Almost all devices I come across nowadays use a common negative earth. Multimeter any device's negative power input and you'll get continuity between it and the case, chassis mounted ports, input/output ports etc.. all have a common negative. So if this is the industry norm, why are power mosfets predominately N-Channel? Surely, if you want common negative earth on whatever you want to switch, you're going to need a P-Channel, lest you're mucking around with charge pumps to kick the gate, extra isolated power sources, bootstrapping etc.., which is a total PITA.
Am I just missing something obvious? Is there an easy way to use N-Channels as a high side switch?
AvidVestRepair:
why are power mosfets predominately N-Channel?
I believe it has much to do with the behaviour of the material doped - and the dopant - for the N and P Silicon. And the manufacturing process, works better with N-substrate material and gives better electrical characeristics.
AvidVestRepair:
Surely, if you want common negative earth on whatever you want to switch, you're going to need a P-Channel, lest you're mucking around with charge pumps to kick the gate etc.., which is a total PITA.
What you want to do in terms of switching things, may not be representative of the majority of electronic applications. You are correct that convention has dictated negative ground - possibly even as a carry-over from valve technology - and most systems are therefore engineered as switching to ground because it is easier to deal with the control (gate) circuits if they reference to ground.
AvidVestRepair:
Am I just missing something obvious? Is there an easy way to use N-Channels as a high side switch?
No, but high-side switching is simply not used anywhere near as much. If your circuit is continuously oscillating, such as a SMPS, charge pumps are no problem.
If high side switching is not used much, how does a device with a single power output relative to ground switch it on and off? There are plenty of power devices, power modules and so forth that do this. If the output is relative to ground how do they switch it on and off if they are not using high side switching? To me it seems counter intuitive to prefer low side switching on everything if you're using a low side common. Sorry, my brain hurts.
EDIT:
To me this seems the most logical way to switch power to a device.
The device can share the same negative as the switching circuit. If I used a low side switch, yes it eliminates the need for the transistor (assuming the logic level is high enough), but now requires the device to share the +ve supply. This creates all sorts of problems if the device also shares other logic and signal lines that requires common -ve. Short circuit, bang crackle pop, or the device spontaneously turns on when anything else is plugged in to it (syncing all the return current through signal wires; melt melt melt, smoke). So how come high side switching is not used very often in a world where the low side is common, it just makes no sense to me at all.
Sorry, what?
I get locked in many things, usually handcuffs, but never in a paradigm. What's a paradigm?
In the sense I'm talking here, ON means "Power is supplied to device" and OFF means "Power is not supplied to device". It's not an unreasonable way to think. With P-Channels HI/1/+V at the gate is usually OFF not ON. Logic high (or 1) is usually considered ON because most devices default to logic states as low (or 0) when they are first powered up. You don't want all your stuff flickering on for an instant while the controller switches them all off by bringing the logics high on power up. Not to mention what would happen if a coding error or controller failure occurs and all the stuff turns on and stays on when powered up. Yes I understand high and low is not exclusively On and Off respectively but it's the most common way to regard them.
A way of thinking.
And you are stuck in a hobbyist frame of mind. It is exactly that frame of mind that has push buttons being connected to the input and 5V with a pull down resistor. A very rare way of doing things in industrial electronics.
In the sense I'm talking here, ON means "Power is supplied to device" and OFF means "Power is not supplied to device". It's not an unreasonable way to think
It is not but you make several unwarranted assumptions.
FETs are not used for turning external devices on an off very much.
As processors power up with pins as inputs they are not providing any power that is exactly what you need to feed into an N-channel FET to have something off. A simple pull down resistor ensures that.
Current sinking for controlling things is by far the most common way, because for a long time devices could always sink more than they could source.
You can get closer to the power rail by sinking than by sourcing.
It is also simpler to provide power to things like solenoids, LEDs, motors, etc. that are running on much higher voltages than the microcontroller that is switching them, using low side switching.
n-channel FETs use electrons to carry the current through the channel,
p-channel FETs use holes to carry the current.
Electrons are 3 times more mobile than holes (in silicon), so n-channel devices
are 3 times as good for the same die area/dimensions.
No-one can ignore a factor of 3, it means p-channel devices either have to
be 3 times worse performers or use 3 times the die area.
Probably 99% of the uses of MOSFETs are in switching converters / inverters/ H-bridges
where MOSFET driver chips drive pairs of n-channel MOSFETs using a
bootstrapped high-side gate supply so an n-channel can be used instead of
a p-channel for the high-side switch. IGBTs are basically the same, n-channel only.
By the same reasoning silicon MOSFETs are going to be gradually superseded for
most applications by GaN MOSFETs - gallium nitride has much higher electron mobility
than silicon.
It is exactly that frame of mind that has push buttons being connected to the input and 5V with a pull down resistor. A very rare way of doing things in industrial electronics.
Sorry to jump in.
This is how I do switches (I'm a hobbyist). My thinking is that I am not using any power until somebody presses the switch.
The other way, connect to 5V and wait until it goes low uses power all the time. Do I have it wrong and if so what is the correct way?
No current flows until the switch is pressed, which ever way up the circuit is - its
a complementary configuration. Internal pull-ups or pull-downs would be used
where possible to reduce component count.
For lots of switches multiplexing is used to keep the pin count low.
Do I have it wrong and if so what is the correct way?
Yes.
There is no correct way, there is a cheap way and an expensive way. Often sinking current is the cheapest way, especially when the control signal is not as big as the voltage you are trying to control.
MartynC:
This is how I do switches (I'm a hobbyist). My thinking is that I am not using any power until somebody presses the switch.
The other way, connect to 5V and wait until it goes low uses power all the time. Do I have it wrong and if so what is the correct way?
Methinks you should put child-proof plugs in your unused power outlets, to prevent leakage.