Testing Power Sources for Stability

Recent advances in measurement technology have made it not only possible to measure stability margins, but made it a quick and simple process. On power sources with remote voltage sensing terminals, gain and phase margins can usually be determined in a few seconds without even removing the cover. These measurements can be made while the source is operating normally, supplying power to the real load. In this way, the effects of the real load on stability margins and power source performance can easily be determined. This paper will explore the causes of problems when the power source is connected to the real load — as opposed to the resistive load normally used by the power source manufacturers — and how to specify and test the power source to avoid those problems. How a Power Source Regulates Voltage In order to understand how to effectively test power sources for stability, one needs to understand what stability means and where the feedback loop is in a power source. Figure 1 shows a simplified block diagram of a typical power source. The power source may have several outputs, but prime consideration is given to the one around which the loop is closed. Figure 1. Block Diagram of Power Source Feedback Loop The POWER PROCESSING CIRCUITRY is the power-handling portion of the power source, typically from the PWM terminal of a control IC to the regulated output. On one popular control IC, the SG1524, this is from pin 9 of the IC to the output of the power source that is being regulated. We call this portion the MODULATOR.