Hi There,

I'm still figuring out MOSFETs and have a question. I have two of them on my board (schematic attached). One for a motor controller which seems to work fine. The other one is used to turn off current from the solar panel by interrupting it's ground connection.

The issue I'm seeing is that current flows from the panel (I'm simulating the panel with my bench power supply) even when the voltage to the MOSFET gate goes low (from 5V to mV's). If I connect a light bulb to that MOSFET I'm able to switch it on and off via the control pin on the MCU.

I think that because the current is going in the opposite direction with the panel, as opposed to the light bulb, that I need to connect the panel ground lead to the MOSFET source pin and the drain pin to ground.

The direction of the current flow is important with these MOSFETs right? Thanks for the help.

 

Power MOSFETs only block in one direction because they have a built in 'body diode'.
So basically you always need the drain at a higher voltage than source.
http://www.electronsbefree.com/uploads/3/3/0/6/3306755/2501852.jpg

You need to add a series diode if you want to block both polarities.

 

You want to check the datasheet to ensure the added diode is in the proper orientation (opposite of the body diode)

 

Hi and thanks for the replies. I forgot to "subscribe" to this thread and didn't see this until now (actually figured out how to do it with a P-channel ).

I'd much rather use the N-channel though. So would I put the blocking diode, opposite direction of the body diode, between the PANEL_GND and the drain or between the source and GND? (battery ground in this case).

-Russ

 

Here's the solution using a P-channel MOSFET. After thinking about it I don't see how adding another blocking diode is going to fix my N-channel solution. The problem I'm having is that the mofet doesn't turn off and block current in the direction that the body diode should. I think it's more related to the fact that I've got the battery positive and panel positive sides connected. I tried swapping drain and source and I still ran into the issue of the diode failing to stop current if the panel voltage got too high.