Re: Disabling the freewheling diode

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

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

Hi,
so the question is, why is the series diode which is used to disable the
internal MOSFET diode rated at 3A, while the MOSFET it's self is rated at
something like 20A?

As I said in a previous email:

"To disable the back-emf or free-wheeling diode that is built into
the power MOSFET, a high speed schottky diode is connected in series with
the MOSFET, either on its source side, or more usually on the drain side.
This device, usually a 1N5822 or similar, allows current to flow through the
MOSFET but will stop any back-emf from reaching the transistors internal
diode. Then external free-wheel diodes (MUR1660CT or similar) are connected
across this network, which are better rated for the job than the MOSFET
internal diode. This is all clear, no problem, however there is this
confusion relating to current ratings. The MOSFET can have a current rating
of 20A, and the two diodes that make-up the external free-wheel diodes can
together handle 32A. Now, the diode used to block the MOSFET internal diode,
usually a 1N5822 or similar is only rated at 3A, yet the MOSFET is set to
handle say 20A. If the MOSFET was to switch say 10A, then the 1N5822
internal MOSFET diode blocker would theoretically be destroyed by the high
current; why doesn't it?

Now I have built a SSTC and it works, and I have followed others design
without question, and the internal diode blocking shottky diode (similar to
a 1N5822 and so similarly current rated) has not died - why?

However, my SSTC has not drawn any more current than 2.5A from the mains
supply, that is 1.25A per MOSFET average (half-bridge switching),
discounting the rms of half wave rectification. I wondered if I have simply
used to big a primary coil, and other researchers systems draw more current.
But, if my SSTC is typical, then the rating of these internal diode blocking
shottkys is O.K. In which case the MOSFETs are over rated.

Well these are my thoughts and questions, perhaps someone more experienced
can explain what is going on."


Cheers,

Gavin


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, September 01, 2003 11:40 PM
Subject: Re: Disabling the freewheling diode


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