LLC resonant converter

On this page RubinoLab.com presents several photos of a prototype and experimental tests of a 3kVA bidirectional resonant DC-DC converter, made for research activities.

The work was commissioned by AIRBUS, a major European company operating in the aerospace and defense sector, and was part of the doctoral thesis of Luigi Rubino, a member of RubinoLAB. The converter is an evolution compared to the classic hard-switching converters in which we participated in 2008-2009 for a European project called “Moet” More Open Electrical Technologies.

The LLC converter, required many hours of work to develop all the mathematical models before the realization taking into account also the parasitic effects of the components. Furthermore, all the parts that are difficult to find, such as transformers, resonant capacities, high thickness copper PCBs, MOS drivers for frequencies up to 300kHz, measurement and control boards have been realized in our laboratories and compared with mathematical models. The result, the measurements are identical to the simulations.

Particular design accuracy was given to the resonant transformer and to the resonant capacities not found by the component distributors.

Only the magnetic parts were purchased for the transformer, while the coils are suitably machined copper plates isolated from each other.

The resonant capacities at the primary and secondary are the most critical components in the system, since they must keep the value stable even when the working temperature changes. A minimal variation in the capacity value varies the resonance frequency of the circuit and therefore we will no longer have the maximum power transfer. The number of SMD capacity to be parallelized was chosen keeping in mind the capacity value and the working current.

To evaluate the correctness of the parameters of the entire circuit, a LabView interface was created where it was possible to characterize the resonant circuit by varying the circuit frequency.

Figure shows the prototype during the test phases.

In the video we can see the typical measurements with varying working frequency.

Scientific articles:

1. Complementarity Model for Steady-State Analysis of Resonant LLC Power Converters
2. LLC resonant converters in PV applications comparison of topologies considering the transformer design