Demystifying capacitor energy loss using LTSpice

The power of LTSpice stems from the fact that it is a free and widely used software for electrical simulation and analysis. It may not be as user friendly as other commercial software for running complicated simulation. However, we can always find some ways if we dig deeper into the vast resources available on the internet as well as LTSpice Help files.

Recently, I was trying to simulate a capacitor charging simulation which I have already done using MATLAB-Simulink.

The simulation was simple, and I was sure that it could be done using LTSpice. I started building my simulation model. Once I placed the two capacitors (C1, C2), one inductor, L1 (ESL), one resistor, R1 (ESR), I had to specify the initial voltage of one capacitor.

With my first google search, I found the spice directive. I specified the initial charge of the capacitor, C1, and C2 as shown below.

I ran the simulation. I was disappointed that I did not see the results I was expecting. My simulation in MATLAB Simulink provided the correct result in a single run. A little later, I realized that I have not specified the initial current in the circuit. Once I specified the initial current, it worked to my satisfaction.

The main objective of this simulation is to show that when a charged capacitor charges an initially discharged capacitor, the energy loss is a combination of conduction and radiated losses. In power electronics, the damping is high, and the frequency of oscillation remains relatively low. Therefore, we tend to ignore the EM radiation losses and say that energy is lost in the ESR of the capacitor. However, for very high frequency of oscillation (i.e. RF/Microwave oscillation), a significant amount of energy loss will be through EM radiation.