Small MOSFETs flyback diode (MBRA210LT3)

[tomi.opt...@gmail.com]

The flyback diode (Onsemi MBRA210LT3) used with Small MOSFETs has reverse voltage rated to max 10V and the operating voltage for the board is defined 12-24V -> there is a mismatch with the diode and operating voltage limits. We noticed this when assembling components for Small MOSFET Q4: during the test run, diode D9 and MOSFET Q4 were destroyed when we turned the MOSFET Q4 ON (we have 4x v1.1 board and 24V VBB in our system. When the MOSFET is turned ON there is 24V reverse voltage over the diode D9 and it burns to short circuit state which leads the MOSFET Q4 to break as well). I can give more details about the issue if needed.

The components (D9, Q4) we assembled were exactly the same as there were with other small mosfets (Q8 and Q9) and we got the part codes/numbers directly from the BOM.

We measured the ordered diodes on the table and they started to conduct (and break) when the reverse voltage reached about 20V. We didn't try the same with the 'factory assembled' diodes but most likely they work because of diode manufacturing process tolerance and thus can tolerate the 24V VBB.

We'd like to propose using flyback diode having reverse voltage rating >30V (f.ex. MBRA140T3G or similar) to make the design functional over the whole operating voltage range of 12-24V: how to get this change done so the boards would be ok when we order new ones in the future?

-tomi

[Arthur Wolf]

Looking into this with the team now.

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Courage et bonne humeur.

[John deGlavina]

Hrm, I did nearly the same thing the other day. That is, I added a small mosfet to my 4x board, and caused the magic smoke to rapidly release from the mosfet with a fairly large flame.

Granted, I reversed the polarity on the voltage input of that mosfet, wouldn't a properly sized diode have prevented the mosfets destruction from being so spectular (and potentially prevented other damage)?