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Schematic of ZVS driver Schematic of ZVS driver

Output waveform of transformer Output waveform of transformer

Determining frequency from time difference between peaks of output waveform at secondary coil of transformer Determining frequency from time difference between peaks of output waveform at secondary coil of transformer

I was simulating a ZVS driver in LTspice and while the frequency of the output waveform matched up with f = 1 / (2π √LC) when the transformer had not been added yet. With L and C being the value of the inductor and capacitor at the output respectively. Taking C as 1 µF and L as 4 µH, f is equal to 79.577 kHz.

After adding the transformer, the frequency changed. By taking the time between the peaks of the waveform to be 17.592 µs, and using the formula 1/T, you get 56.844 kHz as the frequency.

Hence my question: What exactly causes this and how can I calculate the frequency properly?

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Two coupled 2 µH inductors is not equivalent to 4 µH, but 8 µH. By adding the transformer, you also coupled the two primaries, which doubles the total inductance (assuming k = 1 as in your simulation). Calculating the resonant frequency with this in mind, the result is pretty close to your measurement at 56.3 kHz. The remaining error could be from the way you measure frequency by looking at time between the peaks. Looking at timing between zero crossings can be more accurate, as the rate of change there is higher, and measuring over multiple cycles can further improve accuracy.

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