A multiple-sampling circuit is proposed for measuring a voltage signal and a discharge time of a transformer. Sampling signals are used for generating hold voltages by alternately sampling the reflected voltage from the transformer. A buffer amplifier generates a buffer voltage from the higher voltage of hold voltages. A sampling switch periodically conducts the buffer voltage to produce a voltage-feedback signal. The voltage-feedback signal is proportional to an output voltage of the switching circuit. A threshold signal added to the reflected voltage signal produces a level-shift reflected signal. A discharge-time signal is generated as the switching signal is disabled. The discharge-time signal is disabled once the level-shift signal is lower than the voltage-feedback signal. The pulse width of the discharge-time signal is therefore correlated to the discharge time of the transformer. The sampling signals are enabled to generate hold voltages only when the discharge-time signal is enabled.