Frequency Domain Modelling & Design of an LCC Resonant Converter with Capacitive Output Filter

Abstract

Resonant DC-DC converters have been widely discussed with one of the most popular being the LCC topology. It’s application towards low to high voltage converters warrants the use of a capacitive output filter to limit the voltage stress on the rectifier. Designs with a high quality factor (Q) suffer from large resonant component sizes and stresses leading to power losses in the magnetic components. It is then desirable to design a low Q converter to minimize these stresses for compactness and efficiency. Previous time domain analysis shows the converter predominantly operates in one mode so a frequency domain analysis was possible. Due to the voltage charging and clamping action of the parallel resonant capacitor as a result of the capacitive output filter, enhanced fundamental harmonic approximation (FHA) models were used to analyze this topology. These were accurate for high Q designs with approximately sinusoidal waveforms but degraded for light load, low Q conditions. In this thesis, a frequency domain model considering higher order harmonics is presented for the LCC resonant converter with a capacitive output filter. This general model applies to the study of the converter under variable frequency and phase shift modulation control techniques. Converter characteristics can be studied using the provided generalized curves of voltage gain (Vo / Vi), phase shift (ϕ), and non-conduction angle (θ). Using the nth harmonic equivalent circuit, steady state performance is easily obtained. The model is verified against a 380V, 250W experimental prototype with 22-44V input and the commercial simulation software PSIM. A simple design procedure is detailed focusing on low Q designs helping minimize the stress and size of the resonant components. An optimal parallel to series capacitance ratio (k) and Q selection helps reduce conduction and magnetic losses of the converter.