Understanding high and low side mosfet operation

Thread Starter

MCH170

Joined May 13, 2021
22
Hello, I am trying to understand how mosfets work and I created this theoretical configuration.

Lets say I have a load with 2 n-type mosfets, one between vcc (18V) and the load and the other between the load and gnd (0V). Can I use a 5V microcontroller to keep the bottom mosfet (between load and gnd) ON and use a blinking pattern on the top mosfet (between vcc and load) to turn the load on and off?

Driving the low-side mosfet is straight forward as long as the gate-source voltage of the particular mosfet is 5V or similar (please correct if I am wrong).

What about the high-side? When the low side is switched on then the source voltage of the high-side is 0V (pulled low) so the top mosfet should need 5V at the gate? Something doesn't seem right but I can't figure it out.

I appreciate any advice you can give. Thanks.
Notes_240430_190631.jpgNotes_240430_190631.jpg
MCH170
 

crutschow

Joined Mar 14, 2008
34,844
The top MOSFET need 5V from gate-to-source (Vgs) not 5V at the gate, so the gate on-voltage would need to be 23V.

Why do you need two MOSFETs for that function?
Just pulse the lower one with the load going to the supply voltage.
 

Thread Starter

MCH170

Joined May 13, 2021
22
The top MOSFET need 5V from gate-to-source (Vgs) not 5V at the gate, so the gate on-voltage would need to be 23V.

Why do you need two MOSFETs for that function?
Just pulse the lower one with the load going to the supply voltage.
Thanks for replying. I don't need 2 mosfets, I am just trying to understand if that is possible. Also, I saw that high side mosfets are usually p type and wondered if it could be done with n-type as well. What would change if I was to use a p-type for high side?
 

dl324

Joined Mar 30, 2015
17,142
I saw that high side mosfets are usually p type and wondered if it could be done with n-type as well.
It can, but not without complications.

To do a high side switch with an N channel MOSFET requires a voltage higher than Vload+Vgs(th). You can't do that with a 5V microcontroller output unless you add more circuitry.

Additionally, threshold voltages for discrete MOSFETs aren't well controlled. For example, IRFZ44N give a range of 2-4V, and they only conduct 250uA at that voltage.
What would change if I was to use a p-type for high side?
You can't do it with just a 5V microcontroller output. The gate needs to be close enough to the source voltage for it to be turned off.

Something like this:
1714503235861.png
Note that this is operating Q2 near its maximum gate-source voltage (which I'd have no problem doing).
View attachment 321293
 
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