"N channel mosfet" conducts when Vds is negative and body diode is not modelled

I am using "N-channel MOSFET" block from simscape and i have an observation which is not plausible.

When Vds is negative the mosfet conducts. I am using threshold based modelling, parametrization from datasheet, and body diode is not modelled. Using default values when block is inserted. Tried in matlab 2021b, 2023b

Am I missing something here or is it a bug?

Answer 1

[Shivam Gothi]

Hello @Ismail S Marikar,

As per my understanding, you have not modelled the body diode and the MOSFET is still conducting even if you apply positive potential at source and negative potential at drain.

This is the expected behaviour for any "practical" MOSFET.

MOSFET can block the voltage in only one direction and it can conduct current in both the directions, owing to its construction

Let me explain....

The construction of MOSFET is shown below: (source: https://www.shindengen.com/products/semi/column/basic/mosfet/mosfet_structure_and_operation_principles.html

You can see that the P-type body is shorted with the source and a P-N junction is formed between Drain and source. Therefore, the "body diode" is inherent, owing to its construction.

Having selected : "Model Body Diode = No", will not remove that body diode. Instead, when you do not model body diode, Simulink will use the default model for it. When you choose "Model Body Diode = yes", simulink will allow you to enter your desired diode characteristics to be modelled.

Practically, you can never find 3-terminal MOSFET without body diode. Therefore, you are witnessing drain currents even with negative Vds

Consider setting the "Number of terminals" to "four.

When you set the "Number of terminals" properties to "four" as shown below,

the MOSFET will be modelled as a four pin device, whose construction diagram is shown below: (Source: https://en.wikipedia.org/wiki/Field-effect_transistor

You can see that there is change in construction. The P-type body is not shorted with the source, instead, a seperate terminal is connected to the P-type body.

The Simulink block for "N-channel MOSFET" will now look like:

Now, if you provide positive potential between "g" terminal and "b" terminal, the channel will be formed, which can allow the conduction in both the directions, from Drain to source and vice-versa. But when is lower than the threshold voltage, the channel will not be formed and the device will be capable of blocking voltage of either polarity accross drain and source terminals.

Therefore, you can see that in this case, the MOSFET can allow flow of current in both the directions and it can block the voltage of either polarity.

In both the cases, MOSFET allows the current to conduct in both the directions

I hope you find this explaination usefull !

[Ismail S Marikar]

Thanks for the answer.

All the above explanation is correct except for the part where you said Simulink will assume a default body diode model when selecting "Model Body Diode = No".

The correct explanation as I understand is:

In simulink, mosfet is modelled in such a manner that the source and drain terminals are interchangeable. It is Vgb which dictate channel formation, so in this kind of modelling it is as if body is tried to source when Vds>0 and body is tied to drain when Vds<0. And to explain the current flow which I observed when Vds<0 is nothing but the mirrored output characteristics as source and drain and interchangeable.

I hope anyone looking at the query in future finds this explanation helpful.

Thanks again