# Help confirming probable dead flyback diode

I'm a beginner in electronics and not really my field, but I have a dead network switch (Unifi US-24-250W) which, from what I found, tends to have defective PSUs. The PSU on mine seems to be working though, the led on it is on and all the rails (24v and 48v for POE on secondary board, and main board connector) are bringing the right voltage. Everything else, from the status led, to all the port leds, which are supposed to do a pattern when turning on, are dead.

No obvious signs of what could be the culprit, no burn markings or blown caps. The only thing I found is an inductor, near the main processor, that gets hot enough to be uncomfortable to touch rather quickly. Looking on the internet I found about flyback diodes and it seems that a diode is shorted near it. I can't identify the diode but I think is probably something like this: https://www.diodes.com/assets/Datasheets/ds13004.pdf

The markings on it are:

• LT7A30
• B260A

and I wasn't able to find the exact specs online. Here is a picture, sorry if it's not very readable

It's just under the middle of the picture, what's on the left is the main processor with its heatsink. I have not desoldered it yet because I don't have a replacement on hand, but in the upper section of the board you can see an identical inductor that doesn't get nearly as hot, and a resistor that doesn't look shorted both ways with the multimeter.

Could this be the cause of the issue? There is a slight coil whine from another part of the board which I wasn't really able to identify but nothing hot nor blown there. Can add pictures if required, the board is the same as in https://youtu.be/0_t1-uP4Q8Y?t=56 but in the video the PSU was at fault

EDIT:

U103, the IC near the probably-dead diode is an AEOSMD with markings:

• Z1212AI
• ZA7S18

Here is the datasheet

• Most likely the diode is a flyback (a.k.a. freewheeling) diode of a buck converter. If it shows a short then it could be dead. Could you please tell us (I mean, update the post) what is written on U103 (the IC which is near that probably-dead diode) if you are able to? – Rohat Kılıç Mar 13 '20 at 17:14
• @RohatKılıç thank you for stopping by. Not sure if you get notifications when I edit the post, I added U103 as requested – Giorgio Aresu Mar 14 '20 at 18:24
• Thanks for the update. The IC is a buck regulator and the diode is a freewheeling diode. The IC's datasheet shows that there's an N-Channel MOSFET inside which connects to the freewheeling diode in parallel. Regarding your determination it seems that a diode is shorted near it the short-circuit could be either the fw diode or the MOSFET inside the IC or even both. They aren't supposed to be short-circuited. I personally recommend you to remove the diode first then check if it's short. And also check for a short between the pins LX and GND as well. Let's hope only the diode is dead. – Rohat Kılıç Mar 14 '20 at 18:42
• What voltage do you read on the capacitor where it says '3.3V'? – Bruce Abbott Mar 14 '20 at 19:29
• @BruceAbbott it reads about 0.3v :| for comparison the upper one, where it says 5v, reads 5v correctly. Does this help diagnose the faulty component? As this is the part I don't get, assuming it's possible to debug without pulling the components out – Giorgio Aresu Mar 14 '20 at 22:36

The diode appears to be a B260A schottky diode. It is rated for 2 amperes and 60 volts peak reverse voltage.

You need to remove it from the circuit and test it. You can't reliably check parts in circuit.

It is possible that the short is in another part. As it is built, either the IC itself could be shorted, or there could be a short else where on the board.

To figure out what is broken, do this:

1. Remove the diode from the board and test it. It should read about 0.3V on a diode tester in one direction and open circuit in the other direction.
2. If the diode is OK, remove the inductor from the board and measure the resistance between the diode pads on the board.
3. If you still have a short then the IC is bad.
4. If the short goes away when you remove the inductor then there's a problem else where on the board. Finding that will be difficult.

You describe the inductor as getting hot. That could mean the short is somewhere else. If the chip were shorted or the diode were shorted then there'd be no current flow through the inductor, so it wouldn't get hot.

I almost expect you'll find the diode and the chip OK and that the problem is elsewhere.

You can test the diode by putting a blob of solder on one end. Heat it until it melts, then use the point of your iron to push that end of the diode up. Raise it a bit, then use the tip of your iron to remove the last bits of solder between the diode and the pad.

• Hi, thank you very much for the steps, I do also have a rework station if needed. Before pulling components out I'm just waiting confirmation that the seller/vendor won't RMA it. Is there anything else I can test before pulling them? I'll add a comment if/when they accept or refuse it. – Giorgio Aresu Mar 14 '20 at 22:48
• There's not really much else you can do. You're going to have to take some parts out to do any further testing. – JRE Mar 14 '20 at 22:53
• Sorry for the long time, I was busy at work even during weekends. I tried removing the inductor and the short is gone. I suppose it's pretty impossibile to troubleshoot it and fix it, for me at least, given my knowledge and the size of some components. Guess I need to buy a new one. Thank you for the steps :) – Giorgio Aresu May 2 '20 at 17:31