Wireless charging looks invisible, yet it is constantly talking to the phone on a very specific frequency. That band was not picked on a whim, it is the outcome of repeated trade-offs between electromagnetic compatibility and efficiency.
Why the baseline band sits at 100-205kHz
Qi's Baseline Power Profile, the 5W tier, operates at 100 to 205kHz. This low band keeps the coil and compensation capacitors small and cheap, and ferrite shielding is mature; it also sits far below broadcast and communication bands, so outbound radiation is easy to contain and interference risk during FCC or ETSI certification stays low. The cost is that switching loss rises with frequency, so older designs dared not push it high and capped power at 5W.
Why Qi2's magnetic tier moves to 360kHz
For Qi2's Magnetic Power Profile at 25W, the frequency shifts to about 360kHz. Moving up the band sustains high coupling with a smaller coil and tighter alignment, while opening control-signal bandwidth so foreign-object detection and power negotiation respond faster. The cost is stricter demands on the drive circuit and shielding, and a higher chance of crosstalk with nearby NFC and speaker coils, so the board needs finer filtering and isolation. In one line, every frequency shift re-seats the three-way contest between fast charging, staying quiet, and staying small.
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