Hello,

Can anyone explain the operation of this circuit to me? It's from an existing design I'm working off of, and I can't see how the FET turns on, or why the +5V net would be +5V (this is where that net is created it doesn't come from anywhere else and it goes off board). The FET number is wrong, it's a DMG1012T-7

It seems like Vg will always be ~0V, so why would this be anything other than a block?

 

Welcome to AAC!

Can anyone explain the operation of this circuit to me?

What you've shown makes no sense. Is there anything connected to the source?

BTW, it's better for you to refer to this device as a MOSFET. There are many different types of FETs.

 

I agree it makes no sense, that's why I'm asking the question! I don't think this product has actually been built yet (I'm creating a different product with this one as a reference for a client).

The +5V_USB_ISOL comes off an isolated DC/DC source and is expected to generate the +5V net through this circuit, I don't see how. +5V goes to an off-board connector and I don't have visibility into how it's used in the connected device unfortunately.

 

It would make sense if the MOSFET was a P-channel type with its drain source connected to the USB input. The MOSFET would then act to protect against a wrong-polarity input on the USB terminal.

 

It would make sense if the MOSFET was a P-channel type with its drain connected to the USB input. The MOSFET would then act to protect against a wrong-polarity input on the USB terminal.

I agree, a P channel would do the trick. I was wondering if someone would see this and know how it would work with an N channel since this is how I received the circuit, I thought I had a tested circuit and so thought maybe there's a trick I'm not aware of that makes this operate. Given how the parameters are maintained in the file structure thought it won't surprise me if this is supposed to be a P channel and it slipped through the cracks in the design file.

 

Is it meant to be a Series voltage regulator? With a component/s missing from the gate?

 

In this switch-on, the transistor works like a diode.

 

I agree with Alec that its purpose would appear to be as an ideal-diode for reverse polarity protection and, for that, it needs to be a P-MOSFET with the source going to the left (node 3).