Implementation of a Single Stage AC-DC Boost Converter for Low Voltage Micro generator N.Gowthami 1 P.Ravichandran 2 S.Yuvaraj 3 1 & 2 Department of EEE, Surya Engineering College, Erode. 3 PG Scholar, ME-Power Electronics and drives, Vels University, Chennai. ABSTRACT: The conventional power electronic converters used in the micro generator based energy harvesting applications have two stages: a diode bridge rectifier and a dc-dc converter. This paper presents a single stage ac-dc boost converter topology for efficient and optimum energy harvesting from low-voltage micro generators. The proposed Boost converter utilizes the bidirectional current conduction capability of MOSFETs in order to avoid the use of a first end of the bridge rectifier. It is working in discontinuous conduction mode and offers resistive load to the microgenerator. Analysis and modeling of the converter explained in detail is presented. For low power applications, the power consumption of gate drive and control circuits should be minimal. In this paper, they are specifically designed to consume very low power supply.. A low voltage microgenerator is used to verify the performance and operation of the converter and the gate drive circuits and finally simulated by PSIM software package. Index Terms- Energy harvesting, ac-dc conversion, boost converter, low power, low voltage, PSIM I.INTRODUCTION The development of energy-efficient semiconductor devices has reduced the power requirements of electronic circuits. This has led to the development of wireless electronic devices like sensor nodes, medical implants, etc., which require only a few milliwatts for their operation. They can be powered by harvesting ambient energy from the environment in the form of light, vibration, heat, etc. [1] [8]. Electromagnetic microgenerators are particularly popular due to high energy density and are hence considered for this work. Such microgenerators are typically spring mass systems, in which mechanical energy is converted to electrical energy by electromagnetic damping (Figure. 1). The output of an inertial microgenerator is typically around a few hundred millivolts of ac. The conventional power converters [Figure. 2(a)] reported for vibration energy harvesting mostly consists of a front-end diode bridge rectifier followed by a standard buck or boost converter. This arrangement of two-stage power conversion has several disadvantages for the electromagnetic microgenerator.1) Diode voltages in a bridge rectifier are difficult to overcome for low input voltage; 2) input current is much higher than output current, leading to more losses in diodes and 3) a rectifier offers a nonlinear load, which makes the converter unsuitable for energy harvesting. Figure1. Resonance-based inertial electromagnetic microgenerator Figure.2. Block diagram: (a) Two-stage power conversion. (b) Reported dual-polarity boost converter 52 Techscripts
A dual-polarity boost converter [Figure. 2(b)] topology for such ac dc conversion was recently reported in above. This converter uses two inductors, and the output dc bus is split into two series-connected capacitors. In [9], the authors presented a direct ac dc converter which utilizes the bidirectional currentconduction capability of MOSFETs for direct ac dc conversion. The converter utilizes only one inductor and charges the output capacitor paper presented only open-loop simulation and experimental results using an ideal sinusoidal voltage source. The reported converter also utilized continuously without any line voltage sensing. The paper presented only open-loop simulation and experimental results using an ideal sinusoidal voltage source. II. REVIEW OF THE CONVERTER The converter topology utilizes a bidirectional switch for its operation. However, a single semiconductor device capable of both bidirectional conduction and blocking capability does not exist. A MOSFET channel is typically capable of conduction in both directions when it is sufficiently turned ON. However, due to the presence of the inherent body diode, it cannot block current in the reverse direction. A bidirectional switch in the present case is realized by connecting the drain of an n-mosfet to the source of a p-mosfet so that their body diodes block the current in the opposite direction. The MOSFETs are turned on and off at the same instants and thus can conduct and block currents in both directions. This bidirectional switch is referred as S1 throughout this paperthis operation has many advantages: 1) A constant duty cycle extracts constant power from the source, enabling a simple control; 2) A converter operating with a constant duty cycle has only fundamental and switching harmonic frequency components (much higher than fundamental) and thus offers a resistive load to the microgenerator; and 3) DCM operation reduces switching losses which are significant in such low-power applications. The circuit diagram for the split-capacitor topology is shown in Figure. 