Adjust SMPS Voltage - Help Please!

[ssashton]

Hi Everyone,

I have an SMPS laptop style brick that is built to output 30V. I would like to re-propose it to output 15V.

I imagine the SMPS is designed so that a couple of resistor swaps will allow the manufacture to set the output voltage. Can anyone help me work out which parts I need to change, please?

IC1 soic8 chip on the back (looks like primary side) is an OB2269CP - Datasheet HERE

I've read the datasheet but do not fully understand it, so some hand-holding guidance would be appreciated!

There is also a TL431 on the secondary side with what looks like a hand soldered resistor next to it (R13). Where it reads R12A on the image there is a through hole resistor on the other side of the PCB.

I tried putting a trimmer pot in place of that resistor and it did adjust the voltage a little bit, but a small adjustment created an audible whine and if I went too far it would make the SMPS unstable. I could hear a kind of discharge click repeating and the voltage would jump about.

[Gyro]

Please be careful - I don't know your level of experience but if you're not experienced with mains electronics, then please stop now. Those things can really bite (when disconnected and not properly discharged) or kill you.

I think you would be much better picking up another laptop brick off ebay. Yes, it is possible to tweak the output voltage by changing the voltage feedback resistors, but not by 50%, much too far. Also you will be losing a lot of power capability because it won't safely be able to output any more current at 15V than it does at 30V.

Keep it as is - 30V bricks are harder to find when you need one.

You should be able to pick a brand name 15 or 16V brick on ebay very cheaply - avoid Chinese clones.

EDIT: See what I mean:  https://www.ebay.co.uk/sch/i.html?_odkw=dell+laptop+psu+16V&LH_PrefLoc=1&_sop=15&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xlaptop+psu+16V+genuine.TRS0&_nkw=laptop+psu+16V+genuine&_sacat=0  Put a diode or two in series if you want closer to 15V.

[dave_k]

The ratio of windings on the transformer will largely determine the output voltage.
As you've already discovered there is some margin for adjustment, but not by 50%.

[ssashton]

Thanks for the info. I have a number of these I bought for another project but wanted to re-purpose. If it isn't possible though, I'll just have to suck up the wasted money it's not that bad 

I'd still be interested to better understand how this specific item works. A block diagram style description would be appreciated! I thought the controller ICs typically monitor the output voltage via an opto-coupler and then adjust duty cycle of their charging signal. So I'm not sure what the TL431 is doing here? I basically expected a voltage set resistor on the control IC as you might find on an LM317 isn't it that simple?

Are there any EEVBlog videos about how AC/DC SMPS work? I watched one about DC/DC converters.

Cheers!

[strawberry]

Theoretically it is possible to get half voltage but you will get half current. In practice adjusting is not enough and you will find the problem near R8 and it is bias powersupply winding for OB2269CP to start operating. It will go down too and will not give sufficient power.

TL431 is reference regulator or basically adjustable zener diode. Some cheap PSU use zener instead of TL431.

[IanMacdonald]

There are two basic types of SMPS, the flyback converter and the forward, or 'buck' converter. Flyback converters typically have one mains-side power transistor, buck converters two. Though, that may not always be the case.

The flyback converter is quite flexible in its output voltage, the buck converter has a more limited range of adjustment but gets more power out of a given size of transformer.

The flyback converter behaves like a constant current source, the error amp feedback being its voltage control (and hence losing feedback can be bad!) The buck acts more like a traditional transformer, with the turns ratio determining the output voltage. Hence only a small range of adjustment is possible with a buck.

Most laptop 'bricks' are flyback converters because that allows operation from 100v-240v input, which range a buck converter cannot over without manual input switching. Taking one down to half its rated output is maybe stretching things a bit though, and the feedback loop may not have been designed to remain stable under such conditions. Hence the oscillation.