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MOSFET current flow direction...

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kit_714

kit_714

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mosfet reverse current

Hi All,

i just started to study some power electronics courses and face a question about the current direction of N-MOS...

Is it possible that the current direction of N-MOS is from Source to Drain (but not through the body diode)?

I have checked the datasheets of some N-MOS power switches, they didn't mention about this...

all the information and graphs just state the current flow from Drain to Source...

Can anyone help me??

Thanks in advance

Kit
 

mosfet current flow

Yes, this is possible since the channel is on! As long Vgs > Vgs threshold the channel is on and the current can flow in both directions.
 

current flow in mosfet

oh Thanks!

2 more questions~

1. Is there any limitation about the reverse current (source to drain)?

2. In high reverse current condition, would there be the case that current would pass through both the channel and the body diode?

Thanks!

Kit
 

mosfet current flow direction

No limitation.......Max current flow d>s = max current flow s>d

BUT.

The body diode is usually very low quality..it has slow reverse recovery........it can take ages to turn off so usually u dont risk ever turning it on.
Usually, if current could flow in the body diode, you put a fast, quality diode in parallel with the body diode so that the DREADED body diode hopefully doesn't get turned on.

and sometimes u try and stop the DREADFUL body diode from ever turning on by using a series diode in the drain connxion.....(cathode to drain)
 
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    hamedo

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current flow direction

Although it's a only side remark, I want to contradict this point:
The body diode is usually very low quality..it has slow reverse recovery........it can take ages to turn off so usually u dont risk ever turning it on.
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The statement is true for MOSFET parts from the flea market or the museum rather than modern devices used in recent power electronic designs. Driving the body diode into conduction causes higher losses mainly due to Si-diode specific forward voltage, so active controlled backward conduction is preferable of course.

In a 4-quadrant H-bridge converter, the MOSFETs are generally loaded with equal current magnitudes for both directions. In designs, that are utilizing the maximum MOSFET available current (rather unusual in normal operation), I never checked a particular reverse current rating.
 

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