In a fly-back voltage converter that includes a transformer to transform a primary current to a secondary current and a switch serially coupled to the primary winding to switch the primary current in response to a control signal, a detection signal is produced by comparing the secondary current with two threshold values after the primary current is switched off to trigger the next on-time cycle of the switch. Once the secondary current is detected to be greater than a first threshold value, it is determined that the secondary current has been switched on, and until the secondary current is detected to be lower than a second threshold value, it is determined that the secondary current is to be switched off. The hysteresis range of the threshold values prevents error detection of the secondary current.