120214 IEEE Energy Harvesting Vibration

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Vibration Energy Scavenging and Managementfor Ultra Low Power Applications *

Lu Chao, Chi-Ying Tsui and Wing-Hung KiDepartment of Electronic and Computer Engineering

The Hong Kong University of Science and TechnologyHong Kong SAR., P. R. China

{eeluchao, eetsui and eeki}@ee.ust.hk

ABSTRACT

In this work, the design of a mechanical vibration energyscavenging and management system is presented for ultra low

power applications. A new maximum power point tracking

(MPPT) scheme is proposed for piezoelectric conversion. This

scheme consumes very little power and is especially suitable for

ultra low power energy harvesting applications. This design iscapable of self-starting and self-powered, thus eliminates external

battery integration and significantly reduces the system volume.

System modeling, analysis, and VLSI implementation weredeveloped. Various simulations were carried out and the

simulation results show that the proposed MPPT scheme can

achieve an energy harvesting efficiency higher than 90%.

Categories and Subject Descriptors

B.7.1 [Integrated Circuits]: Types and Design Styles –

Advanced technologies, Algorithms implemented in hardware.

General Terms

Algorithms, Management, Design, Verification.

Keywords

Energy scavenging and management, MPPT, batteryless

1. INTRODUCTIONUbiquitous applications have potential to be used in many

areas, where ubiquitous computing, sensing, and perceptionfacilitate the interaction between human and the environment.

Wireless sensor network is a good example. Providing the

required supply voltage and power to hundreds or thousands of

distributed sensor nodes is a challenge. The conventional solution

is to use electrochemical batteries. However, battery has limited

energy capacity, relatively large volume with respect to theelectronic circuits, finite recharging cycle and is difficult to be

replaced regularly in many cases. All the above disadvantages

pose a big limitation on the wide deployment of such systems. In

some ultra low power applications (e.g. picoradio [1], smart dust

[2]) that demand compact, low cost, long lifetime and high

integration, eliminating the battery is much desirable.

For some ubiquitous applications, the average power

consumption can be down to the level of hundreds or even tens of

microwatts. Power scavenged from the environment can be usedas an alternative power source to provide a virtually infinite

lifetime [3]. Mechanical energy conversion is one of the feasible

approaches for these ultra low power applications [5][6]. Low

level vibrations commonly occur in various household or

industrial environments, such as machinery or air-conditioningvibration. It is estimated that mechanical vibrations inherent in the

environment can provide a power density of tens to hundreds of

microwatt per cubic centimeter [4], which is sufficient to sustain

operations of many ubiquitous applications [5][6]. Therefore,

vibration-based energy scavenging systems have drawn many

attentions in the research communities [3]-[7].

Previous studies have found that for piezoelectric conversion,under a given vibration status (magnitude and frequency), there is

an optimal output voltage point where maximum harvested

electrical power can be obtained [9]. Vibration status is often

unstable and varying, and heavily depends on the environment in

which the application is located. Hence, the optimal voltage point

is also changing. In order to harvest as much energy as possible, arun-time adaptive mechanism is required to track the optimal

output voltage with the environmental change. At the same time,

this tracking mechanism should have as low power overhead as possible since the energy harvested is already very small.

A vibration based self-powered wireless sensor using a fixed

voltage band-band control scheme was proposed in [7]. ExistingMPPT schemes for piezoelectric conversion [9][10] are not

designed for low power applications because they employed

complex circuit components and computation-intensive control

algorithms. Hence, they are very inefficient in such low power

level and the power overhead could be higher than the maximum

power harvested from the environment. In [9], MPPT wascontrolled by a DSP through comparing the harvested power

before and after a change in the duty cycle of a buck converter.

The power overhead of the MPPT scheme was not presented. In

[10], an expression for the optimal duty cycle of a buck converter

operating in discontinuous conduction mode was developed and itrevealed that as the level of vibration excitation increases, the

optimal duty cycle becomes relatively constant. Based on the

above analysis results, a simpler and improved MPPT scheme was

developed. The reported power overhead of the tracking unit is

5.74mW and is too much for ultra-low power applications. From

this, we can see that in order to facilitate the use of piezoelectricconversion in ultra low power applications, it is necessary to

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ISLPED’07 , August 27–29, 2007, Portland, Oregon, USA.

