Is there a four terminal MOSFET that can be used to eliminate the body diode ?

Thread Starter

mouseinahouse

Joined Oct 29, 2016
12
Is there a four pin MOSFET where the substrate can be left disconnected from the source, and thus eliminating the body diode effect, but that can still be driven as a normal MOSFET ?
 

ScottWang

Joined Aug 23, 2012
6,933
Since that is the "body diode", I'm sure how can you eliminate it?
What's your application for mosfet, the V/I?
Maybe you could using the IGBT or mosfet plus bjt to build your own IGBT.
 

Thread Starter

mouseinahouse

Joined Oct 29, 2016
12
If I use a four terminal MOSFET, and do not join the substrate to the source, can I still use it for high current applications, as a three terminal device ?
 

AlbertHall

Joined Jun 4, 2014
10,401
You would need to connect the substrate to the most negative point in your circuit (or at least some point which is more negative than the drain will ever get to)

Swap negative for positive for P-channel.
 

ScottWang

Joined Aug 23, 2012
6,933
So you have three ID, ElectronicMotor, Fuk and mouseinahouse?
The Fuk already deleted and you just create the new ID mouseinahouse?

Our forum only allowed one member for one ID, one question for one thread.
 

Thread Starter

mouseinahouse

Joined Oct 29, 2016
12
Thanks AlbertHall, I will purchase one and tie the substrate to ground. I will also do more research on these four terminal devices and find a manufacturer and begin building the next three phase full H bridge, 150 amp electronically commutated motor driver.
 

shortbus

Joined Sep 30, 2009
7,946
I'm pretty sure the 4 pin mosfets are internally connected together substrate and source, at least the ones I'm using are. The ones I'm using are of the "iso-top" type, pretty sure it would be the same in a TO-220 type too.
 

Thread Starter

mouseinahouse

Joined Oct 29, 2016
12
I think that by the very nature of their design, MOSFETs have a body diode ie the positively doped source and drain and the negatively doped channel in an enhancement mode MOSFET. Although MOSFETs are better at controlling high currents, as apposed to BJTs, the body diode poses real problems in three phase electronically commutated motor full H bridge drivers.
Perhaps I will go with BJTs, as they do not pose this problem, and I would imagine that, although they are more difficult to drive, they would be just as able to drive 150 amps and more. Any ideas ?
 

dl324

Joined Mar 30, 2015
11,544
I think that by the very nature of their design, MOSFETs have a body diode ie the positively doped source and drain and the negatively doped channel in an enhancement mode MOSFET.
Both drain and source have parasitic diodes:
upload_2016-10-29_10-13-23.png
In a 3 terminal MOSFET, the diode on the source is shorted.

In a 4 terminal MOSFET, bulk must be connected to the most negative voltage to prevent the diodes from becoming forward biased.
 

Thread Starter

mouseinahouse

Joined Oct 29, 2016
12
Would I be correct to say that, even if I could leave the fourth body terminal of a MOSFET tied to ground or disconnected, it probably would not solve the problem of the parasitic diodes, and that regardless of which type I use, they would be unsuitable for the cct I have proposed ?
 

dl324

Joined Mar 30, 2015
11,544
Would I be correct to say that, even if I could leave the fourth body terminal of a MOSFET tied to ground or disconnected, it probably would not solve the problem of the parasitic diodes, and that regardless of which type I use, they would be unsuitable for the cct I have proposed ?
Most likely. The diodes are always going to be there. You'll get better answers if you post a schematic.

You can't leave bulk floating because that would affect the threshold voltage. If you connect to ground, it becomes a 3 terminal device with the diode on the source shorted; assuming the source is grounded. As a 4 terminal device, bulk needs to be negative enough to prevent the parasitic diodes from becoming forward biased.

As as aside, parasitic diodes and transistors exist in many integrated circuits; could be all - I haven't studied any of the more exotic technologies like SOS, SOI, FDSOI, etc.
 
Last edited:

kubeek

Joined Sep 20, 2005
5,733
Could you explain that "problem of the parasitic diodes"? You see, although a BJT doesn´t have a body diode, the B-E junction can only withstand ~5V before it breaks down, leading to the same behavior as the mosfet, only having 5.7V drop instead of 0.7V, so using BJT won't be any better. But I fail to see how every VFD in the world uses mosfets or BJTs to drive a three phase motor, and only you are having some nondescript issues with reverse voltage on the transistors.
 

Thread Starter

mouseinahouse

Joined Oct 29, 2016
12
I have posted a schematic, under my normal pasona, but had leave my computer due to urgent business. I was unable to get my normal account at risk of safety, and had to invent a new persona. Uforortunatly, the moderator decided to ban me. I am building an electric buggy that will bring ingured people out of the Australian bush, using an electronically commutated motor and battery. I have been continually frustrated a moderator. I will say no more until this is resolved

@jrap, @bertus:
You have to see this message.
 

jpanhalt

Joined Jan 18, 2008
10,251
Your story doesn't pass the sniff test half way around the world in Ohio.

1) There are many designs for electric vehicles that do not use the type of mosfet you seem so desperate for.
2) Why do you need an electric vehicle for such a humane endeavor?
3) Maybe you aren't trying to save sick people so much as you are worried about the noise? Are you a smuggler?

John
 

Thread Starter

mouseinahouse

Joined Oct 29, 2016
12
I am finding it difficult to understand why I have to justify my reasons, as this forum is about answering questions about elelectronic design problems. Very unproffesional, regardless of where you are from.
 

kubeek

Joined Sep 20, 2005
5,733
You should note that this is a public forum, not a Professional answering service. No one here has any obligation to reply, people have their own moods and their own reasoning, and I´d say members here generally dislike people who switch from account to account, start the same threads all over again...

So my advice is: choose one ID and stick to it, tell this choice to the mods, and start the discussion by answering why you think you need such device in the first place, preferably in the old thread http://forum.allaboutcircuits.com/threads/no-body-diode-mosfet.128379
 
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