by Andreas Reiter - Microchip
Establishing reliable and robust control loops of switch-mode power supplies essentially requires detailed knowledge of the frequency domain characteristics of the plant. While for smaller, non-isolated converter circuits simple, first approximations of pole and zero locations may be sufficient for a rough estimate of the stable operation area, allowing to safely power up the circuit for the first time, more sophisticated converter types, high voltage and high efficiency designs may be less error tolerant and therefore considerably more challenging to work with. These designs commonly require detailed modeling of the power stage to derive their characteristics across the intended operating range as well as to identify critical corner cases, which are vital for establishing robust design margins ensuring mass production with minimum failure rates. However, even the most detailed theoretical model eventually requires verification before it can be considered reliable and serve as the basis for the loop design. In this lecture we will walk through practical invasive and non-invasive measurement techniques and procedures used to measure the plant transfer function of digitally controlled power supplies, introduce the procedure, discuss expected accuracy implications while highlighting tricks to avoid and work around typical pitfalls.