3. A single inductor L is used for the boost operation in both half cycles. Positive half cycle: The inductor current increases linearly from zero when switch S1 turns ON. When S1 is turned OFF, the body diodes block the circulating current. Diode D1 is forward biased, and the current flows into capacitor C2 to complete the charging process. Negative half cycle: In the negative half cycle, the current rises in the opposite direction when S1 is turned ON. However, this time, when S1 is turned OFF, diode D1 remain OFF and diode D2 is forward biased. The inductor energy is transferred to capacitor C3. Figure.3. Proposed direct ac dc converter: Split-capacitor topology. The three capacitors share energy through charge recycling. It should be noted that even though voltages across capacitors C2 and C3 show large variations, the duty cycle can be effectively controlled to maintain a steady voltage across output capacitor C1. III. CONVERTER ANALYSIS A. Input Side Analysis A bidirectional switch in the input side is used to build the inductor current. The input current waveform of the converter can be considered as shown in Figure. 4(a). Consider any kth switching cycle of the boost converter as shown in Fig. 4(b), where Ts is the time period of the switching cycle, D is the duty cycle of the converter, dfts is the boost-inductor current fall time (or the output diode conduction time), Vi is the voltage of the microgenerator with amplitude Vp, and V0 is the converter output voltage. The converter switching frequency is much higher than the generator output ac voltage frequency. Therefore, Ts is much smaller than the time period of the input ac cycle (Ti). In the analysis presented henceforth, circuit parasitics are ignored, and it is assumed that output voltage remains constant over a switching cycle. The peak value of the inductor current (ipk) for a general switching cycle can be obtained as in The inductor current fall time can be found as IPk = m1 DTs = VikDTs/L (1) DfTs = ipk/m2 = ipk/ (Vo Vik). (2) 53 Techscripts
During this switching cycle, the energy (Ek) transferred from the input to the output can be obtained as Ek = Vik ipk Ts (D + df)/2. (3) Where Vik = Vp sin (2 k Ts/Ti). Defining N = Ti/Ts, the average input power Pi of the converter can be obtained in Pi= (1/Ti) _Nk=1Ek =(1/Ti) Nk=1 Vik ipk T(D +df)/2 (4) Therefore, the average input power Pi can be derived as in Furthermore, for large N s, the summation in (4) can be approximated as integration. Therefore, the average input power Pi can be derived as in Pi = V 2 pd 2 Ts β 4L1 2 β = (1/ ) sin2 θ [1 (Vp/V0) sin θ] -1.dt (5) 0 Figure.4. Converter input current: (a) Input current over one cycle of microgenerator output voltage. (b) Input current over one switching cycle Figure.5. DCM switch model Figure.6. (a) Port 1: Voltage V1 and current I1 (b) Port 2: Voltage V2 and current I2 B. Output Side Analysis A switch-diode low-frequency model can be formed by averaging over the switching frequency, as shown in Figure. 6 This allows us to study the input-current- and output-voltage-related characteristics. The port currents I1 (t) and I2 (t) can be averaged over the switching cycle according to the DCM operation. Their waveforms are shown in Figure 6. The three time periods DTs, dfts, and dsts correspond to the three states of the converter in each switching cycle. During period DTs, switch S1 is ON. The period dfts corresponds to the time when S1 is turned off but the inductor current is still nonzero, and dsts is the discontinuous state of the converter. The waveforms for the port 1 voltage and current are shown in Figure. 6(a). Their averaged values can be derived as V1avg (t) =df [Vo (t) Vi (t)] + (1 D)Vi(t) as I1avg (t) =0.5ipeak (D + df) Ts... (6) Ts The current I2 (t) corresponds to the diode current. The voltages V2 (t) and I2 (t) are similarly averaged 54 Techscripts
V2avg (t) = Vo (t) I2avg (t) =0.5ipeakdfTs. (7) Figure. 7. Proposed energy-harvesting system using PSIM IV.CONVERTER IMPLEMENTATION In low-power applications, the implementation of auxiliary circuits (gate driver and control circuits) is very important. They should be chosen such that they consume very low power and are able to drive the circuit in steady state. In this section, a gate driver circuit and an auxiliary dc supply with startup circuit are presented. A control strategy with its analog implementation is also described. A. Feedback and Control Circuit Figure.8 Proposed gain values for P MOSFET & N MOSFET A simple PI controller is implemented to regulate the output voltage. As can be seen in Figure. 8, the negative rail of the output voltage is not the same as the ground of the control circuit which is the common node of the split capacitors. Therefore, a specific feedback circuit for the converter has to be designed. For the split-capacitor topology, capacitors C2 and C3 are chosen to have equal value. The voltages across these two capacitors are shown in Fig. 11. It can be found from previous analysis that the average voltage across any of these split capacitors is half of the output voltage. Therefore, this voltage can be used as feedback to the controller B. Gate Driver Circuit The bidirectional switch is realized using an n-mosfet (Mn) and a p-mosfet (Mp) connected in series (Figure. 13).The source of the n-mosfet is connected to the ground. The source of the p-mosfet is connected to the drain of the n-mosfet. Such an arrangement gives switch S1 bidirectional current capability and ability to block the reverse conduction through body diodes. The schematic of the gate driver circuits is shown in Figure. 