Maximum power point tracking converter

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<p>Maximum Power Point Tracking Converter Based on the Open-Circuit Voltage Method for Thermoelectric Generators</p> <p>Maximum Power Point Tracking Converter Based on the Open-Circuit Voltage Method for Thermoelectric GeneratorsSHIVAM SUNDRAMReference: IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 2, FEBRUARY 2015</p> <p>Contents:-IntroductionTE Power Generating DeviceO.C. Voltage MPPT MethodTheoretical AnalysisMPPT Overvoltage Snubber MPPT Synchronous Buck-BoostExperimental ResultsConclusionReferences</p> <p>Introduction:-Thermoelectric generators (TEGs) convert heat energy into electricity.</p> <p>The MPPT method based on the open-circuit voltage is arguably the most suitable for the linear electrical characteristic of TEGs.</p> <p>Theorem of maximum power (MP) transfer if the load matches the internal resistance then MP is produce.</p> <p>Due to relatively high cost and low efficiency --around 5% use has been in the past.</p> <p>TEGs can now be successfully employed to harvest the heat energy rejected by other processes.</p> <p>Low conversion efficiency is not a serious drawback but the power obtain from the device should be maximum.</p> <p>Introduction:-</p> <p>T.E. Power Generating Device</p> <p>Composed of n- and p-doped semiconductor pellets.Electrically connected in series and thermally in parallel.TEGs open-circuit voltage is directly proportional to T.</p> <p>Comparison Of Some Geometrical And Electrical Parameters Between Two TEG Devices</p> <p>TEGs Electrical Characteristic</p> <p>Open Circuit Voltage MPPT MethodA TEG is connected to the input of a Buck or Buck- boost derived converter.Under Normal operation,Mcap is closed and contributes the input current.When the O.C Voltage measurement is required, Mcap gets opened.</p> <p>Timing Diagram For Open-Circuit Voltage Measurement</p> <p>Theoretical Analysis of MPPT EfficiencyThis section presents a theoretical analysis to quantify the losses introduced by :-The additional switch Mcap in series with Cin and;The VOC measurement technique used</p> <p>In order to find the losses</p> <p>It is necessary to calculate the RMS current ICinRMS flowing into the input capacitors, and</p> <p>To understand the converters behavior in response to the measurement technique</p> <p> Plots of the currents in a Buck or Buck-Boost derived converter without Mcap Inductor Current (IL blue, and ILAVG orange); Switch Current (IS red);Average Input Current (Iin light blue); Input Capacitor Current (Iin green)</p> <p> During toff, Cin is charged by the current ICin,off =VOC/2RWhere R= Rint + ESRThe RMS current in capacitor Cin =Power dissipated in Mcap=Where ron is the on-resistance of the switch usedThe switching and body diode losses of Mcap are almost irrelevant because they occur for a few microsecond every Tmeas.</p> <p>MPPT CONVERTEROver Voltage Snubber (DCD circuit)The Non-Inverting Synchronous BuckBoost DCDC Converter.</p> <p>Over Voltage SnubberWhen a TEG is suddenly disconnected from the load,Due to the presence of parasitic inductance LP,A very fast underdamped oscillation with frequency usually in the order of MHz.Such parasitic inductance forms a resonating tank with the parasitic capacitances of the circuit and it is damped by the TEGs internal resistance Rint</p> <p>Experimental Switching Transient TestAt the beginning of the transient,t0 the voltage sharply rises from Vload to VmaxThis increases the switching transition losses on M1 and it also requires M1 to have a higher maximum drainsource voltage rating.</p> <p>Diode Capacitor Diode (DCD) Snubber CircuitTo improve the TEGs transient response</p> <p>A Capacitor (Cs) with two anti parallel connected diodes DS1 and DS2</p> <p>Cs is sufficiently large compare with parasitic components to store the energy.</p> <p>Diodes are used to add some damping due to their conduction resistances.</p> <p> Experimental and simulation results have proven thatThe over voltage is reduced from 18 to 1V in both casesThe settling time is shorter when the two diodes are used</p> <p>Synchronous Buck-BoostA non inverting synchronous Buck-Boost using four switches.Output switch is replaced by a schottky diode (prevents battery discharge in non conduction mode) Microchip PIC16F microcontroller activates the gate drivers with two 180 anti-phase PWMs. The microcontroller measures the TEG voltage at the converters input and the batterys voltage Vb at the output. The algorithm calculates PWMs duty cycle using relation: 2Vb/VoC = D/(1 D).