Copyright 2007 ACM 978-1-59593-709-4/07/0008 …$5.00.* This work was supported in part by the Hong Kong Research

Grants Council under Grant CERG 620305.

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develop a new MPPT method that has very little power loss and

the corresponding circuit implementation should be simple.

For batteryless vibration based energy harvesting applications,

the feature of self-starting is very crucial since the vibration

sources can be very unstable. Sometimes, there is no vibration or

the vibration excitation is too weak to be used. Thus, no energy

can be harvested. The system will dissipate all the energy storedin the system and stop working. As there is no external battery as

the backup power source, later even the vibration excitation

becomes strong enough again; the system cannot be started up as

there is no power left in the system. Thus the capability of self-

starting is crucial for batteryless applications.

In this paper, we tackle the above design challenges. We present anew MPPT scheme and circuit implementation that have a very

low power overhead, which is in the order of microwatts. This is

about three orders of magnitude lower than the existing MPPT

approaches. Based on this MPPT scheme, a batteryless vibration

based energy scavenging and management system is presented.

The system is capable of self-starting and self-powered. It wasimplemented using a 0.35μm 5V CMOS process. Post-layout

simulations were carried out to verify the functionality and the

efficiency of the proposed MPPT scheme and the overall system.

The rest of the paper is organized as follows. Section 2

introduces the proposed MPPT scheme, and the circuit

implementations of the energy harvester and the vibrationtracking unit. Section 3 describes the proposed batteryless

vibration based energy scavenging and management system. In

addition, the circuit implementation of each functional block is

presented in detail. Various simulation results are presented in

Section 4. Finally, the conclusion is provided in Section 5.

2. The PROPOSED MPPT SCHEME

2.1 Electrical Model of Piezoelectric Material

pC p R

0 2 4 6 8 100

1

2

3

4

5

Load Resistance (M Ohms)

O u t p u t v o l t a g e ( V )

Measured

Electrical model

Figure 1 Piezoelectric film model and experimental

verification

Fig. 1(a) shows an equivalent electrical model of a

piezoelectric thin film. It consists of a sinusoidal current source

i(t)=I psin(2πft), where I p depends on the vibration magnitude, size

and material of the film, f is the vibration frequency, C p

and R p

are the internal capacitance and resistance of the film,respectively. Measurement results show that C p is almost constant

under a wide range of vibration frequencies. R p is usually very

large and can be ignored. The output voltage of a piezoelectric

film thus depends on the material’s geometry, piezoelectric

properties, the mechanical vibration strength, and the outputimpedance. We used a commercial piezoelectric thin film [8] to

conduct experiments to verify the above model. A piece of

1.3cm×2.5cm×0.2mm piezoelectric film with C p=0.5nF was

mounted on a variable vibration platform. Various resistive loads

were connected to the film and the peak-to-peak output voltages

were recorded. The measured data are marked in Fig. 1(b). The

output voltages predicted by the model are also plotted. It is clear

that the model fits well with the measurement data. In the rest of

the paper, we use this model for analysis and optimization.

2.2 Energy Harvester

The output of the piezoelectric film is an AC signal. For energyharvesting usage on most CMOS applications, we need an AC-

DC rectifier to convert the input to a DC voltage source for

powering up the circuit. Fig. 2 shows the basic structure of theenergy harvester, which consists of a rectifier and a storage

capacitor Cs. The instantaneously harvested power is small and

may not be able to sustain continuous operations of the

application. Hence, Cs is inserted to accumulate the harvested

power. From [9], it is shown that the time-averaging harvested

power <p(t)> and the output voltage Vs at the maximum input power point are given by

π

π

π

π

π

p s p s s p fC V V fC V V I t p

Δ−−>=<

842)(

2

(1)

V fC

I

V p

p

optimal s Δ−=π 4

, (2)

where ΔV is the forward voltage drop of the diode. For low level

vibration cases, the output voltage of the energy harvester is

comparable to the voltage drop ΔV. The passive diode rectifier causes a significant reduction in the output voltage of the rectifier

and hence the overall power efficiency. In [11], an active diode,

which consists of a large PMOS transistor and a comparator, was

used to reduce the voltage drop and power loss for piezoelectric

conversion. However, an external 3.3V voltage supply was stillrequired for the power supply of the comparator [11]. Thus, this

design is not suitable for batteryless applications since it is not

capable of self-starting. In this work, we propose a hybrid scheme

which uses both passive and active diodes to solve this problem.

