A40mVtransformer-reuseself-startupboostconverterwithMPPTcontrolforthermoelectricenergyha

104 ? 2012 IEEE International Solid-State Circuits ConferenceISSCC 2012 / SESSION 5 / AUDIO AND POWER CONVERTERS / 5.75.7 A 40mV Transformer-Reuse Self-Startup Boost Converter with MPPT Control for Thermoelectric Energy HarvestingJong-Pil Im1, Se-Won Wang1 , Kang-Ho Lee1, Young-Jin Woo2, Young-Sub Yuk1, Tae-Hwang Kong1, Sung-Wan Hong1, Seung-Tak Ryu1, Gyu-Hyeong Cho11KAIST, Daejeon, Korea2Siliconworks, Daejeon, KoreaWhile the demand for micro-energy harvesters ( μ EHs) is increasing (for seam-less energy source in applications such as wireless sensor node), two majorproblems still obstruct versatile use of them. The first problem is the self-start-up capability. Because many wireless sensor nodes are likely to be located wherehuman-maintenance is difficult, starting them up manually can be as difficult asreplacing the battery. Its realization has been difficult because μ EHs must be ableto turn itself on without any stored energy. Some previous works have reportedsuch a function: some needed high voltage [1] or vibration [2] and one used atransformer as a starter [3]. The other problem is the maximum power pointtracking (MPPT) capability. Because it is known that MPPT algorithms usuallyrequire considerable power consumption [4], using them in μ EHs is impractical.This paper suggests a new boost converter architecture and MPPT controlmethod which can bring μ EH into practical use.Figure 5.7.1 shows a block diagram of the proposed converter. A thermoelectricgenerator (TEG), as a possible energy source, is modeled as a voltage source,VTEG, with internal resistance, RTEG. Motivated by the topology in [3], this workemploys a 1:60 (N1: N2) transformer for the boost converter with the proposedtransformer-reusing technique. During the startup mode, in which all of the con-trol circuits are disabled due to a very low supply voltage, the transistor SNS andthe transformer undergo self-oscillation. As SNS has a negative threshold volt-age of -15mV, the thermal noise across R1 is good enough to turn on SNS. Oncesome energy is transferred to the secondary coil (L2), L2 resonates with C1 and,therefore, the VX node starts to oscillate. This maintains the switching of SNS andmakes it possible for VX to swing above the diode on-voltage of D1 to charge theoutput capacitor. The minimum V OUT required for the controller operation isabout 1V and the minimum VTEGis about 40mV for startup in this proposed cir-cuit. If the negative terminal of L2 is connected to the ground as in [3], two seriousproblems arise in the transformer-based step-up operation in Fig. 5.7.1. The firstis the circuit reliability problem. If VTEGincreases above a certain level due to alarge temperature difference on the TEG, the circuitry on the secondary coil suf-fers from high voltage stress over the maximum ratings. The second is that theenergy transfer efficiency via the transformer decreases sharply as VTEGincreas-es. To avoid these problems, we suggest tying the negative terminal of the sec-ondary coil (L2) to the supply voltage VIN instead of the ground where VIN is thevoltage of VTEGdropped by RTEG. By virtue of this configuration, the transformer-based oscillation mode (TOM) is disabled when VIN is larger than a referencevoltage VR1. Then, L2 alone works as an inductor (transformer-reuse) and thenormal boost converter structure is configured to inductor-based boost mode(IBM). The boost converter is then controlled by the proposed MPPT controller. A detailed schematic of the proposed boost converter is shown in Fig. 5.7.2.During the start-up mode, the resonance amplitude tends to be limited by theleakage current of the switch SBM. To prevent this, VG is managed to be negativeas VN2 by the negative voltage generator. If VIN is higher than VR2 which is set to100mV in this work, switch SMC is disabled and switch SBM is controlled for nor-mal boost converter. As long as VOUT VR2.MCGD is to drive the switch SBM for the normal boost conversion operation andgets its input from the MPPT control block as shown on the right side of Fig.5.7.3. The MPPT control is performed by sensing half of the open-circuit voltage(VTEG) as the maximum power point is found around VTEG/2 [5]. Unlike [5], wesense VTEG without detaching the TEG cell from the circuit. When Φ 1 is high, SBMis turned off and the TEG is an open-circuit with zero inductor current. In thisphase, VTEG is sampled on CS1. When Φ 2 is high, the sampled voltage on CS1 isdivided into half by sharing charge between CS1 and CS2. The clock signals, Φ 1and Φ 2 are applied externally for test purpose with a frequency of 50Hz. Comp3compares VIN with VTEG /2 and controls the boost switch SBM via MCGD so thatVIN ≈ VTEG/2 during Φ 2. The waveforms in the lower-right side show the simpli-fied inductor current, IL2, and the sample time of VTEG. The MPPT control worksas long as VOUT .[4] R. Y. Kim, and J. S. Lai , “ A Seamless Mode Transfer Maximum Power PointTracking Controller For Thermoelectric G enerator Applications, ” IEEETransactions on Power Electronics, vol. 23, no. 5, pp. 2310-2318, September2008.[5] S. K. Cho, N. J. Kim, S. S, Park, and S. H. Kim, “ A Coreless Maximum PowerPoint Tracking Circuit of Thermoelectric G enerators for Battery ChargingSystems,” IEEE Asian Solid-State Circuits Conference, November 2010.978-1-4673-0377-4/12/$31.00 ?2012 IEEE105DIGEST OF TECHNICAL PAPERS ?ISSCC 2012 / February 20, 2012 / 4:15 PMFigure 5.7.1: Block diagram of the proposed converter and principle of dual-mode operation. Figure 5.7.2: Detailed schematic of the proposed boost converter.Figure 5.7.3: Detailed circuit of the mode-change block and the MPPT- controlblock.Figure 5.7.5: Measured waveforms of the voltages during TOM and IBM operations.Figure 5.7.6: Efficiency graph and performance comparison between state-of-the-art thermoelectric energy harvesting converters and the proposed converter.Figure 5.7.4: Timing diagram of the proposed boost converter.5? 2012 IEEE International Solid-State Circuits Conference 978-1-4673-0377-4/12/$31.00 ?2012 IEEEISSCC 2012 PAPER CONTINUATIONSFigure 5.7.7: Chip micrograph and small size Transformer.