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Benefits and drawbacks of the LLC and LCC Resonant Power Supplies

The resonant topologies are, to this day, among the most efficient for designing SMPSs.

The market requiring more efficiency in the areas of LED lighting, server farms, gate automation, battery chargers, vending machines, etc. is a widely known fact.

Besides the marketing driven requirements, many standards impose higher and higher efficiency targets.

This has led to a great revaluation of the LLC (and in some cases LCC) resonant topology in the last few years.
See a comparison between LLC and LCC here.

All the major active components manufacturers on the SMPS market have therefore developed controllers allowing the realization of high efficiency converters with low EMI  emissions, with a particularly low circuit complexity.


  • Referred to 24Vdc output, 95-96% LLC stage typical efficiency with low-cost design; up to 98% with synchronous rectification;
  • with correctly designed magnetic components, the design is very quick and simple;
  • high frequency current waveforms are quasi-sinusoidal, with a very limited harmonic content compared to other topologies;
  • ZVS (Zero Voltage Switching) of MOSFETs enables a dramatic reduction in power loss, heatsink size, component stress and EMI, which are among the most hostile design problems;
  • possibility of low power consumption at light-load/standby using the burst mode and PFC stop features implemented in many controllers;
  • high peak power easily achievable even with small dimensions; as an example, our transformer with dimensions equal to the common EF25 size can easily reach 500-1000Wpk;
  • as a result of the previous points, LLC and LCC power supplies have smaller size and considerably reduced thermal and EMI problems, compared to other topologies;
  • for applications sensitive to common-mode current injection through the transformer (i.e. Isolated IGBT and SiC driver supply), the best-in-class primary/secondary capacitance is attained.
Itacoil LLC resonant tank

Critical issues

  • Optimal designing of the SMPS must necessarily consider also the constraints linked to magnetic components, otherwise it may result in a considerable efficiency reduction;
  • the definition of the optimized integrated transformer requires specific skills;
  • the necessity of 2 MOSFETs (half bridge) compared to the single MOSFET in flyback converters;
  • slightly more expensive controller compared to a flyback converter, but thanks to the lower spending for heatsinks, EMC filter, more compact transformer, etc., the total cost is low for powers exceeding 40-50W.

If you want to see some of the simplest circuit solutions and the related comparative tests, open the links below:

Original demo boards

Improved demo boards

Our expertise

We can supply standard transformers with already correctly designed resonant tanks, study optimized variants of these tanks and collaborate with the electronic designer defining custom resonant tanks in order to resolve every specific need.

We invested a lot for the capability of giving a quick and reliable technical support.
With the acquired experience and the support of internally developed proprietary software, we are able to design taking into account skin effect, proximity effect, etc., as well as all the structural constraints.

Do not waste your resources:

Even a well-made power supply project is often widely optimizable when reviewed with our proprietary algorithms.
They take into account at the same time the relationships between the various parameters, all the structural constraints and the losses in the tank elements, in particular the magnetic ones, allowing, as widely demonstrated, the highest level of optimization.

Our support to the electronic designer in the initial phases allows, in addition to a drastic reduction in development time, the best results in terms of efficiency, temperatures, costs and dimensions.

Proprietary design tools

The integrated transformer

This definition refers to a transformer with an integrated resonance inductance obtained through controlled magnification of the leakage inductance (see our technical article published on various european industrial electronic magazines: englishgerman).

Our standard transformers series covers the most common voltage and power requirements.

To this day converters that, for design simplification, use a discrete inductor in addition to a conventional transformer are still quite common, but this choice is inefficient under economic, energy and dimensional aspects.

Another benefit is the robust and high insulation between input and output, side effect of the primary and secondary windings being placed into separate sections of the bobbin for generating a higher leakage inductance.

We can quickly design and sample custom integrated transformers optimized by-design. The need for reiterations and resampling is practically non-existent.


Comparison of the winding layouts in the traditional transformer and in the integrated transformer.
See also