# Using High side mosfet switching for high current apps

Hello,

I'm trying to learn about using mosfets for high current applications and there are a few practical things I don't understand some are about why they get so hot easily if they are such wonder devices and for now what is the advantage and disadvantage of driving n channel mosfets at high and low sides (from what I understand low side switching is way more popular...

here is the general High side switching circuit, could that work?:

That is an N channel MOS FET.
Put the load in the drain leg.
R2 is too low, try 10K.

See

Watch this:

How much current?

What’s your +BATT voltage? I assume -BATT is ground? With high-side switching of an N-channel MOSFET the Gate voltage has to be higher than the Drain voltage, because when switched on the Drain and Source voltages are equal and you need VGS to turn-the device on.

What’s the nature of your 12V switching voltage? When off, is it open/disconnected or pulled-down to ground?

Two equal-value resistors cutting the gate voltage in half. R2 should be a much higher value than R1, if you need it at all.

For high side switch use a P-channel MOSFET, like this:

Pololu makes some nice high side, high current switch modules that can be controlled either by an MCU or an on board switch.

That doesn't explain why you can't use a N channel mosfet for high side switching. and the current varies from 5A to 60A

You can use an N channel MOSFET as a high side switch, but you need the gate voltage to be about 10V higher than the source or output load voltage, in order to turn on the MOSFET.

Decide for yourself how easy that will be to implement.

jremington:
You can use an N channel MOSFET as a high side switch, but you need the gate voltage to be about 10V higher than the source or output load voltage, in order to turn on the MOSFET.

Decide for yourself how easy that will be to implement.

But I managed to make this circuit maybe partly work somehow...:

Any idea?

See your other post. That circuit doesn't work at all.

Sounds like you just want to argue.

You have been told to use a P channel MOS FET, or Schottky diode.

Did you watch the Youtube video?

Enough time spent on this, you have the answer, good luck learning by producing smoke.

larryd:
Sounds like you just want to argue.

You have been told to use a P channel MOS FET, or Schottky diode.

Did you watch the Youtube video?

Enough time spent on this, you have the answer, good luck learning by producing smoke.

Yes I've watched the video for the 10th time...

Shottky diode is just too bulky and inefficient for the application and not suitable because of the voltage drop.

And how do you suggest I use a P channel mosfet for this application as I need to do High side switching with a positive voltage input with a P channel mosfet?

I know you have your N channel transistor already, but it is not what you needed.

If you watched the video then you know your cct. will not work.

This has been offered to you already.

"Shottky diode is just too bulky"
That Schottky diode is similar in size to a power MOS FET.
It has a voltage drop of only .2 to .3 volts, not significant to your application.

Edit:
Have you considered a simple on/off key switch rated at the charge current needed?

MikeLemon:
I’m trying to learn about using mosfets for high current applications and there are a few practical things I don’t understand some are about why they get so hot easily if they are such wonder devices and for now what is the advantage and disadvantage of driving n channel mosfets at high and low sides

Temperature depends on power dissipation and heat sinking.

A MOSFET when on is pretty much a pure resistance, power dissipation is calculated from I-squared-R
like any other resistor. Given knowledge of the dissipation and the heat sink performance you can
calculate the temperature and avoid disaster.

If using PWM you have to worry about switching losses (which will depend on how fast you switch
the MOSFET gates - use a MOSFET driver chip always for high power PWM, they are designed to drive
the highly capacitive gate fast and efficiently.

To switch high side either use p-channel or a high-side driver chip with PWM for a bootstrapped
n-channel high-side switch. For low side use n-channel.

The advantage of using n-channel high side is n-channel devices are inherently 3 times
better due to the 3 times higher mobility of electrons over holes in silicon. This only applies
to silicon. For instance GaNFETs are only available n-channel I believe, so there’s no
decision to make.

