.096V
750k
3.24M
Nmos
Drive
Fig.2. Hysteresis control circuit for the supercapacitor voltage.
To achieve system self-starting, a voltage monitor is utilized
to monitor the voltage of the supercapacitor. The voltage
monitor functions as a comparator, with its minimum reset
voltage set to 2.98V. When VCAP < 2.98 V, the comparator
output enables the EN signal to a low logic level, thereby
disabling the DC/DC and shutting down the downstream
circuitry. Conversely, when VCAP > 2.98 V, the comparator
output enables the EN signal, allowing the DC/DC enable
terminal to conduct and turn on the downstream circuitry, thus
achieving the cold start of the circuit.
At the same time, considering the supercapacitor as an
energy storage component, its charging and discharging
voltages should be taken into account. As a result, a hysteresis
control circuit for supercapacitor voltage regulation was
designed, as shown in Fig.2. The designed maximum and
minimum voltages for supercapacitor charging and discharging
are 5 V and 4 V, respectively.
To satisfy the design requirements for hysteresis control
voltage values, the 5V power supply input is passed through a
reference voltage chip to generate a 4.096 V reference voltage.
R1 is configured as 750 kΩ and R2 is configured as 3.24 MΩ.
Substituting these values into (1) yields the maximum voltage
VH and minimum voltage VL as 5.04 V and 3.88 V, respectively.
(1)
When the voltage across the supercapacitor rises to 5.04V,
the hysteresis comparator outputs a high logic, causing the shortcircuit MOSFET to bypass the rectifier output. As depicted in
Fig.1, the magnetic core induction current is shorted to the
ground after passing through the rectifier bridge. The addition
of an anti-reverse Schottky diode prevents the supercapacitor
from being shorted. At this moment, the supercapacitor supplies
power to the load independently. As the load consumes energy,
the voltage of the supercapacitor decreases. When the voltage of
the supercapacitor drops to 3.88V, the hysteresis comparator
outputs a low potential, causing the short-circuit MOSFET to
close, and the rectifier circuit output current charges the
supercapacitor again.
C. Synchronous Rectifier Circuit Design
A rectifier circuit is designed to rectify the induced AC
current and link to the supercapacitor through an anti-reverse
current diode to charge the supercapacitor.
The rectification circuit mainly consists of four MOSFETs,
four Schottky diodes, and a filter capacitor, as shown in Fig.3.
Among them, 4 parallel Schottky diodes are used to achieve
rectification before the MCU starts the synchronous
rectification.
Before the system cold starts, the rectifier circuit rectifies the
current through four parallel Schottky diodes. The forward
voltage of Schottky diodes is much smaller than that of PN
diodes After the system completes cold-start, when the voltage
of the supercapacitor rises to 5.04V, the discrete signal A from
the MCU outputs a low potential, disabling the rectifier circuit.
When the voltage of the supercapacitor drops to 3.88V, the
discrete signal A from the MCU outputs a high potential,
enabling the rectifier circuit, and the MOSFETs begin rectifying.
The MCU controls the charging of the supercapacitor by
outputting the gate-switching signals of the full-bridge
MOSFETs through the discrete signal A.
PMOS
QE1
Q1
QE2
Q2
iP(t)
+
C1
V0(t)
-
Lμ
Q3
NMOS
Q4
This driver circuit ensures that the MOSFETs of the fullbridge rectifier are in the off state during system startup, thereby
preventing energy consumption by the DC/DC circuit during the
supercapacitor energy storage phase.
D. Supercapacitor Capacitance Design
The supercapacitor capacitance is calculated by the required
input voltage and standard charging current for the charging IC
and thin-film battery in the load circuit. The standard charging
current for the thin-film battery is 15 mA, and the input voltage
for the charging IC is 5 V, resulting in an approximate power
requirement of 75 mW.
QE3 QE4
Fig.3. Synchronous rectifier circuit.
V0(t)
QE1
QC1
A
Q1
cold starts. The upper-side control circuit consists of a power
MOSFET, three non-power MOSFETs, and two anti-shortcircuit resistors. Prior to cold start, when the discrete signal A is
at a low potential, QC3, being an NMOS, is in cutoff region,
resulting in the non-operation of Q1. Passive rectification occurs
at this juncture. Conversely, when output A is at a high potential,
both QC3 and QC1 are conducting. Because QE1 is a PMOS, it is
in cutoff region. At this point, the control circuit self-locks, with
the MCU providing a high potential output to the gate of Q1,
thereby activating MOSFET rectification. The operation
principle of the NMOS driver circuit mirrors that of the PMOS
driver circuit.
