State-Plane Analysis of Anomalous Step-Up Behavior in Series-Resonant Converters

This paper presents the analysis of a bidirectional series-resonant converter (SRC) for light loads working above or below resonance. In normal operation (heavy load), it is well known that the SRCs have a maximum voltage conversion ratio (m) equal to one. This occurs when the switching frequency (f
_S
) equals the resonant frequency (f
₀
) and power losses are neglected. This maximum can also be obtained for f
_S
<; f
₀
when the switches are turned ON during a half-resonant period. Thus, zero-current switching can be achieved at both the turn-ON and turn-OFF processes. However, anomalous stepup behavior is found when the converter is attached to light loads. A comprehensive analysis of this phenomenon in the timedomain and in the state plane is presented. Based on the latter, an expression that estimates m and shows the influences of the parameters is obtained. With the aid of this expression, a solution that mitigates that effect is presented and implemented in a prototype, which is part of a bidirectional power chain for supercapacitors. This solution can be implemented for both f
_S
> f
₀
and f
_S
<; f
₀
.