FSP 19V 180W power supply repair

[lucky13]

I am helping someone to repair his ac adaptor for a LCD TV.

The adaptor is made by FSP. It is sized like a power adaptor for notebook computer.

This thing has a maximum rating of 180W!

Anyways, the problem is the 19V output fluctuates up and down. With a DMM connected on dc, the output varies between 19 and 14v.

Crack opened it and found a small Teapo capacitor that is high on ESR. Replaced it and still the same!

I found the dc across the 400v input cap (Rubycon) is not stable.

ESR meter indicates it is about 0.5 ohm.

It is 82uF 400V. I can't find any on hand to swap this out to verify the source of the problem. The smallest value I have is 100yF/400V. Would this work?

[Krankshaft]

Re: FSP 19V 180W power supply repair

Check each of the line rectifiers with the diode check function are any of them shorted?

Do any of the other small value caps have high ESR?

Specifically ones in the feedback circuit?

[Newbie2]

Re: FSP 19V 180W power supply repair

Yes I think 100uf/400V is suitable for replacing 82uf/400V. The capacitance ratings are close.

[Krankshaft]

Re: FSP 19V 180W power supply repair

A Rubycon in the line section causing this?

Unlikely first it's a Rubycon second usually the only way this cap gets taken out is by lighting strikes and the like.

Even bad cap brands can last as the line cap because they're not heavily stressed.

[Newbie2]

Re: FSP 19V 180W power supply repair

Unusual in normal conditions for a Rubycon to fail, but maybe the guy's just unlucky to experience that.

Try your 100uf/400V replacement capacitor and see what happens.

[lucky13]

Re: FSP 19V 180W power supply repair

I couldn't believe my eyes either but my DMM says the dc voltage on this 82uF/400V cap is jumping up and down (no load).

Yep, found one small Teapo cap with like 0.5ohm ESR on it. However, replacing it with a new cap did not make the dc (no load) voltage stablizes.

I will try the 100uF/400V tomorrow.

[Toasty]

Re: FSP 19V 180W power supply repair

Jumping up and down???

How often does it rise and fall?
What voltages are you seeing?

You didn't answer Krankshaft's suggestion/question:
>> Check each of the line rectifiers with the diode check function are any of them shorted? <<

...or open?

If any of the rectifiers are defective, you could get the effect you're seeing as it will produce 1/2 wave DC. If that's the case, then on the DC scale of the DMM, you are in effect trying to read AC, which may give you the jumping voltage.

OR, you may be seeing the PSU trying to run, and an as of yet unknown component is causing it to shutdown. Is it making any noise or squealing, whining, or tick-tick type of sound?

Toast

[lucky13]

Re: FSP 19V 180W power supply repair

Yes, didoe bridge ohmed OK, in and out.



Using an analog ESR meter, this Rubycon shows up around 0.5ohm.



Checking a 100uF/400V on a LCD mointor, it reads less tha 0.1ohm. This is the obe I planned to "borrow".

That's why I think this Rubycon may be suspect although it does not bulge.

Toasty, the voltage reading on the DMM jumps like 19, 14, 19, 15, 18, 14, etc.

Across the 400V cap, it also jumps (I think 167V, 143V, or similiar).

The same 400V cap on the LCD monitor p/s that works, this voltage is steady.

[lucky13]

Re: FSP 19V 180W power supply repair

OK, swapped in the 100uF/400V and it is the same.

I've measured the dc voltage across the 400V cap, it jumps as high as 207v and as low as 165v.

Any ideas?

[Wizard]

Re: FSP 19V 180W power supply repair

They mean the output diodes are they good?

Cycling is bad start up resistor or bad PWM IC. 19V at 180W is stupid, way too much current at low voltage. 20A, bad design of this TV.

Cheers, Wizard

[lucky13]

Re: FSP 19V 180W power supply repair

Jumping up and down???
Is it making any noise or squealing, whining, or tick-tick type of sound?

