True Grid Independence: Robust Energy Harvesting System for Wireless Sensors Uses Piezoelectric Energy Harvesting Power Supply and Li-Poly Batteries with Shunt Charger

True Grid Independence: Robust Energy Harvesting System
for Wireless Sensors Uses Piezoelectric Energy Harvesting
Power Supply and Li-Poly Batteries with Shunt Charger
George H. Barbehenn
There is an emerging and potentially large market for wireless sensors. By their very
nature, wireless sensors are chosen for use in inaccessible places, or for applications
that require large numbers of sensors—too many to easily hardwire to a data network. In
most cases, it is impractical for these systems to run off primary batteries. For example,
a sensor for monitoring the temperature of meat as it is shipped would need to be
mounted in a tamperproof way. Or, HVAC sensors that are mounted on every source of
conditioned air would be far too distributed to feasibly use batteries. In these applications,
energy harvesting can solve the problem of providing power without primary batteries.
Energy harvesting alone often does not
produce sufficient power to continuously run the sensor-transmitter—energy
harvesting can produce about 1mW–10mW,
where the active sensor-transmitter
combination may need 100mW–250mW.
Harvested energy must be stored when
possible, ready for use by the sensor/transmitter, which must operate at duty cycle
that does not exceed the energy storage
COMPLETE ENERGY
HARVESTING SYSTEM
capabilities of the system. Likewise, the
sensor/transmitter may need to operate
at times when no energy is harvested.
Figure 1 shows a complete system implementation using an LTC3588-1 energy
harvester and buck regulator IC, two
LTC4071 shunt battery chargers, two
GM BATTERY GMB301009 8mAh batteries and a simulated sensor-transmitter
modeled as a 12.4mA load with 1%
duty cycle. The LTC3588-1 contains a
very low leakage bridge rectifier with
Finally, if the stored energy is depleted
and the system is going to shut down, the
system may need to carry out housekeeping tasks first. This may include a shutdown message, or storing information in
nonvolatile memory. Thus, it is important
to continuously gauge available energy.
15k
Figure 1. Complete piezo-based energy harvesting
system is independent of the grid. This design uses
thin film batteries to gather energy collected by
the piezo for a wireless sensor transmitter, which
operates on a 1% duty cycle.
ADVANCED CERAMETRICS PFCB-W14
D1
MMSD4148T1
1µF
6V
22µF
16V
PZ1
PZ2
VIN
PGOOD
CAP
LTC3588-1
VOUT
VIN2
4.7µF
6V
D1
D0
10k
PGOOD
10µH
SW
VCC
NTCBIAS
VOUT
3.3V
100µF
6V
LTC4071
NTC
HBO1
BAT
NTC1
10k
GND
GND
BAT1
VCC
NTCBIAS
10k
LOAD
12.4mA
1% DUTY CYCLE
LOAD
LTC4071
NTC
NTC2
10k
GND
BAT1, BAT2: GM BATTERY GMB301009 Li-Poly
NTC1, NTC2: VISHAY NTHS0402E3103LT
36 | October 2010 : LT Journal of Analog Innovation
HBO
HBO
HBO2
BAT
GND
BAT2
design ideas
With a few easy-to-use components, it is possible to
build a complete compact energy-harvesting power
subsystem for wireless sensor-transmitters.
inputs at PZ1 and PZ2 and outputs at
VIN and GND. VIN is also the input power
for a very low quiescent current buck
regulator. The output voltage of the buck
regulator is set by D1 and D0 to 3.3V.
The LTC3588 is driven by an Advanced
Cerametrics Incorporated PFCB-W14
piezoelectric transducer, which is capable
of generating a maximum of 12mW.
In our implementation, the PFCB-W14
provided about 2mW of power.
