Re: OLTC Update - Primary circuit resistance.

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Terry,

On 19 Aug 2002, at 12:21, Tesla list wrote:

> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> 
> Hi All,
> 
> Eddie mentions that the IGBTs may be able to take higher currents than
> rated.  Darren disagreed since "spot heating" on the die may be a problem.
> Hard to say...  At these points the data sheets and the manufacturers say
> "your on your own.  Good luck! :o))  The "cheap" IGBTs don't use copper on
> silicon or any of that fancy stuff.  Ones that do, appear very similar but
> are rated at 280 amps instead of 204.  I would guess we are nearing the
> point that the conductors in the die will simply fuse.  Spot heating may
> not be too bad since the die are running very cold.  Like 30 C.  So we have
> a lot of room for heating!  A lot depends on how well the IGBT and package
> are made.  IMHO International Rectifier is the best in the business there.
> We really just have to test them and find out...  It will be interesting to
> see how much margin is really there.
> 
> IR usually over rates CE voltage by about 100 volts and that is a known
> hard limit that we can't go over especially with our currents.  Far far
> beyond any avalanche current rating!
> 
> 
> Marco mentions that I am running the gates at pretty high voltage (18V).
> IGBTs like these usually "break" at about 50 volts on the gate.  No problem
> nearing the 20 volt spec there.  The 2 9V batteries will quickly go down to
> about 16 volts.  That is only 6 volts over the full 200 amps on spec and I
> am a little worried about noise getting on the gates too so more margin is
> nice.  Shoot through from the Ccg capacitance is a concern and that is why
> I have 100 ohm gate resistors to slow the turn on way down compared to what
> it "could" be.  
> 
> 
> Bert mentions "As previously mentioned, a possible downside is that, through
> transformer action, some of the secondary's energy will be extracted
> during secondary ringdown, resulting in partial recharging of the tank
> caps through the internal HexFRED diodes."
> 
> The "theory" is:  In order for the primary to take energy back from the
> secondary.  The primary has to be "tuned".  When the IGBT opens, it
> destroys the primary as a tuned circuit.  Thus the primary will not be able
> to take back the energy.   We will really just have to see what happens
> There.  there is a "plan B" of two IGBTs in a CE-EC configuration.  But
> we'll see if we even have to worry about it.

The primary remains coupled to the secondary whether the IGBTs and 
diodes are conducting or not and will generate some voltage as an 
open-circuit winding. In my MOSFET expts I had a look at exactly 
these effects and found that quenching when primary currents were 
zero was the best time to avoid severe spiking. Interrupting the 
primary with current flowing was the worst and generated spikes far 
higher than the supply voltages as the primary was being interrupted 
at a time when it was generating magnetic flux which was mostly 
linked to itself. 

Regards,
Malcolm

> Cheers,
> 
> 	Terry
> 
> 
> 
> 
>