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Autonomous Temperature Sensor Based on a
Photovoltaic Energy Harvesting System
M. Ferri∗, D. Pinna∗, E. Dallago∗, P. Malcovati∗, G. Ricotti†∗Department of Electrical Engineering, University of Pavia, Pavia, Italy
†STMicroelectronics, Cornaredo (Milano), Italy
E-Mail: {massimo.ferri, daria.pinna, enrico.dallago, piero.malcovati}@unipv.it, [email protected]
Abstract—In this paper we present an autonomous tempera-ture sensor supplied by a on chip photovoltaic energy harvester.Then system is realized in a BCD SOI technology. The energyharvesting elements consist of a 34 trench-insulated p-n junctions,while the sensing system consists of a bandgap reference circuit,including an integrated high precision temperature sensor, anda high voltage low drop-out voltage regulator (LDO). The entiresystem operates also at low illumination levels and tolerates awide variation of the voltage produced by the micro-photovoltaiccell chain.
I. Introduction
Energy harvesting technologies and systems are emerging
as the new challenge in the research and industrial field,
growing at rapid pace. A wide range of applications can
involve energy harvesting technologies, including distributed
wireless sensor nodes [1]–[3] for structural health monitoring,
embedded and implanted sensor nodes for medical applica-
tions, battery recharging in large systems, monitoring environ-
mental parameters, monitoring tire pressure in cars, powering
unmanned vehicles, and running security systems in household
conditions. Modern ultra-low-power integrated circuits [4]–
[7] have reached such a level of integration and processing
efficiency that the power consumption of the electronics in
many applications is compatible with the amount of wasted
energy [8]–[11] available in the environment.
Breaking down the barriers of traditional sensors, wireless
devices based on energy harvesting eliminate long cable runs
as well as battery maintenance. Combining processors with
sensors, the wireless nodes can record and transmit data, use
energy in an intelligent manner, and automatically change their
operating mode as the application may demand. Harvesting
energy from the environment in the form of vibrations, strain,
or light, these devices use background recharging of a battery
or a super-capacitor to maintain an energy reserve. Recent
applications include piezoelectric powered damage tracking
nodes for helicopters as well as solar powered strain and
seismic sensor networks for bridges.
Photovoltaic phenomena [12], [13] allows us to retrieve the
highest amount of energy with respect to any other type of
harvesters, but when the source is the sun, power collecting
becomes an intrinsically discontinuous process, forcing the
adoption of storage elements in order to supply the system
during the dark period. Moreover the light energy source
usually features several noise components, such as the 50-
500 mV
1 V
17.5V
Bandgap LDO
TemperatureSensor
3.5 V
3.3 V
Vsensor
Fig. 1. Block diagram of the proposed system
Hz modulation of a light bulb, or, simply, the refraction and
absorption by air molecules.
In this paper, we present a photovoltaic energy harvesting
power source, realized in a 0.35-μm BDC SOI technology
[14], which supplies an autonomous temperature sensor. The
system, whose block diagram shown in Fig. 1, consists of a
series of 34 trench insulated p-n junctions, a bandgap reference
circuit, including an integrated high precision temperature sen-
sor, and a high voltage low drop-out voltage regulator (LDO).
The regulator allows us to deliver a constant 3.3-V power
supply voltage to a load also in low environment illumination
conditions, as the large number of micro-photovoltaic cells in
series ensures that, also considering a degradation of almost
80% of the voltage produced by each cell, the generated
voltage is enough to properly operate the system.
II. IntegratedMicro Solar Cells
In order to convert the incident light power into electrical
power, we designed a chain of 34 micro-photovoltaic cells.
Fig. 2 shows the structure of each photovoltaic cell, imple-
mented in a p-well insulated from the common p-substrate
by an oxide trench. The geometry of the n-diffusion realized
in the p-well consists of a series of short-circuited rows, thus
maximizing both the area and the perimeter of the diodes and
creating several photovoltaic structures connected in parallel.
The BCD SOI technology and the configuration realized allow
us to create series structures and provide a voltage higher
than 3.3 V, eliminating the parasitic diode between each
single cell and the chip substrate. In particular, since the open
circuit voltage Voc obtained for each illuminated cell is almost
530 mV, the entire chain can provide up to 17.5 V. This
voltage value ensures that the system can operate, ideally,
with an open circuit voltage as low as 20% of the nominal
value. This voltage reduction can be caused by a condition of
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275
N-Diffusion
P-Substrate
Trench
Single Solar Cell PhotovoltaicString
500 mV
1 V
17.5 V
Fig. 2. Block diagram of the integrated photovoltaic energy harvesting system
low illumination or by a large power request from the load.
Moreover, the series connection of photovoltaic cells allows
us to obtain directly all the reference voltages required for the
entire system. The geometrical dimensions of each cell are
385 μm × 245 μm. The width of the depletion region in the
p-n junction is given by
xdr =
√3εSiΦbi
q
(1
Na+
1
Nd
), (1)
where Na is the p-well doping concentration, Nd the n-
diffusion doping concentration, εSi is the dielectric permittivity
of silicon, q the charge of the electron and Φbi is the built-in
potential, given by
Φbi = VT ln
⎛⎜⎜⎜⎜⎝NaNd
n2i
⎞⎟⎟⎟⎟⎠ , (2)
where VT = kT/q is the thermal voltage. Substituting the
values of each variable in (1), xdr results equal to 3.2 μm.
