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I have a 3rd-party circuit (dimming 5V bulbs), which suddenly stopped working. The FSMS7658AS mosfet was dead.

Here is the datasheet »

The key parameters are: N-Channel PowerTrench® SyncFET™; 30 V, 176 A, 1.9 mΩ; SyncFET™ Schottky Body Diode


Went to the local store, and the guy gave me this one as a replacement: AON6512

The key parameters are: N-Channel MOSFET; 30V, 210 A, 1.7 mΩ;

Here is the datasheet »


I went back home and replaced them.

All good works well, but after a few seconds, smoke rose.

I have three questions as I am a newbie in the topic:

  1. Was this a suitable substitution?
  2. Does the Schottky Body Diode make a big difference?

I've found these ones, but these don't have Schottky:

  • IRL7833PbF
  • IRL3803PbF
  • IRL3713PbF

Also asked ChatGPT, but just gave me a general answer unfortunately.

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    \$\begingroup\$ Do you know what the fault was which caused the original FSMS7658AS to fail? E.g. a short on the output. If the fault was something other than the FSMS7658AS, then the replacement AON6512 could then failed in the same way. \$\endgroup\$ Commented Nov 14, 2023 at 17:41
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    \$\begingroup\$ Part recommendations are specifically off-topic - so I've edited that part of the question. \$\endgroup\$
    – brhans
    Commented Nov 14, 2023 at 18:00
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    \$\begingroup\$ Do not ask chatgpt for anything that you need a factual answer for. LLMs do not and cannot provide facts--they provide, and are designed to provide, something that looks like an answer. It doesn't have to be an answer, just look like one, and it doesn't even have to look like a correct answer. I don't understand why everyone seems to think of them as some useful resource. \$\endgroup\$
    – Hearth
    Commented Nov 14, 2023 at 19:39
  • \$\begingroup\$ Thank you guys. I have no idea what caused the failure. There are two identical dimming boards—drive and dim 10-10 old-fashioned bulbs. I took a deep breath and put the failed dimmer's bulbs into the working one. All good. So I would exclude the possibility of the failure of the bulbs/wiring. I may have made a mistake while I soldered to the board... but it worked for 10-20 seconds at full brightness. \$\endgroup\$
    – sTE
    Commented Nov 15, 2023 at 18:30

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The AON6512 is a suitable replacement for the FSMS7658AS. In particular...

  • Its on resistance is lower at Vgs=10V and equal at Vgs=4.5V. That's an improvement and will lead to lower conduction losses.
  • Its gate charge is lower, which is also an improvement (lower switching losses, faster switching).
  • The body diode's reverse recovery characteristics (time and charge) are better as well.

The fact that the AON6512 doesn't have a reverse Schottky will lead to slightly higher reverse conduction loss. This will make the FET run slightly hotter when its body diode is used for free-wheeling, but it's not going to make the FET fail, especially not that quickly.

Given that you're dimming LEDs, it's also quite likely that the body diode isn't even used for anything (i.e. if the circuit uses plain PWM).

Whatever killed the original FET also killed your AON6512.

If you have an oscilloscope, you could check the gate signal of the FET. It should be a proper squarewave with an amplitude of at least 4.5V. If it's not, that could explain the death of your FETs. You can replace the FET with a 4.7nF capacitor from gate to source as a test dummy for this measurement.

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  • \$\begingroup\$ Another noteworthy characteristic that makes it suitable: the AON6512 has the same or higher BVdss rating (it's the same in this case, but anything higher would be fine too. Lower would not) as the FSMS7658AS. Also, the Idss rating is much lower, which is an improvement (probably due to the AON6512 not having an integrated schottky diode, which are very leaky). \$\endgroup\$
    – Hearth
    Commented Nov 15, 2023 at 14:14
  • \$\begingroup\$ On the negative side, the AON6512 has slightly higher Rg and Crss than the FSMS7658AS. These would worsen performance in very fast switching circuits. \$\endgroup\$
    – Hearth
    Commented Nov 15, 2023 at 14:17
  • \$\begingroup\$ Thanks, Jonathan, thanks Hearth. Any recommendation for a TO220, as it is much easier and safer to solder that one? (dimming tiny old-fashioned lightbulbs) \$\endgroup\$
    – sTE
    Commented Nov 15, 2023 at 18:39

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