Electronics > Repair
buck converter repair
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PA4TIM:
The original MOSFET is a logic level version. The one you used is not. That could be the problem but that depends of the amount of gate voltage and it's function. But that has no relation to the tVS.

A TVS shorts if the voltage over it is to high. You could log the voltage over it with a scope using one shot triggering at the hight of the max voltage of the TVS. But all that time you can not use the scope for other repairs. But first I would monitor the voltage over the TVS with the scope in normal upgoing flank trigger to see if there are many spikes on the rail it protects.

How good is your electronic knowledge and how sure you are the P-ch mosfet was really dead ? How do you test the mosfet ?
You can not test that in situ and if you charge the gate by accident during your tests the channel will test as a short.

If it is only the TVS that is bad, then there is something that is causing high voltage spikes/levels over the TVS.  Did you use exactly the same TVS or like the mosfet, one you think is the same ?
 
Pinus94:
the originale mosfet has a Vgs of - 1.0 - 2.5 (min max), mine -1 -3, the one used in the application notes of the ic (i.e  Si9435DY) –1  –3
so from my "electronic noobness" i'm pretty confident the aod417 is fine for this application.

Mosfets, both the original one and the replaced one, were completely shorted between all tree pin.

I replaced the TVS with the exact original model.

I have a 20 mhz oscillsocope + a battery one (about 1Mhz), but for now is difficult to take measurement on the board, it is installed and the machine is working right now.

I asked here and i'm studying to understand what can cause this problem, in case it will recur in the next weeks.

for sure "there is something that is causing high voltage spikes/levels over the TVS"
PA4TIM:
This is not how it works. -Vgs(th) is the voltage to start the conduction. Not max/full conduction. Often -Vgs(th) is specified for only 100uA or 1mA.
A logic level mosfet often needs less then -5Vgs to conduct fully. A normal mosfet needs a lot more than that. But again, it depends on things only you know, the -Vgs it really sees. Measure that, then look in the graph of the datasheet of your mosfet and the original and you know the max current it can transfer at the measured -Vgs. The datasheet from the controller also uses a logic level mosfet. Your mosfet replacement is not logic level.

Is the TVS protecting the input or at the output protecting the load? If input then the incident voltage comes from the external power source. If on the output the smps itself has a problem (maybe a dead snubber or bad filtering) Based on your limited info I can not tell more.

But again, I do not see a relation between the TVS and mosfet. Or it is the mosfet that dies and then the output goes high and kills the TVS (If it is at the output) In that case it is simple, you used the wrong mosfet.

20 MHz sounds like an analog scope or a very low budget toy-scope. 1MHz for sure will be a toy scope. (or an old scope meter) both will probably not support  a usable single shot triggering and can be to slow for transient detection.
Pinus94:
thanks for the usefully information,
Comparing the 3 mosfet datasheet (application note, original, replaced) the difference between logic level and not logic level mosfet is clear

the VTS is protection the output
"it is the mosfet that dies and then the output goes high and kills the TVS (If it is at the output) In that case it is simple, you used the wrong mosfet."

yes, the oscilloscope, is an old hitachi analog model

the circuit is drawing 200mA from the 24vdc in input
Pinus94:
the machine worked properly for 10 days, today it didn't turn on, the fuse blew, replaced it and start working again
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