A simple 4-9V to 12V PWM Fan converter using MPS mEZD41501A-B DC-DC step-up converter module.

Gerbers (Gerber_Board_1_2023-06-17.zip and Gerber_Board_1_2023-06-20.zip) for manufacturing the board are attached near the bottom of this page.  The difference between the two is that the latter, has a ground plane pour on both sides of the board, with a few vias to stitch the two together.

BOM from Mouser:

• 1 pcs MPS mEZD41501A-B DC-DC step-up converter module
• 4 pcs Nexperia NX138AKR N-MOSFETs in SOT-23-3 or similar footprint
• 2 pcs 100 Ohm resistors in 0603 or 0805 footprint (the ones with a continuous outline in the silkscreen)
• 3 pcs 10 kOhm resistors in 0603 or 0805 footprint (the ones with a broken outline in the silkscreen)
• 1 pcs NXP PMEG3010ER,115 or similar Schottky diode in SOD-123W footprint (D2)
• Either a Bourns 3296W-1-103LF 10k trimmer potentiometer, or a pair of resistors in 0603 or 0805 footprint, to select or adjust the PWM pull-up voltage.
• Optionally, 1 pcs Vishay BAT54W or similar freewheeling diode in SOD-123 footprint (D1)
• Optionally, 2 pcs Molex 47053-1000 4-pin PWM fan connectors

The safest PWM pull-up voltage is 3.3V.  Some old hosts may require a 5V PWM pull-up voltage, or they cannot control the fan speed at all. After having soldered (any components but including) the trimmer potentiometer and the Schottky diode D2, connect 12V to input pin 5/6 and ground to input pin 2/3, and adjust until the voltage between host-side BLU and BLK reads about 3.3V.

If you don't want the trimmer, for 3.3V pull-up, use R1=7.15 kOhm and R2=2.7 kOhm resistors in 0603 or 0805 footprint.  For 5V pull-up, use R1=5.9 kOhm and R2=4.3 kOhm resistors.

For a low-profile version, use the resistors and omit the trimmer, and omit the 4-pin connectors.  Cut a 4-pin PWM fan extension cable in two, and solder the wires to the board.  The wire colors are listed on the backside of the board.  You can then use heatshrink to wrap the pair of boards into a 27mm wide, ~ 6mm thick, 40mm tall "package".

Modified on 2023-06-17: Added a couple of vias, and updated silkscreen.  The possible wire colors are either (black, red, yellow, blue) or (black, yellow, green, blue) for (GND, +V, Tach, PWM), so replaced colors with the actions.  Marked the connector locking tab location with small lines on the topside silkscreen.

Modified on 2023-06-20: Added ground plane pour on top side of board also, so now both top and bottom are mostly ground.  Bottom plane has wide tracks for 4-9V and 12V, and all signals are on top plane.