Laptop power adapter repair

[OM222O]

I'm troubleshooting a laptop power adapter (acer ADP-180MB K around 6 years old) and I'm fairly certain one of the fets (toshiba K12A60U) is bad. It makes a weird hissing noise when I plug in the adapter and smells a bit too. Resistance check with multimeter shows around 3Mohm Rds for a good fet but it keeps jumping around in the Kohm range for the bad one (between 50k and 80k).

1)Is it safe to assume the fet is bad and replace it? There are no visibe signs of damage and the adapter does create 19.5 under no load, but after connecting it to the laptop, it drops to a few mV.

2)Is it enough to just replace the fet or should I look for something that might have caused the fet to fail?

[Fraser]

When a laptop power supply fails, I tend to find a replacement used power supply on eBay as repairing the old power supply is rarely worth the effort when used units are not expensive. I tend not to buy “replacement” or “compatible” new units as these can be of very variable quality. Original used units have served me well.

Regarding the output voltage change. Are you aware that many laptop power supplies have a self protection mode that is activated if they find a short circuit at the output ? The output is effectively switched off and stays off even if disconnected from the laptop. Laptops can fail in a way that presents a dead short to the connected power supply brick. The power supply protection system is reset by disconnecting it from the mains waiting a few seconds, and reconnecting it (but not to the laptop).

Fraser

[OM222O]

There are no shorts. I unplugged the adapter from mains and laptop, then waited for the caps to discharge. Then checked the 19v rail and no shorts. Plugged the cable to the laptop and still no shorts. I've ordered replacement fets. If that doesn't fix it, I'll probably buy a new one.

[Whales]

It might not be the fet, it might be the thing controlling it.  It's often hard to tell in closed loop systems what part is really at fault, any one part amiss will cause all of the others to appear to misbehave.

Sometimes it's easier to tackle the "easier" possible problems first, even if you don't know their likelihood.  Eg cracked solder joints (visual inspection), cracked/failed resistors (you can measure them with a DMM), electrolytic capacitors (ESR meter), controller chip generating suspicious amounts of heat.  Anything more complicated than that will probably require reverse engineering the schematic for the charger.

[Whales]

It makes a weird hissing noise when I plug in the adapter and smells a bit too.

Lots of my laptop power supplies hiss, often its the coils or capacitors but I guess it could be a fet too under certain conditions.

How can you smell one part and not the others right next to it mm away?  That's really selective.

[OM222O]

I know it's not that accurate but moving the board in all 4 directions , the smell is the strongest right above the fets. There are two so I'm not sure which one, but given that it's less than 10 bucks to replace both it's an easy shot. Caps all look good and there are no signs of visible damage. If it's anything other than the fets (controller IC, transformers, etc.) I can't really fix it and have to buy a new one anyways

[TimNJ]

Often, if a FET dies, the driver/IC attached to it also gets cooked. Not always, but probably 50% of the time at least. Could've been a lightning strike type thing. If the IC is available and not expensive, pick that up to and just replace it. And replace the fuse - presumably dead?

Alas, probably just buy a replacement adapter - If you can't repair it on the first attempt, you'll spend more money on your second shipment from Digikey/Farnell than it would to get the real thing...probably. Could be a good learning experience though, if you are interested in doing it for non-practical reasons.

[OM222O]

Fuse isn't blown, I read 330ish volt on the smoothing cap after the bridge rectifier. The ctontroller has 3 markings, none of which return a correct result (DAP026, CHT944, CHK220) DAP026 has the correct number of pins but seems to be an audio chip? Couldn't find a datasheet anyways.

I just need to have a working adapter ASAP because the battery in my laptop is dead and won't turn on without the adapter. I don't like to throw away easily fixable electronics which is why I'm trying to reapir it, but I won't spend more on replacements or spend a few weeks reverse engineering the whole thing. I tried to find schematics or repair guides for it since the same model is used in other laptops too (asus, msi, etc.) but had no luck with that either.

[TimNJ]

I think DAP is the prefix Delta uses for their custom marked chips. Probably you will see “Delta” or the the Delta logo somewhere else on the board. In that case, those ICs are going to be pretty much impossible to replace. I have not seen too many FETs blow open circuit, so I would not imagine FET is actually shot in that case.

Also, didn’t read your post very closely before, but since it is producing output voltage, this is pretty much rules out dead MOSFET. What is the power rating? Photo of the top and bottom side of the board?

Could be many things, but one idea is to check the VCC capacitor, usually small electrolytic on the primary side. At no load, the MOSFET gate drivers average current requirement is not that high (usually operating in skip/burst mode). At higher loads, gate drive energy requirement is higher, so could explain why the output drops out under load.

Could also just be bad feedback loop component.

[bdunham7]

There's a 99% probability that you have bad capacitor in the unit regardless of how nice they look.  If you can remove and accurately test them, do so.  If you can't, just order replacements by dimensions and install them.

[OM222O]

You guys were right, replacing the fets did not fix it. I've ordered a replacement unit. Replacing the caps is next to impossible as there is a ton of white silicone adhesive on both sides of the board holdong the components in place and covering a lot of the solder joints. It's honestly not worth wasting time without a working laptop. Thanks for the help!

[Whales]

Sorry it didn't work out OM222O.  Best of luck in future repairs 

[gamedog]

When the PSU without load has normal operating voltage, this indicates that both the PWM controller and the switching mosfet are working. You need to check how your power supply works on a resistive load. Connect an electronic load or if it is not available, a block of powerful resistors providing a current close to the maximum marked on PSU. If the voltage sags, then most likely you have a problem with a high voltage (one 400V or 450V, or two 200-250V in series) capacitor after the diode bridge. Measure its capacitance and ESR using LCR meter. Just in case, check the condition of the capacitors at the output of the power supply. But your problem is definitely in electrolytic capacitors, especially in high-voltage one.