Most switching power supplies are not designed to change their output voltage. However, situations arise where a different voltage from the power source is needed. The purpose of this article is to develop a practical interface scheme for digitally controlling the output voltage of a switching power supply with an analog feedback loop. The article presents the results of the research of isolated flyback switch-mode power supply (SMPS) circuits, determines the method of controlling the output voltage of the SMPS, and proposes a digital interface for controlling the output voltage of the power supply. The emphasis was on using readily available, inexpensive components with minimal modification to the existing design. The study revealed limitations in existing switching power supply designs regarding effective output voltage control. Modifying the internal reference source is often not an available option. Therefore, influencing the feedback circuit emerges as a simple and viable control method. All power supply feedback loops include a voltage divider measuring the output voltage of switching power supply. This research demonstrates that controlling output voltage can be achieved by injecting or draining current within or from output voltage divider of power supply. Draining current increases the output voltage, while injecting decreases it. The additional resistor only requires a single connection point within the existing circuit. The proposed method provides a linear dependence of the output voltage increase on the value of the injected/drained current. It is shown that the digital interface scheme depends on the number of discrete levels of the required output voltage. For a small number of levels, field-effect transistors controlled by a positional or binary code can be used to control the power supply, and with a significant number of levels, it is advisable to use ready-made digital-to-analog converters.

switching power supply, control, interface, feedback, digital code

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