11 along with the overall converter system. In this converter, the MOSFETs are driven with respect to the common node of the split capacitors, i.e., C2 and C3. Comparator CP1 is used to drive the n-mosfet. Since the source of the n-mosfet Mn is connected to the ground, it can be driven with a conventional low-side driver. Comparator CP2 is used to drive the p-mosfet Mp using a negative gate pulse. It should be noted that the MOSFET Mp is driven with respect to ground instead of its source Sp. However, since the voltage drop across MOSFET Mn is very small during conduction, the gate drive voltage can turn on the MOSFET Mp properly. C. Startup Circuit It can be observed that the controller and driver circuits require a dual dc supply for their operation. The low ac input voltage cannot be used to start the converter. A suitable circuit, as shown in Figure. 11, is used as startup circuit. The voltage nodes V+ A and V A denote the positive and negative dc voltages which power the controller and gate driver in the converter system. Batteries E1 and E2 provide the startup power to charge capacitors Ca and Cb through diodes Dc and Dd. With the controller and driver circuits operating, capacitors Ca and Cb start getting charged by the microgenerator through diodes Da and Db. This boost 55 Techscripts
mechanism is similar to the charging of capacitors C2 and C3 in the split-capacitor topology. Furthermore, these capacitors are designed to maintain steady dc voltage while powering the auxiliary circuits. In steady state, the voltages across capacitors Ca and Cb can be approximately related as V + A = V0 and V A = V0. The nominal value of battery voltages are chosen to be less than steady-state voltages of capacitors Ca and Cb. V. RESULTS The prototype for the converter was developed to verify its operation. The values of the key components of the converter are presented in Table I. TABLE 1.CONVERTER CIRCUIT COMPONENTS The auxiliary circuits were developed using micropower components. The gate driver circuit is realized using a low voltage comparator TLV2760 from Texas Instruments which has a nominal current requirement of 20 µa per channel. A resonance-based linear microgenerator capable of producing 400-mV ac at 108 Hz is used as input to the converter. An electro dynamic shaker is used to produce vibrations for the electromagnetic microgenerator. The converter is operated in DCM to reduce switching losses. Figure.9 PWM waveforms for MOSFETs Mn and Mp (1 V/div) A resonance-based linear microgenerator capable of producing 400-mV ac at 108 Hz is used as input to the converter. An electro dynamic shaker is used to produce vibrations for the electromagnetic microgenerator. The converter is operated in DCM to reduce switching losses.. PARAMETER VALUE PARASITIC Switching frequency 10000Hz - Output voltage 3.3V - Capacitor C1 22µF Resr=33mΩ Capacitor (C2 &C3) 4.7µF Resr=5mΩ Inductor (L) 10 µh Resr=80mΩ N & P MOSFET 20V,6A Rdson (N&P MOSFET)=30mΩ40mΩ Figure10. Input waveform from microgenerator in PSIM VI. CONCLUSION This paper has presented a split-capacitor-based ac dc boost converter for low-power low-voltage energy harvesting. A bidirectional switch, based on series-connected n- and p MOSFETs, has been proposed in this paper. The converter utilizes this bidirectional switch to boost the low ac microgenerator voltage to a steady dc voltage in both the input half cycles. The modeling and analysis for the converter has been presented. The auxiliary circuits in the energy-harvesting converter the gate driver circuits and the control circuit have been designed for low-power operation. A suitable startup circuit, an auxiliary dc supply, and a feedback circuit are proposed for the implementation of the converter. Experimental results for a lowvoltage microgenerator have been presented to verify the operation of the converter and the proposed 56 Techscripts
auxiliary circuits. The designed auxiliary circuits draw minimal power and are able to operate the converter at a high efficiency. REFERENCES [1] J. A. Paradiso and T. Starner, Energy scavenging for mobile and wireless electronics, Pervasive Comput., vol. 4, no. 1, pp. 18 27, Jan. Mar. 2005. [2] J. Krikke, Sunrise for energy harvesting products, Pervasive Comput., vol. 4, no. 1, pp. 4 5, Jan. Mar. 2005. [3] S. Meninger, J. O. Mur-Miranda, R. Amirtharajah, A. P. Chandrakasan, and J. H. Lang, Vibration-toelectric energy conversion, IEEE Trans. Very Large Scale Integr. (VLSI) Syst., vol. 9, no. 1, pp. 64 76, Feb. 2001. [4] M. El-Hami, P. Glynne-Jones, N. M. White, M. Hill, S. Beeby, E. James, A. D. Brown, and J. N. Ross, Design and fabrication of a new vibration based electromechanical power generator, Sens. Actuators A, Phys.,vol. 92, no. 1 3, pp. 335 342, Aug. 2001. [5] T. M. Thul, S. Dwari, R. D. Lorenz, and L. Parsa, Energy harvesting and efficient power generation from human activities, presented at the Center Power Electronics Systems (CPES) Seminar, Blacksburg, VA, Apr. 2007. [6] J. R. Amirtharajah and A. P. Chandrakasan, Self-powered signal processing using vibration-based power generation, IEEE J.Solid- State Circuits, vol. 33, no. 5, pp. 687 695, May 1998. [7] D. Dondi, A. Bertacchini, D. Brunelli, L. Larcher, and L. Benini, Modeling and optimization of a solar energy harvester system for self-powered wireless sensor networks, IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2759 2766, Jul. 2008. [8] A. Nasiri, S.A.Zabalawi, and G. Mandic, Indoor power harvesting using photovoltaic cells for lowpower applications, IEEE Trans. Ind. Electron, vol. 56, no. 11, pp. 4502 4509, Jul. 2008. 57 Techscripts