Power recovery circuit for Battery-less TPMS Yong Li, Liji Wu, Chun Zhang, Zhihua Wang Institute of Microelectronics, Tsinghua University Beijing, China 100084 Email:
[email protected] Abstract-A 13.56MHz HF power recovery
wave battery-less wireless sensor method, in
circuit was designed for Battery-less Tire
which electromagnetic wave is emitted from
Pressure Monitoring System (TPMS) or wireless
outside of the tire. Such wave is reflected when
sensor networking applications using Chartered
it touches the in-tire module, and the pressure
0.35-μm
data can be brought back.
CMOS
technologies.
It
converts
received HF power to DC and charges a large
The architecture of the second solution is
capacitor, then supplies stable voltage output for
shown in Figure 1, including 13.56MHz signal
other modules. Simulation results show that the
transmitter, tire monitoring module and central
circuit operates normally at electromagnetic field
controlling module. This design uses inductance
strength with induced voltage above 0.3v. The
coupling to implement battery-less TPMS. The
circuit also generates an enable signal for digital
13.56MHz signal transmitter which is installed
circuits when the capacitor has been charged to a
on the automobile generates electromagnetic
certain extent.
field, the receiver antenna which is installed in the tire receives the energy through inductance coupling and provides electric power for the tire
1. Introduction Tire Pressure Monitoring System (TPMS)
monitoring module. As a crucial component of
can be utilized to watch over the tire pressure in
battery-less TPMS, the battery recovery circuit is
a real-time manner when the automobile is
discussed in detail in this paper.
running. It can warn the driver of low tire pressure due to leakage as well as high pressure due to high temperature, thus the safety of driving is ensured. Battery TPMS has already been in use, but the usage of batteries induces severe drawbacks as follows: limited lifespan of batteries,
unable
to
perform
real-time
surveillance, no guarantee of stability and reliability. If the in-tire module can be implemented in a battery-less way, then the aforementioned problems can be solved. There
Figure 1. Architecture of battery-less TPMS
are three solutions to realize battery-less TPMS: (1) Piezoelectricity method, in which the in-tire module has a power-generating scheme with it
2. Architecture of Power Recovery Circuit
which transfers the mechanic energy of the
The power recovery circuit is responsible of
rolling tire into electricity. (2) Electromagnetic
generating stable DC voltage by filtering and
coupling method, in which electromagnetic
stabilizing the AC voltage received from the
wave is driven into the tire to provide the in-tire
antenna; such DC voltage works as the power
module with electricity. (3) Surface acoustic
supply
of
other
modules
in
the
tire.
Consequently, power recovery circuit is one of
at its working frequency of 13.56MHz, CS works
the essential components of battery-less TPMS.
as both filtering capacitor and energy storage
In the design, the receiver antenna is fixed inside
capacitor, thus a large load is ensured.
of the tire which is dozens of centimeters from the transmitter, so the AC voltage of the received signal is not high. Under such circumstance, it is difficult to meet the requirements of output voltage ranging between 2.2V and 3.6V and output current up to 7mA with the use of a common power recovery circuit. As a result, new architecture must be utilized to meet the demand. Battery-less TPMS has a few dozen milliseconds
Figure 3. Full-wave rectifying circuit
for measuring and transmitting the data for one complete working period, and measures the
3.2 Bandgap reference circuit
parameters such pressure and temperature of the
At the temperature 27ºC, the temperature
tire every few seconds. Therefore, we should
coefficient of VEB is approximately -2.2mV/ºC,
make the power recovery circuit discharge and
while such coefficient of VT is +0.086mV/ºC.
provide power for other modules for those
Due to the reverse value of these two
milliseconds, while for the rest of the time it
coefficients, we can use the linear sum of them
should charge the big capacitor. At the same time,
to achieve output voltage of zero temperature
an enable signal is produced to control other
coefficient. CMOS bandgap reference circuit is
modules at two different working modes: run
shown in Figure 4. The reference voltage output
and idle. The block diagram of the power recovery circuit is shown in figure 2.
VR =
R2 VT ln n + VEB 3 R1 , in which the first
is addend has a positive temperature coefficient
while the second has a negative one. By carefully selecting R1, R2 and n to make the absolute value of these two terms equal, we can have a reference voltage with zero temperature coefficient. The circuit utilizes cascode current Figure 2. Block diagram of power recovery
mirror to enhance power supply rejection ratio
circuit
(PSRR), so we have stable output voltage even if the power supply voltage varies within a wide
3. Circuit Design
range. The output voltage of OPAMP works as
3.1 Rectifying circuit
driving voltage, and also provides biasing for the
The
antenna
inductance
receives
the
OPAMP itself. The EB junction voltage drop of
coupling signal. The signal resonates with the
the
capacitor and is then sent to the rectifying circuit.
compatible with CMOS technology provides
Finally it is processed by the filtering capacitor
another biasing. N4, N5, P11, P12, P13 comprise
to gain the working voltage of the circuit. In
the startup circuit. N4 is on upon power up.
order to enhance the rectifying efficiency,
Once the circuit settles down, the gate voltage of
full-wave rectifying circuit is adopted in the
N4 is pulled down and the transistor is cutoff.
design. Diode-connected CMOS transistors (N1, N2, P1 and P2) form the rectifier. LC resonates
PNP transistor,
whose
technology
is
the load. So the pass element must be large enough to obtain high output current and low dropout voltage. Regular N-type pass devices require gate voltage to be higher than their source output. And it makes low-voltage output and lower power efficiency. However, the stability of the kind of LDO can be easily achieved compared with a PMOS transistor as the pass element. The feedback network produces the output voltage to be compared with Figure 4. Bandgap reference circuit
the voltage reference. This voltage is produced by a voltage divider described by the following
3.3 Low dropout regulator equation:
Vout = Vref (1 +
R1 ) R2 .
