MOSFET switch help!

[jlwilson]

Guys I have a little problem that I am hoping someone here can help me with.

The design allows a rechargeable battery to be connected to the charger. It also will not allow a non-rechargeable battery to be connected to the charger. The rechargeable battery used has a tab that is connected to ground, therefore when this ground signal is present, the circuit knows a rechargeable battery is connected and will allow the path from the charger to the battery to be made.

The first version of this battery was designed by someone other than myself. I am now re-designing this (with as little changes as possible) in order to create a dual fault protection system. I would like to redesign this completely but as of now I must try and keep changes to a minimum.

The original circuit works fine, but the modified circuit will not work correctly. I cannot get the MOSFETS to switch the charger to the battery no matter what voltage level is on their gates. The inverters in the circuit are working correctly, it is the MOSFETs that seem to be giving me troubles.

Can someone see what I am doing incorrectly??

For clarification:

Inverters - NL27WZ14DFT2G
MOSFET N/P Dual Package - Si4562
Resistor - 1.3MΩ

 

[jlwilson]

I apologize for the size of the schematic. I will try and resize that.

 

[jlwilson]

Better view of the MOSFETs in the bottom schematic.

 

[BrownOut]

I cannot get the MOSFETS to switch the charger to the battery no matter what voltage level is on their gates.

What are the voltages you have tried? It appears your inverters are powered by the battery, and so will only swing as high as the batterys' voltage. In order to turn on the n-channel MOSFET, you'll need to drive the gate higher than the source, which also appears to be at the batterys' voltage. You'll never get it turned on that way.

 

[jlwilson]

What are the voltages you have tried? It appears your inverters are powered by the battery, and so will only swing as high as the batterys' voltage. In order to turn on the n-channel MOSFET, you'll need to drive the gate higher than the source, which also appears to be at the batterys' voltage. You'll never get it turned on that way.

You are correct sir. I overlooked that completely when I modified it. I chose the N-channel because the package had an unused one already there. Will go another route.

Thanks very much.