Ultrahigh voltage conversion ratio (VCR) DC-DC boost converters are essential for biomedical, optics, sensing and diagnostics applications. For realizing an ultrahigh VCR, the conventional boost converter would result in a large duty ratio D, a large inductor current IL and thus a significantly degradation on power efficiency, and its VCR is limited by the parasitic resistors. Figure 8.10.1 top shows the recently reported hybrid boost converters. The multilevel boost converter (MLBC) has a single inductor at the input, and has the feature of input-series-output-series. Therefore, multilevel boost converters considerably improve the VCR and reduce the device voltage stress with the flying capacitors. However, the MLBC topologies still suffer from the problems of low efficiency and low power density caused by the large IL. Another topology with multipath boost converters (MPBC) has the feature of input-parallel-output-parallel. The MPBC can effectively reduce the IL of each branch by connecting parallel input inductors. But the output-parallel structure does not improve the VCR nor reduce the device voltage stress. To address the above issues, this work presents a hybrid input-parallel output-series high power density ultrahigh-VCR step-up DC-DC converter for electrostatic and piezoelectric actuators used for small-scale flyweight devices. This converter is very challenging due to the low VIN (<5V), high VOUT (>100V), high output power (>200mW) and high power density (volume<10mm3 and weight<100mg). We use both mW/mm3 and mW/mg for the power density units to emphasize the importance of the size and weight for flying robots.