Circuit and method for reducing the size and cost of switch mode power supplies

Abstract

An improved circuit and method for decreasing the size and cost of switch mode power supplies is disclosed. In one illustrative example, a feedback circuit for a switch mode power supply is disclosed that includes two error amplifiers. The input of a first error amplifier is coupled to a voltage source associated with a load for the supply, and the input of a second error amplifier is coupled to a current source also associated with the load for the supply. Each one of the error amplifiers includes an adjustable shunt regulator that functions as an amplifier stage and diode. Also, the outputs of the shunt regulators are connected together. Therefore, the shunt regulator of the first error amplifier can be used to select a voltage limit for the power supply, the shunt regulator of the second error amplifier can be used to select a current set point for the power supply, and the open collector functions of the shunt regulators serve to OR the output signals from the two error amplifiers to produce a signal indicating the loading of the supply. In this manner, an improved feedback circuit for a switch mode power supply is provided that performs the same functions as prior feedback circuits, but has at least three less discrete components (e.g., two diodes and a voltage reference) than the prior feedback circuits. Thus, the disclosed feedback circuit significantly reduces both the size and cost of switch mode power supplies.