Abstract: Fully digital control implemented by DSP for the capacitor charging power supply of TEA CO2 laser was studied in order to improve its applicability and reliability.A real-time resonant period tracking scheme of "comparing-capture" was proposed,based on which comparing studies was conducted with the scheme of "sampling-calculation".Both of them can complete the tracking in 3 switching periods when the resonant current is strong and the signal noise ratio(SNR) is high,but the "sampling-calculation" scheme can not realize tracking when the resonant current is weak or the SNR is low.These experimental results and its insensibility to the waveform distortion of resonance current show that the "comparing-capture" scheme is a better tracking scheme to ensure the zero-current-switching of serial resonant circuit.