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analog_match
Joined: 04 Nov 2008
Posts: 41
Location: USA
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 16 Oct 2009 15:17 Question about sync FET conduction in synchronous buck |
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| In a synchronous buck when the sync FET conducts, the current will have to flow from gnd up to drain because the inductor current will flow in that direction. For this to happen the drain will have to be more negative than the source. Is this the correct explanation. I am usually used to seeing NMOS drain at higher potential than src and current flowing from drain to src.
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FvM
Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany
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16 Oct 2009 16:28 Question about sync FET conduction in synchronous buck |
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Simply assume an almost constant Rdson for |Vds| << Vgs,threshold. The Vds polarity doesn't matter in this region.
For Vds < -0,5V, the substrate diode starts to conduct, of course.
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Mr.Cool
Joined: 20 Jun 2001
Posts: 641
Helped: 24
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17 Oct 2009 15:37 Question about sync FET conduction in synchronous buck |
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you got it.
the trick is to know WHEN body diode starts to connect so that you can quickly turn ON your mosfet..
Mr.Cool
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analog_match
Joined: 04 Nov 2008
Posts: 41
Location: USA
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21 Oct 2009 10:05 Question about sync FET conduction in synchronous buck |
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| I guess what y'all are saying is since the FET is in the linear region when on it behaves as a resistor and current flow can be in any direction. Thanks for help on this.
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eem2am
Joined: 22 Jun 2008
Posts: 753
Helped: 13
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27 Oct 2009 12:24 Question about sync FET conduction in synchronous buck |
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No it is not in linear region.
The excellent sync buck is not that great because you have dead time for the fets...so when the series fet goes on, you will get a momentary large spike of current going through the shunt fet. -giving big switching loss for the shunt fet
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