ReducingIncreasing Tuning Capacitor for Frequency-Modulated Spread-Spectrum Inductive Power Transfer

In frequency-modulated spread-spectrum (FMSS) EMI suppression, the switching frequency of the transmitter is continuously modulated across below and above the nominal frequency. This operation gives challenges to series-resonant inverter, such as zero-voltage switching failure and output current degradation, which occur due to mismatch between the fixed LC resonance frequency and the modulated switching frequency. This frequency mismatch can be solved by tuning the LC resonance frequency using the variable resonant capacitance. However, previous methods of capacitance tuning have limitations - they cannot both increase and reduce the effective capacitance, and they exhibit higher harmonic distortion of coil current. Hence, they cannot be used for FMSS EMI suppression. To solve these issues, we propose a resonant capacitor tuning that can both increase and decrease the capacitance with zero-voltage switching (ZVS). Its harmonic distortion in coil current is less than conventional capacitor tuning methods. The proposed method prevents the degradation of output current at detuned frequencies allowing both the main inverter and the tuning MOSFETS to achieve ZVS. Measurements show that it can improve the overall efficiency by 8%-11% point in spread-spectrum operation across 75-95 kHz. At farthest end of detuned switching frequencies, the maximum achievable output current is increased by 1.75 times.