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In this blog I show how to use Ansys Maxwell to create a 3D FEA model of a DCDC Resonant LLC Converter which is excited with pulses. Resonance is created in the primary winding using the excitation frequency and leakage inductance of the primary winding. The secondary is rectified and connected to a 1kW load. 




The Ansys Maxwell 3D FEA model has a PQ 50/50 core. Below is information for the magnetic core used from the manufacturers specification sheet including the complex permeability vs frequency, nominal values, and the schematic of core used which is made of 3C97 material. It has one primary winding (54 turns), and two secondary windings (split, 6 turns per half). Quarter symmetry is used to decrease the simulation time.

The core loss vs magnetic flux density curves, P vs B, for various temperatures from the manufacturer's data sheet were used in Maxwell to extract the core loss coefficients. The Magnetic Transient solver uses these core loss coefficients in a dynamic core loss model which uses time derivatives of B and H produced by any periodic excitation, and not limited to sinusoidal excitation. 


FERROXCUBE PQ Core: Schematic


FERROXCUBE Core Material 3C91: Specifications


FERROXCUBE Core Material 3C91: P vs B curves for different operating frequencies.




The power electronics circuit below is a DCDC Resonant LLC Converter using a positive pulse and a negative pulse in series to model a Full Bridge Inverter on the primary side. The negative pulse is delayed half cycle relative to the positive pulse and there is a dead time to prevent overlap of the pulses. The secondary is a split-transformer rectifier connected to an output filter capacitor in parallel to a resistive load. Because of the dead time in the excitation, the dominant frequency (resonant frequency) of the square wave is slightly less than the excitation frequency. The resonant capacitor is computed using the resonant frequency.

DCDC Resonant LLC Converter: Square Wave Voltage Input - Split Transformer Rectified Filtered Output.


The electrical circuit variable are fully parameterized as shown in the table below. Some variables are input directly and units are specified, and other variables are computed in terms of variables and the units for these variables are not specified in the variable definition but is specified where they are used. Maxwell always converts any variable expressed in terms of variables to a value in SI units.

Circuit Parameters.


Square wave voltage input excitation.


Positive and negative pulses used to create the square wave voltage excitation.



Here are results showing the output voltage, winding currents, magnetic flux density distribution, and the magnetic fringing effect in the air gap. The average output voltage is about 49.4V and the ripple is about 3.1V, and these values can be adjusted by tuning the duty cycle and output filter capacitor. There is a slight current imbalance with the split secondary windings and this will be investigated and corrected later. Possibly a finer mesh is required to have balanced currents.


Rectified filtered output voltage.


Resonant winding currents.


Magnetic flux density distribution.


Magnetic flux density distribution around the air gap - fringe flux effect.





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Post by David A. Giglio, PhD, PE
June 10, 2024