## Resonant LLC

A resonant converter consists of a voltage or current source, a resonant network, and a voltage or current load. The source and load characteristics determine the most suitable resonant network. Power transfer in resonant topologies by definition happens primarily in the fundamental component of the switching signals. The resulting currents and voltages in the topology are piecewise sinusoidal.

An important property of resonant converters is the fact that circuit parasites can be functionally used in the converter resulting for example in zero voltage switching of the primary side MOSFETs. As a consequence, switching frequency of the converter can increase without certain additional switching losses and the size of the power components can reduce. This is in contrast to pulse-width-modulated converters where the parasites hamper the converter functionality and degrade efficiency. Resonant topologies however, suffer from circulating current not contributing to the real power transfer to the output. The effectiveness of a resonant converter can be expressed in terms of a utilisation factor.

The LLC configuration is an example of a three element resonant converter. High voltage gain capability for long holdup time can be an advantage. On top of that zero current switching is achieved for the secondary side rectifiers. Synchronous rectification is not trivial from a drive perspective. There is no obvious correlation between the primary side driving signals and the required secondary side synchronous rectifier switches.

The two inductors in the LLC topology might be integrated in one and the same transformer. The magnetising inductor functions as the inductor parallel to the load. The leakage inductor functions as the series resonant inductor.

Commonly, the First Harmonic Approximation (FHA) is used to analyse the power transfer characteristics of the LLC converter. The resonant tank is treated as a band pass filter and when operated close to series resonance, the assumption is that only the first harmonic signals contribute to power transfer. Distortion of the resonant current or voltage and discontinuity in the conduction angle of the output rectifiers however result in approximation mismatch.

Resonant LLC converters are very promissing in many applications. ZeoN PowerTec has the knowledge and experience to support customers with the design of power supplies based on this topology including the definition and design of the required magnetic components.