A SINGLE INDUCTOR DIDO CONVERTER WITH ULTRA LOW POWER MPPT AND THIN-FILM LIPON BATTERY FOR PIEZOELECTRIC ENERGY

HARVESTING AND MANAGEMENT

A. Ramond1,2, M. Sanchez1,2, K. Li1,2, H. Durou1,2, B. Jammes1,2, and C. Rossi1,2

1CNRS ; LAAS ; 7 avenue du Colonel Roche, F-31077 Toulouse, France. 2Université de Toulouse ; UPS, INSA, INP, ISAE ; LAAS ; F-31077 Toulouse, France.

*Presenting Author: aramond@laas.fr Abstract: This paper reports the design, fabrication and testing of a Micro Power Management System (MPMS) for autonomous wireless sensor node. The presented MPMS interfaces a commercial piezoelectric energy harvester with a 700µA.h thin-film Lithium Phosphorous Oxynitride (LIPON) battery and a regulated DC bus that supplies a DS2422 temperature sensor and data-logger. The power delivered by the piezoelectric source is shared between the regulated DC bus and the battery. By monitoring the voltage across the input and output decoupling capacitors, the MPMS is able to detect if the power delivered by the source is sufficient to supply the load. In this case the surplus of power is placed into the battery; otherwise the battery is used to recoup the lack of harvested power. The global efficiency tends toward a maximum value of about 70%. 25% of the power is lost in passive elements, and 15% to 30%, depending on the vibration level, correspond to the active control circuit consumption. Keywords: Vibration energy harvesting, DIDO DC/DC, MPPT, thin-film LIPON battery INTRODUCTION

The problematic of Wireless Sensor Nodes for Ambient Intelligence has driven the main recent progresses on two topics. On one side, energy harvesting sources becomes more integrated and efficient [1]. On the other, specific power and/or energy density of energy storage devices are increasing while these devices are being miniaturized [2]. At the same time, the consumption of the electronic applications for sensing and monitoring is decreasing. Finally, considering all of this, we are about to reach the point where the performances of the firsts meet the requirements of the seconds. Consequently the need for an efficient Micro-Power Management System (MPMS) that would bridge the gap between energy harvesting and storage devices and end-user applications is coming to the fore [3,4,5]. In this context we aim at developing a new generic and versatile architecture that handles power coming from multiple energy harvesting sources while managing the usage of a storage device (battery or supercapacitor) in order to provide a regulated voltage that can supply any sensor, data logger, or microprocessor.

This paper reports the design, fabrication and testing of a MPMS for autonomous wireless sensor node. The presented MPMS interfaces a commercial piezoelectric harvester with a 700µA.h thin-film Lithium Phosphorous Oxynitride (LIPON) battery and a 1.2V to 3.3V regulated DC bus. The first part of the paper describes the piezoelectric harvester and the thin-film battery models. Results are compared to our experimental characterizations. The second part develops the architecture and design of the MPMS. The complete principle of the system will be detailed in this part. Afterwards, the main simulation results concerning the Maximum Power Point Tracking (MPPT) and the analysis of the converter efficiency

are presented. Finally, we present the characterization results of a first test circuit

MODELING Piezoelectric harvester

In our work we use two Volture Raw Harvester V21BL from the Midé Technology Corporation. Bimorph beam dimensions are 60x17mm². The resonance frequency is tuned around 60 Hz using a tungsten tip mass. In order to increase the output voltage, the two active piezoelectric layers per beam are connected in series. The two beams are mounted on a frame (81x38mm) that can be fixed onto a shaker for test purpose (Fig. 1).

Fig. 1: Schema of Midé piezoelectric harvesters mounted on the shaker frame.

The VHDL-AMS model of the harvester been presented at PowerMEMS09 [3]. Nevertheless the range of validity has been extended from 0.1 to 6g with an average relative error on the rms-voltage of 8.8% (249mV absolute). The main figures are gathered in Table 1. Table 1: Harvester output power.

LIPON thin-film battery

Our energy storage element is a thin-film LIPON battery called Thinergy Micro Energy Cell (MEC) from Infinite Power Solution (IPS). The battery characteristics are 1 inch² surface, 150µm thickness, and a nominal capacity of 700µA.h (at C/2 discharge rate). In addition we measured a capacity of 900µA.h at C/10 discharge rate. The model we have developed for this battery is based on the runtime-based model presented by Rincon-Mora and al. in [6]. We adapted the model to our battery characteristics and added the temperature influence (between -20°C and 80°C). The model is coded in VHDL-AMS but is fully Spice compatible (Fig. 2). The evolution of the time-constants R1.C1, R2.C2 and R3.C3 versus the State Of Charge (SOC), the current (I) and temperature (T), has been fitted from experimental measurements. In the same way we have experimentally determined the evolution of the Open Circuit voltage of the battery with the SOC and the temperature.

