FSP power brick

[Khron]

Hello "again", good people.

Now, i've had this power brick made by FSP, with the intention of using it as a spare for my laptop. It's heftier than the "stock" one, but i wanna use it as the "home" PSU anyway. It says 19v / 10.53A max (200w).

Last time i tried it out (after a re-cap) and modded the feedback for a 12v output (about 2 years ago), it worked fine. Granted, i didn't have the chance to REALLY put it to work, but it powered a couple fans without a hitch.

But now, it seems to be oscillating from a certain load upwards.

With a 12w/21w bulb in series with two 2R2/5w resistors on the output, which comes out at about 1,7A / 33w load (19v out), the output looks like this.

With the same bulb, but only ONE 2R2/5w in series, which comes out at 1,95A / 38w, the output looks totally wrong (about 300mV ripple, and the "big" oscillation seems to be around 3.6kHz, with which my ears would agree, it just "sings").

The main PWM is an ST L5991D, and it uses a TSM101.

FWIW, on the output there are two Panasonic FR 470u/35v, a sort of "common mode choke", and an FC 470u/25v.

If needed, i've lifted the schematic of the secondary, so i can post that as well. It has two optocouplers; the one i figure is for the voltage feedback is a straight-ahead output->opto->zener->resistor->ground affair, so it does NOT use the TSM101 for voltage feedback. The other one is hooked up to the TSM101 and most definitely deals with the current limiting, in some manner.

I'd really appreciate some ideas how to stabilize this bugger

[Khron]

Re: FSP power brick

I did one extra test - i replaced the two 470u/35v FR's with two 680u/35v FM's. 32w also stable, 38w also unstable, although the waveform is slightly different.

[momaka]

Re: FSP power brick

I don't know, but from what I have seen in other PSUs, 3x 470uF caps won't cut it for 10A output.

Any chance you can post pictures of the power adapter? Is this a synchronous MOSFET rectifier power adapter or does it use just schottky diodes?

[tom66]

Re: FSP power brick

Also, in many types of power supplies you need a greater capacitance after the inductor than before it. At least, that is in my experience.

[Khron]

Re: FSP power brick

I don't know, but from what I have seen in other PSUs, 3x 470uF caps won't cut it for 10A output.

Any chance you can post pictures of the power adapter? Is this a synchronous MOSFET rectifier power adapter or does it use just schottky diodes?

Well, i definitely won't need the whole 10A; the power brick that came with my laptop is only 90w.

And about pictures... The primary heatsink comes like a "shroud" over the whole primary section, and i'd really love to AVOID taking it off again.

Regarding the output, it has three TO220 double diodes in parallel. Can't make out what model / type (due to the light-blue epoxy/glue that's holding the ferrite beads on the cathodes and has been smeared over the top of the casings), but there's an ST logo and "UOK 350", but that's most likely some fab/date code...

I did manage to "lift" the schematic of the secondary, for what it's worth...

[Khron]

Re: FSP power brick

I also traced most of the apparently-relevant part of the primary circuit.

One or two of the diodes are zeners and/or bigger than 4148's, but i couldn't be bothered to "rename" them. Resistor values are what i could read off their tops.