In this paper an airgap length of 0.5 mm has been assumed. At an airgap of 0.5 mm, a maximal error of 5% can be found between the measured and analytical calculated inductances.It should be noticed that rotating the cores with a small airgap in between, requires an accurate assembly of the transformer.
B. Electric model
To complete the electric equivalent circuit, winding resis-tances, Rp, Rs and resonant capacitors, Cp, Cs have been added to the circuit, as shown in Fig. 9.
1) Winding resistance: The voltage applied from the half bridge converter to the rotating transformer has a square-waveform, which gives rise to the AC-losses due to harmonics.An analytical expression for the wire resistance in case of non-sinusoidal waveforms has been derived by [9], based on Dowell’s formula for AC-resistance.
2) Resonant capacitors: On both sides of the transformer,resonant capacitors have been added to overcome the voltage drops across the leakage inductance, by locally boosting the voltage and, thereby, increasing the magnetic flux density.Resonance capacitors can be placed in series or parallel to the winding at either side of the transformer.
On the primary side the resonance capacitor has been placed in series to act as a DC-blocking capacitance and to create a zero crossing resonance voltage. This makes it possible to use zero-current switching, to minimize the switching losses.Placing the primary capacitor in parallel would results in a high current in the resonance loop due to the high frequency input voltage. This current would increase the power losses and should therefore by avoided.
On the secondary side the resonant capacitor has been added to boost the power transfer capability. Figure 10 shows the normalized value of Cp