PWM speed control frequency of modern 4 wire computer fans?

[e100]

The original spec https://wiki.kobol.io/helios4/files/fan/4_Wire_PWM_Spec.pdf from 2003 says the PWM speed control carrier has to be between 21 and 28kHz.
Do modern fans with their onboard driver circuitry still need such a high frequency?
If they are more tolerant of lower frequencies, how low can you go?

[Benta]

If you're talking fans for PC enclosures or other computer cooling, it's 25 kHz nominal (21...28). For interchangeability purposes, I'd expect all such fans to adhere to this specification.

If you want to operate outside this spec, it's on your own risk.

[Doctorandus_P]

Those fans are (usually) very simplistic devices.

They started with two wires, and in the fan there was just a HALL sensor and a transistor, and even those could run on whatever frequency you put in them.

Then came the need for some speed feedback, and for that a third wire was added to break out the signal from the HALL sensor.

This had a little problem though, because there was no speed feedback during the "off" time of the PWM cycle.
So a 4th wire was added, so the HALL sensor always has power and can give back speed information.

But the fan itself is still very simple, not much more then a permanent magnet, an electromagnet a HALL sensor and a transistor as a power switch.

Unless of course you have some kind of fancy fan. They may have built in temperature controllers, LED lighting an all bets are of.

[Siwastaja]

Doctorandus_P described the situation as it was some 15 years ago.

Now, for about a decade, almost all fans have more control than that, including stall detection, auto-retry, some have better modulation for the transistors.

Out-of-spec PWM may still work, though. Or may not.

[SVFeingold]

Now, for about a decade, almost all fans have more control than that, including stall detection, auto-retry, some have better modulation for the transistors.

Not to mention that basically all "DC" fans are brushless and include the requisite driver. Even the cheapest $1 fans on Digikey.

[Siwastaja]

I thought that was obvious to everyone.

But the early BLDC fans worked like Doctorandus_P described, basically just the hall sensors controlling the transistors directly. That of course counts as a "driver".

Modern drivers are a bit more complex because why not, they use ICs anyway so extra features come basically for free.