IAVG =
VIN
2V/DIV
IAVG =
ILOAD
5mA/DIV
0V, 0A
4ms/DIV
Figure 2. Charging with sensor-transmitter load
The LTC4071 is a shunt battery charger
with programmable float voltage and
temperature compensation. The float
voltage is set to 4.1V, with a tolerance
on the float voltage of ±1%, yielding
a maximum of 4.14V, safely below the
maximum float allowed on the batteries. The LTC4071 also detects how hot
the battery is via the NTC signal and
reduces the float voltage at high temperature to maximize battery service life.
set with a self-clocked digital timer and
a MOSFET switching a 267Ω resistor.
The LTC4071 is capable of shunting 50mA internally. However,
when the battery is below the float
voltage, the LTC4071 only draws
~600nA of current from the battery.
Charging-Sending
The GM BATTERY GMB301009 batteries have a capacity of 8mAh and an
internal series resistance of ~10Ω.
The simulated sensor-transmitter is
modeled on a Microchip PIC18LF14K22
and MRF24J40MA 2.4GHz IEEE standard 802.15.4 radio. The radio draws
23mA in transmit and 18mA in receive.
The model represents this as a 12.4mA,
0.98% duty cycle (2ms/204ms) load,
MODES OF OPERATION
This system has two modes of operation: charging-sending and dischargingsending. In charging-sending mode,
the batteries are charged while the
sensor-transmitter presents a 0.5%
load. When discharging, the sensortransmitter is operating, but no energy
is being harvested from the PFCB-W14.
When active, the PFCB-W14 delivers
power at an average of approximately
9.2V × 180µA ≈ 1.7mW. The available
current must charge the battery and
operate the buck regulator driving the
simulated sensor-transmitter. The active
sensor-transmitter draws 12.4mA × 3.3V ≈
41mW at around 1% of the time, or about
0.41mW on average, leaving some current
to charge the battery. Taking into account
the 85% efficiency of the LTC3588 buck regulator, assuming an average VIN of 9.2V (see
Figure 2), and a buck quiescent current of
8µA, the average current consumed by the
system without charging the battery is:
ISENSOR
• DUTYCYCLE + IQ(BUCK )
VIN( AVG)
• ηBUCK
VOUT
12.4mA
• 0.0098 + 8µA ≅ 60µA
9.2V
• 0.85
3.3V
Harvested energy can drive the sensortransmitter at a 0.5% duty cycle with
about 120µA left to charge the batteries. The GMB301009 batteries have a
capacity of 8mAh, so they completely
charge from empty in about 75 hours.
Discharging-Sending
When the PFCB-W14 is not delivering power, the voltage at VIN drops to
approximately:
8.4 + 6.6
= 7.5V
2
So the reflected load current calculation
changes to:
IAVG =
12.4mA
• 0.0098 + 15µA ≅ 78µA
7.5V
• 0.85
3.3V
The quiescent current of the buck regulator is higher because the regulator
must switch more often to regulate from
7.5V versus 9.2V. At 78µA, with no energy
harvested, the battery is discharged in
approximately 115 hours. This indicates
a charge storage capacity of >8.95mAh.
These batteries when brand new could
store approximately 12% more charge
than rated.
A more serious problem is what happens
when the battery is fully discharged. If
current is drawn after the state of charge
reaches zero, and the battery voltage drops
October 2010 : LT Journal of Analog Innovation | 37
MEASURED RESULTS
below 2.1V, the battery is permanently
damaged. Therefore the application must
ensure that the battery voltage never falls
below this limit. For this reason, the battery cutoff voltage is set to 2.7V or 3.2V to
ensure some energy remains in the battery
after the disconnect circuit has engaged.
The system shown in Figure 1 was
measured in both operating modes
discharging-sending (Figure 3) and
charging-sending (Figure 4).
Since the voltage at VIN is now VBAT1
+ VBAT2 + (180µA × 15k) = 6.2V, the
buck regulator on the LTC3588 restarts
and 3.3V is once again available.
Discharging-Sending
In Figure 3 the voltages of the two batteries BAT1, BAT2 and VBUCK are plotted against time with the batteries
supplying all the system energy, none
from the PFCB-W14 piezo.
