FSP 19V 180W power supply repair

[lucky13]

Re: FSP 19V 180W power supply repair

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.

Krankshaft,

Are you thinking the problem is on the high voltage side or the low voltage side?

The start up resistor is near the input bridge? Would it be high ohms (over 1K) or low ohms?

[Wizard]

Re: FSP 19V 180W power supply repair

MOSFETs are switches for the primary side of transformer, chops the 165V into PWM pulses as determined by the feedback from secondary side output via the PWM control IC that you got the schematic for.

This is safer way to diagnose the SMPS if it is start up or load problems I pull the switching main transistors out and check the PWM is starting up and getting sufficient and clean supply from the startup resistor to a cap on that Vcc pin of PWM IC. Then if that doesn't turn up clues, cut power and disconnected, discharge main cap with 100K resistor and unsolder transformer, start measuring resistances on resistors and check all transistors and diodes so you don't get caught by a leaky ones. I had one happen to me. Thank goodness I found that on a 32" generic TV. :P

The start up resistor is high value resistor from the 165V to the Vcc pin PWM IC with a small cap (often can be bad cap too), when SMPS starts, the diode is tapped from one low voltage winding on primary side (hot side) to that Vcc as well to take over the start up resistor and provides sufficient supply that start up resistor cannot provide during the RUN mode.

Start up resistor can be ONE or 2 or more resistors in series around 120K to 300K, even 1 meg. Very meager current, it is just sufficent for one shot at starting up. Otherwise you get looping start ups like a short due to bad resistor or bad cap, or a bad diode.

Cheers, Wizard

[Gianni]

Re: FSP 19V 180W power supply repair

lucky13: in the picture you attached I see something that looks like a blown MOSFET.
Can you check their case to see if they are broken? The 20N60 (on top-left corner) looks like it misses part of the body, the other one has a strange area near pins.

Sometime the PFC controller (L6561) receive the supply voltage from the auxiliary winding that feeds the SMPS controller (L5991).
L5991 it is an evolution of UC384X, it has more function but the core is the same.

If the Mosfets are dead, I think you have to replace also the controllers.

Ciao
Gianni

[Toasty]

Re: FSP 19V 180W power supply repair

Nice Catch Gianni !!

[lucky13]

Re: FSP 19V 180W power supply repair

lucky13: in the picture you attached I see something that looks like a blown MOSFET.
Can you check their case to see if they are broken? The 20N60 (on top-left corner) looks like it misses part of the body, the other one has a strange area near pins.

Sometime the PFC controller (L6561) receive the supply voltage from the auxiliary winding that feeds the SMPS controller (L5991).
L5991 it is an evolution of UC384X, it has more function but the core is the same.

If the Mosfets are dead, I think you have to replace also the controllers.

Ciao
Gianni

That's epoxy of somekind. Both are not blown. I ohmed them outside of the PCB.

[lucky13]

Re: FSP 19V 180W power supply repair

Sometime the PFC controller (L6561) receive the supply voltage from the auxiliary winding that feeds the SMPS controller (L5991).
L5991 it is an evolution of UC384X, it has more function but the core is the same.
Ciao
Gianni

Gianni,

Yes, this I think is a somewhat complicated design with lots of feedback.

AC > L6561 > L5991 > TSM107 > 19vdc

I see two optoisolators PC123 on this board.

So, I am quite confused as to which section is really working. By my pure luck, I did not blow up the p/s when I disconnected those two MOSFETS you questioned.

And, I find the high side dc voltage stablizes. My thinking is everything up to L5991 is working and the issue is on the low voltage dc side.

I am going to repalce the 3 OST caps near the output end and that may be all I can go into this thing w/o getting more "new" parts to swap in.

[Gianni]

Re: FSP 19V 180W power supply repair

That's epoxy of somekind. Both are not blown

Maybe the second one, but the first one seems blown.
Can you post a close-up of it? I have seen a lot of blown Mosfet and Transistor, they very often explode and part of the body "disappears".

I ohmed them outside of the PCB.

TSM107 is a IC reference (fo feedback regulation) and has 2 or 3 operational amplifier, they are usually used for current limitation and overvoltage protection.

To troubleshoot this kind of problems it is necessary to have (IMO) an oscilloscope and one or two lab power supply to feed the IC and check signals and waveforms, at least when nothing is visible like a bulging cap or burnt component.
As I do repair SMPS for job I don't know how to help you because I always use a scope, lab SMPS and active load and without them and without visual signs of the problem it is hard to point you in the right direction.

Ciao
Gianni

Ciao
Gianni

[lucky13]

Re: FSP 19V 180W power supply repair

Maybe the second one, but the first one seems blown.
Can you post a close-up of it? I have seen a lot of blown Mosfet and Transistor, they very often explode and part of the body "disappears".

Ciao
Gianni

Here is a close up:

[Gianni]

Re: FSP 19V 180W power supply repair

Thanks lucky13 you are right. Looking at the first picture I was convinced it was cracked.
I apologize because reading again all your post I saw you wrote the output is present but it's not stable, so the mosfet should be ok.

I googled and the only thing I found with L5991 and L6561 is 80W AC/dc adapter probably similar to your SMPS.
Like Wizard wrote "the Vcc pin PWM IC with a small cap (often can be bad cap too), " check the small lytics caps near the controllers. I attach here the schematic from AN just as an example.
I have put a red sign near the components I would check: the small lytics on controllers Vcc and the sensing resistor (R25). If for some reason it is damaged (high value) the SMPS start but even without load the current signal can reach the threshold and shuts down the SMPS.
Another problem could be the feedback: if the voltage is "out of control" probably the OVP shuts down it. Usually DMM are not so fast to show you quick event, so maybe you read 18-19V but the real voltage reaches 20-21V and than the SMPS shuts down.
Your SMPS should be a little bit complicated respect to this one if it has 2 optocouplers.

