Abstract: The disclosed power source apparatus includes a direct current (DC) source connected to the primary side of a transformer through a main switching element, a shunt circuit connected in parallel to the main switching element the shunt circuit being a serial circuit having an auxiliary switching element and a capacitor, and a control circuit connected to the main switching element so as to operate the main switching element at a duty ratio for generating a pulse-width-modulated output voltage at the secondary-side of the transformer depending on the duty ratio. The control circuit is also connected to the auxiliary switching element so as to turn off the auxiliary switching element when the main switching element is on or "closed" and to turn on the auxiliary switching element when the main switching element is off or "open". The auxiliary switching element and the capacitor in the shunt circuit are such that the shunt circuit has a small impedance at low switching frequencies of the main switching element.