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Recently I have noticed high freq. screeching noise coming out from my laptop's adaptor. I opened it up and found that it came from somewhere around a diode named egp10a. I checked it (unplugged) with a multimeter disoldering only one end and saw that it works fine (no reverse voltage leak detected).

Later, I wanted to power the board without soldering the cathode to the board to see if the noise really caused by this diode. As I plugged the adapter the fuse blew and gave that burning smell. I found another identical fuse and replaced it and soldered the diode completely back again. Hovewer the new fuse also blew.

As someone who is very keen on learning electronics I would like to know what would be the problem and what I did wrong. I would appreciate if you guys guide me through learning the basics of electronics or share a source where I can learn this stuff.

Update:

I have noticed a new component with a burn mark on it located near the FETs (?). I do not know how it's named.

This adapter is becoming my learning kit now I guess since I consider buying a new adapter.

Do you guys think that the SMD caps under the board close to the diode legs I removed would be the cause of the noise I mentioned before?

Edit & Update 2:

  • I have replaced components shown in the pictures with the equivalent once (I suppose(!))
  • ***Used 11N60C3 instead of damaged STP11NM60AFP
  • With the new parts put together I get no output voltage at all. (Fuse don't blow anymore)
  • Do shorted caps might cause this?
  • Please refer to the picture below where I get 308v output. Is it normal? (standard for my country is 220v)

Note: The power supply's model is ADP-65YB B and rated as 19V @ 3,42A. (Would it be okay if I used 19V @ 4,74A which I already have lying around? enter image description here enter image description here enter image description here enter image description here enter image description here trying to draw the diagram using multimeter on continuity enter image description here enter image description here enter image description here

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    \$\begingroup\$ You'd need to reverse engineer the design and produce a schematic, then find out where in the design the problem is, it's probably not in the diode, but too much current through the diode. \$\endgroup\$ – Voltage Spike Jul 14 at 16:14
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    \$\begingroup\$ removing the flyback diode prevented the ferrite noise but damaged the driver semiconductor. Did it smell like carcinogenic fuming epoxy? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 14 at 17:44
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    \$\begingroup\$ Smells like an epoxy resistor \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 14 at 20:22
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    \$\begingroup\$ Welcome to SE EE . Do keep asking questions. As others have noted, this is probably in the TOO HARD category and also rather dangerous. You now have a great source of components :-). \$\endgroup\$ – Russell McMahon Jul 15 at 10:51
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    \$\begingroup\$ It needs to be said : DON'T LEARN ON POWER ELECTRONICS. You can kill yourself too easily. Learn on low voltage. Don't touch the dangerous stuff until you know wtf you're doing. \$\endgroup\$ – J... Jul 15 at 15:01
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The answer is clear: Buy a new supply and find something else much simpler to "learn" on. Switching power supplies are outrageously complex and not something you're gonna be able to wrap your head around without a good understanding of electronic theory.

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  • \$\begingroup\$ Your are probably right! The backside of the adapter is too complicating for me. \$\endgroup\$ – Karel Capek Jul 14 at 20:22
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    \$\begingroup\$ Good and simple answer. These power supplies give a nasty shock and bang with tiny mistakes.+1 \$\endgroup\$ – user105652 Jul 15 at 1:02
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    \$\begingroup\$ Yes, absolutely! I know it feels like learning, but it’s a positively atrocious lesson plan. Your time matters, and you will be far, far more productive learning eletronics in some sort of organized training course. What ever happened to Heathkit... \$\endgroup\$ – Harper - Reinstate Monica Jul 15 at 4:42
  • \$\begingroup\$ @Harper-ReinstateMonica Almost real Heathkit still exist !!! see here see also here and Wikipedia and ... . \$\endgroup\$ – Russell McMahon Jul 15 at 10:49
  • \$\begingroup\$ Many devices include components whose purpose is to safely shut them down should a dangerous condition arise. Sometimes the safety-shutdown components may fail in a way that shuts down the system even when everything else is operating normally, but even such erroneous shutdown serves a useful purpose. While one could easily restore circuit operation by simply bypassing the emergency shutdown device, the purpose of a shutdown device is to prevent device operation under dangerous circumstances, and the lack of an effective shutdown device is a dangerous circumstance in and of itself. \$\endgroup\$ – supercat Jul 15 at 16:17
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Powering up with a random component removed is a bad idea unless you know its function and understand what will happen without it in the circuit.

It could be part of a clamp that prevented overvoltage damage to your switching FET for example. So by removing it you could cause damage to other components. (Which it sounds like you did.)

Diodes don't create noise in power supplies, noise comes primarily from ceramic capacitors and magnetic components.

Without a schematic of you converter, or at least some good photos we won't be able to comment much further.

