US Patent 7768292 Non-invasive power supply tester

The output current of a switching regulator in operation on a printed circuit board can be accurately measured without opening the circuit, and without specially dedicated sensing components in the regulator. The inherent series resistance of an inductor or similar resistive component can be used—even though it's resistance and its temperature are unknown. The apparatus is easily extended to make other measurements such as transient response and output impedance. These measurements are generally easier to perform and more convenient than other commonly used methods. Key elements are an injector circuit (312), a voltmeter circuit (310), a control unit with arithmetic capability (316), a user input unit (324) and a user output unit (326).