A 30mV input battery-less power management system

Bulletin of Electrical Engineering and Informatics
Vol. 8, No. 4, December 2019, pp. 1169~1179
ISSN: 2302-9285, DOI: 10.11591/eei.v8i4.1614  1169
Journal homepage: http://beei.org/index.php/EEI
A 30mV input battery-less power management system
Jim Hui Yap, Yan Chiew Wong
Micro and Nano Electronic (MiNE) research group, Centre for Telecommunication Research and Innovation (CeTRI),
Fakulti Kejuruteraan Elektronik dan Kejuruteraan Komputer (FKEKK), Universiti Teknikal Malaysia Melaka, Hang
Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
Article Info ABSTRACT
Article history:
Received Mar 29, 2019
Revised Apr 24, 2019
Accepted Jun 30, 2019
This paper presents a fully-integrated on chip battery-less power
management system through energy harvesting circuit developed in a 130nm
CMOS process. A 30mV input voltage from a TEG is able to be boosted by
the proposed Complementary Metal-Oxide-Semiconductor (CMOS) voltage
booster and a dynamic closed loop power management to a regulated 1.2V.
Waste body heat is harvested through Thermoelectric energy harvesting to
power up low power devices such as Wireless Body Area Network. A
significant finding where 1 Degree Celsius thermal difference produces a
minimum 30mV is able to be boosted to 1.2V. Discontinuous Conduction
Mode (DCM) digital control oscillator is the key component for the gate
control of the proposed voltage booster. Radio Frequency (RF) rectifier is
utilized to act as a start-up mechanism for voltage booster and power up the
low voltage closed loop power management circuit. The digitally control
oscillator and comparator are able to operate at low voltage 600mV which
are powered up by a RF rectifier, and thus to kick-start the voltage booster.
Keywords:
Battery-less power management
CMOS voltage booster
Dynamic closed loop power
management
RF Rectifier
Thermoelectric energy
harvesting Copyright © 2019 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Yan Chiew Wong,
Micro and Nano Electronic (MiNE) research group,
Centre for Telecommunication Research and Innovation (CeTRI),
Fakulti Kejuruteraan Elektronik dan Kejuruteraan Komputer,
Universiti Teknikal Malaysia Melaka,
Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.
Email: ycwong@utem.edu.my
1. INTRODUCTION
Energy harvesting topic has been widely discussed as it has become a potential alternative way to
replace conventional battery. This is due to the demand of low power, small size and maintanence free
operation of electronic devices has been increased. Wireless Body Area Network (WBAN) is one of the
applications that expected to operate for long durations but energy supply becomes a severe bottleneck and
much effort has been spent on the efficient use of battery energy. A bulky battery is not desirable as the size
making the node unwearable. While a small battery needs a frequent charging and replacement which reduce
the wearable compliance. Another example is the pacemaker. One of the main problems about pacemakers is
their batteries. As the capacity of the batteries is limited, they limit the lifetime of pacemakers [1]. In
addition, implantable medical devices and stock tracking devices face a difficulty to do placement of battery
beneath the body as the leakage of mercury of the battery has raise to health issue. A battery is not
recommended to be placed inside the human body as it may contact with blood (such as pH sensor) [2].
Thermoelctric energy harvesting field has become one of the great interests recently. Thermoelectric
Generator (TEGs) is a solid state device converts the temperature gradient into electrical energy. A
thermoelectric generator consists of an array of pairs of semiconductor materials connected electrically in
series and thermally in parallel [3]. TEG works on the principle of the Seebeck effect, when the junctions
formed by joining two dissimilar current carrying conductors are maintained at different temperatures, an

