Measuring micro-amp inductor currents in switched-inductor DC-DC power supplies

Wireless microsystems can add performance-enhancing, energy-saving, and networked intelligence to inaccessible places like the human body and large infrastructures like factories, hospitals, and farms. For this, they require an onboard source and a power-conditioning circuit that supply microwatts about a prescribed dc voltage. And since tiny dc batteries store little energy, switched-inductor dc-dc converters are popular in this respect, because they dissipate less power than linear regulators and are more accurate than switched capacitors. To monitor how they operate and ultimately meet these expectations, engineers monitor the current flowing through the inductor. In the case of miniaturized supplies, however, inductors switch at 100 kHz - 1 MHz to produce micro-amp currents that are difficult to sense. Although series resistors and magnetically coupled probes are normally viable options, the series components they introduce into the conduction path alter the currents being measured and noise energy obscures the results. But as experimental measurements further show, characterizing and extracting current from the terminal voltages of the inductor is less obtrusive and less sensitive to noise.