The passive diode rectifier is used for the self-starting purpose,while the active diode rectifier kicks in to replace the passivediode when the harvester has started up and it can provide a

smaller voltage drop and higher power efficiency.

)2sin()( ft I t i p π =

Figure 2 Energy Harvester


Figure 3 Schematic of the proposed energy harvester

Fig. 3 shows the schematic of the proposed rectifier [12]. Itintegrates the passive and active diode structures together. During

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the starting up of the system, vibration starts to occur at the

piezoelectric film and AC voltage is generated at the output of the

film. Initially Vs is still zero and cannot drive the operation of the

amplifiers (AMP1, AMP2) and thus the active diodes are not

working. At this time, the PMOS cross-coupled, NMOS diode-connected passive diode rectifier operates and provides

rectification for the AC voltage. The current generated from the

piezoelectric film charges up Cs through the passive rectifier andVs starts to increase. When Vs reaches a level that the amplifiers

can operate, the active diode rectifier is activated. The active

diode will bypass the diode-connected NMOS and reduce thevoltage drop across the rectifier. For the proposed active rectifier,

ΔV is close to zero. Substituting ΔV=0 into (1) and (2), <p(t)>

and optimal Vs are equal to

π

π

π

p s s p fC V V I t p

242

)( >=< (3) p

p

optimal s fC

I V

π 4, = (4)

Expression (4) indicates that the optimal voltage for maximum power harvesting depends on the vibration magnitude I p and

frequency f, which vary with the deployment environment and

time. Thus, the maximum power point is also changing with the

vibration status.2.3 Tracking Unit

The tracking unit is designed to detect the vibration variations

and generate a reference voltage Vref for the MPPT control.Vibration usually does not change very fast and it is not necessary

to track the vibration status continuously. Here we propose a

time-multiplexing scheme to track the vibration status. A tracking

unit, which can reflect the vibration status, is periodically

connected to the piezoelectric material to update the referencevoltage Vref . It is time-multiplexed with the AC-DC rectifier that

during tracking, the power is not harvested from the piezoelectric

film through the rectifier, as the rectifier is disconnected from the

film. To reduce the power overhead of the tracking unit, we use a

very low duty cycle for tracking purpose so that most of the time

the film is providing power through the rectifier. Themultiplexing structure is shown in Fig. 4(a). When the tracking

signal Φ is low, the rectifier is connected to the film and the

system performs energy harvesting. When Φ goes high, the

tracking unit will connect to the film to perform vibration tracking.

The equivalent circuit in this mode is shown in Fig. 4(b).

)2sin()( ft I t i p π = )2sin()( ft I t i p π =

Figure 4 (a) The time-multiplexed structure for the energy

harvester and the tracking unit (b) The vibration tracking

circuit

Figure 5 Illustrative waveforms for the tracking process

In the tracking mode, suppose initially the direction of the

piezoelectric current flow is along the dashed line, as shown in

Fig. 4(b). C p and Cd are discharged, and finally Vd decreases to

zero due to the reverse biased active diode D3. In the next half

cycle, the direction of the current flow is reversed. Hence, C p andCd is charged up again until Vd reaches its peak value. This

positive peak value is maintained as Vref by the capacitor Cr and

the active diode D5, which is similar to the active diode used inthe AC-DC rectifier. Fig. 5 illustrates the tracking process. During

the time period T1, the piezoelectric current discharges Cd. Vd is

reduced and maintained at zero by the active diode D3. Then, inthe next half cycle T2, the current reverses its flow direction, and

Vd is charged up again. At the end of T2, Vd is charged up to its

positive peak value and this value is maintained at Vref .

Let us denote the time instant that i(t) starts to reverse the

flow direction as t=0. The transient response of Vd is analyzed

and expressed as (5),

C Rt

d

C Rt p p

d p p eV e

fC

I ft

fC

I t V

//)0(

2)

22sin(

2)(

−−++−=

π

π

π

π

(5)

where C is the sum of C p and Cd, and Vd(0) is zero. Since R p is

very large and R pC is also very large, the peak value of Vd can beobtained at t=T/2, where T is the cycle time of the vibration and

equation (5) becomes

fC

I t V

p

peak d π

=)( (6)

If we use Cd=15C p, we have V ref = V d (t ) peak =1/4V s,optimal . The time

needed to wait for the piezoelectric current to reverse the flow

direction is at most one vibration cycle. In addition, one half of

the cycle time is needed to obtain the positive peak value of Vd.

