H bridge using all n fets

kinarfi

Well-Known Member

• Sep 25, 2013

• #1

I was looking at a 12V H Bridge motor controller and it has only N FETs (4 IRF3205). I did find that there was 20 volts on board which I presume is to fire the N FETs, but when I tried to simulate, I could keep the Fet from conducting through the Fet diode. I tried to see how they did it, but it is a multi layer board.
Any idea of how this works?
Thanks,
Kinarfi

 

ronv

Well-Known Member

Most Helpful Member

• Sep 25, 2013

• #2

The source of the top FET goes towards the center of the H.

 

dr pepper

Well-Known Member

Most Helpful Member

• Sep 25, 2013

• #3

You need to drive the high side fet gates with enough voltage to make sure you get the rds on.
For a 12v automotive application you could either do this by building a dc to dc converter to step up to a higher voltage or use gate transformers on the fets.

 

M

Mike odom

Active Member

• Sep 25, 2013

• #4

They also make high side drivers that have a 'boost charge circuit' to produce a higher than source gate voltage to turn on the high side N-Fets.

 

MikeMl

Well-Known Member

Most Helpful Member

• Sep 25, 2013

• #5

Mike odom said:

They also make high side drivers that have a 'boost charge circuit' to produce a higher than source gate voltage to turn on the high side N-Fets.

Click to expand...

I just posted this in the Student forum:

https://www.fairchildsemi.com/an/AN/AN-6076.pdf

 

ronv

Well-Known Member

Most Helpful Member

• Sep 25, 2013

• #6

He has a 12 volt H-Bridge, but also has 20 volts on the same board. He has 8 volts left to turn on the high side FETs. He doesn't need a charge pump or bootstrap circuit.

 

kinarfi

Well-Known Member

• Sep 26, 2013

• #7

I figured out what I was doing wrong, I had 2 of the FETs upside down, the two tied to +12v had the source to + instead of the drain, I also plan to use 10V for the control circuit and 4N35 opto isolators to fire the FETs