System and method for conversion of high voltage ac to low voltage dc using input voltage gating

An apparatus for converting high voltage alternating current (AC) power input to a low voltage direct current (DC) power output, comprising:

• a rectifier for receiving a high voltage AC line power input and for outputting a full wave rectified waveform;
• an electronically actuated switch coupled to receive the full wave rectified waveform and provide a voltage-modulated pulse output in response to a gating signal;
• a input voltage detector for providing an input voltage signal corresponding to the voltage of the AC power input;
• a threshold voltage generator connected to receive the low voltage DC power output and provide a threshold voltage signal that varies in proportion to the low voltage DC power output;
• voltage comparator for comparing the input voltage signal to the threshold voltage signal and providing a control signal when the input voltage signal is below the threshold voltage;
• a pulse generator for generating the gating signal at a predetermined frequency in response to the control signal;
• a transformer having a primary coil coupled to the switch to receive the voltage-modulated pulse output and a secondary coil with rectifier that provides the DC power output;
• and
• a filter capacitor for smoothing the DC power output,
• whereby the circuit provides a low voltage DC power output using a transformer, and filter capacitor having voltage ratings less than the voltage peak of the AC power input by only gating power though the switch to the transformer and filter capacitor when the AC power input voltage is below a predetermined value.