Maximising power-transfer efficiency in low-power DC–DC converters

Wireless microsensors can add energy- and life-saving intelligence to remote and inaccessible places like factories, hospitals and so on. For this, they normally house sensors, data converters, digital processors, memory, transmitters, and power supplies. Although some of these functions can demand milliwatts at a time, the events monitored are typically so sporadic and sparse in time that average consumption is in microwatts. Still, tiny batteries cannot supply power for long, so power supplies cannot afford to burn much power. This is why reducing the power that switched-inductor supplies lose when delivering microwatts is critical. For these power levels, balancing switches to deliver fixed energy packets in discontinuous-conduction mode and adjusting their frequency to modify power level is the most efficient means of managing a DC-DC converter. In other words, fixing the inductor´s peak current and adjusting frequency is more efficient than fixing frequency and adjusting peak current. In fact, experimental measurements show that fixing peak current to 6 mA and adjusting frequency to supply up to 250 μW is 2-10% more efficient than fixing frequencies at 40, 80, and 120 kHz and 1.4-7% more efficient than fixing currents at 5 and 10 mA.