L. Palma

An inverter output filter to mitigate dV/dt effects in PWM drive system

High switching frequencies and small rise times are common with modern switching devices. These parameters, especially the rise time, are known to have undesired side effects such as excessive voltage stress on windings, motor leakage currents, bearing currents, conducted EMI, etc. Furthermore, if long cables are used to connect the inverter and the motor, the rapid rise times cause the transmission line effect, which can double the voltage applied to the motor terminals. In this paper a new inverter output filter to mitigate these problems is presented. The filter topology has the advantage of being easy to implement on existing ASDs as an add-on option and it reduces both common and differential mode dV/dt simultaneously. Its effectiveness is shown analytically via simulations and by experimental results on a 20 kW ASD.

A High Gain Transformer-Less DC-DC Converter for Fuel-Cell Applications

In this paper analysis and design of a new transformer-less high gain DC-DC converter for fuel cell inverters is presented. Due to the low input voltage in fuel cell applications a high voltage gain DC-DC converter is required. Typically to meet this requirement transformer based converters are used, with the consequent cost and size increase. As an alternative to the traditional solution the proposed converter provides a voltage gain of 5 in per unit without the need of a transformer. This contributes to a significant reduction in size and cost while maintaining high conversion efficiency. Moreover the proposed converter produces a split DC-link which can be independently controlled. This feature is ideal for bi-phase inverters to produce an AC output from a fuel-cell energy source. Converter analysis, and experimental results obtained from a 1 kW 45 Vdc to 200 Vdc prototype are presented in this paper

Design considerations for a fuel cell powered dc-dc converter for portable applications

Fuel cells are emerging as primary power source for portable applications. The output voltage from a fuel cell suffers from a 2 to 1 variation from no load to full load. Furthermore, staking fewer cells in series results in a less complex fuel cell stack. In view of this a boost type dc-dc converter becomes necessary for powering the electronics. The purpose of this paper is to explore design considerations for a fuel cell powered dc-dc converter for portable applications. First, methods to determine electrical equivalent circuit for small portable fuel cells is discussed. The equivalent circuit is then employed to evaluate the system transient response for load changes and determine the effects of continues and discontinues conduction operation of the dc-dc converter on the fuel cell performance. Finally, experimental results on 20W and SOW PEM fuel cell powered dc-dc converters are presented.

An approach to improve battery run-time in mobile applications with supercapacitors

In this paper an approach to improve battery run-time in mobile applications with supercapacitors is explored. The performance of a battery-supercapacitor combination is analytically described using simplified equivalent circuit models. It is shown that there is an overall reduction in the internal losses and this translates into increased run-time. Three possible approaches are explored: (a) supercapacitors connected directly across the battery; (b) battery-inductor-supercapacitor connection; and (c) supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented. These results show an increase in runtime of 4-12% is achievable. Also from these results it is shown that the use of a DC-DC converter appears to be a cost effective option, since it allows the use of reduced number of capacitors while maintaining a comparable performance.

Analysis and evaluation of a series-combined connected boost and buck-boost dc-dc converter for photovoltaic application

In this paper it is presented an approach for power conditioning of a PV (photo-voltaic) cell array. In particular, an array of solar cells is employed as an alternative renewable resource (ARR) to provide clean electric energy to remote rural residential or industrial nonlinear loads. The approach employs a series-combined connected boost and buck-boost dc-dc converter for power conditioning of the dc voltage provided by a photo-voltaic array. The input voltage to the combined converters is 100 V provided from two series-connected PV cells, which is converted and increased to 400 V at the dc output voltage. Series-combined connected boost and buck-boost dc-dc converters operate in alternate fashion each other. This helps to reduce the input ripple current and provide the required 400 Vdc on a sinusoidal PWM three-phase inverter. Analysis of the two series-combined dc-dc converters is presented along with simulation results. Evaluation of the combined and series connected topologies is carried out to compare their performance against individual and separate power conditioning structures. Simulations of the series-combined dc-dc converters are presented with an output dc voltage of 400 V and a maximum output load of Po = 600 W

Analysis of DC-DC Converter Stability in Fuel Cell Powered Portable Electronic Systems

