Mosfet conduction vs Drain voltage

  • #1
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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
CWatters
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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
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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
CWatters
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Normally the body diode will start conducting first. I don't think the drain can go low enough for the get to turn on.
 
  • #5
rbelli1
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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
 
  • #6
Baluncore
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I have attached a ckt to make it clear.
Without a clearly marked ground or reference voltage it is cannot be clear.
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
 
  • #8
Borek
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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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