Laptop switching power supply overheating

[delmadord]

Greeting, I am long time watcher, not that active on the forums here, but maybe someone will know what to do. My laptop charger  power supply suddenly started to overheat, after six years of perfect service. After getting to ridiculously hot temperature, laptop (without battery, I do not have one) switches off completely, so I am assuming that the power supply just fails. When cools off, it is working again, until it gets too hot again. With another supply, from my brother's laptop everything seem to work just. Brother's suply is rated for 1.3A less than mine (the one which overheats), but I think it is ok since there is no battery charging stuff going on on my laptop, so that excess current is not needed.

The reason I am writing here is, what could cause the actual overheating? I have torn it apart, but I cant see any suspicious things going on. There are 4 electrolytic caps, 1 big beefy, I cant see the ratings of it and three smaller. Two of that smaller ones have bulky top. I believe the problem is in capacitors, since they had been in quite a high temperature for a long time. I will buy new adater for the laptop anyway but in a free time, I would like to repair that one, just to gain more knowledge. I could provide a foto of it if needed. What do you think about it? Thanks!

[delmadord]

I have replaced the two badly looking electrolytic caps but it doesn't sem to do anything with the overheating, now this is strange.

[ablacon64]

Bulky top = bad. Replace all electrolytic capacitors, not just the strange ones, including the bigger one. Six years is a long time for that ac adapter.

[delmadord]

I have replaced also the thir smaller one, all three was 470uF 25V, I had some with the same rating lying around. But they are only cheapo ebay ones. The bigh one is 120uF, 400V and this one will need  a radioshack visit. What if it still keeps overheating after capacitor replacement? Are there some other common steps to resolve?

[delmadord]

The main heatsink, without all the shielding (which thermally connects everything and spread the temperature all over - I have removed it temporarily) has 110°C. There is only a 11N60C3 Infineon Cool MOS™ Power Transistor on it. Its datasheet says its working temp on junction is up to 150°C. I will not try to somehow calculate the junction temperature of it, but my question is: what could cause it to make excess heat?

[IvoS]

Try to measure how much current is really being drawn by the laptop.

[delmadord]

I will do it, but the thing to note is that the overheating adapter is rated 90W and laptop works fine with 65W adapter, which is only around 40°C on the outer surface. This makes me think that there is not really a problem with laptop's current draw.

[Alana]

Common fault with laptop PSUs is broken or partly shorted DC cable. Check for that as well.

[delmadord]

Thanks for the good point but I have desoldered the cable for making sure and triple checked it - it has no problem whatsoever. Nevermind, at least I forced myself to deeply understand the workings of the switch-mode PSU.

[SeanB]

If the capacitors you had were not the right types then it would not work with the new ones.

[delmadord]

What do you mean by right types? The only thing that comes to my mind that could be different is their ESR, which I am not able to measure (unless I make some weird arduino ESR meter or so). They are all three connected in parallel and they are output filtering capacitors. I have matched their capacity and voltage ratings thou. But virtually nothing changed. Still makes me wonder if there is a way to bring it back to normal operation, but wont be using this one for laptop more.

[ablacon64]

Check for cold solder. AC adapters are made to be dropped on the floor.

[amyk]

Partially shorted transformer? See if you can trace a schematic from it and post here, or pictures, at least we can see what other components there are.

Does it get hot even with no load?

[Psi]

It's quite common for laptop psu's to get damn hot under normal operation.

They design them within an inch of their life.

[IanB]

I had a cellphone power adapter once that did the same thing. When it was working normally it ran cool, but one day after a spell of running on 240 V it developed a fault and started becoming very hot. It still delivered the normal output voltage, but the plastic case would become too hot to touch and then it would hit the thermal cut-out. An identical replacement worked fine.

I never learned what caused the problem, but the symptoms sound almost identical. I would guess it might be a common failure mode.

[XOIIO]

It's quite common for laptop psu's to get damn hot under normal operation.

They design them within an inch of their life.

Yeah, all of the Lenovo 20v ones with the big grey/yellow plug run hot constantly unless there is nothing connected, though if an adapter just started getting warm it is odd.

[delmadord]

So I removed almost everything that impaired a clear view to the devices and made some photos, added comments to top view too. That power MOS is overheating still after I replaced that three output caps on the right.

[ablacon64]

I can't recall seeing 2 together, but the TO220 are diodes.

[delmadord]

Why would there be so many diodes? I mean there is main bridge rectifier at the input, then there in the middle there are another 4 diodes and then two more?

[SeanB]

Change that little blue topped capacitor next to the opto isolator, it likely is also high ESR, and it is the power supply for the SMPS controller.

One poor solder joint on the input filter by the bridge rectifier as well.

The TO220 devices are used in parallel so as to handle the peak current pulses to charge the capacitors. 2 devices of around half the rating are a lot cheaper than a single device that can handle the current, and likely there are more suppliers of it as well to shop around for the lowest price.

I am still suspicious of the output capacitors you have used, they are way too small for the current they have to handle. What brand, type and value of capacitor did you put in there.

[delmadord]

Change that little blue topped capacitor next to the opto isolator, it likely is also high ESR, and it is the power supply for the SMPS controller.

One poor solder joint on the input filter by the bridge rectifier as well.

The TO220 devices are used in parallel so as to handle the peak current pulses to charge the capacitors. 2 devices of around half the rating are a lot cheaper than a single device that can handle the current, and likely there are more suppliers of it as well to shop around for the lowest price.

I am still suspicious of the output capacitors you have used, they are way too small for the current they have to handle. What brand, type and value of capacitor did you put in there.

I have used some really bad cheapo ebay ones, of the shelf (the barnd says JVCO or TVCO, 25V, 470uF), just for seing the result but the device acts the same - before and after I have changed them: the voltage was is stable...really, nothing changed....they probably have high ESR even thou they are new, and maybe this is causing that top transistor to work harder and then overheating. From what I have read about switching-mode PSU, the is far more stress put on to the output capacitors, but I am still wondering if they are causing the overheating. Will look at that poor solder joint tomorrow. Thanks for all replies so far.