Fig. 2: Schematic of the battery model.

The model has been successfully compared to experimental data. The average relative error on the battery voltage is less than 1% over the most part of the range of validity of the model. But, for relatively high Depth of Discharge (typically more than 80%), the results are correct from a qualitative point of view only (Fig. 3 around 7000sec at end of discharge).

Fig. 3: Comparison between experimental data and model for a pulsed discharge at ambient temperature.

The behavior of the battery, in terms of voltage

level, dynamic and current response, is completely different at low or high temperature. Our model response is accurate over the considered temperature range (Fig. 4).

Fig. 4: Comparison of the simulated maximum current versus temperature with datasheet specifications.

DESIGN Principle of functioning

The architecture of the proposed MPMS is based on a Single Inductor Dual Input Dual Output (SI DIDO) DC/DC buck converter (Fig. 5). The main idea with SI DC/DC converters is to use the maximum of the inductor current carrying capacity by time-interleaving several DC/DC converters around a single inductor [4,5].

Fig. 5: Schematic of the architecture of a SIDIDO buck DC/DC converter.

The incoming charges from the harvesters are first

rectified trough a full-wave AC/DC convertor (not shown in Fig. 5). The voltage delivered by each beam is rectified separately to avoid phase canceling. Then the charges (from VIN node) are injected into the inductor before being shared between the regulated output (VOUT1) and the battery (VBAT). The principle is the same than for the SI Dual Output (SIDO) DC/DC converter presented at PowerMEMS09 [3]. The harvested charge extraction is enhanced by a Maximum Power Point Tracking (MPPT) system that controls the VIN voltage in order to place it around the MPP voltage VMPP. The output voltage is regulated around Vref1 (Fig. 6). The effective switching frequency is sub-kHz limiting the switching losses and thus increasing the global yield of the converter.

Fig. 6: Input signals for the control circuit of the converter.

That is the normal functioning mode for light load, ie. when Pload < Pharvest. In this mode, the DC/DC converter works in Discontinuous Current Mode (DCM) due to the low frequency and the low amount of charge harvested. Hence, the inductor remains most of the time unused. Consequently, during this time, it is possible to use the same inductor for another DC/DC converter. Now for heavy load mode, when Pload > Pharvest, the battery is used to recoup the lack of harvested power. This is done by making the charges flow from VBAT to VOUT1 during these ‘dead-times’, using the same inductor. Thus we interleave the previous SIDO converter with a traditional buck converter to form the presented SIDIDO converter (Fig. 5).

Control circuit – Asynchronous State Machine

Given that the switches on Fig. 5 are power MOSFET transistors, the main task for the DC/DC converter control circuit is to provide the correct gate voltages at nodes A, B, C and D.

As stated in the previous section, the control is based on an hysteretic observation of VIN and VOUT1 voltages. VIN is maintained around the sampled and hold value of the Maximum Power Point voltage VMPP. VOUT1 is regulated around a reference voltage (between 1.2V and 3.3V) with a small hysteresis (typically 10mV). This provides two input signal, ‘in’ and ‘1a’ for the control circuit (Fig. 6). A third signal, ‘1b’, is necessary to determine whether the load mode is low or heavy. As proposed by [5], we use the output decoupling capacitor on node VOUT1 as a power balance. This is done only by comparing the output voltage VOUT1 with a triggering value slightly lower than the lower bound of the hysteresis range of ‘1a’.

Fig. 7: Flow graph and associated transition table for the control ASM.

Fig. 7 presents the flow graph of the control

circuit. States 1, 2, 3 and 4 are related to the SIDO DC/DC converter, ie. light load mode, whereas states 5, 6 and 7 refers to the heavy load mode. States 8 and 9 are added to verify the Single Input Change hypothesis stating that only one input can change at time. The Huffman synthesis formalism allows us to write the transition table of an Asynchronous State Machine (ASM) that has the expected behavior. Note that after the state minimization step simply one state variable (here called x) can be used. Actually this variable is referring to the load mode (heavy ‘1’ or light ‘0’). This transition table can easily be implemented on a Programmable Logic Device or a microcontroller. Finally the ASM runs parallel to the MPPT sample and hold procedure previously presented [3].

