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I'm using the Infineon BTS117 smart low side switch to drive a DC-DC power regulator in my car. Here's the schematic of the switch board:

schematic

simulate this circuit – Schematic created using CircuitLab

I don't think all the stuff around BTS117 is relevant to the issue, but then, I'm not an electrical engineer so I thought I'd better lay everything out.

I was testing the device in the car a few days ago and it was working properly, but I couldn't finish the work because one of the input leads was missing. So I took the device home and soldered the lead (just a small piece of wire). Next morning I put it back in the car, but it was no longer working. It wouldn't disengage the load with both inputs floating. As soon as I connect the positive terminal, the load (which, I remind you, is a power supply) would power up. The said PSU is able to work with as low as 4V input according to its specs.
I can't recall for sure (it was 2 days ago), but I believe I did try grounding the gate of BTS117 to the ground, and it made no effect.

I took the switch board home and tested it with a 12V 5W light bulb for a load. It again worked perfectly. Connected a milliammeter in series with the load and measured 0 mA with inputs floating at the same 12V across the load (and hence the switch).
Also tried measuring the channel resistance with inputs floating and power supply disconnected. My DMM has the upper limit of 2 MOhm and that wasn't enough for it to sense any conductance.

Any idea what can possibly be going wrong when I put the switch in the car? Why does it conduct when it should be off?

Update: I have once more experienced the faulty behavior, and this time the switch got considerably warm, i. e. it didn't turn on all the way and had decent voltage drop across it. I have disassembled everything and reassembled back again half an hour later; the problem went away and has not repeated since.

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  • \$\begingroup\$ Have you connected the MOSFET the wrong way around (S-D) instead of (D-S)? \$\endgroup\$ – JIm Dearden Sep 10 '16 at 9:05
  • \$\begingroup\$ @JImDearden: No, I was very careful about the polarity and the connectors on the switch board won't let me connect it the wrong way. \$\endgroup\$ – Violet Giraffe Sep 10 '16 at 9:11
  • \$\begingroup\$ Can you provide a schematic or picture how it's connected on the car? Can you measure 12 V across the input terminals? \$\endgroup\$ – winny Dec 16 '16 at 19:01
  • \$\begingroup\$ I believe all the connections were sane. The thing has only 3 leads, how many ways can I possibly get it wrong? Anyway, I had to throw the whole schematics into the trashcan. Turns out a low-side switch just cannot work reliably in a car. It's very difficult to prevent ground from creeping into your load around the switch, so I had to use a high-side switch instead. I went with an Infineon unit and everything worked great at last. \$\endgroup\$ – Violet Giraffe Dec 16 '16 at 19:14
  • \$\begingroup\$ Car grounds (and +12) are notoriously noisy. \$\endgroup\$ – TLW Jan 20 '17 at 2:46
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"It wouldn't disengage the load with both inputs floating"

The offset drain current should be <5uA. Assuming there are no shorts or semi-conductive paths, (given the 12V supply) the only way the BTS117 could be conducting is if it damaged.

The input rather ESD sensitive w/only 3k volts of protection. I'd recommended replacing it.

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  • \$\begingroup\$ I have replaced it. Had the same issue briefly with the second unit, but it quickly went back to normal. I couldn't tell what's causing this floating issue. \$\endgroup\$ – Violet Giraffe Oct 7 '16 at 19:47
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    \$\begingroup\$ Then there must be something about the hardware that is different that the schematic. Possibly a loose connection, or extra connection. To figure it out one needs to know the voltages at (esp around the BS117). Also you say that the load is a power supply - could you provide more details? How does the power supply output interact with the schematic? (Is there a connection between its output and ground?) \$\endgroup\$ – brainfog Oct 7 '16 at 22:42

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