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This paper describes and evaluates 3 original step-up converter architectures able to harvest energy from low-voltage and low-power generators. Design and sizing are made according to specifications issued from the stringent characteristics of microbial fuel cells. The maximum harvested power is 10mW under input voltage Vin=0.3V (33mA input current). The considered converters include self-oscillating circuits for autonomous operation. The 2 first topologies are respectively adapted from boost and flyback topologies. The 3rd topology uses a Greinacher voltage-lift circuit. Energy is transferred to the load both directly (forward transfer) and indirectly (flyback transfer). PSPICE simulations enable evaluation and comparison of the 3 topologies in term of efficiency, robustness, step-up ratio, control and cost. Best efficiency of 80.6% is achieved by the boost-derived circuit which also enables a feedback action to harvest energy at maximum power point.