DIY switching power adapter repair

An ordinary laptop power supply is a very compact and rather powerful switching power supply.

In the event of a malfunction, many simply throw it away, and buy a universal power supply unit for laptops for replacement, the cost of which starts from 1000 rubles. But in most cases, you can fix such a block with your own hands.

It's about repairing a power supply from an ASUS laptop. It is also an AC / DC power adapter. Model ADP-90CD... Output voltage 19V, maximum load current 4.74A.

The power supply itself was working, which was clear from the presence of a green LED indication. The voltage at the output plug corresponded to that indicated on the label - 19V.

There was no break in the connecting wires or breakage of the plug. But when the power supply was connected to the laptop, the battery did not start charging, and the green indicator on its case went out and glowed at half the original brightness.

It was also heard that the unit beeps. It became clear that the switching power supply was trying to start up, but for some reason either an overload or short circuit protection was triggered.

A few words about how you can open the case of such a power supply. It is no secret that it is made sealed, and the design itself does not imply disassembly. For this we need several tools.

We take a manual jigsaw or a canvas from it. It is better to take the canvas on metal with a fine tooth. The power supply itself is best clamped in a vice. If they are not there, then you can contrive and do without them.

Next, with a manual jigsaw, we cut into the depth of the body by 2-3 mm. in the middle of the body along the connecting seam. The cut must be done carefully. Overdoing it can damage the circuit board or electronics.

Then we take a flat screwdriver with a wide edge, insert it into the cut and unclip the halves of the case. There is no need to rush. When separating the halves of the case, a characteristic click should occur.

After the case of the power supply is opened, we remove the plastic dust with a brush or a brush, we take out the electronic filling.

To inspect the elements on the printed circuit board, you will need to remove the aluminum radiator bar. In my case, the bar was attached to other parts of the radiator with latches, and was also glued to the transformer with some kind of silicone sealant. I managed to separate the bar from the transformer with a sharp blade of a pocket knife.

The photo shows the electronic filling of our block.

The fault itself did not take long to look for. Even before opening the case, I was doing test turns. After a couple of connections to the 220V network, something crackled inside the block and the green indicator indicating work was completely extinguished.

When inspecting the case, a liquid electrolyte was found, which leaked into the gap between the network connector and the elements of the case. It became clear that the power supply unit ceased to function normally due to the fact that the electrolytic capacitor 120 uF * 420V “slammed” due to exceeding the operating voltage in the 220V power grid. Quite an ordinary and widespread malfunction.

When the capacitor was dismantled, its outer shell crumbled. Apparently it lost its properties due to prolonged heating.

The safety valve at the top of the housing is "swollen" - this is a sure sign of a defective condenser.

Here's another example with a faulty capacitor. This is a different laptop power adapter. Pay attention to the protective notch on the top of the condenser housing. It broke open from the pressure of the boiling electrolyte.

In most cases, bringing the PSU back to life is pretty easy. First you need to replace the main culprit of the breakdown.

At that time, I had two suitable capacitors at hand. I decided not to install a SAMWHA 82 uF * 450V capacitor, although it was ideally sized.

The fact is that its maximum operating temperature is +85 0 C. It is indicated on its body. And if you consider that the power supply case is compact and not ventilated, then the temperature inside it can be very high.

Long-term heating is very bad for the reliability of electrolytic capacitors. Therefore, I installed a Jamicon capacitor with a capacity of 68 μF * 450V, which is designed for operating temperatures up to 105 0 С.

It is worth considering that the capacity of the native capacitor is 120 uF, and the operating voltage is 420V. But I had to put in a capacitor with a smaller capacity.

In the process of repairing laptop power supplies, I encountered the fact that it is very difficult to find a replacement for the capacitor. And the point is not at all in the capacity or operating voltage, but in its dimensions.

Finding a suitable capacitor that would fit into a cramped case proved to be a daunting task. Therefore, it was decided to install a product of suitable size, albeit a smaller capacity. The main thing is that the capacitor itself is new, of high quality and with an operating voltage of at least 420

450V. As it turned out, even with such capacitors, the power supplies work properly.

When sealing a new electrolytic capacitor, you must strictly observe the polarity connect the pins! Typically, the PCB has a “+" or "–“. In addition, a minus can be marked with a black bold line or a mark in the form of a spot.

On the negative side of the capacitor case, there is a mark in the form of a strip with a minus sign “–“.

When turning on for the first time after repair, keep a distance from the power supply, because if the polarity of the connection is reversed, the capacitor will "pop" again. This can cause the electrolyte to get into the eyes. This is extremely dangerous! Wear protective goggles if possible.

And now I’ll tell you about the "rake" that it is better not to step on.

Before you change anything, you need to thoroughly clean the board and circuit elements from liquid electrolyte. This is not a pleasant occupation.

