Tine Konjedic

Simplified Mathematical Model for Calculating the Oxygen Excess Ratio of a PEM Fuel Cell System in Real-Time Applications

The oxygen starvation phenomenon is a dangerous operating condition that reduces the lifetime of PEM fuel cells. The detection and prevention of this undesired phenomenon require estimation of the oxygen excess ratio λO2. The mathematical complexities of the reported methods for obtaining λO2 complicate its real-time calculation and require high-performance computational devices, which significantly increase the costs of the system. In this paper, a mutual information approach is used in obtaining a simplified mathematical model for the calculation of λO2. The usage of such a simplified model requires much less computational power for real-time monitoring of the variable λO2, while it provides comparable results to those obtained by using the complex model. Therefore, it represents a cost-effective solution, suitable for usage within applications that require high sampling frequencies, like emulators, converter and air compressor control loops, simulations, etc. In order to validate the accuracy of this simplified λO2 calculation model, a real-time monitoring system was built and experimentally tested using both the simplified and complex models. The matching experimental results validate the proposed simplification and justify the use of this simplified model within real-time monitoring applications.

Fast Transitions Between Current Control Loops of the Coupled-Inductor Buck–Boost DC–DC Switching Converter

An already published current control strategy for the coupled-inductor buck-boost converter is able to change its aim from controlling the input current to controlling the output current, and vice versa, depending on the instantaneous operation point and the applied current references. The main drawback of the two PI-based control implementation is its slow response when the control aim is changed from one current to the other. Due to the magnetic coupling, the converters control-to-input and control-to-output current small signal transfer functions exhibit similar first-order characteristics. Therefore, it is possible to transform the previous control scheme to a PI-based one that exhibits faster and, in certain cases, much faster transitions between input and output current control operation. The presented experiments also show that the steady-state behavior of the converter is unaffected by the new control implementation.

Hysteretic Transition Method for Avoiding the Dead-Zone Effect and Subharmonics in a Noninverting Buck–Boost Converter

A new hysteresis window method is proposed as a solution for avoiding the operational dead zone that exists at the transition between buck and boost operating modes in all noninverting buck-boost converters. In addition, this method also eliminates the discontinuities in the converters steady-state output voltage transfer characteristic, which is a function of the duty cycle. The converters output voltage function is surjective and, therefore, smooth mode transitions are achieved. The negative effects of operating within the dead zone are shown by the presence of subharmonics in the output voltage, increased output voltage ripple, poor regulation, and the instability of the converter during the transition between buck and boost operating modes. The dead-zone avoidance technique proposed in this paper eliminates all these issues while at the same time ensures highly efficient operation of the converter. An additional advantage of the technique is its simplicity, which allows for implementation into low-cost digital signal controllers, as well as into analog control circuits. The advantageous features of the proposed approach were evaluated on the basis of comparisons with three other dead-zone avoidance approaches and the initial case, which does not utilize any dead-zone avoidance technique. All the experiments were carried out on a purpose-built prototype of a noninverting buck-boost converter with magnetically coupled inductors.

Identification of a Proton-Exchange Membrane Fuel Cell’s Model Parameters by Means of an Evolution Strategy

This paper presents the parameter identification of an equivalent circuit-based proton-exchange membrane fuel cell model. This model is represented by two electrical circuits, of which one reproduces the fuel cells output voltage characteristic and the other its thermal characteristic. The output voltage model includes activation, concentration, and ohmic losses, which describe the static properties, while the double-layer charging effect, which delays in fuel and oxygen supplies, and other effects provide the models dynamic properties. In addition, a novel thermal model of the studied Ballards 1.2-kW Nexa fuel cell is proposed. The latter includes the thermal effects of the stacks fan, which significantly improve the models accuracy. The parameters of both, the electrical and the thermal, equivalent circuits were estimated on the basis of experimental data using an evolution strategy. The resulting parameters were validated by the measurement data obtained from the Nexa module. The comparison indicates a good agreement between the simulation and the experiment. In addition to simulations, the identified model is also suitable for usage in real-time fuel cell emulators. The emulator presented in this paper additionally proves the accuracy of the obtained model and the effectiveness of using an evolution strategy for identification of the fuel cells parameters.

DCM-Based Zero-Voltage Switching Control of a Bidirectional DC–DC Converter With Variable Switching Frequency

This paper investigates a control approach for achieving reliable zero-voltage switching transitions within the entire operating range of a conventional nonisolated bidirectional dc-dc converter that utilizes synchronous rectification. The approach is based on operation in the discontinuous conduction mode with a constant reversed current of sufficient amplitude, which is achieved by load-dependent variation of the switching frequency. This paper focuses on the obtained resonant voltage transitions and provides analytical models for determining the reversed current and timing parameters that would ensure safe, reliable, and highly efficient operation of the converter. In addition, the proposed approach solves the synchronous transistors spurious turn-on and body diode reverse recovery induced issues, does not require any additional components or circuitry for its realization, and can be entirely implemented within a digital signal controller. The effectiveness and performance of the presented control approach was confirmed in a 1-kW experimental bidirectional dc-dc converter that achieved 97% efficiency over a wide range of output powers at switching frequencies above 100 kHz.

An Efficiency Comparison of Fuel-Cell Hybrid Systems Based on the Versatile Buck–Boost Converter

This paper extends the use of the versatile buck–boost converter to power manage a parallel hybrid system topology as an alternative to the well-known serial hybrid (SH) topology and the most recent series–parallel hybrid (SPH) topology. These systems utilize a proton exchange membrane fuel cell as the primary source in combination with an auxiliary storage device (ASD), and the selected converter is in charge of the power management between the sources (fuel cell or ASD) and the load. Therefore, the converter has a very important role in the system since it is responsible of ensuring a dc-bus voltage regulation with a safe and reliable operation of the entire system while also guarantee a high power conversion efficiency. Hence, this is the third topology, where the coupled-inductor dc–dc buck–boost converter is studied to demonstrate and exploit its advantages such as noninverting voltage step-up and step-down, high efficiency, regulation of input and output currents and low ripple values, and the ability to change from input to output current regulation loop, suddenly and smoothly, and vice versa. In order to determine which topology (SH, PH, or SPH) exhibits the highest power conversion efficiency under a certain load profile, it is important to ensure a fair efficiency comparison that will only reflect the properties of the topology and not its individual components. Therefore, the same design criteria, the same control, and the same components were used for all the studied topologies. Simulation and experimental results have been validated on a 48-V 1200-W dc bus.

Microinverter with pulse-density modulated HFAC-link and active decoupling circuit

This paper proposes a pulse-density modulated flyback-based microinverter with high-frequency ac link and active decoupling circuit. The converters structure and basic operating principle are analyzed. The advantages of utilizing an active decoupling circuit and the demand for using pulse-density modulation are explained. The operation of the proposed microinverter was verified by experiment.

Implementation of voltage-to-frequency converter in digital based control for step-down DC-DC converter

The measurement units based on the voltage-to-frequency conversion for voltage and current control in step-down DC-DC converter is considered in this paper. This approach enables the digitalization of the both control loops; i.e. voltage and current control loops. The voltage current measurements are performed by using appropriate voltage control oscillators (VCO) and counters, which represent the digital “integrators”, and enables full digitalization of current and voltage control loop.