 ISSN: 2302-9285
Bulletin of Electr Eng and Inf, Vol. 8, No. 4, December 2019 : 1169 – 1179
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electro motive force (EMF) is generated in the circuit. The two dissimilar current carrying conductors are
formed out of n and p-type semiconductor materials [4]. TEG utilizes heat flow across a temperature gradient
to power an electric load through the external circuit [5]. When the open circuit voltage is proportional to
temperature gradient, the constant of proportionality is called Seebeck coefficient [6]. Therefore, recovering
from the waste heat energy like body heat to electrical energy is one of the solutions to replace batteries. The
Fraunhofer scientists believe that future medical sensors may be able to draw the power they need from the
warmth of the human body, with the data transmitted wirelessly to a central monitoring station [7]. In
ambient environments,where the thermal difference is 1-2K, TEG generates several micro watts and tens of
mV to the boost converter [8]. A thermoelectric energy generator is able to generate an output voltage of
40mV with a thermal gradient of 6°K [9]. It is a challenging task to convert such a low voltage to a DC
voltage in order to power current electronic circuitry [10]. In this paper, thermal energy harvesting circuit is
mainly discussed. Body heat is converted into useful electrical energy via some of the key circuitries that
forms the battery-less power management system. The innovations in CMOS technology and smart circuit
design have led into an era of ultra-low power systems which in-turn enabled the batteryless systems [11].
Voltage booster is the highlighted circuitry as it plays a role to boost millivolts from output of TEG to a
certain desire voltage.The power management system is completed with voltage kick-starter from RF
rectifier, load regulation circuitry from low dropout voltage regulator, and power on reset. To design a boost
converter with self-start-up at low input voltage, designers need to trade off the choice of circuit topology,
external component and CMOS process [12].
2. RESEARCH METHOD
Two different sizes of commercial thermoelectric generator modules TEG 1 and TEG 2 as shown in
Figure 1 have been compared. The characterization has been performed by applying temperature difference
between hot and cold side of module and measuring the output open circuit voltage. The data of the
behaviour of TEG towards temperature difference which is collected through a simple thermoelectric
generator experiment is presented in Table 1.
(a) (b)
Figure 1. Measurement of, (a) TEG 1(30 ), (b) TEG2 (
Table 1. Behaviour of TEG towards temperature difference
TEG 1
T1 (° C) T1 (° C) T1 (° C) T1 (° C)
27.4 27.4 27.4 27.4
26.2 26.2 26.2 26.2
24 24 24 24
25.6 25.6 25.6 25.6
25.7 25.7 25.7 25.7
25.2 25.2 25.2 25.2
28 40 12° C 259
TEG 2
T1 (° C) T2 (° C) ΔT (° C) Vout (mV)
30 31 1° C 3
27 29 2° C 10
27.4 33.4 4° C 14
25 31 6° C 23
31 39 8° C 33
38 48 10° C 52
38 51 12° C 59
The main circuit blocks to form a battery-less power management system is discussed in detail in
section 3. Figure 2 shows the block diagram of proposed energy harvesting circuitry. The challenging part in
developing and designing analog integrated circuit is by defining its key parameters such as the transistor„s
width and length. In order to ensure the transistor is operating in an optimize region, parametric study and
sizing process have to be carefully defined to avoid mismatching. Study on [13] to [14] to make sure the
performance of measurement is good.

Bulletin of Electr Eng and Inf ISSN: 2302-9285 
A 30mV input battery-less power management system (Jim Hui Yap)
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Figure 2. Power management system
3. RESULTS AND ANALYSIS
As expected, when the temperature difference increases, the open circuit output voltage of TEG
increases. Even though the temperature difference is the same, the surface area is the key that affecting the
amount of output voltage produced. From Figure 3, it shows that TEG 1 has produced higher output voltage
compared to TEG 2 in the same temperature gradient.
Figure 3. Comparisons of TEG 1 and TEG 2 towards temperature difference
3.1. Voltage booster
Voltage booster is a DC-DC converter to step up small signal DC to an output DC signal which is
greater than input signal. A conventional power switch based voltage booster creates higher static and
dynamic power losses of the switches which cannot be neglected. By replacing power switches with diode is
another way but using diode at high side switch causes a high forward voltage drop. MOSFETs are good
alternative components to replace power switches and diode in the conventional voltage booster. In order to
minimize the conduction loss, very large size of NMOS and PMOS switches are required and external
inductor with small DCR should be chosen [15]. This can reduce the power losses as well as the size
consumption in a chip. Voltage booster operates in one of the two modes: continuous conduction mode
(CCM) or discontinuous conduction mode (DCM) [16]. The limitation of the CCM is that the inductor
current can flow reverse way if the load is small. While in DCM, the inductor current is prevented to flow
backward which it minimizes the switching loss.
The architecture presented in the paper is inductor based voltage booster. In [17], the primary
converter provides a stable output voltage of about1.2 V achieved by output node hysteresis PFM control.
However, the system does not have any power management functionality. The proposed inductor based
voltage booster in Figure 4 operates well with a minimum 30mV from the output voltage of TEG with the
operation of DCM. The DCM is more efficient than the CCM as the PMOS turns off before the current flows