Simply stopping the transmitter or
disconnecting the load will not protect the battery, as the LTC4071 draws
a quiescent current of approximate
600nA. Although this is extremely low,
the total load, including the LTC3588‑1,
is nearly 2µA. A fully discharged battery
will only be able to supply approximately 100µA before its voltage drops
enough to damage the battery.
CONCLUSION
With a few easy-to-use components,
it is possible to build a complete compact energy-harvesting power subsystem for wireless sensor-transmitters.
In this particular system a piezoelectric transducer supplies intermittent
power, while two batteries store energy
for use by the sensor-transmitter. An
integrated disconnect switch protects
the batteries from overdischarge.
The batteries slowly discharge until
BAT1 activates the LBO threshold of its
LTC4071, whereupon the disconnect
circuit activates and disconnects BAT1
from all circuitry except the LTC4071
itself. This causes the voltage at VIN of the
LTC3588 to drop below the UVLO for the
regulator, and the regulator shuts off.
A disconnect circuit is necessary to ensure
that the battery does not discharge in a
reasonable amount of time. The LTC4071
provides an internal low battery disconnect circuit. This disconnect circuit was
measured to provide <2nA of battery load
at room temperature when activated.
This leakage is typically dominated by
PCB leakage. With only 2nA of battery
drain current, the battery could survive for 50,000 hours in the disconnect
state before the battery is damaged.
The load on BAT2 is the 2µA quiescent current of the LTC4071 and the LTC3588. This
small load slowly discharges BAT2 until
the low battery disconnect of LTC4071
is activated and BAT2 is disconnected.
This system can fully charge the battery
in 75 hours, even while operating the
sensor-transmitter at 0.5% duty cycle.
The batteries allow the system to continue operating the sensor-transmitter
at 0.5% duty cycle for 115 hours after
the PFCB-W15 stops providing power. If
longer battery operating time is required,
the sensor-transmitter duty cycle can be
reduced to accommodate this need. n
Charging-Sending
When the PFCB-W14 once again starts
delivering power to the system, VIN rises
to 7V, which forward biases the body
diodes of the disconnect FETs in the
LTC4071. This charges the batteries
until the reconnect threshold is reached,
In Figure 3, the second battery
(BAT2) is seen to disconnect 50 hours
after BAT1 due to the 2µA load.
Figure 3. Discharge with battery undervoltage disconnect
Figure 4. Battery disconnect recovery on charge
9.5
8
8.5
6.5
VIN DROPS TO VBAT2 WHEN BATTERY 1 IS
DISCONNECTED. THE 3.3V REGULATOR
SHUTS OFF DUE TO THE LTC3588 UVLO.
5.5
4.5
VCC2
3.5
2.5
BATTERY 1 DISCONNECTED
BY THE LTC4071.
1.5
20
40
60
80
100
TIME (HOURS)
38 | October 2010 : LT Journal of Analog Innovation
VIN RISES ABOVE BATTERY STACK
VOLTAGE TO RECHARGE THROUGH THE
DISCONNECT MOSFETs BODY DIODES.
5
4
3
VIN
120
140
160
180
0
0
AT RECONNECT, VIN SNAPS
DOWN TO THE BATTERY
STACK VOLTAGE.
LTC3588 UVLO SATISFIED AND
3.3V REGULATOR STARTS.
RIPPLE ON VIN FROM REFLECTED
CURRENT FROM 3.3V LOAD.
1
VCC1
0
VBAT1+VBAT2
6
2
0.5
–0.5
7
VOLTAGE (V)
SHUTDOWN LOAD
CONTINUES TO
SLOWLY DRAIN
BATTERY 2, UNTIL IT
TOO IS DISCONNECTED.
VIN
7.5
VOLTAGE (V)
allowing batteries BAT1 and BAT2 to be
reconnected. Looking at Figure 4, this
can be seen as the voltage at VIN snaps
down to the battery stack voltage.
2
4
6
8
10
12
14
TIME (SECONDS)
16
18
20
22
24