[lpmay]

Down to DC? That should be a surprise, because it's certainly not true! Faraday's law of induction only applies to varying fields, which means AC or physical movement. Remember, it's perfectly normal and common to have a square wave with a DC component of zero!

[cpcallen]

I think he means the response frequency, not the frequency of the AC drive (which he states was 10 kHz).

[tonyarkles]

Yeah, they’re almost definitely referring to the control signal. Since it’s controlling the saturation of the core, it’ll happily go down to DC just fine.