A single-ended primary-inductor converter (SEPIC) circuit has at least a circuit input node and a circuit common node, and includes an inductor, a first coupling capacitor, an isolation transformer, a controllable switch, a second coupling capacitor, and a clamp diode. The inductor is electrically connected in series between the circuit input node and the first coupling capacitor. The first coupling capacitor is connected in series between the inductor and the first primary input terminal. The controllable switch is electrically connected in series between an internal circuit node and the circuit common node, and the internal circuit node is located between the inductor and the first coupling capacitor. The second coupling capacitor is electrically connected in series between the second primary input terminal and the circuit common node. The clamp diode is electrically connected in series between the internal circuit node and the second primary input terminal.