Sudip K. Mazumder

Theoretical and experimental investigation of the fast- and slow-scale instabilities of a DC-DC converter

We use an exact formulation based on nonlinear maps to investigate both the fast-scale and slow-scale instabilities of a voltage-mode buck converter operating in the continuous conduction mode and its interaction with a filter. Comparing the results of the exact model with those of the averaged model shows the shortcomings of the latter in predicting fast-scale instabilities. We show the impact of parasitics on the onset of chaos using a high-frequency model. The experimentally validated theoretical results of this paper provide an improved understanding of the dynamics of the converter beyond the linear regime and this may lead to less conservative control design and newer applications.

A Ripple-Mitigating and Energy-Efficient Fuel Cell Power-Conditioning System

We describe an energy-efficient, fuel-cell power-conditioning system (PCS) for stationary application, which reduces the variations in the current drawn from the fuel-cell stack and can potentially meet the

Master–Slave Current-Sharing Control of a Parallel DC–DC Converter System Over an RF Communication Interface

40/kW cost target. The PCS consists of a zero-ripple boost converter (ZRBC) followed by a soft-switched and multilevel high-frequency (HF) inverter and a single-phase cycloconverter. The ZRBC comprises a new zero-ripple filter (ZRF), which significantly reduces the input low- and high-frequency current ripples, thereby potentially enhancing the durability of the stack. A new phase-shifted sinewave modulation of the multilevel HF inverter is proposed, which results in the zero-voltage switching (ZVS) of all four switches without the use of any auxiliary circuit components. For such a sine wave modulation technique, >90% ZVS range is obtained per line cycle for about 70% of the rated load. Further, the line-frequency switching of the cycloconverter (at close to unity power factor) results in extremely low switching losses. The intermediate dc bus facilitates the inclusion of power systems based on other forms of alternative-energy techniques (e.g., photovoltaic/high-voltage stack). A 5 kW prototype of the proposed PCS is built, which currently achieves a peak efficiency of 92.4%. We present a detailed description of the operation of the PCS along with its key features and advantages. Finally, experimental results showing the satisfactory performance and the operation of the PCS are demonstrated.

Robust control of parallel DC-DC buck converters by combining integral-variable-structure and multiple-sliding-surface control schemes

Using analog wireless communication, we demonstrate a master-slave load-sharing control of a parallel dc-dc buck converter system, thereby eliminating the need for physical connection to distribute the control signal among the converter modules. The current reference for the slave modules is provided by the master module using radio-frequency (RF) transmission, thereby ensuring even sharing of the load current. The effect of delay due to RF transmission on system stability and performance is analyzed, and regions of operation for a stable as well as satisfactory performance are determined. We experimentally demonstrate a satisfactory performance of the master-slave converter at 20-kHz switching frequency under steady state as well as transient conditions in the presence of a transmission delay. The proposed control concept, which can potentially attain redundancy that is achievable using a droop method, may lead to more robust and reconfigurable control implementation of distributed converters and power systems. It may also be used as a (fault-tolerant) backup for wire-based control of parallel/distributed converters.

Solid-oxide-fuel-cell performance and durability: resolution of the effects of power-conditioning systems and application loads

We develop a robust controller for parallel DC-DC buck converters by combining the concepts of integral-variable-structure and multiple-sliding-surface control. The advantages of the scheme are its simplicity in design, good dynamic response, robustness, ability to nullify the bus-voltage error and the error between the load currents of the converter modules under steady-state conditions, and ability to reduce the impact of very high-frequency dynamics due to parasitics on the closed-loop system. We describe a method for determining the region of existence and stability of the sliding manifolds for such parallel converters. The results show good steady-state and dynamic responses.