Therefore, the minimum duration that the tracking unit needs to be connected to the piezoelectric film (i.e., the pulse width of the

tracking signal Φ) is 1.5T.

3. OVERVIEW of the PROPOSED SYSTEMFig.6 shows the block diagram of the proposed batteryless

vibration based energy scavenging and management system [13].

As we mentioned before, MOS switches (N1, N2, P1) are used toimplement the time-multiplexing MPPT. The duty cycle directly

affects the time that the piezoelectric material can be used for

energy harvesting. Therefore, the duty cycle should be kept as

low as possible. The control unit implements a band-band control

strategy to maintain the output voltage of the energy harvester Vs

at the optimal voltage value by turning on/off the switch S1. The buck converter is used to convert the unregulated voltage Vs into

a regulated output voltage Vout, for the operation of the

application load. Vs is used as the supply voltage for the control

unit, the pulse generator, the tracking unit and the buck converter.

If the harvested power is larger than the power overhead of these blocks, the system can be self-powered without the need of a

battery.

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AC-DC

Rectifier

Buck

Converter

Load

Control

Unit

Vibration

Tracking

Unit

Vs

Cs

S1Piezo

Film

Environmental

Vibration

Vout

con

Vref

enable

. .Vin

Pulse

Generator

.

.

R 1

R 2

Voltage Dividers

Energy Harvester

Tracking

Signal

N1

P1

Refreshing

Unit

.

R 3

R 4

V1

V2

.

.

R 5

R 6

V3

V4

Figure 6 Block diagram of the proposed system

3.1 Tracking Pulse GeneratorSince the vibration frequency varies with the environment, so it

is a challenge to generate a tracking signal Φ with a fixed duty

cycle, while at the same time consuming very low power. In thisdesign, we directly use the AC signals from the piezoelectric film

to generate the tracking signal. The tracking pulse generator is

shown in Fig. 7. We connect one end of the piezoelectric film to a

comparator with a DC bias voltage that is generated by a bandgap

reference and a resistor divider. A clock-like signal, which has thesame frequency as the environmental vibration, is then generatedat the output of the comparator. A counter and combinational

logic gates are used to generate a tracking pulse with a fixed duty

cycle of 1/64, i.e., around 1.56%. In addition, the tracking pulse

width is generated to be 2T to satisfy the minimum requirement of

1.5T for normal tracking.


Figure 7 Block diagram of the tracking pulse generator

3.2 Refreshing UnitIn Fig. 6, there is a refreshing unit located between the tracking

unit and the control unit. It is used to periodically refresh the

stored reference voltage Vref and provide a correctly updatedvoltage V4 to the control unit. As shown in Fig. 4, Vref is kept and

maintained by a capacitor Cr and Vref is updated through an active

diode D5. When the vibration becomes weaker, the active diode is

cut off; and the Vref cannot decrease to reflect the variations of the

vibration. Therefore, we need to refresh the value V ref so that the

tracking unit can generate a correct value of V4 to the control unit.

Fig. 8(a) shows the proposed refreshing unit, which consist of

five MOS switches and an internal capacitor C0. Fig. 8(b) shows

the sequence of control signals that turn on/off the switches.When a new tracking starts, MN2 is on and charge sharing occurs

between Cr and C0. The value of C0 is chosen to be much less than

that of Cr and hence the voltage at C0 is very close to Vref . Note

that the control signal for MN2 is driven by Vs which is about 4

times higher than Vref and hence there is no threshold drop across

the NMOS switch. Before the charge sharing action, C0 isdischarged to 0 by the switch MN1 driven by Con1. After the

charge sharing is completed, the output of the refreshing unit is

connected to C0 through MN4. Cr is cut off from the output. A

Con3 signal is generated to turn on MN3 to discharge Cr . After it

is discharged, it is ready for another tracking update. The tracking

pulse width is 2T and the above action occurs in the first 0.5T.

This leaves enough time for Cr to be charged up to a new V ref .

When the tracking process finishes, i.e., tracking pulse Φ returns

to zero, Vref is already updated and kept by Cr . The output of therefreshing unit is connected back to Cr through MP1 and

disconnected from C0 by turning off MN4.

