VIN VOUT Hot Side RTEG NN Cool Side RLOAD (a) RTEG Stage 1 Meissner Oscillator + - Transformer On-Chip Transformer NMOS Rectifier Depletion-Mode NMOS PP I + Low-Voltage Oscillator Charge Pump Cold-Start System (b) S∆T - Control Circuitry CSTART COUT (c) (d) NMOS Transformer MIT EH Cold Start System 3.1 mm 0.6 µm With Magnetics 0.18 µm 7.8 mm (e) 3.1 mm On-Chip Transformer Discrete Transformer 6 mm 6 mm Bulky Solution Proof of Concept (g) FIGURE 6: A low-voltage cold-start system for thermal energy harvesting. (a) A thermoelectric generator providing a voltage proportional to the thermal gradient across its plates; (b) thermal energy-harvesting circuits with a boost converter, which needs a low-voltage start-up oscillator; (c) a Meissner oscillator topology with integrated magnetics; (d) an optimized and fabricated depletion-mode NMOS and on-chip transformer; (e) a test-printed circuit board with copackaged transformer, NMOS, and read-out circuits for a proof of concept, with (f) the measured results showing start-up (as indicated by VOK level) from a 25-mV input voltage to the start-up circuit; and (g) a vision for the proposed system to replace bulky, discrete transformers [20]. MIT EH: Massachusetts Institute of Technology Energy Harvesting. 28 FALL 2021 IEEE SOLID-STATE CIRCUITS MAGAZINE Energy Harvesting With Monolithic Integration Switched Cap 0.35 µm With Magnetics 0.18 µm NMOS 3.1 mm EnergyHarvesting System On-Chip Transformer Switched Cap 0.18 µm With Magnetics Boost Converter NMOS 0.028 0.026 0.024 0.0 0.02 0.018 -20 -10 0 Time (s) (f) VIN = 25 mV 1.5 1 VIN VOK 0.5 -0.5 10 20 VTEG 3.5 mm SwitchedCap dc-dc 3.1 mm VIN (V) VOK (V) 3.1 mm 3.1 mm