Ciao
Gianni

[lucky13]

Re: FSP 19V 180W power supply repair

Wow thanks Gianna.

I will use your schematic to do some more poking around.....

[lucky13]

Re: FSP 19V 180W power supply repair

Gianni,

I followed your schematic and replaced all the electrolytic capacitors in the locations you marked. I also saw a 221/1kv ceramic cap (see post#8 left side) and replaced it even though the measured capacitance is more like 300pF.

Ha, it works (at least no load)!

The o/p voltage is now steady at 19.5v.

So, I don't really know which one of the caps I changed was the culprit. Oh well.

Will hook up to TV tomorrow to see if it really works....

[lucky13]

Re: FSP 19V 180W power supply repair

Damm!

I checked again this morning and see that the o/p voltage is steady at 19.5v.

So I connect it to the TV and plug in the ac. It ran for about a minute then it started to cycle again.

I shut it off and pulled off the ac adaptor off the TV. I ran the ac adaptor w/o load but now it is back to the situation before; the o/p voltage fluctuates between 19 and about 15.

So, what could it be?

[japlytic]

Re: FSP 19V 180W power supply repair

When measuring the 400/450V capacitor, connect the multimeter leads across the capacitor, with the negative lead to the negative terminal of this capacitor. You should not get any significant voltage variation.

[Toasty]

Re: FSP 19V 180W power supply repair

General question - Could this be a bad zener?

What are voltages at pins 4, 8, 9, & 14 of the L5991A? (DMM only here)

Voltage at pin 8 of L6561?

Steady or following output swings?

Toast

[lucky13]

Re: FSP 19V 180W power supply repair

When measuring the 400/450V capacitor, connect the multimeter leads across the capacitor, with the negative lead to the negative terminal of this capacitor. You should not get any significant voltage variation.

I measured the voltage across the capacitor again, it is jumping like it did before!

I had previously found that this voltage is stable if I removed the MOSFETS (as they are mounted on the heatsink) but I learned that it could have blown up the p/s!

[Gianni]

Re: FSP 19V 180W power supply repair

Hi lucky13

I think it is normal that the voltage on the big caps fluctuates because the PSU (PFC side) is cycling between on/off from what I understand.

About caps: probably the 221PF 1KV is part of a snubber/clamp circuit across a MOSFET to reduce peak voltage and DV/DT voltage on it.
An advice: when you are repair something change one part per time if you are not sure what's going on, so you can identify which part makes difference in the circuit.

It is strange that the PSU has worked for a minute and than restarted again with the problem.

I would suggest to check the small diodes feeding the controllers through the transformer auxiliary winding.
Follow also Toasty suggestion, check the voltages on controllers pin.

About the caps you changed: can you wrote their values and voltage?

Ciao
Gianni

[lucky13]

Re: FSP 19V 180W power supply repair

General question - Could this be a bad zener?

What are voltages at pins 4, 8, 9, & 14 of the L5991A? (DMM only here)

Voltage at pin 8 of L6561?

Steady or following output swings?

Toast

None of them are steady. They all follow the output swinging.

Which zener? Remember the schematic Gianni found was a generic one; not 100% matching this unit I am having problem with.

[lucky13]

Re: FSP 19V 180W power supply repair

Hi lucky13

I think it is normal that the voltage on the big caps fluctuates because the PSU (PFC side) is cycling between on/off from what I understand.

About caps: probably the 221PF 1KV is part of a snubber/clamp circuit across a MOSFET to reduce peak voltage and DV/DT voltage on it.
An advice: when you are repair something change one part per time if you are not sure what's going on, so you can identify which part makes difference in the circuit.

It is strange that the PSU has worked for a minute and than restarted again with the problem.

I would suggest to check the small diodes feeding the controllers through the transformer auxiliary winding.
Follow also Toasty suggestion, check the voltages on controllers pin.

About the caps you changed: can you wrote their values and voltage?

Ciao
Gianni

This is the 1st p/s that I am working on has PFC so your suggestion regarding the p/s on that cap might well be correct. I just know on monitor p/s, the voltage across the input large filter cap is rock steady.

I was changing one cap at a time. Until the last cap, I changed both; a electrolytic and the ceramic. Everytime I have to unsolder the MOSFETs and the heatsink as they cover up the board. I am a bit reluctant to unsolder too many more times as the solder side looks to begin to deterioate.

I did DMM diode check on all the diodes (I think I did not miss any) but they all seems to be fine (conducting one way with some forward voltage drop).

I will find my notes and read my own handwriting for thew cap values and post later.

[lucky13]

Re: FSP 19V 180W power supply repair

Input cap Rubycon 82uF/400V (sb'd with 100uF but same result, so not replaced)

C6 100uF/25V Teapo SEK 105°C (repalced)
C11 4.7uF/50V (Chang C1 105°C) replaced with 10uF/50V (I really thought this was my hole-in-one)
C? 221/1KV Ceramic (repalced with another ceramic)

Outputs (all checked for ESR and none repalced):
C46 470uF/25V OST 8*20mm
C28 1000uF/25V OST RLS
C29 1000uF/25V OST RLS

[Toasty]

Re: FSP 19V 180W power supply repair

>>the schematic Gianni found was a generic one<<
I asked 2x for the make & model of the TV so we could attempt to find a schematic.

I asked for the voltages because both these chips have an internal zener which limits their Vcc+. The voltages tell me if it's exceeded, it's causing the IC to clamp down.

I am thinking that the L6561 chip is bad. Any freeze spray?

Toast