A circuit includes a transformer configured with a primary winding and a secondary winding that are driven from a voltage supplied by a thermoelectric generator (TEG). The circuit includes a bipolar startup stage (BSS) coupled to the transformer to generate an intermediate voltage. The BSS includes a first transistor device coupled in series with the primary winding of the transformer to form an oscillator circuit with an inductance of the secondary winding when the voltage supplied by the TEG is positive. A second transistor device coupled to the secondary winding of the transformer enables the oscillator circuit to oscillate when the voltage supplied by the TEG is negative. After startup, a flyback converter stage can be enabled from the intermediate voltage to generate a boosted regulated output voltage.