Fuel cell is an emerging power source for portable electronic systems. The steady state DC output of a fuel cell suffers from a 2 to 1 voltage variation from no load to full load. A boost type DC-DC converter is employed to generate a regulated output voltage. The internal impedance of the fuel cell consists of a membrane resistance R m , and two parallel resistor/capacitor (R p1 -C 1 ; R p2 -C 2 ) elements related to the electron transport phenomena in the anode and cathode. It is shown that this internal impedance can influence the dynamic response of the DC-DC converter, often in a manner that degrades regulator performance. In this paper the effect of the fuel cell internal impedance on the dynamic performance of the power converter is fully analyzed. Design inequalities are reviewed in per-unit quantities to better understand the interaction between the converter, fuel cell and potential instability conditions. An approach to utilize supercapacitors to enhance stability and improve dynamics is explored. A method to calculate the value of the supercapacitor required is also detailed. Finally, experimental results obtained on a 30W PEM fuel cell system powering a DC-DC boost converter are discussed in detail.

A Hybrid DC-DC Converter for Fuel Cells Powered Laptop Computers

In this paper a hybrid dc-dc converter for a fuel cell powered laptop computer is proposed. In the proposed system the ac-dc adapter, the fuel cell and its components are assumed to be packaged into one external unit. The purpose of the hybrid dc-dc converter is to suitably control the energy flow from fuel cell, battery and supercapacitor to enable all day computing. A design example is presented to appropriately size the fuel cell stack, Li-Ion battery and supercapacitor modules required for a typical electrical load on a laptop computer. Analysis, design and control aspects of the hybrid dc-dc converter are presented to meet performance requirements. Simulation results verify the performance of the system under various input and output power conditions. Experimental results obtained with the prototype system employing a 30 W fuel cell show that the hybrid converter is capable of successfully interfacing the internal dc-link with the multi-input power source. The source switching between the fuel cell and battery is seamless from the dc-link standpoint as the voltage remains stable regardless of the employed source.

A converter topology to interface low voltage Solar/Fuel Cell type energy sources to electric utility

Solar and fuel cells are some of the most promising energy sources for future energy generation. However due to their variable and low magnitude voltage a power conditioner capable of providing a large voltage gain is required. This is especially true in applications supplying power to AC loads or the electric grid in which a voltage gain around 8 p.u. is required. The traditional approach to implement the power conditioner in single phase applications is by cascading an isolated DC-DC converter with a DC-AC inverter. However this results in a system of larger size and cost due to the need of a high frequency transformer and the presence of a large second harmonic component in the DC link current. An alternative to the traditional approach is presented in this paper in which a high gain transformer-less DC-DC converter and Z-source inverter are used. It is shown that due to inductor losses the maximum voltage gain of the Z-source inverter is limited. And therefore a two stage approach is a more suitable solution. It is also shown, by means of simulations, that by combining the Z-source inverter with the transformer less DC-DC converter a system with improved performance is obtained.

A cost effective power converter to improve CO tolerance in PEM fuel cell power systems

In fuel cell systems reformers are used to obtain hydrogen from fuels such as natural gas producing a relatively high concentration of carbon monoxide as by-product (up to 500 ppm at start-up). This poses a serious problem to the operation of PEM fuel cell stacks reducing its active area due to poisoning; and thus its terminal voltage and output power. This paper presents a cost effective solution for the CO poisoning problem, based on a low cost, reduced power rating converter. It is shown that the proposed system allows the operation of the stack with contaminated fuel for extended periods of time with minimal output power degradation. Additionally the proposed system has the advantage of being an add-on solution that can be connected to existing power conditioner units, being rated only for 10% of the nominal power of the main converter.

An Integrated Silicon Carbide (SiC) Based Single Phase Rectifier with Power Factor Correction

Silicon carbide (SiC) based power devices exhibit superior properties such as very low switching losses, fast switching behavior, improved reliability and high temperature operation capabilities. These properties contribute toward the ability to increase switching frequency, decrease the size of passive components and switches, and reduce the need for cooling, thus making the devices an excellent candidate for AC/DC power supplies. In this paper a SiC based integrated single phase rectifier with power factor correction (PFC) is presented. The proposed topology has many advantages including fewer semiconductor components; the presence of AC side inductor resulting in reduced EMI interference, and higher performance. This approach takes advantage of the superior properties of SiC devices and the reduced number of devices in the proposed converter to achieve higher efficiency, smaller size and better performance at high temperature. A performance and efficiency evaluation of the rectifier is presented and the results are compared with benchmark Si solutions