Author Topic: p-mosfet voltage leakage from gate to source  (Read 1016 times)

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Offline silas15Topic starter

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p-mosfet voltage leakage from gate to source
« on: February 15, 2020, 10:02:06 am »
My circuit works on battery or USB power. I want USB to be primary and 4xAA secondary source. When USB is present, batteries are not dichargged. Even batteries are not needed when USB power is present. Schema attached. I uderstand that voltage from USB close P-MOSFET (BSS84). When i plug USB, on mosfet source is voltage (without batteries). It gives about 23mA short to ground. This come from drain! I dont understand why. I measured mosfet with diode test. New mosfet gate to source 0V, on used mosfet gate to source 0,9V. I dont get why is this hapening. Voltages are far below maximum ratings in datasheet.
 

Offline fcb

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Re: p-mosfet voltage leakage from gate to source
« Reply #1 on: February 15, 2020, 10:43:46 am »
Circuit looks like it should work out. No gotchas in the data sheet either.

Check layout obviously, blown/mislabelled components.

Possible try measuring current when gate is tied to source (off).
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Offline Mr Evil

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Re: p-mosfet voltage leakage from gate to source
« Reply #2 on: February 15, 2020, 03:33:01 pm »
Fully charged alkaline batteries can be up to 1.6V each. USB voltage can be as low as 4.4V and still be in-spec. That means that the MOSFET's Vgs might be as high as (4*1.6)-4.4=2V when USB power is connected. The datasheet for the BSS84 says that Vgs(th) can be as low as 0.8V, so 2V is enough to turn it on.

Assuming that's the problem, then a simple solution would be to choose a MOSFET with a higher threshold votlage.
 
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Offline silas15Topic starter

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Re: p-mosfet voltage leakage from gate to source
« Reply #3 on: February 15, 2020, 05:01:30 pm »
Thank you. But I think there is more. When there is no battery (only USB), current goes from gate to source. Multimeter from source to ground showing about 23mA, voltage in open circuit 4,9V. From drain to ground 0mA. That tells me that discharged batteries will be charging.
 

Offline fcb

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Re: p-mosfet voltage leakage from gate to source
« Reply #4 on: February 15, 2020, 05:48:52 pm »
Unless the BSS84 is damaged/wrong way round then what you are describing shouldn't happen.

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