Battery protection

Despite that the solid state LIPON technology used in Thinergy MEC has no risk of explosion; a protection must be deployed to avoid battery destruction. Contrary to bigger capacitance module, the Thinergy MEC can be charged with constant voltage from any SOC. The recommended charging voltage is 4.1V. Furthermore the minimal discharge cutoff voltage is 2.1V to avoid deep overdischarge that could be damaging. Finally we decided to implement

the LTC4070 shunt battery charger, as well as a 500µF decoupling capacitor, as a protection device. The battery voltage VBAT is clamped to 4.06V at ambient temperature and a Low battery Output (LBO) signal is provided to the control circuit as far as the voltage drops below 3.2V, that allowed us to inhibit the output ‘B’ of the ASM.

SIMULATION

The complete circuit has been simulated at component level through SPICE/VHDL-AMS co-simulations. Although that simulation plays an integral part in our design methodology, this section gathers the main simulation results with an associated discussion.

Fig. 8: Maximum Power Point voltage tracking efficiency.

Fig. 8 shows that the estimation made of the MPP

voltage by the proposed switched-like capacitor ultra low power MPPT scheme [3] is quite good. The circuit actually starts working at 0.35g. Then the efficiency in terms of harvested power is above 90%. The Zener diode, implemented on node VIN for overvoltage protection of comparators, starts to clamp the voltage as it reaches 11V, ie. above 0.7g. This directly impacts the MPPT efficiency due to an estimation based on the clamped OC voltage rather than on the real OC voltage. Note that the hysteresis width doesn’t affect the efficiency too much. This can be explained with the flatness of the power characteristic of the harvester around of the MPP voltage. This kind of behavior is not expected with other sources. With solar cells, for example, the power rapidly decreases with the voltage, above the MPP, due to an exponential I/V curve.

Fig. 9: MPMS efficiency and losses repartition.

Fig. 9 presents the efficiency of the power chain

between the harvester and the battery and the losses repartition (in percent of total harvested power). Thus, the AC/DC efficiency is around 94% on the considered range of vibration. The DC/DC converter actually starts working around 0.35g where the harvested power is equal to the global overhead consumption of the circuit so that there is no power stored in the battery. As the acceleration increases, the harvested power increases too, contrary to the overhead consumption of the active part of the circuit that mainly remains constant (slightly increase of the conduction looses with the current). Consequently the global efficiency rises toward a maximum value of about 70%. 25% of the power is lost in passive elements, and 15% to 30% are related to the active control circuit consumption. Note that for low level of vibration the circuit voltages are such that the conduction looses in the battery NMOS are not negligible. Otherwise the dynamic and static consumption are limited (less than 2%).

TESTING 1st test circuit – MPMS basic principles validation

Fig. 10: First test circuit for the DIDO converter with delocalized control circuit.

We realized a first circuit to validate the principles

of the presented MPMS (Fig. 10). This circuit implements two AC/DC full-wave rectifiers, the MPPT sampler and holder, and the SI DIDO DC/DC converter with a 500µF (5x100µF) ceramic multilayer capacitance as storage element. The control circuit was realized with a PIC18F development board and several LMC7215 Micro-Power Rail-to-Rail CMOS Comparators. In this configuration the system can’t be autonomous in term of power because of the consumption of the PIC18F microcontroller. But this way we have been able to validate the MPPT procedure (Fig. 11) and the ASM functioning.

Fig. 11: Result waveform capture for MPPT sample and hold procedure.

At the time we write this paper a new circuit including the control circuit is being fabricated. We decided to use a PIC16LF1827 as the core of the control circuit. Consumption measurements made on this microcontroller show that it is possible to run with only 5µA in active mode. We also choose LTC1540 Nanopower Comparator, instead of LMC7215, because its consumption is only 300nA. The load is a DS2422 temperature sensor and data-logger. We are hopeful to present the experimental characteristics of this fully autonomous MPMS at the conference.

CONCLUSION AND PERSPECTIVES

This paper tends to shows that Single Inductor Multiple Inputs Multiple Outputs DC/DC converters can be efficiently used in power harvesting and management applications. The presented MPMS interfaces a commercial piezoelectric energy harvester with a 700µA.h thin-film Lithium Phosphorous Oxynitride (LIPON) battery and a regulated DC bus that supplies a DS2422 temperature sensor. The MPMS has been virtually validated. The efficiency of the power chain between the harvester and the battery tend toward a maximum value of about 70%. The polarization voltage of the harvester is controlled to more of 90% of the MPP thanks to our ultra low power MPPT scheme. The proposed MPMS can be adapted to other harvesting sources. A new version of our MPMS that will hybridize vibration energy harvesting with solar cells is being designed.

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