Quick-n-Dirty Stock PSU Notes

A project log for Monoprice Select Mini Electro-Mechanical Upgrades

Endeavoring to build upon the existing fanbase work of this $200-ish printer.

Michael O'BrienMichael O'Brien 08/15/2016 at 11:513 Comments

The Big Ass Cap for input filtering is a Nippon Chemi-Con 400V 220 uF snap in cap. I wouldn't bother with pricing out a replacement...

The PSU has a small step down transformer to rectified DC voltage via a 4A, though unmarked bridge rectifier that is then fed into a TI UCS3843P-controlled DC-to-AC (?) conversion using the big transformer that is fed by a Fairchild FCP11N60F MOSFET. Without a closer look with a scope or meter, I cannot say what the input voltage is for the SO-8 controller, but this is the blow IC that some have pictured and it rated for a 30V input.

This output is rectified by two V30100S schottky diodes, as far as I can tell, in ITO-220AB packages and this goes into 4 filter caps.

The 4 small output filtering caps at the end are 1000 uF, 25V, 105 ˚C, JH CP263 electrolytics. There are two optional replacements I'd trust from mouser:

If both are run at 95 ˚C, their effective lifetime rating will be 32K hrs & 20K hours respectively. However, the higher ESR and lower ripple rating will cause the former to heat more due to the ripple from the steppers and heating elements. As for the latter, you'll get a a beefier filter with 20% more capacitance and up to 11 amps of ripple at 100 kHz; even though the steppers operate at a third of that.

The output voltage is controlled through a TL431A and the 2.47K & 10K 1% resistors. If I'm interpreting the datasheet and the resistors correctly, excluding error tolerances, the output voltage is then programmed for ~12.6V. Toss in a 0.3V drop from the schottky diodes and you have the 12.3V I measured previously.

In addition to the part identification, the MOSFET on the high side of the larger transformer has a max power dissipation of 125W and a max continuous current rating of 11 amps. I'd love to see some additional photos pointing to the failure mode of this PSU, cause it seems like 11 amps in and 10 amps out, assuming it's 12 V AC we're dealing with, is ~91% and that's a tall order. Even with my rudimentary knowledge of this topology, this seems too close to be coincidence.

Toshiba makes a nice MOSFET that I think will work, the TK16E60W,S1VX. I've not compared all of it's specs and a freewheeling diode will probably help keep things in shape for it too. Swap this guy out and the four filter caps.

If you want to tweak the PSU for say, 15 V output, change I'd advise new caps like this one:

The 2.47K resistor needs to be swapped out with a 1.96K resistor. With the schottky's that should leave you at about 14.96 V. All in all, for ~$15 w/ shipping you can beef up this PSU (have it supply more than 10 A?), make it run cooler, and have it upped to a higher voltage if you want.


Michael O'Brien wrote 08/15/2016 at 20:57 point

Minor upgrade with this guy, no? TK16E60W,S1VX:

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K.C. Lee wrote 08/15/2016 at 14:38 point

Your 91% estimate of the MOSFET is not valid.  These type of power supply typically has a step down transformer, so the input current will be smaller than the output current.

Simply swapping the input MOSFET to a larger one may not work as you intended. Larger MOSFET will have larger gate capacitance and require higher (dynamic) gate current for driving it. This will put more stress in the gate driver the the uCS3843 and the MOSFET will have more losses because it will switch slower.

You have to be careful about modding the supply for a higher voltage as it means that the primary side will see a higher voltage stress.  You are better off getting a correctly rated supply.

Secondary side feedback is taken after rectifier, so the 12.3V you are seeing is after the diode drop.  Schottky drop will be much higher than 0.3V at higher loads.  See datasheet.

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Michael O'Brien wrote 08/15/2016 at 19:34 point

I meant to remove that section before putting that up. Wrote it up just before heading to bed. Thank you for the catch reguardless.

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