Switching powersupply controllable by arduino - looking for an IC

[charliehorse55]

Hello, I have an arduino and I want to control the pump and fans for my PC watercooling. They are currently running from small buck converters I bought from amazon (https://www.amazon.ca/DROK-Regulator-Converter-Electronic-Stabilizer/dp/B00SY37S00/ref=cm_cr_arp_d_product_top?ie=UTF8).

I've thought about a few ideas:

a) Replace the pots on the existing circuit boards with digital pots - this would work, but the potentiometer is exposed to Vout which can be as high as 12V. Most of the digital pots I've seen either have a Vmax < 12V or don't have a through hole package.   
b) Buy a new pre-made circuit from amazon or ebay that is somehow controllable by ardunio - I've looked, and been unable to find anything. A key requirement here is that the output not be PWM'd, as it interferes with the RPM measurement.
c) Build my own buck converters using an IC in a way that could be controlled by arduino

So, unless I can find a solution to a) or b), it looks like I am going to be building my own converters. I've never done that before, and I'm wondering what IC would be best. I'm looking for something with a minimal amount of external components, and able to meet these requirements:

Vin: 12V
Vout: 3-12V, controlled by arduino (I2C, serial, or just an RC filter on a PWM output into a reference pin, whatever works)
Imax: 3A
2 channels (can be 1 IC or 2, don't care)

Ideas? What ICs should I be looking for?

[t1d]

Drive the analog pot stem, with a small motor? Motor driven pots have historical precedence, IIRC. Servo motor, rubber band, code to manage it all. Add monitoring alert, for failure.

[Simon]

why not power PWM them? i take it these are brushed motors?

[charliehorse55]

why not power PWM them? i take it these are brushed motors?

They are permanent magnet brushless motors, the pump rotor and impeller are joined together and sit inside the water:

Low-freq PWM does work to control their speed. The problem is, the RPM reading is generated by a hall effect sensor, so if you PWM them, anytime the magnet passes the sensor while the device has no power, there will be no pulse generated. This makes the RPM reading nearly useless.

[Simon]

Do you need the RPM reading? You could characterize your pumps or use a flow meter. What about high speed PWM? I suppose that will make a mess of your speed sensor output.

You could use voltage control if your speed sensor still works at the lower voltage. What you can do is use a voltage controlled regulator, or you could do your own SMPS. All you need is a high side mosfet, inductor, capacitor and schottky diode. Your output voltage will be the "input voltage" x "the duty %"

[charliehorse55]

I ended up modifying the feedback path of the existing buck converter using an op-amp. The arduino generates a voltage between 0-5V which is used as a bias point for the feedback loop. End result is that the output voltage can now be varied from 2.6v->11.6V.