A power converter for an LED lighting device includes a primary side circuit, a secondary side circuit, a detecting circuit, a load-dependent circuit, and a feedback circuit. The feedback circuit generates a feedback signal according to a detected signal which the detecting circuit generates according to a second node of the secondary side circuit. The primary side circuit adjusts a duty cycle according to the feedback signal. The secondary side circuit outputs a first output voltage at a first node of the secondary side circuit and outputs a second output voltage at a third node of the secondary side circuit, according to the duty cycle of the primary side circuit. The load-dependent circuit receives the second output voltage and controls an electrical strength between the first node and the second node of the secondary side circuit according to the first output voltage.