Electronics > Repair
buck converter repair
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Pinus94:
Hi, everyone. Ten days ago, the board of an old metal bender failed, it refused to turn on. I extracted the pcb and started checking components. I founded a p-mosfter and a tvs diode shorted, replace it and the machine started again. The repair lasted 10 days, (each day the machine has been on for 8 hours. Everything seemed to be going well); until Today, the machine failed to start, again the same components shorted.

Let’s me try to explain the schematic and the problem.

The section to be repaired is the power supply section, it is a buck converter, it converts 24vdc to 5vdc and 12 vdc. The busk converter is based on the LTC1267CG-ADJ5, the shorted components are

1) the p-mosfet (originally it was 10P03L, replaced by me with AOD417)
2) a TVS diode marked “DX” from ST Component (placed between the 12vdc output and ground, it is a tvs diode with 14,5v threshold)

Output voltages seem to be perfect; output capacitor seems to be ok (checked only the capacity). No other components seem to be shorted or bad.

What can I check? What can cause these two components to fail?

For now, I re-replaced these two components, I put a 24vdc power supply exclusively to power this board, added a proper fuse (before, a wrong “size” fuse was used)

I hope I explained it well and sorry for my bad English. About my electronic knowledge, I’m an automation engineer, for hobby I like to repair electronic boards.
Hope I can receive some useful tips
madires:
Is the shorted p-ch MOSFET also for the 12V output? Could you please provide some details about the 12V load?
Pinus94:
yes, the p-mosfet is for the 12v output, looking at the typical application schematic (first page of the datasheet) it is in place of P-CH Si9435DY

the load is a mix of memories, microcontroller and other stuff. The board has a serial communication with a "cn main board" and a video output.

PA4TIM:
Did you buy the parts direct from China (ebay, amazon, Aliexpress, temu) If so, there is a huge chance they are not what they are labelled.

If they are 100% sure original then there is something other wrong that kills the parts. It can be very difficult to find things if they are not constant wrong.

Are fans working like they should ? 
Is the PCB clean ? I had to repair a scope that was used by service technicians who serviced all kinds of metal transforming machines (I do not know the correct English term, things like drilling, turning, cutting, milling)
When I opened the scope there was a lot of greasy/sticky dirt inside and in the there where several flakes of metal stuck to the dirt. One of those had caused a short.
Also had a bow thruster controller that had a conformal coating that was so dirty it was conducting causing all kinds of (destructive) problems.
Pinus94:
I bought replacement parts from "tme" (Poland online shop) they are genuine.
The fan is working and the pcb is very clean, no dust, no metal shavings. Neither the mosfet nor the diode are warm to the touch.

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