The advantage of p-channel high side switching is simplicity and you don’t need PWM and
you can turn the load on at 100% duty cycle. Simple n-channel with bootstrapped
driver limits PWM duty cycle to less than 100% as there must be transitions to generate
the bootstrapped gate supply (a few drivers overcome this limitation though, for instance
the HIP4081)

MarkT:
The advantage of using n-channel high side is n-channel devices are inherently 3 times
better due to the 3 times higher mobility of electrons over holes in silicon. This only applies
to silicon. For instance GaNFETs are only available n-channel I believe, so there's no
decision to make.

First of all thank you very much for the detailed explanation but I don't understand that.

What do you mean by 3 times better? noway the resistance is just a third of what the datasheet says?

And I don't understand How is it like that and then why would not most circuits use that high side drive?(sins the low side is way more common?

larryd:
I know you have your N channel transistor already, but it is not what you needed.

If you watched the video then you know your cct. will not work.

This has been offered to you already.

"Shottky diode is just too bulky"
That Schottky diode is similar in size to a power MOS FET.
It has a voltage drop of only .2 to .3 volts, not significant to your application.

Edit:
Have you considered a simple on/off key switch rated at the charge current needed?

OK then I'll try that out I was hoping to get away with a single FET.

And do you need a zener diode for that circuit sins my system uses around 50V and VGS max is 20V? if so where?

thanks for helping again!

MikeLemon:
First of all thank you very much for the detailed explanation but I don't understand that.

What do you mean by 3 times better? noway the resistance is just a third of what the datasheet says?

I mean electron mobility in silicon is three times better than hole mobility - thus any n-channel device
with the same geometry and doping as a p-channel device will be 3 times less on-resistance - basically its much
easier to get high performance with n-channel devices, so they are very commonly used even though
you may need more complex circuitry to drive them, because they are commonly available and cheaper.

Third thread on the same topic already? Same question, pretty much same answers. How many threads will it take for you to actually accept the reality is different from what you want it to be?

Also I can't help to notice that you still didn't give much if any information on what it is you're actually trying to switch, other than it's current. A load can do pretty interesting things to a power supply when switched, especially a 50-something A load.

From what he's been writing, he wants to switch 50V at up to 60A - that's 3000W of power.

For a load of 60A you'll need to find a P-Channel FET with RDSon of only a few milliohm.
For example, if you had a FET with and RDSon of 3 milliohm, it would still need to dissipate over 10W of power.
Keeping that cool is something that definitely need to be planned for, a simple small heatsink won't cut it.
And this calculation is only true for more or less static, non-PWM operation.

3 kilowatts of power is not child's play anymore, that's pretty serious stuff.
Not sure if this is a suitable project to start learning the ins and outs of MOSFET technology...

wvmarle:
Third thread on the same topic already? Same question, pretty much same answers. How many threads will it take for you to actually accept the reality is different from what you want it to be?

Also I can't help to notice that you still didn't give much if any information on what it is you're actually trying to switch, other than it's current. A load can do pretty interesting things to a power supply when switched, especially a 50-something A load.

And I don't want to give to much info to get answers like "just quite and go with that mechanical solution instead" sins it can't just mess up the other elements so I'm giving just all electrical requirements for the system.

Shuzz:
From what he's been writing, he wants to switch 50V at up to 60A - that's 3000W of power.

For a load of 60A you'll need to find a P-Channel FET with RDSon of only a few milliohm.
For example, if you had a FET with and RDSon of 3 milliohm, it would still need to dissipate over 10W of power.
Keeping that cool is something that definitely need to be planned for, a simple small heatsink won't cut it.
And this calculation is only true for more or less static, non-PWM operation.

3 kilowatts of power is not child's play anymore, that's pretty serious stuff.
Not sure if this is a suitable project to start learning the ins and outs of MOSFET technology...

That's why I'm using a few of those in parallel so the overall power loss gets lower and the left power to dissipate is being dissipated over more thermal mass.

Are you driving a single load? Or several?

Shuzz:
Are you driving a single load? Or several?