In this paper, a current sensorless adaptive secondary-side control is proposed for the series resonant converter (SRC), making it a good candidate for MHz 48 voltage regulators. By means of varying ac equivalent resistance, the output voltage regulation is achieved by controlling the duty cycle of the synchronous MOSFET. By taking advantage of the secondary-side control, SRCs can operate under zero-voltage switching at any input voltage and load conditions. A current sensorless adaptive digital control is proposed to control the current-type synchronous MOSFETs, which automatically compensates the delay caused by the current-sensor circuit and the gate driver, and also eliminates the current sensor in the power path. A hybrid control strategy is introduced to overcome the efficiency degradation caused by the secondary-side control.