In order to avoid overlapping between the depletion regions,
the width of the n-diffusion strips and the space among the
strips have been set to 5 μm.
In order to estimate the photogenerated current available
for the design of the system, we realized several micro-
photovoltaic cells in a 0.35-μm standard CMOS technology
on a test chip. The structure that we tested features as an
area of 0.5 mm × 0.5 mm. The power curve that we obtained
with 300 W/m2 of incident light power at 30 ◦C is shown
in Fig. 3 with a short-circuit current of 8.5 μA. Since the
doping concentrations can be assumed as the same in both
technologies, while the area of the BCD SOI photovoltaic
cell is almost 2.8 times smaller than the measured standard
CMOS photovoltaic cell, for the BCD SOI photovoltaic cell
we can estimate a short-circuit current (Isc) of about 2.5 μA.
Measurements on the realized BCD SOI photovoltaic cell are
in good agreement with this estimation. Indeed, Fig. 3 shows
also the power curve of the BCD SOI cell with 300 W/m2 of
incident light power and constant temperature of 30 ◦C.
0 100 200 300 400 500 6000
1
2
3
4
5
6
7
8
9
Photogenerated Voltage [mV]
Pho
toge
nera
ted
Cur
rent
[μA
]
Standard CMOSBCD SOI
Fig. 3. Power curve of photovoltaic cells realized in standard CMOS(0.25 mm2 of area) and BCD SOI (0.09 mm2 of area) technologies with300 W/m2 of incident light power
Vdd
VB,1
VB,2
M6
M4 M5
M7
M3M8
M1 M2
R1
R2
M9
Q1 (x16) Q2 (x2)
Vbandgap
Ban
dgap
Ref
eren
ce C
ircui
t
Ms
Tem
pera
ture
Sen
sor
Vsensor
Fig. 4. Schematic of the bandgap reference circuit
III. Bandgap Reference Circuit
The bandgap circuit is necessary to generate a temperature
independent voltage reference. The circuit operates on the
principle of compensating the negative temperature coefficient
of Vbe with the positive temperature coefficient of the ther-
mal voltage VT . The temperature coefficient of Vbe, at room
temperature, is −2.2 mV/◦C, while the positive coefficient
of the thermal voltage is 0.086 mV/◦C. Therefore a full
compensation, at room temperature, is obtained by combining
the two terms to achieve Vbandgap = Vbe +mVT , where m must
be equal to 25.6. If this condition is satisfied, the resulting
output voltage, approximately equal to 1.2 V, is at first order
temperature independent. Fig. 4 shows the schematic of the
bandgap reference circuit used in the proposed system. The
circuit is particularly critical, since it has to manage the vari-
ations of the supply voltage, due to illumination reduction or
output power changes, providing a constant reference voltage,
equal to 1.2026 V, to the LDO. The current that flows in
transistors M4 and M5 is mirrored in transistor M3, thus biasing
the two external branches with Id,M3= Id,M8
+ Ib,Q1+ Ib,Q2
.
Since Id,M4= Id,M5
, it results that Vgs,M6= Vgs,M7
. The bipolar
transistors, with emitter area ratio equal to 8, drain the same
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276
25 30 35 40
Temperature [°C]
45 50 55 601.2
1.22
1.24
1.26
1.28
1.3
1.32
1.34
Sen
sor
Out
put V
olta
ge [V
]
IdealExperimental
Fig. 5. Temperature sensor output voltage
current, leading to a ΔVbe = VT ln (8). The resulting current
flowing trough the bipolar transistors is
IR1=
VT ln (8)
R1
. (3)
At 27 ◦C IR1is equal to 1 μA. Since the same current is
mirrored in M3 and M8, the total power consumption of the
circuit is
Ptot =(Id,M3
+ Id,M4+ Id,M5
+ Id,M8+ Ic,Q1
+ Ic,Q2
)Vdd. (4)
The power supply voltage of the bandgap reference circuit
(Vdd) corresponds to the photogenerated voltage of the 7th
micro photovoltaic cell of the series chain (about 3.5 V). The
used bandgap reference circuit does not require any opera-
tional amplifier. The output voltage is fixed by the feedback
loop including transistor M8, which compensates any eventual
variation of Vbe,Q1,Q2. The cascade transistors M6 and M7 are
used to increase the gain of the loop. The voltage drop across
resistor Rs (Vsensor) is proportional to the absolute temperature
and it is used as temperature sensor in the proposed system.
Fig. 5 shows the value of Vsensor as a function of temperature.
The variation of Vsensor is closely related to the temperature
dependent current IR1, which is mirrored in the Rs branch. The
sensitivity achieved by the temperature sensor is 3.8 mV/◦C.
