This time we will use LTspice to run the simulation of the linearized model LLC converter.
In order to calculate the value of each element on the linearized model above, we use the document “Designing an LLC Resonant Hlaf-Brdige Power Converter” from Texas Instrument. The figure below shows the implementation of those calculations into LTspice.
As can be seen from the picture below the value of each element is assigned by the equation on the top side. All we need to do before running this simulation is to set all of the parameters on the left side based on our specifications. The Ln and Qe values will be tuned several times until we find the best values.
VDC is the input voltage of the LLC resonant converter.
Vout is the output voltage of the LLC resonant converter.
Pout is the output power of the LLC resonant converter.
n is the transformer turn ratio.
RL is the load of the LLC resonant converter which can be calculated based on the power rating.
Qe is the quality factor of the LLC tank.
fres is the resonant frequency of the LLC tank, it can be chosen arbitrarily.
Ln is the normalized value of the resonant inductor over magnetizing inductor.
k is the gain factor which depends on the MOSFET configuration either Full_bridge or Half-bridge.
After inputting all of the parameter values above now, we can start to run the simulation.
Simulation type: AC sweep
Parametric analysis: Ln and Qe
As we want to see the voltage gain of the resonant tank over different frequencies, we need to plot the waveform of the output (Voe) over the input (Vge) as can be seen from the picture below.
We can play around with the value of the Ln and Qe until we decide the values that fit our design criteria. The simulation file can be downloaded below.