The fact is that when an electrolytic capacitor slams, the electrolyte inside it breaks out under great pressure in the form of splashes and steam. It, in turn, instantly condenses on the nearby parts, as well as on the elements of the aluminum radiator.

Since the installation of the elements is very tight, and the case itself is small, the electrolyte gets into the most inaccessible places.

Of course, you can cheat and not clean out all the electrolyte, but this is fraught with problems. The trick is that the electrolyte conducts electric current well. I was convinced of this from my own experience. And although I cleaned the power supply very carefully, I did not begin to solder the choke and clean the surface under it, I hurried.

As a result, after the power supply was assembled and connected to the mains, it worked properly. But after a minute or two, something crackled inside the case, and the power indicator went out.

After opening it, it turned out that the remaining electrolyte under the throttle closed the circuit. The fuse has blown because of this. T3.15A 250V on the input circuit 220V. In addition, in the place of the short circuit, everything was covered with soot, and the wire of the choke burned out, which connected its screen and the common wire on the printed circuit board.

The same choke. The burned-out wire was restored.

Soot from a short on the printed circuit board just below the choke.

As you can see, it jumped out decently.

The first time I replaced the fuse with a new one from a similar power supply. But when it burned down a second time, I decided to restore it. This is what the fuse on the board looks like.

And this is what he has inside.It can be easily disassembled, you just need to squeeze the latches at the bottom of the case and remove the cover.

To restore it, you need to remove the remnants of the burnt wire and the remnants of the insulating tube. Take a thin wire and solder it in place of your own. Then assemble the fuse.

Someone will say that this is a "bug". But I disagree. In the event of a short circuit, the thinnest wire in the circuit burns out. Sometimes even the copper tracks on the PCB will burn out. So in which case our self-made fuse will do its job. Of course, you can also do with a thin wire jumper by soldering it to the contact dimes on the board.

In some cases, in order to clean out all the electrolyte, it may be necessary to dismantle the cooling radiators, and with them active elements such as MOSFETs and dual diodes.

As you can see, liquid electrolyte can also remain under coil products, such as chokes. Even if it dries out, in the future, due to it, corrosion of the leads may begin. An illustrative example is in front of you. Due to electrolyte residues, one of the capacitor leads in the input filter completely corroded and fell off. This is one of the power adapters from the laptop that I have been repaired.

Let's go back to our power supply. After cleaning it from electrolyte residues and replacing the capacitor, it is necessary to check it without connecting it to a laptop. Measure the output voltage at the output plug. If everything is in order, then we assemble the power adapter.

I must say that this is a very time-consuming business. First.

The PSU cooling heatsink consists of multiple aluminum fins. Between themselves, they are fastened with latches, and are also glued with something resembling a silicone sealant. It can be removed with a pocket knife.

The upper radiator cover is fastened to the main part with latches.

The bottom plate of the heatsink is fixed to the PCB by soldering, usually in one or two places. A plastic insulating plate is placed between it and the PCB.

A few words about how to fasten the two halves of the body, which at the very beginning we sawed with a jigsaw.

In the simplest case, you can simply assemble the power supply and wrap the halves of the case with electrical tape. But this is not the best option.

I used hot melt glue to glue the two plastic halves together. Since I don't have a thermal gun, I cut off pieces of hot melt glue from the tube with a knife and put them in the grooves. After that, I took a hot air soldering station, set about 200 degrees

250 0 C. Then he heated pieces of hot melt glue with a hair dryer until they melted. I removed the excess glue with a toothpick and once again blew it with a hairdryer on the soldering station.

It is advisable not to overheat the plastic and generally avoid excessive heating of foreign parts. For me, for example, the plastic of the case began to brighten with strong heating.

Despite this, it turned out very soundly.

Now I will say a few words about other malfunctions.

In addition to such simple breakdowns as a slammed capacitor or an open in the connecting wires, there are also such as an open circuit in the choke output in the line filter circuit. Here is a photo.

It would seem that the matter is trifling, I rewound the coil and sealed it in place. But it takes a lot of time to find such a malfunction. It is not possible to detect it immediately.

Surely you have already noticed that large-sized elements, like the same electrolytic capacitor, filter chokes and some other parts, are smeared with something like a white sealant. It would seem, why is it needed? And now it is clear that with its help large parts are fixed, which can fall off from shaking and vibrations, like this very choke, which is shown in the photo.

By the way, initially it was not securely fixed. Chatted - chatted, and fell off, taking the life of another power supply from the laptop.

I suspect that thousands of compact and rather powerful power supplies are sent to the landfill from such banal breakdowns!

For a radio amateur, such a pulsed power supply with an output voltage of 19 - 20 volts and a load current of 3-4 amperes is just a godsend! Not only is it very compact, but also quite powerful. Typically, the wattage of power adapters is 40

Unfortunately, in case of more serious malfunctions, such as failure of electronic components on a printed circuit board, repair is complicated by the fact that it is rather difficult to find a replacement for the same PWM controller microcircuit.

