Jonathan Kimball with adviser Philip T. Krein
Many solutions have been proposed for power converters with high conversion ratios. The performance and efficiency of nonisolated converters degrades significantly when the ratio between input and output voltage exceeds 4:1. Isolated converters can operate with an arbitrary ratio by changing a transformer turns ratio, but higher turns ratios yield lower efficiency. The input-series-outputparallel (ISOP) topology uses several isolated converters, each with a low transformer turns ratio, connected so that they equally share input voltage and output current.
Previous control schemes for the ISOP topology are complicated and sacrifice performance. This project used a modified sensorless current mode (SCM) controller to force the converters to share input voltage equally. Stability has been shown analytically. The method was demonstrated experimentally on a five-phase converter with an overall conversion ratio of 30:1 (see Figure 42).
As part of this project, SCM modeling methodology was extended to a multiphase system (see Figure 43). A model composed of a standard SCM model multiplied by a comb filter matches experimental results well enough for feedback design. This technique is appropriate for any multiphase SCM controller.