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I got a couple of SMC network switches (EZ Switch, SMCGS5, HW rev. A1), of which one is broken on account of something applying 12VDC to the 5VDC input. Looking at the broken switch's PCB, I can see that one component in the voltage input section is fully charred and basically vaporised. Comparing with an intact PCB I can glance the following data:

  • Silkscreen name: Q3
  • Package type: SOT-23
  • Markings (on intact version): N59M.
  • Further PCB markings: D (near the top), G, S (two pins on the lower end).

I have done extensive searching to find anything resembling a match, but other than that it is a JFET/MOSFET transistor I have only drawn a blank.

Anyone have a clue what transistor this may be, and what to replace it with?

U2 is marked FP5453 and seems to be a Dual PWM Controller for DC/DC circuits.

Here is a picture of the board (click to zoom):

top bottom

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  • \$\begingroup\$ It might be a PFET being used as a power switch or a soft-start device. Difficult to say without a schematic. \$\endgroup\$ – Daniel Mar 23 '16 at 21:35
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    \$\begingroup\$ @bitsmack Q3's source pin is connected via a 470 uF, 35V electrolytic capacitor to ground. The drain goes to a coil (L1) and to another cap (C9). From the looks of it is likely used as a voltage regulator? \$\endgroup\$ – MayaPosch Mar 23 '16 at 22:17
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    \$\begingroup\$ Hmm, hard to say. It could well be the control FET on a switching regulator. Is there an IC nearby to control the FET? If so, you could look it up and see what kind of switching topology it uses... \$\endgroup\$ – bitsmack Mar 23 '16 at 22:30
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    \$\begingroup\$ A hi-res Picture would be nice, if not mandatory. Not just of the IC but of the surrounding circuit. \$\endgroup\$ – Passerby Mar 23 '16 at 22:44
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    \$\begingroup\$ Great! Here's the datasheet for U2. On page 19 there's a circuit that looks similar to yours. Now we can be sure that Q3 is a PFET. (Spehro and JRE were right!) I'd give one of them a try :) \$\endgroup\$ – bitsmack Mar 24 '16 at 21:26
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If you can't find a better match I would suggest giving it a try with an AO3401A 3A 30V p-channel MOSFET. That matches the voltage rating of the APM9435 MOSFET used in the other SMPS channel (sources are tied together according to the PCB layout), at lower current (~3-4A) and is also logic-level gate.

Of course it could make things worse than they are now, but I doubt it.

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  • \$\begingroup\$ Thank you. I hadn't looked at U3 yet, but it looks like it's indeed a very similar arrangement, with U3 and Q3 both controlled by what seems to be an MCU (U2) and then providing power via smoothing cap and coil to components. I'll give this p-channel MOSFET a try, I think :) \$\endgroup\$ – MayaPosch Mar 24 '16 at 21:10
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    \$\begingroup\$ I replaced the busted MOSFET with the suggested AO3401A part. Unfortunately upon applying power the power LED of the switch does not light up. It seems that more parts may be broken. I'm marking this as the answer for now. Thanks! \$\endgroup\$ – MayaPosch Apr 8 '16 at 18:04
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Possibly a Si2305CDS P-Channel MOSFET (Datasheet)

Fits the description, and according to the link would be marked N5 with suffixes, so N59 would fit.

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  • \$\begingroup\$ Unfortunately Vishay SOT-32 markings have 5 positions, according to this PDF. N5 would match, but the '9M' would be out of place. Unless Vishay changed their markings? These switches are about 5-6 years old. \$\endgroup\$ – MayaPosch Mar 24 '16 at 16:12
  • \$\begingroup\$ I couldn't tell you. I just had a look around, and found what seemed to be a good match. \$\endgroup\$ – JRE Mar 24 '16 at 16:18
  • \$\begingroup\$ That's only an 8V part- seems a bit low, but then that would explain why it didn't like 12V. \$\endgroup\$ – Spehro Pefhany Mar 24 '16 at 17:51

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