What PWM frequency should I use for DC cooling fans?

Use a frequency in the 20–30 kHz window — 25 kHz is the common convention (and Intel's 4-wire fan specification) because it sits above audible range. Below roughly 18–20 kHz the switching becomes audible as a whine that tracks duty cycle; far above the window, some fans' input filters distort the command. Always verify the specific model's supported range on a sample.

Why 25 kHz became the convention

The switching signal itself excites the fan's windings and structure at the drive frequency. Anything below ~18 kHz lands inside human hearing and shows up as tonal noise that changes with duty — one of the most common field complaints. Intel's 4-wire specification standardized 25 kHz for PC fans, and industrial DC fan families overwhelmingly followed. The fan's internal driver (not your controller's edge rate) does the motor commutation; your PWM only commands speed, which keeps the interface simple and robust.

What to verify on a specific model

The supported frequency range (some industrial models accept 20–30 kHz, some wider), the logic level and pull-up (3.3 V vs 5 V drive), the duty floor (minimum duty for guaranteed rotation, commonly 10–20%) and the duty-to-speed map. Scope the fan's PWM input while commanding from your controller: an RC filter on the host side or a long unshielded lead can round the edges enough to shift the effective duty.

• Confirm the range from the datasheet, then sweep 0–100% on a sample.
• Check edge integrity on the actual harness, not the bench fly-lead.
• If the host cannot reach 25 kHz, prefer a model with a wider window over running audibly low.