IV. LDO Circuit
The proposed LDO circuit is shown in Fig. 6. The circuit
consists of an error amplifier (M6, M7, M4, M5, M10), an
output stage (M1, M2, M8, M9, Ma, Mb), a pass transistor
(Mc) and a resistive divider (R1, R2). The circuit is basically
an operational amplifier with resistive feedback. The output
voltage (VLDO) is an amplified version of the bandgap voltage
(Vbandgap). Transistor M1 mirrors the current of the bandgap
circuit, biasing the output stage composed of transistors M8
and M9.The power supply voltage of the error amplifier is
the photogenerated voltage of 7th photovoltaic cell series
chain (Vdd), while the output stage and the pass transistor
are supplied by the maximum voltage generated by the pho-
tovoltaic cell chain, corresponding to the 34th cell (Vdd,high).
The measured value of Vdd,high is 17.44 V. Transistors M1 and
M8 are standard transistors with a breakdown voltage between
drain and source Vds,break equal to 3.3 V. Therefore, in order
to protect them from the high voltage, we used a cascode
topology realized with the high-voltage DMOS transistors Ma
and Mb. These DMOS transistors, indeed, can withstand up
Vbandgap
Vdd
Vdd
VB,2
VB,1
Vdd
Vdd,high
M2 M9
M10
Ma Mb
Mc
M4 M5
M1 M6M7 M8
R1
R2
VLDO
IL
Fig. 6. Schematic of the LDO circuit
Fig. 7. Layout of the system, corresponding to the microphotograph reportedin Fig. 1
to 80 V of Vds. Transistor Mc is a DMOS too, in order to
avoid breakdown in high illumination conditions (or low load
power request). The transfer function of the LDO circuit can
be written asVLDO
Vbandgap=
A1 − Aβ
, (5)
where
A = gm,M5
(rds,M5
||rds,M7
)gm,M8
rds,M9, (6)
and
β =R2
R1 + R2
. (7)
Assuming Aβ � 1, we obtain
VLDO =
(1 +
R2
R1
)Vbandgap, (8)
and, hence, starting from a bandgap reference voltage of 1.2 V,
we obtain an output voltage equal to 3.3 V. The total current
consumption of the proposed LDO is 3.3 μA, while the total
power consumption depends on the incident light.
V. Measurement Results
The proposed system has been implemented in a 0.35-
μm BCD SOI technology. The chip area is 4 mm2. Fig. 7
shows the layout of the realized chip, corresponding to the
microphotograph reported in Fig. 1. Fig. 8 shows the short-
circuit current (Isc) as a function of the incident light power.
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277
0 100 200 300 400 500 600 700 800 9000
1
2
3
4
5
6
Incident Light Power [W/m2]
Pho
toge
nera
ted
Cur
rent
[μA
]
Fig. 8. Short-circuit current (Isc) as a function of the incident light power
0 0.5 1 1.5 2 2.5 3 3.52
2.5
3
3.5
4
Load Current (IL) [μA]
Reg
ulat
ed V
olta
ge (
VLD
O)
[V]
Fig. 9. Regulated voltage (VLDO) as a function of the load current (IL) with600 W/m2 of the incident light power
The regulated voltage (VLDO) as a function of the load current
(IL) is shown in Fig. 9. The system delivers a regulated
output voltage of 3.3 V for load currents up to 500 nA with
600 W/m2 of the incident light power (3.3 μA are consumed
by the LDO circuit). The value of IL can be increased simply
by using micro-photovoltaic cells with larger area or by
associating to the system an accumulation device (battery or
supercapacitor). Tab. I summarizes the transistors dimensions,
while Tab. II reports the achieved performance in simulation
and measurements.
TABLE ITransistor Dimensions
Transistor W [μm] L [μm] Circuit Block
M2, M9 6 4 LDOMb, Ma 6 2 LDOMc 24 2 LDOM1, M3, M10 10 4 LDOM4, M5 10 2 LDOM6, M7 4 4 LDOM8 0.5 0.35 LDOM3, M4, M5, M8 10 4 BandgapM6, M7, M9 5 2 BandgapM1, M2 5 4 BandgapMs 20 4 Sensor
TABLE IIPerformance Summary
Parameter Simulation Measurement
Bandgap voltage (Vbandgap) 1.199 V 1.202 VOutput voltage (VLDO) 3.357 V 3.369 VVdd,high 17 V 17.4 VVdd 3.5V 3.8 VTemperature sensor sensitivity 4.2 mV/◦C 3.8 mV/◦CMaximum load current (IL) — 500 nA
VI. Conclusions
In this paper we presented an autonomous temperature
sensor with a photovoltaic energy harvester and a voltage
regulator. The voltage regulator consists of a bandgap ref-
erence circuit and a high voltage LDO circuit. The realized
chip has been extensively simulated and measured, showing a
good agreement between simulated and experimental results.
Presently, we are designing an integrated solution in order to
obtain a completely autonomous wireless sensor node.
Acknowledgments
This work was supported by the Italian Ministry of Uni-
versity under FIRB project RBAP065425 “Analog and Mixed-
Mode Microelettronics for Advanced Systems”. The BCD SOI
technology has been provided by the R&D Department of
STMicroelectronics, Cornaredo (Milano), Italy..
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