Figure 8 Schematic of the refreshing circuit and control

signals

3.3 Control UnitThe control unit implements a band-band control strategy to

maintain the output voltage Vs of the energy harvester at theoptimal value. It mainly consists of a Schmitt trigger and a

voltage comparator, as shown in Fig. 9(a). Resistors R 1~R 6 of Fig.

6 form 3 voltage dividers to provide V1= (1/3.95)Vs, V2=

(1/4.05)Vs and V3= (1/3.96)Vs to the control unit. Note that the

refreshing unit provides a reference voltage V4 = (1/4)Vs, optimal tothe control unit as well. Two control signals “con” and “enable”

are generated to turn on/off the switch S1 to track the output

voltage of energy harvester to be optimal and to enable the

application, respectively. When Cs is charged up to a value higher

than 1.0125Vs, optimal, “con” becomes low and turns on S1. It will

trigger the application to wake up from sleep mode and starts an

atomic operation. Power is then transferred to the load through the buck converter. When Vs is lower than 0.99Vs, optimal, “enable”

becomes low to disable the application, which should store the

operation state for next operation before going to sleep mode

again. When Vs is lower than 0.9875Vs, optimal, “con” becomeshigh and turns off S1. Power transfer to the buck converter is

stopped and one operation round is finished. Once the harvested

power charges up Cs and Vs reaches 1.0125Vs, optimal, the operation

cycle repeats again. The control unit maintains Vs between

0.9875~1.0125Vs,optimal.

Figure 9 (a) Control unit diagram (b) Comparator

architecture

The speed of the control unit does not need to be very high and

low power consumption is the most important design factor. Thecomparators are operated in subthreshold region [11], and its

schematic is shown in Fig. 9(b). It contains a two-stage

319



operational amplifier and a current source, where the bias current

remains constant even when the power supply changes. Thus, it

guarantees that the bias current of the comparator is small enough

for normal subthreshold operation, even when the supply voltage

changes in a wide range. The operating current for eachcomparator is 60nA. Assume the supply voltage changes in the

range of 2V~6.5V, then, the power dissipation of each comparator

is 120nW to 390nW and the total power overhead of the proposedcontrol unit is about 360nW to 1.17μW.

3.4 Micropower Buck ConverterThe power consumption of ubiquitous applications is in the

range of tens to hundreds of microwatts, while conventional buck converters, which are dedicated for existing portable electronics,

usually consume several hundred milliwatts. Conventional buck

converters are very inefficient in ultra low power level due to

their complicated control algorithms, high power control circuits

and large conduction loss. Consequently, it is necessary to

optimize the structure of buck converters for ultra low power applications.

In this work, an optimized structure for ultra low power

applications is proposed and shown in Fig. 10. Since PWM

control has large power overhead and is inefficient for low power level, so PFM control scheme is used. A ring oscillator is adopted

to perform current-limiting function to reduce the conduction

power loss by the ESR of the inductor. As the averaged current

through the power transistors is only tens or hundreds of microamperes, the sizes of power transistors have to be carefully

chosen to reduce the switching and conduction power loss. In this

design, the optimal W/L ratio of power transistors is

500μm/0.5μm. Besides, the current source is carefully designed to

reduce the static biasing power loss.

Figure 10 Micropower buck converter

3.5 System Start-up and Regular OperationThe self-starting of the energy harvester has been described in

Section 2.2. Fig. 11 illustrates the voltage waveforms for system

starting up and regular operation. When Vs is high enough todrive the tracking pulse generator, the tracking process is

activated and the corresponding V4

is provided to the control unit.

This time instance is marked as t in Fig. 11. From then on, the

system begins the regular operation. In each operation cycle, the

energy transferred to the buck converter and the load can be

estimated as E = ½Cs[(4×1.0125V4)2 – (4×0.9875V4)

2]. Cs should be sized such that it is large enough for the execution of an atomic

operation of the application even under the worst case (i.e. the

lowest allowable output voltage from the rectifier).

Figure 11 Illustrative graphs for overall system operation

4. SIMULATION RESULTSThe proposed energy scavenging and management system was

implemented in AMS 5V 0.35μm CMOS technology and the

layout is shown in Fig. 12(a). We used a piezoelectric film with a

dimension of 1.6cm×6.15cm×3mm. C p is measured to be 33.36nF

and Cs is 5μF. A 47μH inductor was used for the buck converter.The output voltage of the buck converter is set to 1.2V and the

output ripple requirement is less than 25mV. In the active mode,the power consumption of the load is assumed to be 144μW.

