Does RTCA/DO-254 apply to power electronics?
The description of AC 20-152 answers your question:


  This advisory circular (AC) applies to manufacturers and installers of products or appliances incorporating complex custom micro-coded components with hardware design assurance levels of A, B, and C. These complex custom micro-coded components include application specific integrated circuits (ASIC), programmable logic devices (PLD), field programmable gate arrays (FPGA), or similar electronic components used in the design of aircraft systems and equipment. This AC provides a means (but not the only means) to gain Federal Aviation Administration (FAA) approval by showing the equipment design is appropriate for its intended function. Further, using this AC will help you satisfy airworthiness requirements when these types of electronic components are implemented


"Power Electronics" is a term so broad as to be nearly meaningless, but it doesn't matter: If the devices in question are designed to be installed on an airplane; have a DAL of A, B, or C (a failure would be Catastrophic, Hazardous, or Major respectively); and incorporate ASICs, FPGAs, or other circuitry with custom microcode then the FAA's guidance indicates you should comply with DO-254.



Note that anything you intend to install on an aircraft needs to demonstrate to the FAA's satisfaction that it is not hazardous to safety of flight in both normal and failure conditions, and that it does what you say it does.
Documented compliance with DO-254's guidance is one acceptable means of demonstrating the first of those elements (by showing robustness in the design, and graceful failure modes), though it is not the only means. (In other words if you're building a panel-mount cell phone charger or something equally ridiculous nothing indicates that you must comply with DO-254, but the FAA may ask you to comply if they feel your device is such that DAL A, B, or C is appropriate.)