Re: Disabling the freewheling diode, mk.2

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

Original poster: Finn Hammer <f-h-at-c.dk> 



  "Mccauley, Daniel H"
Wrote:
 >
 > The reason for this is that the freewheeling current is much less than
 > the current that the MOSFET conducts when it
 > is ON.
 >
"Jan Wagner"
Wrote:

Because luckily it conducts only for a short period. If the SSTC is
in-tune
and the drive signals have ~50% duty, the diode ideally
conducts no current at all.

So I ask:

Are we talking about the same diode?
Seems to me, that you are both talking about the external freewheling
diode, the hyperfast one, which is rated at 20A.

My question is still, what about the schottkey diode which is conducting
as long as the MOSFET, but is only rated at 3 amps?

I ask again,  because

"claude masetto"
Wrote:

  Surely this schottkey diode in series with the mosfet conducts at the
same
time that the mosfet does so why isn't it rated for the same current. Am
I
missing something.
Claude

 >  > All,
 >  >
 >  > In a SSTC H-bridge, the internal freewheeling diode of the MOSFET is
 >  > disabled by a clever arrangement of external diodes.
 >  > A schottkey diode above the drain keeps the internal diode
 >  > from getting
 >  > forward biased, so that an ultrafast recovery external diode
 >  > can provide
 >  > the path of any reverse EMF from the load.
 >  > Any current passing trough the MOSFET must pass trough the schottkey
 >  > diode.
 >  >
 >  > In a typical design, I see MOSFETS capable of handling 20A continous,
 >  > with a 3 A schottkey diode on top.
 >  >
 >  > Why is the schottkey diode not dimentioned for a similar 20A?
 >  >
 >  > Cheers, Finn Hammer
 >  >
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