The major source of RFI from switching power supplies is not the switching frequency or its harmonics. It is a parasitic LC oscillator formed by the capacitanc

The major source of RFI from switching power supplies is not the switching frequency or its harmonics. It is a parasitic LC oscillator formed by the capacitance across the switching transistor, and the parasitic inductance between the capacitor and its DC voltage source. Every time the power supply transistor switches, that parasitic LC tank is struck and rings like a bell. 1-10 MHz is a common range for this ringing in a big supply (10's of kW and up). As the components get smaller, the first parasitic ringing frequency goes up, too.