Which FET should I choose?

Thread Starter

John A Bonilla

Joined Mar 11, 2017
66
Hi Everyone,

I am currently working on a project and am developing a charging circuit. My goal is that whenever I plug the charger into my circuit, the rest of the circuit cuts power and the battery charges. I have done some preliminary research and have come with using a depletion type n-channel mosfet at the positive source of the battery so that when the charge plugs in, positive end is applied to the gate of the mosfet and the power from the battery is cut off. I have the following mosfet in mind:

https://www.digikey.com/product-detail/en/infineon-technologies/BSD840NH6327XTSA1/BSD840NH6327XTSA1CT-ND/3196629

The charger will be from a USB outlet so it will supply 5V @ 500 mA. If anyone has any suggestions I would appreciate it, and I am open to any questions as well. Thank you!
 

MaxHeadRoom

Joined Jul 18, 2013
20,403
The enhanced model has a higher rating, I have been using the 2n7000 from its inception and always keep a stock on hand, nice device when used in logic level circuitry.
Max.
 

Thread Starter

John A Bonilla

Joined Mar 11, 2017
66
The enhanced model has a higher rating, I have been using the 2n7000 from its inception and always keep a stock on hand, nice device when used in logic level circuitry.
Max.
Maybe I am misunderstanding the use cases of a depletion vs enhancement type mosfets. My understanding that when an enhancement mosfet is supplied power to the gate, it allows power to flow from source to drain. Depletion is when power is applied to the gate, no power is allowed to flow through the mosfet. Am I wrong?
 

MaxHeadRoom

Joined Jul 18, 2013
20,403
Well the ones I have used so far (1980's) till now require a gate voltage to conduct, if gate is kept at power common (GND) then no conduction occurs.
Max.
 

dl324

Joined Mar 30, 2015
10,462
My understanding that when an enhancement mosfet is supplied power to the gate, it allows power to flow from source to drain. Depletion is when power is applied to the gate, no power is allowed to flow through the mosfet. Am I wrong?
A depletion mode N channel MOSFET is like a JFET. With 0V gate-source bias, it will conduct. You can turn it off with sufficiently negative GS. You can turn on harder by applying positive GS voltage.
 
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