0
\$\begingroup\$

I made a mistake when I designed my PCB and a lot of P-MOSFET are placed with wrong orientation:

enter image description here

As you can see, I inverted the drain/source and in this way Q1 is actually by-passed by the built-in DS diode. The package of BSS-84 is SOT-23.

Because I have dozens of these MOSFET in my PCBs I'm looking for a solution to save the prototypes.

I searched for a P-MOSFET with inverted D-S but I find nothing suitable. Any idea?

\$\endgroup\$
2
  • \$\begingroup\$ Looking at the comments, it means there are no SOT-23 P-MOSFETs with inverted G-S or without the built-in "diode", is it right? \$\endgroup\$
    – Mark
    Commented Apr 3, 2021 at 12:59
  • 1
    \$\begingroup\$ Maybe there are suitable pnp bjts? My impression was that pins vary more for BJTs and for this simple case, a pnp might work. \$\endgroup\$
    – tobalt
    Commented Apr 3, 2021 at 19:27

4 Answers 4

7
\$\begingroup\$

Flip it upside down, bend the pins (dangerous, they can crack), rotate and insert a small wire (blue) to connect the gate:

enter image description here

Fasten it with glue to prevent an ability to vibrate around the axis D-S.

If you happen to have more room it can be only shifted and rotated, but the gate wire is long:

enter image description here

Glue is even more important in this case.

I have no idea does the board space allow one of these. In addition check if there exists elsewhere good places to insert the mosfet so that at least 2 of the pins fit - for ex. the backside of the board? If the backside is free you may use version 1 without bending the pins, but you need wires through the board.

Finally: If no place which need only 1 wire exist, use more wires or scrap the original mosfets and find a part which has long wires.

\$\endgroup\$
3
  • \$\begingroup\$ This is the best solution so far. I managed to solder all the three pins on the pads (like in the first picture) without using any wire! \$\endgroup\$
    – Mark
    Commented Apr 3, 2021 at 15:59
  • 1
    \$\begingroup\$ Someone would have written it in 10 seconds if they had a photo of your system. I made pure guesses, but luckily one was good enough. \$\endgroup\$
    – user136077
    Commented Apr 3, 2021 at 16:32
  • \$\begingroup\$ Classic! 123456 \$\endgroup\$
    – winny
    Commented Apr 3, 2021 at 18:12
5
\$\begingroup\$

Change it by cutting, bending and soldering a through hole TO-92 transistor.

\$\endgroup\$
1
  • \$\begingroup\$ This is probably the easiest fix for a beginner. \$\endgroup\$
    – ChrisD91
    Commented Apr 3, 2021 at 18:12
1
\$\begingroup\$

Make a little daughter board that can be soldered to the existing BSS84 SOT-23 pads and correct the pin-out on that daughter board. It's not a high power circuit so small tracks shouldn't be much of a problem.

\$\endgroup\$
1
\$\begingroup\$

See Andy aka's answer.

My previous answer was wrong.

If you got no time and you can't design Andy's adapter boards than you have to use AWG 30 wires for inverting drain with source.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.