Power control system

A pulse-width modulation (PWM) controller 103 toggles a control switch TR1 between ON and OFF states. The voltage across a charge storage unit such as capacitor C5 increases as it is charged and is detected at an input RT/CT. A voltage sustain unit 105 holds the voltage of the charge storage unit C5 at a holding voltage which is less than the timing voltage threshold of the pulse-width modulation controller 103. The sustain unit can be connected in parallel with the capacitor and comprise a resistor R4 whose resistance sets the holding voltage and a switch such as transistor TR6 which is toggled in response to the output of the PWM controller. The PWM controller can be used in a switch mode power supply which it enables to operate using near to one hundred percent duty cycle, eliminating dead time, and at high efficiency where the desired output voltage is close to the input voltage.