Mosfet conduction vs Drain voltage

In summary, the conversation discusses the operation of an N-ch Mosfet with varying drain and source voltages. It is mentioned that FETs can be used "upside down" with drain and source swapped, but there may be differences in characteristics due to protection diodes. The question is raised about the turn on sequence when the drain voltage changes, and it is confirmed that the body diode will conduct first. The drain voltage is not as important as the gate-source voltage in controlling conductance. The conversation also mentions the use of a level shifter to connect a 3.3V sensor to a 5V Arduino.
  • #1
likephysics
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When does an N-ch Mosfet conduct when drain voltage varies from 0V to Vdd(like 5V).
In case of Source voltage changing between 0V to 5V, all you do is check if VGS > Vth, with Vdrain fixed at 5V.
But in case of Drain Voltage changing between 0V to 5V, how does it work?
 
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  • #2
Not sure I understand the question but...

FETs are semi symmetrical so in theory they can be used "upside down" with drain and source swapped over. However some have protection diodes in them which may prevent this. The characteristics when used upside down will be slightly different as drain and source aren't usually identical.
 
  • #3
I have attached a ckt to make it clear.
Assume gate threshold is 1v.
mosfet_level_converter.jpg

In the fig, if Source (Low side) is 0V, then Vgs > Vth, FET is ON.
Let's say Source is 3.3V, Gate is 3.3V and Drain goes from 3.3V to 0V.
At what drain Voltage does the FET turn ON?
What is the turn on sequence. Does the body diode conduct first and drop the source voltage to Vdrain+diode drop?
 

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  • #4
Normally the body diode will start conducting first. I don't think the drain can go low enough for the get to turn on.
 
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  • #5
CWatters said:
I don't think the drain can go low enough for the get to turn on.

With 2n7000 almost certainly not. For some of the very low threshold devices you can find these days you may be able to turn it on to some extent.

BoB
 
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  • #6
likephysics said:
I have attached a ckt to make it clear.
Without a clearly marked ground or reference voltage it is cannot be clear.
likephysics said:
At what drain Voltage does the FET turn ON?
Drain voltage is not very important. It is the voltage between gate and source that controls conductance.
With reversed drain-source voltage, the FET will still conduct given sufficient gate-source voltage. But there are three conduction paths.
1. Through the FET channel. This will dominate and can be used to reduce power disipation by lowering diode voltage during reverse conduction.
2. If there is an integrated fast parallel Schottky diode, that will conduct at a low diode voltage, maybe -0.5V
3. Through the slow inherent substrate diode when reverse biassed, maybe -0.9V
 
  • #7
  • #8
rbelli1 said:
It appears to be one of these:

And it actually works (not that it should be surprising). I have a very similar one here that I just used to connect BME280 sensor (3.3 V) to Arduino (5V) and it is fast enough to allow I2C to pass through.

Atmel processors are capable of running at 3.3V, so once I decide to make a final version I will perhaps consider getting rid of a level shifter - but atm I am also using LCD display that requires 5V, so it won't be a simple and obvious change.
 
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1. What is a MOSFET and how does it conduct?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of transistor used in electronic devices. It conducts by controlling the flow of current between the source and drain terminals through a voltage applied to the gate terminal.

2. How does the drain voltage affect MOSFET conduction?

The drain voltage directly affects the channel between the source and drain terminals of a MOSFET. As the drain voltage increases, the channel widens, allowing more current to flow through the transistor. This means that a higher drain voltage results in a higher conduction of the MOSFET.

3. What is the relationship between MOSFET conduction and drain voltage?

The relationship between MOSFET conduction and drain voltage is directly proportional. As the drain voltage increases, the conduction of the MOSFET also increases. This is because the drain voltage controls the width of the channel, which determines the amount of current that can flow through the transistor.

4. How does the gate voltage affect MOSFET conduction?

The gate voltage is used to control the conduction of a MOSFET. When a positive voltage is applied to the gate, it creates an electric field that attracts electrons to the surface of the transistor, allowing current to flow between the source and drain terminals. The higher the gate voltage, the wider the channel and the higher the conduction of the MOSFET.

5. What is the difference between MOSFET conduction and drain voltage?

MOSFET conduction refers to the flow of current through the transistor, while drain voltage refers to the voltage applied to the drain terminal of the MOSFET. The two are directly related, as the drain voltage directly affects the conduction of the MOSFET by controlling the width of the channel between the source and drain terminals.

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