(a)
QC3
vCORE+ (t)
η0 is defined as the efficiency of the DC/DC ICs and η1 is
the efficiency of the charging IC. For most of the DC/DC ICs,
when the load current is greater than 10 mA, its efficiency η0 is
above 90%. For simplicity, the loss of the charging IC chip can
be ignored. Substituting these numbers into (2) yields an
average power consumption of the load as 84 mW. Substituting
84 mW into (3), the calculation indicates that the capacitance
of the supercapacitor should not be less than 325 mF. Therefore,
this design adopts two capacitors with a rated voltage of 5.5 V
and a capacitance of 1 F connected in series to form a 500 mF
supercapacitor.
𝑃=
V0(t)
vCORE-(t)
75mW
𝜂0 𝜂1
(2)
1
(3)
𝐶 (𝑉 2 − 𝑉L2 )
2 CAP H
The charging current ICHG is determined by (4). The
maximum charging current for the battery is 40 mA. According
to the battery manual, the minimum discharge voltage of the
battery is 3 V. Therefore, in this design, a 100 Ω currentlimiting resistor is used. In this configuration, the maximum
charging current is 20 mA, and the maximum power loss of the
current-limiting resistor is 0.04 mW.
5 − 𝑉B
(4)
𝐼CHG =
𝑅C
𝑃LOAD =
Q4
QE4
A
QC2
(b)
Fig.4. Driver circuits for MOSFETs in the synchronous rectifier: (a) PMOS;
(b) NMOS.
The MCU controls the rectification circuit through driver
circuits. The driver circuit is designed to disable the energy
management system during system startup. This control circuit
is shown in Fig.4.
The control circuit primarily governs the switching between
diode rectification and MOSFET rectification before the system
III. MODES OF ENERGY HARVESTING SYSTEMS
The scenarios mentioned above are all predicated on the
assumption of a sufficiently large induced current ΙP, with the
critical current being Ιmin. However, it is necessary to consider
the situation when the induced current ΙP < Ιmin, where the
magnetic field energy harvesting may not provide sufficient
power to charge the battery. Due to the small induced current,
the supercapacitor struggles to maintain its voltage at 2.98 V.
TABLE I. THREE OPERATING MODES OF THE POWER MANAGEMENT CIRCUIT
Supercapacitor Charging
Supercapacitor Discharge
Hybrid Power Supply
Enabling conditions
Ip> Imin
2.98 V ≤ VCAP < 5.04 V
Ip> Imin
VCAP  5.04 V
Ip < Imin
NMOS drive
l
1
0
MCU control
On
Off
On
Diode rectifier
Off
On
Off
In this scenario, the magnetic field energy harvesting
system, together with the supercapacitor, contributes to
charging the battery as the supercapacitor voltage continues to
decrease. When the voltage of the supercapacitor drops to the
minimum input voltage of the voltage monitor, 2.88 V, the
voltage monitor outputs a low-potential enable signal EN,
causing the load circuit to shut down. The energy harvesting
system then re-enters the cold start phase, implementing
intermittent charging for the battery. In practical magnetic field
energy harvesting operations, due to variations in induced
currents, the system may switch between continuous and
intermittent power supply modes. The system automatically
switches between different operational states by monitoring the
changes in the supercapacitor voltage.
IV. EXPERIMENT RESULTS
A. Hardware Circuit Design
The magnetic field energy harvesting system which includes
the power management circuit designed in this paper is
illustrated in Fig.5. After completing the system cold start, 5 V
and 3.3 V voltage regulation circuits will be activated to supply
power to the MCU and charging IC. The 4.096 V reference
power supply is generated from the 5 V power input through a
reference voltage chip. The embedded program of the MCU
realizes the rectification mode switching. The charging IC
manages the battery charging.
Upon completion of the system cold start, under different
induced current and supercapacitor voltage conditions, the
energy source for the load circuit varies. Throughout the system
control, there are three supply modes for the load circuit, as
depicted in Table I. When the induced current Ip exceeds the
critical current value Imin and the supercapacitor voltage VCAP
satisfies 2.98 V ≤ VCAP < 5.04 V, the NMOS Drive signal of
the supercapacitor voltage hysteresis comparator circuit is at a
low level, causing the MOSFET to close, enabling the MCU to
activate MOSFET rectification. The rectified induced current
independently powers the load circuit.
When the induced current IP exceeds the critical current
value Imin and the supercapacitor voltage VCAP exceeds 5.04 V,
the NMOS Drive signal of the supercapacitor voltage hysteresis
comparator circuit is at a high level, leading to the opening of
the MOSFET and the MCU deactivating rectification, thereby
enabling diode rectification. The supercapacitor independently
powers the load circuit. When the induced current IP is less than
the critical current value Imin, the output signal NMOS Drive of
the supercapacitor voltage hysteresis comparator circuit is at a
low level, allowing the MCU to activate rectification. The
rectified induced current and supercapacitor collectively power
the load circuit. It is noteworthy that during the system
operation, automatic switching between the three supply modes
is achievable.
