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I am repairing an IP camera that has a blown transistor in the power supply section. The transistor looks like a SOT23 transistor, but it has 6 pins (how is that package called, anyway?), 4 of them connected together. I tried reading the marking, but the best I got was "HC??" or at least I think it's HC, could be HG, BC, BG, 8C, 8G or something like that.

It is a N channel MOSFET (judging by the fact that there is no resistor in series with the gate) with 4 pins connected to the drain (two middle and two end pins), like this (looking from the top):

G   D   D
+--------+
|        |
+--------+
S   D   D

If nobody can identify it, perhaps you could help me choose another suitable transistor. It is used in the primary side of PoE power converter in the device, so the working voltage is 48V, but could be higher. I will try to measure the current trough it (I have connected a TO220 transistor now to find what is causing too much current to be used and burning the small transistor).

EDIT: I managed to read the marking - it's FHC83 or EHC83. Still can't find anything in any marking database.

Also, I identified the driver chip - it's CD40106 - a few of its gates are connected in parallel.

There is also another transistor in there - looks like it's connected in series with the power input, the marking is "61" which makes it "FMMT618", but that is only 20V and would be connected backwards - current going into the emitter and out of collector (for a NPN transistor).

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I imagine there are a bunch with the same pinning. They usually call this package a SOT-23-6 (for 6 leads..).

Generally you'd want 100V or so for PoE switch, for when the mosfet lets go of the coil, it will spring up some. If there is some snubbing maybe not too much, but I think 100V would be a safe bet.

What IC is used to control the FET? They usually are current mode and will sense the current in a small resistor from the switching FET's source to ground, <1 ohm usually. If we know the sense resistor and the SMPS IC, we can figure what the maximum current is also, to select a fet. :)

Edit: maybe I misinterpreted - is this switching the primary of the transformer? or just randomly somewhere in the circuit on the primary side? Might be for enabling PoE power to downstream parts, for example, after uh.. initiation. In that case, the protocol IC most likely limits current too - have to see it's datasheet to see if it switches the high side or the low side, etc.

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  • \$\begingroup\$ The IC that controls the FET is a SOIC14 chip with I think "40" written on it (almost impossible to read). Gate output is pin 8. Yes, there is a 1.5R resistor in series with source, average current is ~0.16A, peak about 0.8A, but this may be too large, since the original transistor failed (and a series diode gets really hot). \$\endgroup\$ – Pentium100 Apr 25 '13 at 20:01
  • \$\begingroup\$ I have updated the question with new information \$\endgroup\$ – Pentium100 Apr 28 '13 at 6:47

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