Re: Disabling the freewheling diode

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gavin Dingley" <gdingley-at-ukf-dot-net> 

Hi Claude,
I have been trying to get this confusion sorted out too. I think the answer
is simple, it's not that the series disabling diode is under rated, it's
just that the MOSFET is over rated. The avarge current through the primary
is divided between the two MOSFETs in a half-bridge, and by four MOSFETs in
a full-bridge circuit. Now if only 2A passes through the primary, an average
current of 1A will pass through the series diode in a half-bridge
arrangement, and only 500mA in the case of a full-bridge. 2A (max) is the
current passing through the primary of my SSTC, and this gives a power of
around 600W (240V rms line voltage in the UK), which sounds O.K. So the
rating for this series diode is fine. The reason for a 20A MOSFET is due to
the fact it is a complex device getting allot of abuse, and so it is over
rated. Well this is how I figure it, perhaps I am wrong?

Regards,

Gavin


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, September 02, 2003 3:07 PM
Subject: Re: Disabling the freewheling diode


 > Original poster: "claude masetto" <claudmas-at-optusnet-dot-com.au>
 >
 > We still do not seem to be getting the answer as to why the series diode
is
 > not rated for the same current as the mosfet
 > Claude
 > ----- Original Message -----
 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > To: <tesla-at-pupman-dot-com>
 > Sent: Tuesday, September 02, 2003 11:09 AM
 > Subject: Re: Disabling the freewheling diode
 >
 >
 >  > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
 >  >
 >  > On 1 Sep 2003, at 16:40, Tesla list wrote:
 >  >
 >  >  > Original poster: dhmccauley-at-spacecatlighting-dot-com
 >  >  >
 >  >  >
 >  >  > There are two diodes both of which typically are Schottky diodes.
 > There is
 >  >  > the schottky ultrafast diode in parallel with the FET which is used
to
 >  >  > bypass the intrinsic diode of the FET, and there is also the series
 > schottky
 >  >  > diode of which you speak of in series with the drain of the FET
which
 > is
 >  >  > used to help keep the internal FET diode turned off during reverse
 > current
 >  >  > (freewheeling)
 >  >  >
 >  >  > Dan
 >  >
 >  > You will find that the parallel diode is not a Schottky and could
 >  > never be. Schottkys are not built with the necessary voltage ratings.
 >  > It appears then that the diode you are talking about with lower
 >  > current ratings is in fact the ultra-fast recovery parallel diode.
 >  > The one I am speaking about and which I think most recognize as being
 >  > a Schottky is the diode in series with the drain.
 >  >
 >  > Malcolm
 >  >
 >  >  >
 >  >  >  > I confess to being absolutely lost. Series Schottky in parallel??
Is
 >  >  >  > not the Schottky diode in series with the drain? What other ON-
 >  >  >  > conduction path is there?
 >  >  >  >
 >  >  >  > Malcolm
 >  >  >  >
 >  >  >  >    The Rds of the FET is much lower (and less voltage drop)
 >  >  >  >  > then the parallel Schottky
 >  >  >  >  > diode, therefore, most of the current goes through the FET.
On
 > the
 >  >  >  >  > otherhand, for reverse freewheeling current, the Schottky
diode
 > will
 >  >  >  >  > conductor first and take the bulk of
 >  >  >  >  > any reverse freewheeling current in the circuit.
 >  >  >  >  >
 >  >  >  >  > The Captain
 >  >  >  >  >
 >  >  >  >  >
 >  >  >  >  >
 >  >  >  >
 >  >  >  >
 >  >  >  >
 >  >  >  >
 >  >  >  >
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 >  >  >
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 >