A modified zero-current-switched quasi-resonant (ZCS-QR) boost converter is employed in a power factor correction (PFC) application. The main goal is to achieve a small-size low-noise PFC circuit based on single-switch boost topology. A clamp diode has been added to avoid the voltage ringing problem that is originally generated across the ZCS-QR switch during its turn-off period. The converter operation states after clamp diode application are presented. The PFC circuit control scheme implementation is also described. An experimental circuit with 100 V rms ac of 50 Hz input and 330 V dc output has been built. A high efficiency of 90% and the proof of compliance to the IEC61000-3-2 class D have been confirmed by experiment.