MOSFET backfeeding?

Discussion in 'General Electronics Chat' started by Tobias, Nov 6, 2009.

  1. Tobias

    Thread Starter Active Member

    May 19, 2008
    Attached are three PDF's; the PCB, schematic and also a drawing of the busbar. The busbar doesn't show up to well on the PCB so I threw it in as a separate drawing.

    This board uses two SUM110P04-05 mosfets in series to power four solenoids. The busbars are on opposite sides of the board, thats how I can get them in series pretty easily. There are four stages of solenoids, so I have eight MOSFETS. Each is activated by an opto-isolator. The wiring of the MOSFETS on the schematic are on the top right part of the schematic.

    Everything works fine when connected correctly. I sent a board to a guy to test about a month ago. He said that the fourth stage would never turn off. I couldn't figure it out, alot of sleepless night. It works fine on my test bench. So this week I sent him another one to test. He called me this afternoon and said he figured out that he was screwing up when he was connecting the VIN and four outputs to the solenoids. He didn't read the manual and was hooking up VIN to stage 4 output and then Stage 1,2,3 and VIN to the solenoids.

    So, first off, how does the MOSFET getting voltage on its Drain side send power to its Source? Secondly, is there anything I can do, besides more obvious documentation to prevent this from happening again. I am going to laser etch the housing above each busbar its function so I am pretty sure I shouldn't have this happen. Yet I am curious to know how MOSFETS backfeed and also how to prevent this by changing around my components.

    Thanks in advance
    Last edited by a moderator: Nov 7, 2009
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Look at the SUM110P04-05 datasheet. MOSFETs have an intrinsic body diode that connects from source to drain. This is how Vin backfeeds from the drain to the source.