As
above
equation shows, the value of Vout can be controlled by changing the ratio R1/R2. 3.4 Lower power schmitt trigger circuit
Figure 5. Low dropout regulator Low Dropout Regulators are composed of four basic components: a voltage reference, an error amplifier, a pass element, and a feedback network. Figure 5 shows a kind of LDO, in which the pass element is an NMOS transistor. The
voltage
reference
required
Figure 6. Lower power schmitt trigger circuit
by
regulators, is set by a bandgap circuit with poor
The main application of schmitt trigger
load driving capability to provide precise voltage
circuit in the design is to monitor the voltage
output of the regulator. An 1.2V voltage bandgap
level of storage capacitor and produce enable
reference is used in the design. This voltage is
signal. Generally speaking, the aspect of low
fed to the error amplifier and compared with the
power operation is not considered because this
output feedback through two resistors in series.
aspect is irrelevant to the targeted application.
The error amplifier produces an error signal
Such circuits are suitable for medium power
when the feedback output differs from the
applications, but the emergence of applications
reference voltage. The error output is used to
which can be sustained for many years on the
control the amount of current into the load and
energy available from a small battery or from a
set the value of output voltage to a level where
rectified RF signal, means that it’s more
the error signal is close to zero.
advisable to develop and use low power circuits.
The pass element provides the output
One prototype of lower power CMOS
current needed to drive the load. In the design,
schmitt trigger circuit is shown in Figure 6. The
the pass device supplies a maximum of 10mA to
operation of the circuit can be described as
follows. When at first Vin is low, N1 is OFF, P1
recovery circuit can work in other types of
is ON, which causes N2 to be ON, P2 OFF, and
wireless sensor networks to provide required
N3 OFF. As the input voltage rises, the output
power supply.
switches from low to high depends on the voltage at drain node of N3 and switching point
References
of the inverter structure which consists of N1
[1] “Intelligent tire Systems – State of the Art
and P1. The Schmitt trigger circuit is buffered
and Potential Technologies,” The APOLLO
with two inverters in order to drive the output
consortium, IST-2001-34372, May, 2003
node.
[2] Jianyun HU, Hao MIN, “A Low Power and High Performance Analog Front End for Passive RFID Transponder”
4. Simulation results The power recovery circuit is simulated
[3] Mohamad Sawan, Yamu Hu, Jonathan
under the Chartered 0.35-um technology. In the
Coulombe,
simulation, DC voltage source, resistor and
Bidirectional Data Exchanged in Smart
capacitor are used to simulate the charging
Medical
circuit. Result of simulation shows that when the
CUSTOM
voltage of capacitor reaches 7.23v, it would
CONFERENCE
activate the enable signal. The circuit drives the
[4]
Pedro
“Wirelessly
Powered
Microsystems,”
M.
IEEE
INTEGRATED
and 2005
CIRCUITS
Alicea-Morales,
Carlos
J.
load until the voltage of capacitor drops to 5.5v,
Ortiz-Villanueva,
then discharge ends and charging begins again.
Palomera-Garcia, Manuel Jiménez, “Design
At the output load of 60uA and 7mA, output
of an Adjustable, Low Voltage,Low Dropout
voltage fluctuates between 2.458v and 2.459v,
Regulator,” Proceedings of the Fifth IEEE
performing a high level of stability.
International Caracas Conference on Devices,
Raúl
Pérez,
Rogelio
Circuits and Systems, Dominican Republic, Nov.3-5, 2004 [5] M.A.T. Sanduleanu, A.J.M. van Tuijl, and R.F.
Wassenaar,
“Accurate
low
power
bandgap voltage reference in 0.5um CMOS technology,” Electronics Letters, vol. 34, May, 1998. [6] A. Varzaghani, “Bandgap voltage reference design,” Technical Report, Emad Semicon, 2000 Figure 7. Simulation result of power recovery circuit
[7] S.F. Al-Sarawi, “Low power Schmitt trigger circuit,” ELECTRONICS LETTERS, August 2002, VOL. 38. NO. 18, pp.1009-1010
5. Conclusions The power recovery circuit proposed in this
[8] I. M. Filanovsky, H. Bakes, “CMOS Schmitt Trigger Design,” IEEE TRANSACTIONS
paper can be adopted in battery-less TPMS with
ON
high current consumption. Without the use of
FUNDAMENTAL
battery TPMS systems, data monitoring and
APPLICATIONS, VOL. 41. NO. 1, JAN.,
transmitting can be accomplished with higher
1994
efficiency, and the safety of driving is ensured. As a power generating module, the power
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AND
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