A multi-mode 3.5-0.5V input, 1.0V output power transformer based on switched-capacitor DC-DC converters is designed to extract 98% of the energy from a 80mF supercapac-itor, to power a periodically active RFID tag. Six converter modes (3 step-up and 3 step-down) are automatically selected depending on the input voltage. The system usage time is maximized by charging the supercapacitor to 3.5V while device breakdown is avoided via novel circuit techniques. The circuit fabricated in TSMC's 65nm GP process occupies an area of 0.48mm2, supports a 3.3μA load current and operates at 2.0MHz. The gradually decreasing 3.5V to 0.5V supercapacitor voltage is transformed to a steady 1V output with a peak efficiency of 70.4%.