4.1 Power Harvesting EfficiencyTo verify the expressions (3) and (4), we simulated the

proposed energy harvester with various resistive loads to find its

power harvesting characteristics. Two sets of vibration status, i.e.,

(60Hz, 70µA) and (60Hz, 100µA) were used and the simulation

results are marked in Fig. 12(b). The harvested power values

predicted by the expression (3) are also plotted in Fig. 12(b). It

can be concluded that the expressions (3) and (4) fit well with the

circuit simulation results.

Buckconverter

Energy

Harvester

Tracking

Unit

Pulse

Generator

0 1 2 3 4 5 6 7 8

0

20

40

60

80

100

120

140

Output Voltage of the Energy Harvester (V)

H a r v e s t e d

P o w e r ( u w )

Power Harvesting Characteristics

(a) (b)

Figure 12 (a) Layout of the proposed overall system (b) Power

harvesting characteristics of the energy harvester

The power harvesting efficiency is defined as the ratio of the

actual harvested power divided by the theoretical maximum

harvested power from the energy harvester. Different sets of

vibration status were used for measuring the power harvestingefficiency. For each set of vibration status, the theoretical

maximum power point was achieved by using the corresponding

optimal resistive load as the load and disabling the MPPT control

blocks and the buck converter. The actual harvested power is

obtained by enabling the MPPT control blocks and providing a

non-optimal resistor as the load. Post-layout HSPICE simulationsof the design were carried to obtain the actual harvested power.

Two vibration status, i.e., (60Hz, 100μA) and (60Hz, 70μA), were

simulated, and the resultant voltage waveforms are shown in Fig.

320



13. The harvested power and efficiencies are summarized in

Table 1. The proposed MPPT scheme is also compared with a

fixed voltage band-band control scheme. Here, fixed optimal Vs is

used and it is set at 2.8V that is the optimal voltage for the

vibration status of (60Hz, 70µA). Table 1 shows its actualharvested power.

Figure 13 Vibration tracking waveforms

From Fig. 13 and Table 1, we can see that the tracking unit

tracks well with the vibration status and the obtained optimalvoltages are within 5% of the theoretical values. The power

harvesting efficiency of the proposed MPPT scheme is above

90%. It is also verified that the system is capable of self-starting.

Comparing with the fixed voltage control, the efficiency of the

proposed MPPT scheme is about the same when the fixed voltageis also the optimal voltage. It has about 7% improvement in

efficiency for the case (60Hz, 100μA) that the fixed voltage is not

the optimal voltage. For some other vibration status, the

improvement will be higher. This shows the advantage of the

proposed MPPT that the power overhead of is not high.

Table 1 Tracking Performance of the proposed MPPT scheme

60Hz,100µA 60Hz,70µA

Desired optimal Vs 3.98v 2.78v

Obtained Vs 3.95v 2.81v

Theoretical maximum harvested power 118.1µw 60.07µw

Actual harvested power and energy

harvesting efficiency

110.05µw

93.18%

55.09µw

91.71%

Actual harvested power by fixing Vs=2.8V

and energy harvesting efficiency

102.3µw

86.62%

55.6µw

92.56%

4.2 Performance of the micropower converterThe definition of the power conversion efficiency of the

converter is given by η =loss power power output

power output+

. As the power

dissipation of the ubiquitous applications is usually in the range of

hundreds of microwatts, therefore, very little power overhead canseriously degrade the efficiency of micropower converter. The

buck converter was simulated at various input voltages from 2.5V

to 6V and the power loss curves are plotted in Fig. 14. The power

loss is mainly due to the current source, the conduction loss and

the switching loss of the power transistors. Fig. 14 indicates thatthe power conversion efficiency is about 70%.

2.5 3 3.5 4 4.5 5 5.5 60

20

40

60

80

100

120

140

160

supply voltage (V)

p o w e r ( u w )

Power Characteristics

total power loss

conduction loss

biasing lloss

switching loss

output power

Figure 14 Power loss performance of the buck converter

5. CONCLUSIONWe present a vibration-based power scavenging and

management system for ultra low power ubiquitous applications.

This system is capable of completely self-powered by the

harvested energy and starting up automatically. Circuit

implementation and simulations were carried out to verify the

tracking performance and the feasibility of the proposed platform.

Post-layout simulation shows that the power harvesting efficiency

of the proposed MPPT system is higher than 90%.

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