 ISSN: 2302-9285
1172
negative. The discontinuous conduction mode (DCM) operation is preferred because it allows the use of
small inductors where this can reduce the area consumption of power management unit in portable devices
[18]. In DCM, energy is stored in the inductor when switch is closed and all energy is transferred to load
when switch is opened. The voltage booster is able to boost 30mV to 1.2715V with current drawn 2.1193uA
under 600kΩ load condition. Ideal gate control pulse for inductor based converter as shown in Figure 5.
Figure 6 is transient response of voltage booster.
Figure 4. Inductor based voltage booster
Figure 5. Ideal gate control pulse for inductor based converter
Figure 6. Transient response of voltage booster
3.2. Digitally control oscillator
A basic control circuitry which consists of pulse signal with a fix frequency and duty cycle is
needed to drive the switches. The precise timing of turning on and off the switches is critical for high

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conversion efficiency [19]. In [20], the control circuitry is all digital and with the efficient Zero Current
Switching (ZCS) circuit that supports a wide dynamic range and fine tuning of delay stages for zero current
switching using only 3-bit design. The advantage of this approach is where the detection of ZCS point is
evaluated using a simple D-FF where VD level state is detected after the PMOS is turned off [16]. In [21],
the voltage booster is integrated with the digitally control circuit. The control circuit consists of power FET,
voltage supervisor, voltage divider, oscillator, frequency control, one shot, VDD filter, and zero compare
block. Example in [22], the designer utilize the Maximum Power Point Tracking and ZCS schemes to
improve the efficiency of converter.
The proposed digitally control oscillator consists of five sub-blocks which are self-generated ring
oscillator, clock generator, shifted register ring oscillator, frequency divider and one shot. The clock is
enabled when the signal from comparator transits to high. The shifted register ring oscillator consists of four
D flip-flop to generate more than 50% duty cycle square wave. This shifted square wave is then entering the
frequency divider. Frequency divider is used to control the wide frequency of the input pulse from the shifted
register ring oscillator. The one shot circuit sets the on time of the switching of PMOS. Figure 7 shows the
block diagram of digitally control oscillator. This digitally control oscillator produces 1.136MHz of pulse to
the gate of PMOS and NMOS for inductor based converter. As shown in schematic, the output of frequency
divider enters NMOS gate while the output of one shot enters PMOS gate.
Figure 7. Digitally control oscillator
Figure 8 shows the transient result of digitally control oscillator. The red colour square waveform is
produced from the frequency divider. While the blue colour square waveform is produced from one shot
block. The magnitude of the gate pulse is obtained as expected which is 600mV.
Figure 8. Transient response of digitally control oscillator
3.3. Low voltage comparator
The comparator is a low voltage comparator as it is able to operate at 600mV. It is an essential block
in the proposed dynamic closed loop power management as it regulates the output of voltage booster at
QPI
QP2

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certain level. All circuits are idle until the comparator activates the on-chip clock. If the output voltage of the
converter drops or rises across the reference voltage, the comparator will send an active signal to an AND
gate to do a comparison with self-generated pulse before it enters the clock generator to power on the rest of
the digital logic circuits. Figure 9 shows the block diagram of the comparator.
Figure 10 shows the transient response of comparator with voltage divider. An ideal pulse switching
has been used to model the output voltage of voltage booster. Blue colour line is the reference voltage
produced by the voltage divider while the red colour line is the response of comparator. From the graph, it
shows that when the output voltage of voltage booster drops below the reference voltage, the comparator
transits high and vice versa.
Figure 9. Comparator
Figure 10. Transient response of comparator
3.4. RF rectifier
In the previous research, the start-up solution for voltage booster is through a pre-charge load
capacitor which has been implemented in [21]. The disadvantage of using a storage element is that converter
can be disabled for long periods of time. One of the major factors limiting the electrical startup is the
threshold voltage of MOSFETs [23]. Thus, in this work, the RF rectifier has been used to kick start the
voltage booster. From [24] and [25], it is known that diferent topologies affect its sensitivity and efficiency
especially for a better energy harvesting system. In this paper, the rectifier operated at 2.45GHz with incident
RF power at 0dbm. The cross-coupled rectifier is used to have a better threshold voltage reduction as
compared to the conventional rectifier. An L-matching circuit is constructed to support fully differential RF
input. The entire RF rectifier is an on-chip component except the balun component. Figure 11 shows the
diagram of RF rectifier with ideal balun. The balun is the only off chip component in the RF rectifier. The
balun is used to create a fully differential RF signal before entering the matching circuit.
In Figure 12, the AC source is able to be converted into DC voltage for 5 stages rectifier. With the
incident RF power of 0dBm, the output voltage reaches steady state response at 8 micro second with 3.6519V
with no load condition.

