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Sci.Electronics.Basics -> Does current flow when Vds is negative in power mosfets?
There are 4 messages in this thread.
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Author: kyujinDate: 17:09 04-10-06
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I'm asking this question, because of the diode in the MOSFET models.
I know these diodes are to prevent spikes, but looking at it, makes me
wonder.
If Vsource > Vdrain by say 20Volts, can a current flow through the
MOSFET even if the gate is not turned on?
This would be a reverse current I guess.
And if it does, how much?
Normally there is a limit to Vds. but I'm wondering about the negativ
value of Vds.
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Author: John PopelishDate: 18:16 04-10-06
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kyujin wrote:
> I'm asking this question, because of the diode in the MOSFET models.
> I know these diodes are to prevent spikes, but looking at it, makes me
> wonder.
> If Vsource > Vdrain by say 20Volts, can a current flow through the
> MOSFET even if the gate is not turned on?
> This would be a reverse current I guess.
> And if it does, how much?
> Normally there is a limit to Vds. but I'm wondering about the negativ
> value of Vds.
>
The drain to source diode in the mosfet is a parasitic diode in the
structure that is normally reverse biased during normal use, when the
drain is more positive than the source. If the drain is driven more
negative that the source, the diode becomes forward biased and will
conduct as any silicon diode. If your circuit drives the fet into
this sort of reverse conduction, make sure that the reverse recovery
time of the diode does not cause you problems.
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Author: kyujinDate: 18:21 04-10-06
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What exactly is a reverse recovery time?
Can this diode conduct large current like 6 Amps?
Small current conduction is okay for me, like in ther order of mAs.
And my switching frequency is in the order of 20Hz, so it's not a fast
switching at all.
John Popelish wrote:
> kyujin wrote:
> > I'm asking this question, because of the diode in the MOSFET models.
> > I know these diodes are to prevent spikes, but looking at it, makes me
> > wonder.
> > If Vsource > Vdrain by say 20Volts, can a current flow through the
> > MOSFET even if the gate is not turned on?
> > This would be a reverse current I guess.
> > And if it does, how much?
> > Normally there is a limit to Vds. but I'm wondering about the negativ
> > value of Vds.
> >
> The drain to source diode in the mosfet is a parasitic diode in the
> structure that is normally reverse biased during normal use, when the
> drain is more positive than the source. If the drain is driven more
> negative that the source, the diode becomes forward biased and will
> conduct as any silicon diode. If your circuit drives the fet into
> this sort of reverse conduction, make sure that the reverse recovery
> time of the diode does not cause you problems.
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Author: John PopelishDate: 19:19 04-10-06
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kyujin wrote:
> What exactly is a reverse recovery time?
It is the time it takes for a diode that has been suddenly reverse
biased after it has been forward biased, to recover its non conducting
state. In brief, it is the time the diode will conduct, even though
it is reverse biased. For some diodes, this recovery can end in a
viciously fast snap into non conduction, that can generate large and
sometimes problematical voltages across small inductances in the
current path, like PCB traces and device leads.
> Can this diode conduct large current like 6 Amps?
The diode has essentially the same area as the mosfet, so it can
conduct about as much current as the fet is rated for, assuming the
heat generated by the conduction is does not over heat the die.
> Small current conduction is okay for me, like in ther order of mAs.
> And my switching frequency is in the order of 20Hz, so it's not a fast
> switching at all.
How fast is the process that takes place between a conducting and a
non conducting state? The duration of the essentially steady states
do not matter as much as how fast the transition between states is.
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