[SOLVED: Broken mosfet] Low-side switching of resistive load not working

[ppff]

Hi everybody,

Doing some research on the web about my problem, I found out about this forum and decided to give it a try.

As you can see on my schematic, I have a battery connected to a heating resistor, their low sides are separated by a NMOS (https://www.infineon.com/dgdl/IPB039N10N3+G_Rev2.1.pdf?folderId=db3a304313b8b5a60113cee8763b02d7&fileId=db3a30431ce5fb52011d1ed1fd3915e0)

BATT- is then connected to the GND of my circuit through a sensing resistor (10 mOhms) for other purposes.

I tried this circuit with a power supply in place of the battery and noticed it would immediately drain current. To be sure it wasn't a command problem, I removed the gate resistor (R12), but it's still happening. When my power supply is at 20V, Vds = 2V and Vgs = 300mV, and the mosfet acts as a very small resistor. But it should act as an open circuit!

I tried with R13=10k, R13=0 and without R13, and it's always conducting.

What is going on? Everything I've found elsewhere displayed the exact same schematic, even a board I've already used worked the same way. What am I missing here?

Thank you for your help, strangers!

[wraper]

Probably your MOSFET already failed. Also from your circuit is not obvious if BATT- is connected to GND as it should be to work properly.

[ppff]

God damn that was it. It had no reason to be broken, but it was

Thanks for the idea!

[Ian.M]

Leave a MOSFET gate floating and it can get biased into its linear region by static charge.  If you powered it up without a gate pulldown resistor or good gate drive it probably fried itself due to excessive dissipation in a fraction of a second.