Re: Disabling the freewheling diode

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

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

Hi,
I made some mistakes here!


----- Original Message -----
From: "Gavin Dingley" <gdingley-at-ukf-dot-net>
To: "Tesla list" <tesla-at-pupman-dot-com>
Sent: Tuesday, September 02, 2003 6:57 PM
Subject: Re: Disabling the freewheling diode


 > 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.

Actually the current passes through either MOSFET, halving the avarge
current for each transistor in a half-bridge, while in a full-bridge the
current passes through either of two opposite MOSFETs, in both cases the
avarage current passing through the transistors is half of the current
passing through the primary coil.

 >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.

Actually it is 1A in both the half-bridge and the full-bridge arrangements.

 >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.

My SSTC is half-bridge, so the peak voltage is 150V. Also I am using
half-wave rectification, so in the end the power is actually 150W maximum
(so far), but usually is half this as the primary draws about 1A, thus
giving a usual power of 75W.

 >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
 >

Next time I will make sure I am more awake when I write emails!

Cheers,

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
 > >  >  >  >  >
 > >  >  >  >  >
 > >  >  >  >  >
 > >  >  >  >
 > >  >  >  >
 > >  >  >  >
 > >  >  >  >
 >