14-12-2012, 02:58 PM
THE RESONANT CONVERTERS
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INTRODUCTION
In fact the introduction of new regulations, both voluntary and mandatory, has brought about a revaluation of the LLC series resonant topology.
In LED lighting, to name just one of the infinite applications, the need for more and more efficient power supply systems is pushing power designers in this direction.
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) reduced EMI/EMC problems in comparison to other topologies thanks to the "Zero Voltage Switching" and to the substantially sinusoidal high frequency currents.
"Generally speaking, resonant converters are switching converters that include a tank circuit actively participating in determining input-to-output power flow."
The operating principle is based on the characteristic gain curve of the resonant tank", which allows to change the gain by a moderate variation of the switching frequency, thus resulting in an effective regulation of 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 discrete inductance.
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 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.
BENEFITS
- typical range of efficiency for the simplest circuitry 94-96%, with possible improvements through synchronous rectification and other small adjustments;
- utilising correctly sized magnetic components, the design is rapid and noteably simplified;
- the current waveform at high frequencies is basically sinusoidal with significant reductions in harmonics with respect to other topologies;
- MOSFET commutation ON "ZVS" (Zero Voltage Switching) with associated elimination of commutation loss, reduction/elimination of dissipators and reduction of stress and EMI, which are often the causes of the hostile design problems;
- the possibility to reduce consumption with low/zero load by utilizing the functions "burst mode" and "PFC stop" implemented on many controllers;
- the possibility of optimal sizing for continuous and temporary power including some significant improvements on conventional solutions (eg. an optimised transformer with volume similar to a classic EF25 can supply to 200-250W);
- with respect to other topologies, with the attributes described above, LLC supplies have reduced dimensions with a noteable reduction in EMI/EMC issues.
- typical cost savings on dissipators, EMC filters, smaller transformers etc.
DRAWBACKS
- one necessary factor to be considered for the optimised design of power supplies is the magnetics, the ommission of which means to renounce a significant potential in improved efficiency;
- the design of the optimised integrated transformer requires specific competence;
- two MOSFETs are required (half bridge) rather than the single MOSFET for the Flyback;
- the controller is slightly less economic than the flyback (this cost increase tends to cancel itself, when it is not transformed into a saving, thanks to the minimal spendings on other components).
Lots of documents are available to extend the topic, let's take a look on some examples.