Considerations for Measuring Loop Gain in Power Supplies

Stability margins and transient response are two important characteristics of a power supply that are closely related to loop gain. Nowadays most power supplies use feedback to maintain good output regulation. The bode plot of loop gain indicates system stability and stability level, for which stability criteria are set up and compensation network should be designed to meet. Designers also use bode plot measurement as guideline to improve output transient response.

While loop gain can be calculated or simulated, PCB parasitic inductance and resistance, component variation could greatly affect frequency response and stability margin. It is still necessary to measure loop gain experimentally. This paper discusses the method to measure open-loop transfer function and considerations to be taken for accurate results. Following this paper on how to setup equipment and how to prepare a power supply for accurate loop gain measurement.