Laser Copy Machine

The efficiency of switch mode power converters has increased with improvements in the switches used in the conversion process. However, while synchronous rectification has been utilized on step-down or buck converters in the output stage of DC-DC converters, rectification at the front-end of the converter where the primary AC-to-DC conversion occurs, has been ignored. Although rectifiers of various forms have been developed, power converters continue to utilize the classic four-diode bridge rectifier to perform the AC to DC conversion.

When a four-diode bridge rectifier is in operation, two of the four diodes are conducting at all times, resulting in continuous conduction losses through the bridge diodes that manifests itself in the form of waste heat. The generation of waste heat, in turn, typically necessitates the use of fans and large heat sinks to provide forced convection cooling of the converter. The diode conduction losses and the power to operate the convection fans represents energy that could otherwise be conserved or utilized for other purposes.

Many electronic devices, including laser printers and copy machines, are at or near their maximum design limit for power use allowed by a single power cord and a standard 15 ampere receptacle and will require two power cords if this limit is exceeded. Environmental Protection Agency “low power limit” requirements are also becoming increasingly difficult to satisfy.

Electronic devices, including laser printers and copiers, would benefit from a more efficient primary AC-to-DC conversion process that conserves energy, requires less space, and eliminates the need for heat sinks and forced convection cooling to remove waste heat.

The present invention provides a synchronous full-wave AC rectifier connectable to an AC voltage source. In one embodiment, the synchronous rectifier includes a full-wave diode bridge having four bridge diodes. A power switch is coupled in parallel with each bridge diode, and each power switch has a power switch control gate. A power switch control circuit is coupled in parallel with each bridge diode. Each power switch control circuit provides a first control voltage to its associated power switch control gate to cause the power switch to turn on when the parallel-connected bridge diode is forward biased. Each power switch control circuit provides a second control voltage to its associated power switch control gate to cause the power switch to turn off when the parallel-connect bridge diode is reverse biased. The power switch control circuits turn their associated power switches on and off synchronously with the AC voltage source to provide the full-wave rectified AC output.

In a second embodiment, the synchronous rectifier includes a full-wave diode bridge having four bridge diodes. A power switch is coupled in parallel with each bridge diode, and each power switch has a power switch control gate. A comparator circuit is coupled in parallel with each bridge diode and is coupled to the power switch control gate of the parallel-connected power switch. Each comparator circuit compares the voltage level across the parallel-connected bridge diode to a reference voltage. Each comparator circuit provides a first control voltage to the power switch control gate when the voltage level across the bridge diode exceeds the reference voltage level to turn on the parallel-connected power switch. Each comparator circuit provides a second control voltage to the power switch control gate when the voltage level across the bridge diode is less than the reference voltage level to turn off the parallel-connected power switch. Three charge pump circuits provide a proper biasing voltage to the comparator circuits so that the comparator circuit can properly bias their associated power switch control gate. The first charge pump circuit is coupled to the first comparator circuit, the second charge pump is coupled to the second comparator circuit, and the third charge is coupled to the third and fourth comparator circuits. The comparator circuits turn their associated power switches on and off synchronously with the AC voltage source to thereby provide a full-wave rectified output.

One aspect of the present invention provides a method for providing a full-wave rectified AC output from an AC voltage source having an AC voltage level. The method includes receiving the AC voltage level for the AC voltage source and applying the AC voltage level to a full-wave diode bridge rectifier having four diodes.