Spacecraft Power Supply Frequency
I've built payloads for a dozen suborbital shots and one satellite. AC was never used. As our missions were not really long-duration interplanetary trips, we used commercially-available DC-DC converters built to aerospace standards. The satellite, I believe, is still functioning after about 6 or seven years. Converter frequencies were, I believe, about 550 KHz.

What NASA does for its missions, I don't know, although I expect they roll their own.