It is not even possible to find a datasheet for a specific microcircuit. Among other things, the repair is complicated by the abundance of SMD components, the marking of which is either difficult to read or it is impossible to purchase a replacement element.

It is worth noting that the overwhelming majority of laptop power adapters are made of very high quality. This can be seen at least by the presence of winding parts and chokes that are installed in the network filter circuit. It suppresses electromagnetic interference. In some low-quality power supplies from stationary PCs, such elements may be absent altogether.

The switching power supply is built into most household appliances. As practice shows, it is this unit that often fails, requiring replacement.

The high voltage constantly passing through the power supply does not have the best effect on its elements. And it's not about the manufacturers' mistakes. By increasing the service life by mounting additional protection, you can achieve the reliability of the protected parts, but lose it on the newly installed ones. In addition, additional elements complicate the repair - it becomes difficult to understand all the intricacies of the resulting scheme.

Manufacturers have solved this problem radically, reducing the cost of the UPS and making it monolithic, non-separable. Such disposable devices are becoming more common. But, if you are lucky - the collapsible unit has failed, self-repair is quite possible.

The principle of operation is the same for all UPSs. The differences concern only schemes and types of parts. Therefore, it is quite simple to understand the breakdown, having basic knowledge of electrical engineering.

You will need a voltmeter for repairs.

It measures the voltage across an electrolytic capacitor. It is highlighted in the photo. If the voltage is 300 V, the fuse is intact and all other related elements (power filter, power cable, input chokes) are in good order.

There are models with two small capacitors. In this case, the normal functioning of these elements is evidenced by a constant voltage of 150 V on each of the capacitors.

In the absence of voltage, you need to ring the diodes of the rectifier bridge, the capacitor, the fuse itself, and so on. The insidiousness of the fuses is that, having failed, they outwardly do not differ in any way from the working samples. The fault can only be detected through a dial tone - a blown fuse will show a high resistance.

Having found a faulty fuse, you should carefully examine the board, since it often fails at the same time as other elements.

• power or rectifier bridge (looks like a monolithic block or may consist of four diodes);
• filter capacitor (looks like a large block or several blocks connected in parallel or in series) located in the high-voltage part of the block;
• transistors installed on the radiator (these are field switches - power switches).

Important. All parts are soldered and replaced at the same time! Replacing in turn will lead to burnout of the power unit each time.

For certain purposes, a switching power supply can be assembled independently from scrap parts. Read more about this here.

Burnt-out elements must be replaced with new ones. The radio market offers a rich assortment of parts for power supplies. Finding good options at the lowest prices is pretty easy.

• voltage drops;
• lack of protection (there is space for it, but the element itself is not installed - this is how manufacturers save).

Solution this malfunction of switching power supplies:

• install protection (it is not always possible to find the right part);
• or use a mains voltage filter with good protective elements (no jumpers!).

Another common cause of a power supply malfunction has nothing to do with a fuse. We are talking about the absence of an output voltage with a fully functional such element.
Solution:

1. Swollen Condenser - Desoldering and replacement required.
2. Failed choke - it is necessary to remove the element and change the winding. The damaged wire is unwound. In this case, the turns are counted. Then a new wire of a suitable section is wound on the same number of turns. The part is returned to its place.
3. Deformed bridge diodes are replaced with new ones.
4. If necessary, the parts are checked with a tester (if no damage is detected visually).

It is quite possible to build a hot air soldering station yourself. A fan is used as a blower, and a spiral is used as a heater. The best option for a temperature controller for a soldering iron is a thyristor circuit.

Breakdown reasons:

• do not block ventilation openings;
• provide optimal temperature conditions - cooling and ventilation.

Things to Remember:

1. The first connection of the unit is made to a 25-watt lamp. This is especially important after replacing diodes or transistor! If a mistake is made somewhere or a malfunction is not noticed, the passing current will not damage the entire device as a whole.
2. When starting work, do not forget that a residual discharge remains on electrolytic capacitors for a long time. Before soldering the parts, it is necessary to short-circuit the capacitor leads. You cannot do this directly. It should be short-circuited through a resistance with a rating higher than 0.5 V.

Depending on the causes and types of breakdowns that have occurred, different types of tools may be required, it is imperative to have:

• a set of screwdrivers with various types of working tips and sizes;
• insulating tape;
• pliers;
• knife with a sharp blade;
• soldering machine, solder and flux;
• braid designed to remove unnecessary solder;
• tester or multimeter;
• tweezers;
• nippers;

In the most difficult cases, when the exact cause of the problem cannot be determined, an oscilloscope may be needed.

After carrying out diagnostics, and identifying the reasons for the incorrect operation of the switching power supply, you can start repairing it: