This document discusses different control methods for offline power factor correction (PFC) converters, including average current mode control, peak current mode control, and analytical control law methods. It then focuses on describing a borderline current mode PFC design that uses constant on-time control to achieve power factor correction. The converter senses the inductor current and uses a multiplier and error amplifier to impose a sinusoidally varying peak current setpoint that is proportional to the rectified input voltage, causing the input current to follow a sinusoidal envelope and correcting the power factor. The on-time is kept constant over a line cycle in borderline mode, resulting in frequency variations as the off-time changes to maintain a constant switching period.