Fig.5. Hardware prototype: (a) front view;(b) back view.
Fig.6. Experiment platform setup
Super Cap
3
EN
Voltage ( V )
2.5
Fig.8. Key waveforms of the power management circuit at IP = 17.8 A.
2
B. Experimental Platform Construction
The experimental platform constructed in this paper is
depicted in Fig.6.
1.5
1
0.5
0
90
95
100
105
110
115
Time (ms)
(a)
Super Cap
3
EN
Voltage ( V )
2.5
2
1.5
1
0.5
0
90
95
100
105
110
115
The 220 V power supply is transformed by a transformer to
drive high-power loads, thereby generating alternating current.
A CT measurement device can display the effective value of the
induced current in real-time. By adjusting the transformer, it is
possible to observe the magnetic field energy harvesting status
under different induced currents.
C. Experimental Results
Fig.7 illustrates the steady-state operation of the magnetic
field energy harvesting system under the condition of battery
load, with IP = 9 A and 13.4 A, respectively. The system
switches on intermittent power supply mode in case of
insufficient inductive current. Upon the connection of the load,
the induced current rectified and controlled by the MCU, along
with the supercapacitor, provides power until the
supercapacitor voltage drops to 2.88 V, at which point the load
is disconnected, and the system re-enters cold start. As the
induced current increases, the duration of power supply to the
load gradually extends, while the duration of load
disconnection shortens.
Time (ms)
(b)
Fig.7. Waveforms of the power management circuit when the current is
insufficient:(a) IP = 9A; (b) IP = 13.4A.
When the induced current grows to meet the battery
charging consumption, the energy harvesting system will
achieve continuous charging of the battery. As shown in Fig. 8,
the steady-state operation process of the system is carried out
when the value of the induced current is 17.8 A.
After the system completes the cold start, the enable signal
EN is high, the downstream circuit is turned on, and the MCU
controls the full-bridge rectifier, at which time the induced
current charges the supercapacitor. When the supercapacitor
voltage reaches 5.04 V, control signal A outputs a low potential;
MCU control is turned off; NMOS Drive signal outputs a high
potential; the short-circuit MOSFET is turned on, and the
supercapacitor charges the load battery until the supercapacitor
voltage drops to 3.88 V. When the supercapacitor voltage is
equal to 3.88 V, the NMOS Drive signal outputs a low potential,
and the short-circuit MOSFET is turned off. restart the MCU
control rectifier to charge the supercapacitor.
In the experiment, the initial voltage VB of the load battery
is 3 V and the maximum charging current is 20 mA. The power
consumption of the load circuit at the maximum charging
current can be found to be about 100 mW.
Neglecting the energy loss of the rectifier bridge in the
energy management circuit, the loss of the energy management
system is the energy released by the supercapacitor minus the
energy consumed by the load circuit. The calculation gives
WLOSS = 7.78mW. From this result, the minimum efficiency of
the energy management system can be approximated as 92.8%.
From the experiment results, it can be seen that the power
management circuit composed of supercapacitor and MCU can
not only achieve the self-starting of the system, but also
effectively ensure the power supply to the load, and its
efficiency can reach 92.8% when carrying out the continuous
power supply, which achieves the steady state operation of the
system. At the same time, the design provides continuous
power supply to the load battery for 24 s, which meets the
expected design requirements of more than 20 s.
V. CONCLUSION
In this paper, a novel and highly efficient power
management circuit is proposed for magnetic field energy
harvesting systems. that is different from the traditional energy
harvesting system. The proposed power management circuit
uses a supercapacitor as the energy storage element, an
improvement compared with the traditional design which uses
a battery. This design has a longer service life and saves costs
compared to the traditional battery backup. The system uses an
MCU and a supercapacitor to form a management circuit,
which not only enables the system to start autonomously but
also enables the energy harvesting system to switch between
different power supply modes under different levels of induced
currents. After further experimental verification, it is verified
that the system can achieve stable energy supply to the load
circuit. Its energy efficiency can reach 92.8% when carrying out
continuous power supply, and can achieve the longest 24
seconds of continuous power supply. The design proposed in
this paper not only can process energy at a lower primary
current but also effectively increases the efficiency for
magnetic energy harvesting with a relatively simple and robust
circuit design. The energy management circuit will promote the
development of sensing technology in smart power grids.
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