How to identify this burned device?

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Hello,

I have a board that has a burned smd component to the point that I can't identify the mark. I have drawn a fragment of the circuit where the component got burnt. It's attached below. A piece of the device that I could identify had the mark RK54. I think it should be a freewheeling diode. What do you think it can be?
 

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ebp

Joined Feb 8, 2018
2,332
The circuit is a snubber for discharge of leakage inductance in the primary of a flyback converter. Typically it would be an unltrafast recovery diode with a reverse breakdown voltage rating at least equal to the supply voltage. The continuous forward current rating is probably 1 A but might be higher - you can judge by the package size and the diodes that are available in that size. There should be many unltrafast diodes that will work satisfactorily.

In rare cases a conventional slow-recovery diode is used in that position with the intent that the slow reverse recovery actually pulls charge from the snubber capacitor when the FET switches on. This reduces the power the resistors have to handle by shifting it to the FET.

It is very odd that a diode like that would burn because the resistors and capacitors in series with it limit the forward power. If might have been breaking down in reverse, which would cause it to overheat. There is a possibility that bad filter capacitors on the output of the converter would prevent delivery of the energy as intended, forcing too much to be dealt with on the input side. Electrolytic caps on the output of flyback converters are subject to a lot of stress (high ripple current) and the do "wear out." Look for capacitors with bulging tops or that look like they might have been overheated. It is important to replace bad capacitors with types with low equivalent series resistance (ESR).
 

MisterBill2

Joined Jan 23, 2018
11,941
ebp is right! A device in that position will not burn up because IT is defective. So before you replace it the trouble must be repaired. So no matter what the part is, there is probably a cause for the failure in another part, unless you know that something outside the circuit caused the problem, such as short circuiting a power supply, or loose screws rolling around on the circuit board
 
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