Jari Leppäaho

Issues on Solar-Generator Interfacing with Voltage-Fed MPP-Tracking Converters

Abstract The large-scale harvesting of solar energy is an important action to decelerate the observed climate changes and ensuring the availability of energy also in the future. Reliably operating solar-energy systems composing of solar arrays and their interfacing converters are of prime importance to maximize solar-energy harvesting. The main obstacle for the improvements in the reliability is turned out to be the lack of full understanding of the solar-generator interfacing. The paper investigates the interfacing in terms of voltage-fed maximum-power-point (MPP) tracking converters. The investigations confirm that there are two reasons for the observed problems of which the one is the violation of Kirchhoff’s current law governing the operation of a solar generator and the other is the phenomenon known as negative resistance oscillation (NRO) causing instability. In practice, this means that the voltage-fed converter can reliably operate only at the voltages higher than the maximum-power-point (MPP) voltage. Practical evidence is provided based on a coupled-inductor superbuck converter supplied by a real solar panel and loaded with a storage battery.

Implementing current-fed converters by adding an input capacitor at the input of voltage-fed converter for interfacing solar generator

The concern on observed climate change has increased the utilization of renewable energy sources. The harvesting of solar energy is recognized as one of the key issues in reducing green house gas emission. Reliable solar-energy systems composing of solar arrays and their interfacing converters are of prime importance in uninterrupted solar energy production. The interfacing maximum-power-point converters are implemented usually by modifying the conventional voltage-fed converters. Actually, the modifications change the converter into a current-fed converter with corresponding steady-state and dynamic properties. The paper investigates the true properties of these transformed converters based on theory and practical measurements. As an example a direct-duty-ratio-controlled voltage-fed buck converter is shown to be transformed into a current-fed boost-type converter.

Characterizing the Dynamics of the Peak-Current-Mode-Controlled Buck-Power-Stage Converter in Photovoltaic Applications

It is well known that the application of peak-current-mode (PCM) control produces highly desirable properties in the conventional or voltage-fed converters but requires to compensate the inductor-current loop for extending the active duty ratio beyond 50%. This paper investigates the dynamical properties and operational constraints of a PCM-controlled buck-power-stage converter in the photovoltaic (PV) applications. It has been already earlier noticed that the application of PCM control can make the converter unstable in the PV applications. The investigations reveal that the inductor-current feedback forces the open-loop converter to adopt the dynamic properties of a voltage-fed converter, and consequently, the stable operation is limited to the constant-voltage region of the PV generator. The developed small-signal models reveal that a right-half-plane pole appears in the converter dynamics, when the operation point enters into the constant-current region leading to instability. The stability can be restored by heavily compensating the inductor-current loop, which seriously removes the desired properties materialized in the conventional buck converter. The provided experimental evidence fully supports the theoretical findings.

Issues on solar-generator-interfacing with voltage-fed converter

Reliably operating solar-energy systems composing of solar arrays and their interfacing converters are of prime importance to maximize the solar-energy harvesting. The paper investigates the solar-generator-interfacing problems in terms of voltage-fed maximum-power-point tracking converters and shows that there are two main reasons for the observed stability problems of which the one is the violation of Kirchhoffs current law governing the operation of the solar generator and the other the phenomenon known as negative resistance oscillation. In practice, this means that the voltage-fed converter can reliably operate only in the voltages higher than the maximum-powerpoint voltage. Practical evidence is provided based on a coupled-inductor superbuck converter.

Dynamics of current-fed converters and stability-assessment of solar-generator interfacing

Large-scale harvesting of solar energy is an important action to decelerate the observed climate changes and ensuring the availability of energy also in the future. It has been observed that the maximum-power-point (MPP) tracking converters have high contribution on the reliability problems in the photovoltaic energy systems because of misunderstanding of their real nature. The paper shows that most of the applied converters are current-fed converters but usually assumed to be conventional voltage-fed converters. The general dynamic properties of a current-fed converter and the method to assess the stability of an interconnected system composing of a solar generator and a MPP-tracking converter are introduced. Theoretical findings are supported with experimental evidence.

On EMI-filter interactions in a regulated converter - stability and load-transient performance

The EMI-filter design rules based on certain impedance parameters are developed in the late 1970s by means of a small-signal equivalent circuit valid only for the converters operating under direct-duty-ratio control in continuous conduction mode. Basically the design rules are valid for an arbitrary switched-mode converter but the implicit nature of some of the critical parameters has prevented the full understanding of the design process. The practical design rules usually dictate that the EMI-filter output impedance has to be designed less than the open and closed-loop input impedances of the converter. The paper shows consistently based on theory and practical evidence that the critical impedances are the closed-loop and short-circuit input impedances of which the short-circuit input impedance usually dominates in the interactions. The voltage and peak-current-mode buck converters are used as the source of practical evidence.

Dynamic Properties of PCM-Controlled Superbuck Converter – Discrete vs. Coupled Inductor Implementation

Abstract Fourth-order converter known as superbuck converter is used in the applications requiring continuous input current such as interfacing solar generators or improving power factor of a converter. Peak-current-mode control is used to reduce the resonant behaviour of the converter for facilitating the control design. The input-current ripple can be reduced substantially by applying coupled-inductor technique. The paper investigates the dynamic effects the application of the coupled-inductor technique causes in a superbuck converter. The dynamic features of the higher-order converters are usually not well known due to the complexity of the analysis. The coupled-inductor technique further increases the complexity and therefore, its real dynamic effects are not properly analyzed. The investigations show that the peak-current-mode-controlled superbuck converter with coupled inductors will become almost useless even if the original converter has features resembling that of an optimal converter. Experimental evidence is provided to validate the theoretical findings.