1175
(a) (b)
Figure 11. RF rectifier, (a) RF rectifier with ideal balun, (b) single stage rectifier
Figure 12. Transient response of rectifier
3.5. Dynamic closed loop power management
The design of the dynamic closed loop power management in this work is the integration of
rectifier, digitally control oscillator, comparator, voltage reference and inductor based voltage booster. By
referring to Figure 2, it shows the integrated circuit of dynamic closed loop power management. The dynamic
closed loop power management starts to operate when there is sufficient power from RF energy for digitally
control oscillator and comparator. The entire dynamic closed loop power management is a feedback loop
between the output voltages. The comparator transits to high when the output voltage of voltage booster is
below 1.2V. Once the output of voltage booster reaches 1.2V, it triggers the comparator to disable the
digitally control oscillator.
The response of the dynamic closed loop power management is shown in Figure 13. The system
takes 110us for start-up and start to stabilize at 1.2V after 110us. While when the output voltage drops to
1.15V, the voltage booster starts to charge up again.
3.6519V
RF_POS
RF_NEG

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Figure 13. Regulated output voltage from dynamic closed loop power management
3.6. Low dropout voltage regulator
An ultra-low power CMOS Low-dropout voltage regulator is deployed to regulate the given output
voltage to avoid the potentially damaging over discharge of the circuit. This LDO comprises a bandgap
reference, error amplifier, a feedback-voltage divider, and a pass device. Figure 14 shows the schematic
design of LDO. In Figure 15, the LDO manage to regulate 1.2V when sweep with input supply from 1.2V
to 2.5V.
Figure 14. LDO
Figure 15. DC analysis
1.2V

1177
3.7. Power on reset
A low voltage Power on Reset (PoR) has been developed in this work which can operate at 1.2V.
PoR generates a reset pulse signal when it detects a power down trigger in the system until input voltage is
stabilized. Figure 16 shows the schematic of PoR. Figure 17 illustrates the behaviour of PoR when there is
power down in the system, the PoR is activated to high to reboot again the system. The power on reset is off
when the input voltage reaches stable state.
Figure 16. PoR
Figure 17. Transient response of PoR
3.8. Layout of battery-less power management system
Figure 18 shows the entire layout of battery-less power management system which includes of
rectifier, voltage booster, digitally control oscillator, comparator, voltage reference, LDO and PoR. The area
size of this integrated layout is 1.005928 .

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Figure 18. Layout of battery-less power management system
4. CONCLUSION
A significant finding in this work is where one degree Celsius temperature difference between body
heat and ambient temperature is able to provide 30mV. The battery-less power management system is able to
produce a regulated 1.2V from 30mV harvested energy from TEG by having a dynamic closed loop power
management system. The optimum frequency of the pulse to control the switching period in inductor based
voltage booster is in a range of 666.67 kHz to 1.136MHz with a duty cycle of 50%. Instead of using external
electrical start-up, RF rectifier is sufficient to power up the circuitry in dynamic closed loop power
management. This work demonstrates feasibility of self-powered power management system by utilizing
thermal energy and RF energy to obtain a stable 1.2V from 30mV harvested from ambient environment.
ACKNOWLEDGEMENTS
The authors acknowledge the technical and financial support by Universiti Teknikal Malaysia
Melaka (UTeM) and Ministry of Science, Technology and Innovation Malaysia‟s grant no.
01-01-14-SF0133//L00029.
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BIOGRAPHIES OF AUTHORS
Jim Hui Yap received the B.Eng. degree in Electronics from Universiti Teknikal Malaysia
Melaka in 2018.She is currently pursuing the M. Sc. Degree at the Faculty of Electronics and
Computer Engineering, Universiti Teknikal Malaysia Melaka (UTeM), Malaysia. Her research
interests include CMOS voltage booster and low power application.
Yan Chiew Wong received her doctorate from The University of Edinburgh, United Kingdom on
2014.She is currently a senior lecturer in Universiti Teknikal Malaysia Melaka. She has more
than 5 years industry experience in semiconductor process design in Infineon Technologies and
Intel. Her research field involving of RF and mixed signal circuitries, adaptive impedance
matching and power management control system. She also produced a number of research
papers in the field of adaptive algorithm, RF reconfigurable design, EDA and etc. She has been
successfully design and develop high voltage CMOS control chips and RF circuitries.

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