Electronics > Repair

Mosfet went Rocketdyne - HOW TO - Replacing nearby componets that were toasted

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ThermallyFrigid:
Hello,
I am working on a PCB where an SMD Mosfet burned furiously, taking out a few other nearby very small (maybe 0201) capacitors and very small SMD resistors so there is no hope of identifying them.
No schematic is available.

Obviously a careful cleaning with 90% alcohol and cotton swabs is a first step, but when that does not reveal any information?

My question is what do you normally do in situations like this?

Does it take a EE to determine what suitable component goes there?   Do you guess?   Of course, SMD caps usually have no markings any way and if you're replacing them it may be because they are defective and any readings you get are not necessarily accurate in identifying them anyway.  Do you give up?

Thanks

[attachimg=1]

T3sl4co1l:
Easiest: replace board.

Medium: replace identifiable components until it works.  Mind, you'll need to replace everything that's toasted; likely the controller is dead, maybe the supply to it as well (sometimes there's an aux supply, as simple as a transistor or two, or as fancy as a whole other controller/regulator).  Plus whatever caused this to die, which could be anything from overloaded output to bad electrolytics or tin whiskers.  (Most SMPS should tolerate a shorted output, but it's a design issue that's easy enough to overlook.)

Hard: trace the circuit, compare with known circuits.  Offhand, it looks like it might be a bog standard UC3842 based offline SMPS.  I don't get what the two series diodes are doing there, and the driver transistor (likely the resistors and transistor by the gate pin) probably aren't necessary, but eh, it depends, right?  Failure could've been various things, including just overheating.  '3842 is a current mode controller, safe against overload -- on an immediate (cycle by cycle) basis, but the transistor can still overheat and blow up.  It's a control method that solves several ills, but far from every possible one.

Tim

ThermallyFrigid:

--- Quote from: T3sl4co1l on January 19, 2022, 02:04:07 pm ---Easiest: replace board.

Medium: replace identifiable components until it works.  Mind, you'll need to replace everything that's toasted; likely the controller is dead, maybe the supply to it as well (sometimes there's an aux supply, as simple as a transistor or two, or as fancy as a whole other controller/regulator).  Plus whatever caused this to die, which could be anything from overloaded output to bad electrolytics or tin whiskers.  (Most SMPS should tolerate a shorted output, but it's a design issue that's easy enough to overlook.)

Hard: trace the circuit, compare with known circuits.  Offhand, it looks like it might be a bog standard UC3842 based offline SMPS.  I don't get what the two series diodes are doing there, and the driver transistor (likely the resistors and transistor by the gate pin) probably aren't necessary, but eh, it depends, right?  Failure could've been various things, including just overheating.  '3842 is a current mode controller, safe against overload -- on an immediate (cycle by cycle) basis, but the transistor can still overheat and blow up.  It's a control method that solves several ills, but far from every possible one.

Tim

--- End quote ---

Extremely helpful reply.
Thank you much !

MegaVolt:

--- Quote from: ThermallyFrigid on January 19, 2022, 01:22:34 pm ---My question is what do you normally do in situations like this?

--- End quote ---
Search for photos of the working device can help. Internet, forums ...

ThermallyFrigid:

--- Quote from: MegaVolt on January 20, 2022, 01:11:25 pm ---
--- Quote from: ThermallyFrigid on January 19, 2022, 01:22:34 pm ---My question is what do you normally do in situations like this?

--- End quote ---
Search for photos of the working device can help. Internet, forums ...

--- End quote ---

Thanks.
Not only did I find photos....
I found a NEW replacement board for about $11.00 delivered.

No point in even a attempting repair.

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