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Problem with hard switching a MOSFET. does not turn off.

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charliepuch

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Hi I am having some trouble with with a seemingly simple circuit (see attachment 1). I am using a N-channel MOSFET to control power to a wireless bridge connected to connector X5. If JP4 is left open, the gate of the FET is pulled high by resistor R36, and the FET is turned on allowing power to flow to the wireless bridge. If pins 1 & 2 on JP4 are jumpered together, the gate of the FET is pulled to GND and the FET should turn off, causing the wireless bridge to turn off. However, this does not seem to be the case. When I jumper pins 1 & 2 together on JP4 the FET does NOT turn off, and the bridge remains on. This seems like a very simple circuit to me, but maybe I am missing, or overlooking something. I have checked the gate pin with a voltmeter and scope to ensure that it was actually at ground when JP4 is jumperd. Everything seems to check out but the FET does not seem to want to turn off. Does anyone have any Ides on what may be causing this or what I may be overlooking?

Also on the same 18V line in this circuit is the input side of a DC-DC converter that supplies other voltages to other parts of the circuit, (see attachment 2) I am wondering if these could affect anything?

Thanks for your help,

Charlie
 
Thoughts:
P-MOSFET the SQ2360 is N so not that problem.
D-S reversed?
Bad part?

I don't know the PWM you are using jck3012s05. Is pin 3 correct.
 
Yep,did a double check on the pins they are okay.

Pin 3 is correct on the DC/DC. The only reason I posted that portion is I though it could be something to do with the input filter to the converter affecting the 18V line...

here is the link to the converters datasheet. Although i don't think it matters since that is for a complexly different portion of the circuit.

https://www.electro-tech-online.com/custompdfs/2012/02/SF_JCK30.pdf

Thanks for you help guys.
 
If the FET conducts when the gate and source are shorted together the FET is faulty.
 
Here is the strange part. If no load is attached (bridge is not plugged into X5) and I connect the gate and source together (jumpper JP4) then it is shows up as an open circuit on my voltmeter. However, as soon as I connect the load (plug in the bridge) the FET switches on. I tried soldering in a new FET and had the same problem. :(
 
Hey guys I figured out the problem. The antenna of the bridge is always connected to ground of the circuit so there is always a return path for the current. It looks like I will need to change the design to a high side switch instead of a low side. I feel kind of silly for overlooking this. thanks again for your help.

Charlie
 
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