This article by Steve Sandler discusses the measurement of DC-DC converter negative input resistance; a critical parameter impacting a converter’s input filter stability. The new way to measure the negative input resistance is detailed using a new device (Picotest’s J2121A line injector) which makes this measurement easier to perform. This measurement can be used to verify (or design) a stable input filter for your converter.
Measuring the negative resistance of a DC-DC converter’s input requires introducing a perturbation of the input voltage while measuring the input voltage and the input current. Using a frequency response analyzer (FRA) or vector network analyzer (VNA), the modulated voltage divided by the resulting modulated current provides a direct measurement of input impedance. The Picotest J2121A line injector simplifies this measurement by integrating a modulator and a hall current monitor into a single device.