body diode and mosfet orientation

[PG1995]

Hi,

In this PDF, https://www.maximintegrated.com/en/app-notes/index.mvp/id/636 , under the section "Replace Bipolar Transistors with MOSFETs", the following is said:
"Pay careful attention to a MOSFET's orientation in the circuit. MOSFETs have an intrinsic body diode that conducts current under forward-bias conditions. This current flows from the drain to the source for a PMOS FET and from the source to the drain for an NMOS FET. Whether using an NMOS or a PMOS FET as a low- or high-side switch, orient the device's body diode in the direction of normal current flow. Then, a reversed battery reverse-biases the diode and blocks the flow of current."

If one has access to the fourth terminal, i.e. body terminal, then a mosfet is a four terminal device and is a reversible device. To which terminal of the two (excluding gate terminal) one connects the body terminal determines the 'source' and 'drain' sides. This connection would constitute a body diode or internal diode of mosfet. Most, if not all, mosfets come with the connection already made.

In a PMOS current flows from source to drain and body diode is directed from drain to source as shown below.

How can one decide to orient the device's diode in the direction of of normal current flow? For example, one cannot simply flip a pmos and make the current flow from drain to source. Could you please help me with this?


Note to self:
low- or high-side switch:

What is a high-side/low-side driver in electronics?

Answer (1 of 6): What is a high-side/low-side driver in electronics? First, realize that we say “high side”, we generally mean the power supply hot or +Vcc/Vdd side of the load. And when we say “low side”, we generally mean the neutral or ground/common/return side of the load. On the left is a ...

www.quora.com

The page explains that for a high side driver a pmos is preferred though it requires an additional 'simple' circuitry. An nmos could also be used in a high side driver but additional circuitry required is quite complex to that for pmos. For a low side driver, an nmos is preferred.

 

[Diver300]

In a PMOS current flows from source to drain and body diode is directed from drain to source as shown below.

How can one decide to orient the device's diode in the direction of of normal current flow? For example, one cannot simply flip a pmos and make the current flow from drain to source. Could you please help me with this?

You can run a MOSFET backwards.

If the Gate - Source voltage is low, then the MOSFET doesn't conduct and the body diode is the only part that can conduct.

If the MOSFET is turned on with a suitable Gate - Source voltage (Gate more negative than the Source for a P-MOSFET) then the MOSFET conducts in either direction.

This is often used to give a lower voltage drop than a diode.

Have a look at this device:- https://omronfs.omron.com/en_US/ecb/products/pdf/en-g3vm_61g1.pdf

I realise that it is tiny, and turned on optically instead of with a voltage, but the diodes are bypassed when the device is turned on. The graph titled "Continuous load current vs. On-state voltage" shows a purely resistive characteristic. The voltage drop at low currents is too small to be turning on one of the body diodes, so the current must be going through the MOSFETs. The two MOSFETs are in reverse-series, so any current must be going backwards through one of them.

 

[Diver300]

Figure 3 in https://www.maximintegrated.com/en/app-notes/index.mvp/id/636 is an example of giving a lower voltage drop than a diode would give.

A diode in place of the MOSFET would work as reverse battery protection, but there would be a drop of 0.7 V or so. Using a MOSFET puts Rds(on) in parallel with the diode and results in a lower voltage drop.

 

[PG1995]

Thank you!

So, a MOSFET could conduct in either direction as long as right gate threshold voltage is present. For example, in case of a P-MOSFET, gate-source voltage should be equal or less than the given threshold voltage and for current to flow from drain to source, the drain should be at higher potential than the source. Normally, it is said that in a PMOS current flows from source to drain and body diode is directed from drain to source.

But I see there is a problem. If you orient the device's diode in the direction of of normal current flow then it's like having an FET and diode in parallel. So, when the FET is not turned on but the source-drain potential is more than 0.7 V, the diode will conduct. For example, in an MPPT circuit I once made, I couldn't orient the body diode in direction of current flow because it would made the current flow even when FET was turned off therefore MPPT point would never be reached. I have PMed you the link to schematic; for some reason I wouldn't like to copy the link here.

Thanks a lot.

 

[AnalogKid]

How can one decide to orient the device's diode in the direction of of normal current flow? For example, one cannot simply flip a pmos and make the current flow from drain to source.

Yes, one can.

A fully enhanced FET channel is basically a resistor. Current moves both directions with equal ease. This can be a problem in applications where you want to use the FET not only as a low voltge drop diode but as a switch, because there is no way to turn off the body diode.

This is why some applications have two MOSFETs in series with the two sources connected in the middle. It is the only way to have a voltage controlled switch that blocks current in both directions when off. Example:

https://www.analog.com/media/en/reference-design-documentation/design-notes/dn160f.pdf

ak

 

[Diver300]

But I see there is a problem. If you orient the device's diode in the direction of of normal current flow then it's like having an FET and diode in parallel. So, when the FET is not turned on but the source-drain potential is more than 0.7 V, the diode will conduct.

That is correct. Having the diode that way round means that the circuit can't really turn off, so arrangements like that are less common. However in circuits like the reverse polarity protection, having current go in the same direction as the body diode can be useful.

 

[crutschow]

I couldn't orient the body diode in direction of current flow because it would made the current flow even when FET was turned off

One way around that is to use two MOSFETs in series connected source-to-source, with the gates in parallel.
That way they will block in both directions when the MOSFETs are off since the diodes are back-to-back.

 

[PG1995]

Thank you, everyone!

This is why some applications have two MOSFETs in series with the two sources connected in the middle. It is the only way to have a voltage controlled switch that blocks current in both directions when off.

This webpage, https://electronics.stackexchange.c...derstanding-two-mosfet-with-sources-connected , explains it well.