What is in this article?:
Part 3 will review ac behavior and stability of hysteretic-mode (HM) compared with voltage-mode (VM) and current-mode (CM) control in depth because stability is a major concern for voltage-regulator design quality and reliability.
Three tools will be used to review regulator stability:
- Open-loop response/Bode plots
- Small-signal transient response
- Output impedance
These three analysis methods are linked together, so this section is organized by control method.
In contrast to the large-signal transient response that was reviewed in part 2, we will perform ac analysis without changing (or with minimal changes to) the operation/bias points of regulators by using very small perturbations.
Power-electronics engineers are very familiar with Bode plots. The small-signal transient response and output impedance are identical analyses from different domains: the time domain and frequency domain, respectively. The output-impedance measurement is known as a noninvasive method and can measure the loop stability of virtually all regulator systems, including ones with the entire loop inside their integrated circuit (IC) packages where a Bode-plot measurement is not possible because there is no way to open the loop. [1, 2]