I want to implement a reverse polarity N FET or P FET to protect a low power PIR circuit. I expect the PIR circuit to use ~26uA, so I need a protection MOSFET that is very efficient when turned on, otherwise it will be wasting more power than the circuit is using.
From looking online, a low R(DS) ON resistance at 3.3 volts appears to be ~ .013 Ohms. Somehow I keep messing up my current calculation and therefore my power loss calculation. I think it’s because I am calculating the current for the FET and not using the 26UA for the power loss calculation?
Power losses are found by the R(DS) ON of the device and current load:
If I have a 3.3v supply and the R(DS) ON is 13 mOhms
I=3.3/.013 ohms = 253.8A? Clearly this is wrong
Power loss = (I)^2 *(R)= (253.8)^2*(.013) = 837 Watts
There’s no way that current calculation is correct… Mostly I’m confused why this gives me the wrong answer.
I think this calculation is correct using the actual current the circuit will be using but I’m not sure. It’s much closer to what I’d expect, but I’m still confused why the previous current calculation doesn’t work:
Power loss = (I)^2 *(R)= (.000026 A) ^2*(.013 Ohms) = 8.7X10^-12 Watts
Should I also be taking into account the current the MOSFET consumes just to stay on? Thanks in advance
From looking online, a low R(DS) ON resistance at 3.3 volts appears to be ~ .013 Ohms. Somehow I keep messing up my current calculation and therefore my power loss calculation. I think it’s because I am calculating the current for the FET and not using the 26UA for the power loss calculation?
Power losses are found by the R(DS) ON of the device and current load:
If I have a 3.3v supply and the R(DS) ON is 13 mOhms
I=3.3/.013 ohms = 253.8A? Clearly this is wrong
Power loss = (I)^2 *(R)= (253.8)^2*(.013) = 837 Watts
There’s no way that current calculation is correct… Mostly I’m confused why this gives me the wrong answer.
I think this calculation is correct using the actual current the circuit will be using but I’m not sure. It’s much closer to what I’d expect, but I’m still confused why the previous current calculation doesn’t work:
Power loss = (I)^2 *(R)= (.000026 A) ^2*(.013 Ohms) = 8.7X10^-12 Watts
Should I also be taking into account the current the MOSFET consumes just to stay on? Thanks in advance