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An active power-factor correction (PFC) circuit is presented that employs a newly proposed resistor-free zero-current detection (ZCD). While the conventional ZCD requires either a sensing resistor or auxiliary transformer, the proposed ZCD requires only one OFF-chip capacitor. The active PFC circuit with the proposed resistor-free ZCD has been implemented in a 0.35-μm BCDMOS process and the power factor is improved up to 9% from the one employing the conventional ZCD. The proposed resistor-free ZCD scheme can be applied to any type of switch-mode dc-dc power converter.