A supply voltage generating circuit for generating a supply voltage signal to supply the active elements of an AC-DC voltage converter. The supply voltage generating circuit has a charging switch, a charging diode and a charging capacitor. When a main switch of the AC-DC voltage converter is turned on, the charging switch is turned on. Primary current flows through the charging switch and the main switch to the logic ground. When the main switch is turned off and the voltage across the charging capacitor is smaller than a charging threshold, the charging switch is kept on for a period of time and the primary current flows through the charging switch and the charging diode to the charging capacitor. When the period of time is expired or the supply voltage signal is larger than the charging threshold signal, the charging switch is turned off.