Toast

No to all of these situations.

It just quietly produces this fluctuating voltage.

Cycling is bad start up resistor or bad PWM IC.

Any idea what/where to look?

The board uses L5991D PWM controller and L6561D PF correction ICs.

[Toasty]

Re: FSP 19V 180W power supply repair

Follow Wizard's lead on this. Get the spec sheets for the PWM IC and check it out.

A scope on that output would be sweet. It sounds like it's half-cycling somehow. Loading on half the time and unloaded or freewheeling the other.

Whose make / model PSU is this?

Toast

[lucky13]

Re: FSP 19V 180W power supply repair

Here is the link to the L6561 PF chip:

L6561

So, it looks like that 82uF is C5 in the sample diagram:



And the problem may be this controoler or some feedback stuff.

Any pointers?

Sorry, no scope. Only DMM & ESR meter.

[lucky13]

Re: FSP 19V 180W power supply repair

Here are a few findings:

I have 167v dc steady if the mosfets are removed (the red arrorws are where a heat shriked disc cap? on the other side):



Same here, the voltage is stabe at 165vdc with the mosfets out (yellow circles and the red arrows are where the 82uF/400V is on the other side):





Output caps have not been replaced. The ESRs seem to be less than 0.1ohm.

[Krankshaft]

Re: FSP 19V 180W power supply repair

Whose make / model PSU is this?

Says its an FSP.

I second Wizard if the FETs removed stabilize the voltage then it could be a PWM problem. Or the supply isn't getting the proper kick to start up (startup resistor).

The startup resistor is usually metal film (higher wattage than most on the board) is supplied from the line rectifier and is usually not far from it.

[Toasty]

Re: FSP 19V 180W power supply repair

I was looking for the TV info. Perhaps a schematic would clarify.

[Wizard]

Re: FSP 19V 180W power supply repair

Oh oh. This is MOSFETs on primary side. I mean the DIODES on the secondary side.

and .01 ohms can't be. Pull caps and confirm again with ESR meter on removed caps. That is why I wanted to see if you are checking the diodes too with multimeter set to ohms, 20A is usually 2 large diodes on heatsink that feeds to the capacitors.

Never disconnect diodes or capacitors and power up. No feedback and PSU might blow up!

Cheers, Wizard

[gg1978]

Re: FSP 19V 180W power supply repair

Does this PSU have an integrated load resistor?? Switching power supplies dislike being run without a load on them, be it a minimal one, or the rated current..

[lucky13]

Re: FSP 19V 180W power supply repair

Does this PSU have an integrated load resistor?? Switching power supplies dislike being run without a load on them, be it a minimal one, or the rated current..

Rated output is like 9A so I don't think so. At the present stage, the TV cannot power up for sure.

No idea. Can I just ohm the output? It odes have a little green LED at the end of the connector that plugs into the TV. Does that count?

It also has a tiny fan to suck outside air and blow across the inside to the opposite end.

[lucky13]

Re: FSP 19V 180W power supply repair

Oh oh. This is MOSFETs on primary side. I mean the DIODES on the secondary side.

and .01 ohms can't be. Pull caps and confirm again with ESR meter on removed caps. That is why I wanted to see if you are checking the diodes too with multimeter set to ohms, 20A is usually 2 large diodes on heatsink that feeds to the capacitors.

Never disconnect diodes or capacitors and power up. No feedback and PSU might blow up!

Cheers, Wizard

Wizard,

What do you mean diodes on the secondary side? Which picture?

Do you want me to pull the 3 output caps and check their ESR?

I guess I was lucky this thing didn't blow up... Would it go boom?

Because those two MOSFETS are mounted on the heatsink, I didn't want to disassemble them to test as the heat sink is like a lid covers up alsmost everything on the component side. I thought if I leave the MOSFETs out, that would isolate the low voltage end and see if the voltage still fluctuates.