1 Introduction

The DC-DC converter is a typical closed-loop control system. Loop performance is critical for the system to properly work. In a closed loop, response in output is determined by input interference and closed-loop control.

Figure 1-1 shows the block diagram of a closed-loop system. From the diagram, loop performance is directly determined by closed-loop transfer function expressed in Equation 1.

Calculation with closed-loop transfer function from s-domain to time-domain is the most straightforward method to check if the response is oscillating. A more simple judgement method on loop response is frequency stability criterion which is equivalent to Nyquist stability criterion. The frequency stability criterion checks Bode plot of open-loop transfer function as Equation 2.

To obtain a Bode plot of an actual DC-DC converter, a frequency response analyzer is often used to perform the loop test. Figure 1-2 shows the typical connection of the loop test in a DC-DC converter.

In a loop test, the analyzer will generate a small disturbance voltage signal at different frequency sweeping by certain step and collect the response at the same time. When the frequency sweep is done the analyzer will process the data and show Bode plot of open loop transfer function of the converter under test.