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August 09, 2001

Stability with Battery Load

Question: Why does adding a battery load cause power supply instability?

Original Question: I was testing a rectifier and I am having a problem. Both the voltage and current loops are stable when I load the rectifier with resistive load. But when I connect a battery at the output, the current limit loop gets very unstable. It becomes ok when I slow down the response but then it becomes unstable on resistive load.

Answer: It is tough to give an answer without more details. I assume the "rectifier" is some type of power supply that regulates voltage and current and has an output capacitor. With the resistive load, the power supply sees a resistor that breaks with the output capacitor at some frequency. When you add the battery, it brings the break point way in (lower frequency), perhaps to the origin. To solve the problem, you need to look at the Bode plots of both the voltage and current control loops, either with a network analyzer in the lab, or with a mathematical or Spice model. Looking at the two types of load in the lab or with the models and examining the results should give some idea of how to solve the problem.

Posted by Jerrold Foutz at August 9, 2001 04:11 PM