Logic Level MOSFET - High or Low?

I think I have a pretty good grasp of low vs high side switching with MOSFETs and their gate drive requirements. My question is how does this work with logic level MOSFETs? Since they require a 5V logic level input, does it matter if they are used in low/high side configurations?

I have a circuit and it would be very convenient if I could use a single logic level MOSFET, but have it work in either low side or high side (the mode would be selected via jumpers).

This is for automotive use for low speed (on/off only) switching of low current loads like lights or small motors. Since automobile circuits are split between providing power (high side) or a ground (low side) to a device, I would like to have a single circuit operate with both using only a single MOSFET.

Logic level MOSFETs only differ from ordinary MOSFETs in that the turn-on threshold is lower, so that they are fully turned on by 5 V.

If you want to high-side switch and your supply voltage is larger than 5 V, which I think is what you want, then logic level MOSFETs don't really help at all. You would probably want a P-MOSFET for high side switching, which would keep the gate voltage in the 0 - 12 V range. A logic level P-MOSFET would still need a transistor or something to convert the logic output to the gate voltages required.

If you are low-side switching, then a logic-level N-MOSFET can be useful as it can be run directly from the logic. If you are high side switching something running from a 5 V supply, a logic-level P-MOSFET can also be driven directly from the logic.

So to do what I want I should use both an N channel and a P channel logic level MOSFET, and use a jumper to connect my load to the appropriate MOSFET and leave the other MOSFET unused.

I was hoping to save some space and money by using a single MOSFET, but if I have to use two, then I'll get by.

I think you should do some research on high side and low side switching. There ARE devices which have a P chan and N chan in a single package, to save space, but probably more expensive than using two single devices.

To use a single N channel mosfet for both high and low side switching you would need to be able to change the gate voltage. You need to provide a gate voltage of 'source voltage + threshold' plus some headroom. (at the threshold, the FET isn't fully switched on, so acts as a resistor). So for low side where the source is connected to ground, the 'on' gate voltage will be > threshold voltage. For high side, you will need to provide a voltage higher than your supply voltage. Since when 'on', the voltage between the source and drain is quite small, effectively making the source at your supply voltage. But you would need 'source v + threshold v' to fully turn on the mosfet.

For low frequency applications (where you're not switching the FET at >kHz) the current required for the gate is tiny, so a small boost converter which doubles your supply voltage can be used.

But, ultimately, is it FAR easier to just use seperate P channel, and N channel MOSFET's. As this is low frequency, you can simply use a small transistor to invert the gate signal for the P channel, meaning that both MOSFET's could use the same control signal - provided they are not connected at the same time.

Use a dual N/P channel package for both devices in one unit.