Patent ID: 6995481

Claim:
A power control device for connection to AC mains electric power supply having a power supply frequency and an AC load, comprising: a bi-directional electric power switch capable of being switched at a switching frequency that is a multiple of the power supply frequency, connected in series between the AC mains power supply and the load and comprising two switching elements connected in series and oriented in opposite polarities, each switching element having an “off” condition in which the switching element substantially blocks a flow of current in one direction, a bi-directional electric circulating switch capable of being switched at the switching frequency, for parallel connection to the load, comprising two switching elements connected in series and oriented in opposite polarities, each switching element having an “off” condition in which the switching element substantially blocks a flow of current in one direction; and a control module for controlling the switches, comprising an oscillator running at the switching frequency, a duty cycle control circuit controlling the oscillator, the duty cycle controlled by a reference signal and having a high portion and a low portion, a power switch driver coordinated to turn at least one element of the power switch “on” during one of the high and low portions of the duty cycle and to turn the elements of the power switch “off” during the other of the high and low portions of the duty cycle, and a circulating switch driver coordinated to turn one element of the circulating switch to the “on” condition when a positive current is flowing in the load and the other element of the circulating switch to the “on” condition when a negative current is flowing in the load, and during a crossover lag zone encompassing zero crossing points of the voltage and current and during which the voltage and current have opposite polarities, both elements of the power switch being switched to the “on” condition, whereby the circulating switch is operative to supply a circulating current to the load during intervals when the mains power supply to the load is interrupted.