Abstract
A general approach for the analysis and design of self-oscillating resonant converters is presented in this paper, for a particular class of circuits in which the change of input voltage polarity is caused by the zero-crossings of the input inductor current. The key features of the method are an analytical description in the time-domain of a spiral that eventually converges into an ellipse, and a frequency–domain analysis that explains the behavior of the ellipse as a limit cycle. On a theoretical basis, this class of circuits behaves as loss-free resistors (LFR) because in steady-state the input inductor current is in phase with the first harmonic of the input voltage. The proposed analytical procedure predicts accurately the amplitude and frequency of the limit cycle and justifies the stability of its generation. This accuracy is reflected in the close agreement between the theoretical expressions and the corresponding simulated and measured waveforms. Third and fourth order resonant converters are designed following simple guidelines derived from the theoretical analysis.
Highlights
A variable structure system (VSS) is characterized by the change of its physical configuration depending on its internal state
An experimental prototype of the LCC resonant converter has been implemented taking into account the set of parameters employed in the simulations of Figures 4 and 5, and a controller based on the sign of the inductor current
The delay effect can be compensated by the use of appropriate networks as it has been recently reported in the design of a 6.78 MHz self-oscillating resonant converter [35]
Summary
A variable structure system (VSS) is characterized by the change of its physical configuration depending on its internal state. Switching converters are a clear example of VSS because their change of structure and, the subsequent repetitive sequence of circuits take place when a function of the converter variables attains a certain value. This function usually depends on a combination of the state-variables and an external signal establishing the switching period as, for example, in pulse-width modulated (PWM) converters. As in PWM converters, the corresponding change of polarity is forced in most of the cases by means of an external signal given by the control system setting the frequency of the excitation [1]. The change of polarity can be determined by the change of sign of some converter variables, or a function of them, this being the basis of the self-oscillating resonant conversion mode
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