LLC Resonant power supply - The inductive components - LLC RESONANT POWER SUPPLY

The need for high performance power supply systems for LED lighting and, in general, the increasingly greater request for energy efficiency, both regarding running costs and compliance with recent Standards, has brought about a revaluation of the LLC series resonant topology. (SRC).
All the main manufacturers of active components currently available on the SMPS market have included efficient chips in their product catalogues. With an effectively contained degree of circuit complexity, they allow the realisation of power supplies with 90-96% efficiency, (which can be improved further using synchronous rectifiers instead of output diodes) and reduced EMI/EMC problems in comparison to other topologies thanks to the "Zero Voltage Switching" and to the substantially sinusoidal high frequency currents.
The operating principle is based on the characteristic gain curve of the resonant tank".

Fig. 1 – Inductance current wave form of a Transition Mode PFC (simulation from ITACOIL software)

which allows to change the gain by a moderate variation of the switching frequency, thus resulting in an effective regulation of output voltage or current in relation to load and input voltage changes.
The resonant "tank" is a set of two inductive elements and one capacitor (LLC). Even if the use of three different components, i.e. a discrete inductor, a conventional transformer and a capacitor is technically possible, poor results would be obtained on all fronts: cost, size and energy efficiency.
The use of an integrated transformer is much more convenient, which has specific features and integrates resonant inductance as described below.
To give an idea of the advantages, if a 150 W integrated transformer is well-dimensioned it can have dimensions of less than 28x29x23 mm, with costs that are obviously more competitive with respect to solutions with discrete inductance.

Fig. 2 - 150W integrated transformer

It is therefore evident that the only reason that can lead to this solution is the design difficulty of a coherent tank.
While the designs of the largest manufacturers of electronic equipment generally show a correct structuring of the integrated transformer and other inductive components, mid-sized or small manufacturers - even if all SMPS manufacturers enjoy the same technology advantages in relation to active and passive components - very often do not have the same benefits in regards to fundamental components, such as the integrated transformer and PFC stage inductor, given the specificity of the inductive components and the limitation of resources destined to the project.

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