A switch is shown which operates at a rather high frequency (in the region of 20 kHz) and its duty-cycle sets the magnitude of the output voltage. The inductor is alternately connected to the dc supply and then across the output via the diode. The circuit configuration allows the inductor to charge and store energy. This happens when the switch is closed. When the switch opens, it discharges into the resistor capacitor combination in such a way that the output voltage is reversed. The voltage is reversed because the direction of the current in the inductor cannot change; it carries on in the downward direction. The current is decreasing because the battery is not connected to it any more. With the current having a negative rate of change, the voltage across the inductor is reversed. Hence the voltage across the inductor is reversed; it is opposite to the battery polarity voltage. The presence of the diode ensures that the capacitor will only charge with the polarity shown. In the steady state, the capacitor is charged to a reverse voltage determined by the on-period of the switching function. A small ripple voltage exists on the capacitor voltage and hence the output. The choice of the capacitor is such as to keep this ripple as low as possible.