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  • \$\begingroup\$ Do you mean Field-effect transistor by FET? That is what I found on google. Please check the photos. There is a burnt component there which's name I do not know. Thanks \$\endgroup\$ – Karel Capek Jul 14 at 20:26
  • \$\begingroup\$ Yes, by FET I meant Field Effect Transistor. The burnt component in the pictures looks like a resistor. Unfortunately there's not enough info in the pictures to know what the root cause of the failure was, or what damage removing the diode might have caused. \$\endgroup\$ – John D Jul 14 at 23:30
  • \$\begingroup\$ I don't think this diode is part of a clamp, in fact, I have never seen over-voltage clamps on FETs in SMPS. \$\endgroup\$ – Dmitry Grigoryev Jul 15 at 7:37
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    \$\begingroup\$ @DmitryGrigoryev The comment was just an example, I don’t think we can tell the purpose of the diode. However, RCD clamp circuits are common in SMPS to deal with transformer leakage inductance: ridleyengineering.com/design-center-ridley-engineering/… \$\endgroup\$ – John D Jul 15 at 11:20
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First of all, consider that fiddling with mains voltage can kill you should the discharge go through the heart. SMPS have large caps inside which will keep deadly voltage for hours after you pull the plug. Are you sure you can work safely with it?

If yes, you should check if the hot (high voltage) side of the PSU still works. My guess would be that by removing the diode you cut the power coming from the auxiliary winding to the SMPS IC. That would be D4 in the schematic below (keep in mind it's a reference design, not the schematic of your particular supply). A good PSU should have survived that (providing output voltage for a split second every once in a while), but considering the blown fuse, yours was not as forgiving.

First of all, if the short persists after you have put back the diode, you should localize it. Perhaps you have just accidentally shorted something and the short is already gone, in that case you should see the big 400V cap charging to nominal voltage after power up.

enter image description here

If there's no AC voltage on the transformer, check the gate of the MOSFET: if there's a PWM signal there, the SMPS IC is still alive, and replacing the FET can bring the PSU back to life. If not, you need to replace the IC, and I wouldn't bother trying to repair it.

If you see AC voltage on the transformer's primary but there's nothing on the output, the problem is likely on the cold (low-voltage) side of the schematic.

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    \$\begingroup\$ I agree that removing D4 should not kill the supply. I suspected OP removed D3, the inductive kickback killed (shorted) Q1, shorted Q1 fried R11 (See burnt resistor picture). R11 went open, full mains at the current sense input fried IC1. The short circuit N1, Q1, IC1 blew the fuse and possibly the rectifier as well. Or in short: A lot of damage at the hot side, so experience and knowledge is needed for a safe and proper fix. \$\endgroup\$ – Michael Karcher Jul 15 at 14:50
  • \$\begingroup\$ @MichaelKarcher Yeah, it all fits together now! \$\endgroup\$ – Dmitry Grigoryev Jul 17 at 7:49
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    \$\begingroup\$ I know the pattern because I fixed a DELL 2407WFP power supply. The main PFC output cap in these supplies is prone to fail open (corrodes pin breaks). As the PFC is a boost converter, failure of the output cap causes the inductive spike to not be caught, and then the things happen I described in my previous comment. \$\endgroup\$ – Michael Karcher Jul 17 at 8:07
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Some additional general comments meant as a complement to what's already posted.

  1. High-frequency 'squealing' noises coming from power supplies generally implies incorrect / abnormal behaviour. Magnetorestriction caused by high pulsed currents (i.e. the power supply hiccupping energy into a shorted part) can cause acoustic noise. When you hear this, DISCONNECT THE INPUT POWER IMMEDIATELY as bad things are bound to happen in short order. (This noise will make the hair of an experienced power-supply designer or repairperson stand on end.)

  2. The long dashed line on the silkscreen (top to bottom) is a reprentation of the isolation barrier between the mains-connected lethal part of the power supply and the galvanically-isolated non-lethal part. By non-lethal, I mean: if the power supply is working correctly (and designed correctly) on the secondary (isolated) side there should not be sufficient voltage present to produce a shock hazard. The mains side must always be considered lethal and appropriate precautions must be taken: never live-probe, always discharge HV capacitors before handling, etc. The other dashed lines seem to be following the two AC input traces are are likely meant to indicate to the PCB designer to keep separation between them to avoid flashovers.

  3. Based on the appearance of the power electronics (single transformer, no boost inductor, no output filter inductors) I agree with others that this power supply is a flyback converter with no active PFC - the mains voltage is rectified and the DC voltage is down-converted via a high-frequency (10s to 100s of kHz) flyback converter topology. The primary switch will see high voltage during the off-time of the converter due to the coupled-inductor action of the transformer (energy is stored during the on-time and supplied to the load during the off-time).

  4. MOSFET replacement only from a datasheet for a switching power supply is a big no-no. You need to carefully consider gate charge, switching loss, avalanche energy handling capability, reverse recovery time and Rds_on among many other parameters, many of which require in-situ measurements to qualify. The datasheet may give you a good starting point, but you really need to take a lot of measurements to be sure. That said, your replacement choice - on paper - looks to me like a good candidate for trials. (I still would not blindly swap it in).

  5. Bridge rectifier substitution by datasheet is a little bit safer than MOSFET substitution. As long as the diodes are rated for comparable-or-higher current, comparable-or-higher reverse voltage, and have comparable forward voltage drop, it's not unreasonable to try a different bridge. I would still keep an eye on device temperature though.

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