Wireless PWM control of a parallel DC-DC buck converter

We describe methodologies for comprehensive and reduced-order modeling of solid-oxide-fuel-cell (SOFC) power-conditioning system (PCS) at the subsystem/component and system levels to resolve the interactions among SOFC, balance-of-plant subsystem, and power-electronics subsystem (PES) and application loads (ALs). Using these models, we analyze the impacts of electrical-feedback effects (e.g., ripple-current dynamics and load transients) on the performance and reliability of the SOFC. Subsequently, we investigate the effects of harmonics in the current, drawn from the SOFC by a PES, on the temperature and fuel utilization of the SOFC. We explore the impacts of inverter space-vector modulation strategies on the transient response, flow parameters, and current density of the SOFC during load transients and demonstrate how these two traditionally known superior modulation/control methodologies may in fact have a negative effect on the performance and durability of the SOFC unless carefully implemented. Further, we resolve the impacts of the current drawn by the PES from the SOFC, on its microcrack density and electrode/electrolyte degradation. The comprehensive analytical models and interaction-analysis methodologies and the results provided in this paper lead to an improved understanding, and may yield realizations of cost-effective, reliable, and optimal PESs, in particular, and SOFC PCSs, in general.

A Soft-Switching Scheme for an Isolated DC/DC Converter With Pulsating DC Output for a Three-Phase High-Frequency-Link PWM Converter

We demonstrate a new concept for wireless pulse-width modulation (PWM) control of a parallel dc-dc buck converter. It eliminates the need for multiple physical connections of gating/PWM signals among the distributed converter modules. The new scheme relies on radio-frequency (RF) based communication of the PWM control signals from a master to the slave modules. We analyze the system stability and demonstrate the experimental effectiveness of the wireless control scheme for a two-module parallel buck converter for 10-kHz and 20-kHz switching frequencies and for channel lengths of 1.5 and 15ft, respectively. The proposed control concept may lead to easier distributed control implementation of parallel dc-dc converters and distributed power systems, and may lead to redundancy that is achievable using droop method. It may also be used as a backup for wire-based control of parallel converters to provide fault tolerance.

A novel discrete control strategy for independent stabilization of parallel three-phase boost converters by combining space-vector modulation with variable-structure control

This paper outlines a soft-switching mechanism based on zero-voltage-zero-current-switching (ZVZCS) principle for the front-end isolated DC/DC converter of an isolated three-phase rectifier-type high-frequency-link bidirectional power converter. In conjunction with a back-end DC/AC converter operating with a novel patent-filed hybrid modulation scheme outlined in , , and that reduces the number of hard-switched commutation per switching cycle, the proposed ZVZCS scheme can lead to less overall switching losses than other conventional switching schemes. The proposed ZVZCS scheme is effective for various load conditions, operates seamlessly with a simple active-clamp circuit, and is suitable for applications where low-voltage dc to high-voltage three-phase ac power conversion is required.

Continuous and discrete variable-structure controls for parallel three-phase boost rectifier

We propose a discrete nonlinear controller, developed in a synchronous frame, for a parallel three-phase boost converter consisting of two modules. The basic idea, however, can be extended to a system with N modules. Each of the closed-loop power-converter modules operates asynchronously without any communication with the other modules. The controller stabilizes the currents on the dq-axes and limits the flow of the pure-zero sequence current. It combines the space-vector modulation scheme with a variable-structure control, thereby keeping the switching frequency constant and achieving satisfactory dynamic performance.

A Universal Grid-Connected Fuel-Cell Inverter for Residential Application

We describe three nonlinear control schemes for a parallel three-phase boost rectifier consisting of two modules. The basic idea, however, can be extended to a system with N modules. All of the control schemes are developed in a synchronous frame. Moreover, each of the closed-loop power-converter modules operates asynchronously without any communication with the other module. Based on the dynamical equations of the parallel converter, we find that independent control of both of the modules on the DQ axes is not necessary and possible. Consequently, we develop control schemes that stabilize the dq axes and limit the zero-axis disturbance by preventing the flow of the pure zero-sequence current. One of the control schemes is developed purely in the discrete domain. It combines the space-vector modulation scheme with a variable-structure control, thereby keeping the switching frequency constant and achieving satisfactory dynamic performance. The performances of the other control schemes are also satisfactory.