A kilowatt-scale high-frequency electrical power conversion system adapted for variable output power delivery is described, in which switching frequency of switchable power delivery circuitry is dynamically modulated in transition of the system to a low power regime that is substantially below maximum power output, in order to at least partially attenuate thermal overload from heat generation of switching components in such transitions. The system is useful in high-efficiency charging of rechargeable batteries of electrical vehicles, e.g., e.g., automotive, industrial, and other motive power vehicles.