Power limit circuit for computer system

Abstract

A method for controlling the power level of a computer system or the like provides a dual power range feature which allows a power supply to be used over a wide range of AC input voltages. The power control circuit employs a detector for the RMS voltage of the input power line, and a detector for the AC input current at the power line. These detected levels are applied to a multiplier to provide an indication of power input, multiplier output is compared with a reference to produce a control for the output power of the power supply. The duty cycle of an oscillator for an AC-to-DC conversion for a power factor control circuit within the power supply is the controlled element, using pulse width modulation. A dual power range is provided so that for low AC line voltages, where the AC line current would tend to be high, a lower power limit is in effect, then for higher AC line voltages, where AC line current is proportionally lower for a given power level, a higher power limit is allowed. This is accomplished by a simple and efficient circuit, in an example embodiment; when the RMS voltage of the AC input exceeds a certain level, the values of resistors in a voltage divider within the RMS voltage detector circuit are changed so that a different power limit is provided which is in excess of the first power limit. The switchover from one power limit to another is done by a non-linear switch device such as a diode turning on and adding a resistor in the voltage divider.