</p> <p>19</p> <p>Synchronous Buck-BoostThe converters electrical efficiency with a power supply in series with a fixed 6- power resistor is 92.6% when tested at 30.4W (13.5V, 2.25A) input.</p> <p>After 45 s Mcap is switched off the input voltage goes to VOC , during toff , after an overshoot of less than 6V when using a DCD snubber. The ADC measurement starts 2 s after the PWM goes low.</p> <p>RESULTS Experiments were designed to test the steady state and transient performance of the proposed MPPT converter.</p> <p> First, the steady-state performance is measured using TEGs. Next, effect of VOC transient is measured. Finally, effect of a thermal transient was tested to analyze TEGs characteristics.</p> <p>A. Steady-State PerformanceExperiment is to compare the power extracted by the MPPT converter to the MP available from the three TEMs, maintained at the same temperature difference.The three modules are electrically connected in series. 1) open-circuit voltage, VOC,S = VOC,1 + VOC,2 + VOC,3 2) theoretical current for MP: IMP = VOC, S ____________ 2(R1 +R2 +R3 ) 3) the theoretical MP: Pmax = VMPIMP = V2OC, S ____________ 4(R1 +R2 +R3 )</p> <p>A. Steady-State PerformanceThe MPPT converter has an accuracy (tracking efficiency) of 99.85%.</p> <p>Comparison of the steady state tracking performance of the MPPT converter with the maximum available power from the series connected array.</p> <p>B. Sudden-Transient PerformanceAllows characterizing the settling response of the converter after a step change in the open-circuit voltage.TEGs have been replaced by a power supply in series with a power resistor of 4.7.VOC step from 10 to 20 VThe MPPT converter regulates the input voltage to half of VOC in 8 ms.</p> <p>B. Sudden-Transient Performance</p> <p>Converters input voltage after a VOC step-up from 10 to 20 V. Time: 1 ms/div (x-axis); Voltage: 5 V/div (y-axis).</p> <p>C. TEG Transient PerformanceExperiment assesses the ability of the MPPT converter to respond to changes of temperature gradient.The TEGs are initially maintained at 200 C, the temperature difference diminishes at a rate of 0.25 C/s.This experiment is effectively a continuous series of steady-state experiments because the thermal time constant of the TEG system is much slower than the transient response of the converter.the average tracking efficiency of the MPPT converter is 98.7%.</p> <p>C. TEG Transient Performance</p> <p>Thermal transient from T = 200 C to T = 100 C across the three TEGs. Available and extracted output power on the left y-axis and temperature difference on the right y-axis.</p> <p>CONCLUSIONThe MPPT algorithm is programmed to a low-cost microcontroller and does not require expensive sensors;It checks the open-circuit voltage every 500 ms and accurately adjusts the optimum operating point in less than 10 ms. The converter used is a dcdc non inverting synchronous Buck-Boost (93% efficient), which can work in Boost, Buck-Boost or Buck mode; this guarantees the harvest of power over a wide range of temperature differences across the TEG.It is able to harvest close to 100% of the MP. </p> <p>ReferencesAndrea Montecucco, Andrew R. Knox (October 7, 2014.). Maximum Power Point Tracking Converter Based on the Open-Circuit Voltage Method forThermoelectric Generators. IEEE transactions on power electronics, Vol. 30, NO. 2, february 2015. pp 828-839. [Online] available at Digital Object Identifier 10.1109/TPEL.2014.2313294.D. Rowe, Thermoelectrics, an environmentally-friendly source of electrical power, Renewable Energy, vol. 16, pp. 12511256, 1999.M. Orellana, S. Petibon, B. Estibals, and C. Alonso. (2010, Nov.). Four switch buck-boost converter for photovoltaic DCDC power applications. in Proc. 36th Annu. Conf. IEEE Ind. Electron. Soc. [Online]. no. 1, pp. 469474. [Online]. Available: Montecucco, J. Siviter, and A. R. Knox, Simple, fast and accurate maximum power point tracking converter for thermoelectric generators, in Proc. IEEE Energy Convers. Congr. Expo., 2012, pp. 27772783.</p>


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