25.1 A 50.7% peak efficiency subharmonic resonant isolated capacitive power transfer system with 62mW output power for low-power industrial sensor interfaces

Low-power sensor interfaces play a key role in monitoring industrial system reliability in hostile environments. They are exposed to destructive surge voltages, which are caused by ground current spikes from operation transitions of machinery and motor drives, endangering circuit functionality. Employing galvanic isolation is thus imperative to protect the sensor analog front end (AFE) from severe damage. However, the use of an isolation barrier presents several challenges. Without a wired connection to the sensor AFE, power/data must be transferred through the barrier via magnetic or capacitive coupling. Moreover, size and cost are constrained in such applications. In [1], an isolated DC/DC converter is used, which delivers up to 2W with 80% efficiency, but at the cost of bulky external micro-henry transformers. To avoid a large form-factor solution, the transformers can be integrated, but efficiency is sacrificed and cost is added. In [2–3], the operating frequency of the integrated transformers is ∼300MHz for size reduction. However, at such a high frequency, the switching power loss (P
SW
) is greater than 10s of mW, reducing efficiency below 30%. On the other hand, capacitive isolation, usually used in digital links [4], replaces polyimide-based transformers with integrated thick dielectric capacitors. It represents a major improvement due to size and cost savings as a result of integration on silicon.